Can someone explain the use of a heater INSIDE a mechanical (bimetal strip) room thermostat? (It usually just seems to be a resistor placed near the bi-metallic strip).

All it seems to do for me is make the thermostat very inaccurate. If the temperature is only a bit lower outside than inside, the temperature inside is what I set the stat to. As the temperature outside drops further, the inside temperature strays further and further from what I set it to (in the colder direction). This is presumably because whenever the heating is on, the stat thinks it's warmer than it really is, because of the internal heater. If I disconnect this internal heater, the stats functions properly.

I've replaced the offending pile of crap with a digital one anyway, but I wanted to know why they design them like this.

--
http://petersparrots.com
http://petersphotos.com

In a recent survey 40% found they didn't have time to answer the
question, 25% hung up the phone when the question was being
asked, 20% couldn't speak English, and 15% gave answers that
weren't asked.

On Sun, 04 Mar 2012 14:42:26 +0000, Lieutenant Scott wrote:

Can someone explain the use of a heater INSIDE a mechanical (bimetal
strip) room thermostat? (It usually just seems to be a resistor placed
near the bi-metallic strip).

All it seems to do for me is make the thermostat very inaccurate.

[ ... ]


I've replaced the offending pile of crap with a digital one anyway, but
I wanted to know why they design them like this.

It's an anticipator, and should be set so as to introduce exactly the
right amount of heat inside the thermostat casing so that it doesn't have
to wait for the heat to percolate from a radiator across the other side
of the room, thereby tailoring the response so that it stabilises sooner.

It's a bit like trying to throw a Yo-Yo away whilst still holding onto
the string. And yes it's a crap plumbers idea.

DerekG

Lieutenant Scott wrote:

Can someone explain the use of a heater INSIDE a mechanical (bimetal
strip) room thermostat? (It usually just seems to be a resistor placed
near the bi-metallic strip).


It is to make the thermostat/heating more efficient by allowing the stat
to 'predict' a rise/fall in temperature.
If the stat is in good condition, it does work better than a plain
on/off stat, where the temperature can fall/rise a lot more than one
supplied with a neutral.
It wil also save you money by turning off/on the boiler more
efficiently.

Alan.

--
To reply by e-mail, change the ' + ' to 'plus'.

DerekG wrote:

It's a bit like trying to throw a Yo-Yo away whilst still holding onto
the string. And yes it's a crap plumbers idea.


It isnt. I've been on a training course that shows the graphs of heat
and gas used. The heated stats use less gas, and keep the room at a
steadier temperature than the standard on/off stats.

Alan.

--
To reply by e-mail, change the ' + ' to 'plus'.

On Sun, 04 Mar 2012 15:21:19 -0000, DerekG wrote:

On Sun, 04 Mar 2012 14:42:26 +0000, Lieutenant Scott wrote:

Can someone explain the use of a heater INSIDE a mechanical (bimetal
strip) room thermostat? (It usually just seems to be a resistor placed
near the bi-metallic strip).

All it seems to do for me is make the thermostat very inaccurate.

[ ... ]


I've replaced the offending pile of crap with a digital one anyway, but
I wanted to know why they design them like this.

It's an anticipator, and should be set so as to introduce exactly the
right amount of heat inside the thermostat casing so that it doesn't have
to wait for the heat to percolate from a radiator across the other side
of the room, thereby tailoring the response so that it stabilises sooner.

It's a bit like trying to throw a Yo-Yo away whilst still holding onto
the string. And yes it's a crap plumbers idea.

DerekG

I've found that the convection of the room air from the heat source (a radiator) heats the room very evenly anyway. The stat switches off when the room reaches the required temperature without that bloody heating thingy!

As you said, it just doesn't work as designed - it's guessing! All that happens is when the heating is needed to be on more, it thinks it's going to be warmer than it is, so it doesn't switch it on enough.

--
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http://petersphotos.com

What does a Polish woman do after she sucks a cock?
Spits out the feathers.

On Sun, 04 Mar 2012 15:24:52 -0000, wrote:

Lieutenant Scott wrote:

Can someone explain the use of a heater INSIDE a mechanical (bimetal
strip) room thermostat? (It usually just seems to be a resistor placed
near the bi-metallic strip).


It is to make the thermostat/heating more efficient by allowing the stat
to 'predict' a rise/fall in temperature.
If the stat is in good condition, it does work better than a plain
on/off stat, where the temperature can fall/rise a lot more than one
supplied with a neutral.
It wil also save you money by turning off/on the boiler more
efficiently.

Seems to work a LOT better without the "heater" connected. It simply turns on a degree below what I set it to, and off at the temperature I set it to.

--
http://petersparrots.com
http://petersphotos.com

Why is the front of an aeroplane called a cockpit?
If you have female pilots do you call it a pussypit?

In article op.wanhs0x7ytk5n5@i7-940,
"Lieutenant Scott" writes:
Can someone explain the use of a heater INSIDE a mechanical (bimetal strip) room thermostat? (It usually just seems to be a resistor placed near the bi-metallic strip).

The bimetal strip will have a fair measure of hysteresis, i.e. it might
switch on your heating at 18C, but not switch it off until 22C, so the
room will swing between these two.

The accelerator heater works to reduce this hysteresis. In the example
above, if the resistor is chosen to heat the bimetal strip by 3C, it
will reduce the hysteresis of the thermostat from 4C down to 1C.
Additionally, it adds a bit of a time lag, so that even if the
hysteresis is reduced to nothing (which might lead to rapid switching),
the lag extends both the on and off times a bit to reduce rapid switching,
which is not a good idea for many types of heating, or the life of the
contacts if it's switching a large load.

All it seems to do for me is make the thermostat very inaccurate. If the temperature is only a bit lower outside than inside, the temperature inside is what I set the stat to. As the temperature outside drops further, the inside temperature strays further and further from what I set it to (in the colder direction). This is presumably because whenever the heating is on, the stat thinks it's warmer than it really is, because of the internal heater. If I disconnect this internal heater, the stats functions properly.

I have found cases where the supplyu and load are connected the wrong
way around, which can really screw up thermostats with accelerator heaters.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

On Sun, 04 Mar 2012 15:40:18 -0000, Andrew Gabriel wrote:

In article op.wanhs0x7ytk5n5@i7-940,
"Lieutenant Scott" writes:
Can someone explain the use of a heater INSIDE a mechanical (bimetal strip) room thermostat? (It usually just seems to be a resistor placed near the bi-metallic strip).

The bimetal strip will have a fair measure of hysteresis, i.e. it might
switch on your heating at 18C, but not switch it off until 22C, so the
room will swing between these two.

Mine seems to be about 2C. It's getting replaced with a digital one anyway.

The accelerator heater works to reduce this hysteresis. In the example
above, if the resistor is chosen to heat the bimetal strip by 3C, it
will reduce the hysteresis of the thermostat from 4C down to 1C.
Additionally, it adds a bit of a time lag, so that even if the
hysteresis is reduced to nothing (which might lead to rapid switching),
the lag extends both the on and off times a bit to reduce rapid switching,
which is not a good idea for many types of heating, or the life of the
contacts if it's switching a large load.

I've found that bi-metal strips simply can't switch some loads, like my central heating. There must be a lot of inductance in the load or something - it's just a motorised valve. As it switches (I think it's more at the "off" switching) an arc forms on the gap in the bi-metal strip, which obviously heats it and causes it to stay on. This makes a worrying noise and interferes with radio and TV reception.

All it seems to do for me is make the thermostat very inaccurate. If the temperature is only a bit lower outside than inside, the temperature inside is what I set the stat to. As the temperature outside drops further, the inside temperature strays further and further from what I set it to (in the colder direction). This is presumably because whenever the heating is on, the stat thinks it's warmer than it really is, because of the internal heater. If I disconnect this internal heater, the stats functions properly.

I have found cases where the supply and load are connected the wrong
way around, which can really screw up thermostats with accelerator heaters.

Presumably you mean swapped supply live and switched live - which would make the internal heater stay on continuously. I would think this would make it like a stat without a heater, but with the temperature shunted off by a few degrees.

--
http://petersparrots.com
http://petersphotos.com

Never have I seen a word as accurate as politics.
Poly meaning many, and tic being a blood-sucking thing.

On Sun, 04 Mar 2012 14:42:26 -0000, "Lieutenant Scott"
wrote:

Can someone explain the use of a heater INSIDE a mechanical (bimetal strip) room thermostat? (It usually just seems to be a resistor placed near the bi-metallic strip).

All it seems to do for me is make the thermostat very inaccurate. If the temperature is only a bit lower outside than inside, the temperature inside is what I set the stat to. As the temperature outside drops further, the inside temperature strays further and further from what I set it to (in the colder direction). This is presumably because whenever the heating is on, the stat thinks it's warmer than it really is, because of the internal heater. If I disconnect this internal heater, the stats functions properly.

I've replaced the offending pile of crap with a digital one anyway, but I wanted to know why they design them like this.


It reduces the hysteresis of the system by forcing it to shut off the
heat earlier than it would do naturally.

Hysteresis is a bit like mechanical backlash when a gear chain goes
into reverse.


--
Graham.
%Profound_observation%

DerekG formulated on Sunday :
On Sun, 04 Mar 2012 14:42:26 +0000, Lieutenant Scott wrote:

Can someone explain the use of a heater INSIDE a mechanical (bimetal
strip) room thermostat? (It usually just seems to be a resistor placed
near the bi-metallic strip).

All it seems to do for me is make the thermostat very inaccurate.

[ ... ]


I've replaced the offending pile of crap with a digital one anyway, but
I wanted to know why they design them like this.

It's an anticipator, and should be set so as to introduce exactly the
right amount of heat inside the thermostat casing so that it doesn't have
to wait for the heat to percolate from a radiator across the other side
of the room, thereby tailoring the response so that it stabilises sooner.

It's a bit like trying to throw a Yo-Yo away whilst still holding onto
the string. And yes it's a crap plumbers idea.

DerekG

A plumber could not come up up with such a clever idea :')

It was a widely used idea to prevent over reaction of the heating
system, due to the wide hysteresis of a mechanical thermostat in the
days before modern electronic stats. The stat switches on, which then
powers a resistor, which warms the bi-metal sensing element, so the
stat gets both the warmth of the heating system and the warmth from the
resistor.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk

On Sun, 04 Mar 2012 16:38:14 -0000, Graham. wrote:

On Sun, 04 Mar 2012 14:42:26 -0000, "Lieutenant Scott"
wrote:

Can someone explain the use of a heater INSIDE a mechanical (bimetal strip) room thermostat? (It usually just seems to be a resistor placed near the bi-metallic strip).

All it seems to do for me is make the thermostat very inaccurate. If the temperature is only a bit lower outside than inside, the temperature inside is what I set the stat to. As the temperature outside drops further, the inside temperature strays further and further from what I set it to (in the colder direction). This is presumably because whenever the heating is on, the stat thinks it's warmer than it really is, because of the internal heater. If I disconnect this internal heater, the stats functions properly.

I've replaced the offending pile of crap with a digital one anyway, but I wanted to know why they design them like this.


It reduces the hysteresis of the system by forcing it to shut off the
heat earlier than it would do naturally.

Hysteresis is a bit like mechanical backlash when a gear chain goes
into reverse.

I know about hysteresis in electronics, but you've lost me with the gears :-)

--
http://petersparrots.com
http://petersphotos.com

What lives in the sea, and goes dah di dah dit, dah dah dit dah?
A morse cod.

On Sun, 04 Mar 2012 16:55:27 -0000, Harry Bloomfield wrote:

DerekG formulated on Sunday :
On Sun, 04 Mar 2012 14:42:26 +0000, Lieutenant Scott wrote:

Can someone explain the use of a heater INSIDE a mechanical (bimetal
strip) room thermostat? (It usually just seems to be a resistor placed
near the bi-metallic strip).

All it seems to do for me is make the thermostat very inaccurate.

[ ... ]


I've replaced the offending pile of crap with a digital one anyway, but
I wanted to know why they design them like this.

It's an anticipator, and should be set so as to introduce exactly the
right amount of heat inside the thermostat casing so that it doesn't have
to wait for the heat to percolate from a radiator across the other side
of the room, thereby tailoring the response so that it stabilises sooner.

It's a bit like trying to throw a Yo-Yo away whilst still holding onto
the string. And yes it's a crap plumbers idea.

DerekG

A plumber could not come up up with such a clever idea :')

It was a widely used idea to prevent over reaction of the heating
system, due to the wide hysteresis of a mechanical thermostat in the
days before modern electronic stats. The stat switches on, which then
powers a resistor, which warms the bi-metal sensing element, so the
stat gets both the warmth of the heating system and the warmth from the
resistor.

But it's wildly inaccurate. And there seems to be no adjustment for them.

--
http://petersparrots.com
http://petersphotos.com

Some "chinese english" instructions (with software for a camera card). This is the contents of a file named "English installs the elucidation.doc", quoted in its entirety:

The high regard's customer:
How are you!
Thank youing can use the pico product of my company, and please press below the operation order install, and thank!
A,The software installs in proper order
1, install the good WIN2000 system;
2, open the software light the dish;
3, double click the SETUP.EXE
4, the model number of the choice gearing
a)PICO2000_104( PALApplication) this model number can at most support 4 roads see the frequency signal the importation
b)PICO2000_208( PALApplication) this model number can at most support 8 roads see the frequency signal the importation
c)PICO2000_416( PALApplication) this model number can at most support 16 roads see the frequency signal the importation
5, after finishing installing, three documents that light patch in the dish the catalogue descend: The msdxm.ocx, odbc32.dll, odbcint.dll beat arrives the c:\ windows\ system inside.
6, square version of usage hero , please double click the light the English Pack in the dish the document.
7, the copy resemble the regulating of appearance tone must install the VideoSetup software to proceed to regulate, install the procedure under the light dish root the catalogue VideoSetup the blank page clip setup.exe.
Plank card gearing
1.Insert the plank card arrive the main plank PCI the slot;
2.Start the calculator, and the auto install the plank card the drive to move the procedure( position:Light dish driver catalogue bottom)
Change the compression method
Beginning the ? circulate the ? the importation the " REGEDIT", and make sure the ?? enter the registration watch the editor, and open the HKEY ? CURRENT ? the USER\ software\ univision Canada Linited\ the pico2000 double click the " CODEC" can is worth this key to change to" MP42" or" IV50"
Note:
MP42 the MPEG4 compress the way (suggestion adoption MPEG4 compress way, should compress the way the compression the rate to is high)
IV50 the INDEO compress the way

A.Lee wrote:

Lieutenant Scott wrote:

Can someone explain the use of a heater INSIDE a mechanical (bimetal
strip) room thermostat? (It usually just seems to be a resistor placed
near the bi-metallic strip).


It is to make the thermostat/heating more efficient by allowing the stat
to 'predict' a rise/fall in temperature.
If the stat is in good condition, it does work better than a plain
on/off stat, where the temperature can fall/rise a lot more than one
supplied with a neutral.
It wil also save you money by turning off/on the boiler more
efficiently.

Alan.


Or another way to look at it is it narrows the thermostat's hysteresis, ie
makes the off-on temperature closer to the on-off point.



--
Tim Watts

Lieutenant Scott wrote:
On Sun, 04 Mar 2012 16:55:27 -0000, Harry Bloomfield
wrote:

A resistor in the room thermostat housing

But it's wildly inaccurate. And there seems to be no adjustment for them.

It's a very cheap way to improve the responsiveness of the system. I
once disconnected one, and found out that re-connecting it it made a
noticeable improvement to my comfort in the room with no noticeable
increase in heating costs.

It was a single small room with a blown air calor gas powered heating
system, but the same principle applies to a central heating system as
used in my house.

--
Tciao for Now!

John.

On Sun, 04 Mar 2012 17:08:02 -0000, Tim Watts wrote:

A.Lee wrote:

Lieutenant Scott wrote:

Can someone explain the use of a heater INSIDE a mechanical (bimetal
strip) room thermostat? (It usually just seems to be a resistor placed
near the bi-metallic strip).


It is to make the thermostat/heating more efficient by allowing the stat
to 'predict' a rise/fall in temperature.
If the stat is in good condition, it does work better than a plain
on/off stat, where the temperature can fall/rise a lot more than one
supplied with a neutral.
It wil also save you money by turning off/on the boiler more
efficiently.

Alan.


Or another way to look at it is it narrows the thermostat's hysteresis, ie
makes the off-on temperature closer to the on-off point.

The trouble is it makes the on-off point drift depending on how much the heating is used.

--
http://petersparrots.com
http://petersphotos.com

"Americans will always do the right thing when they have exhausted all other alternatives." -- Winston Churchill

On Sun, 04 Mar 2012 17:42:30 -0000, John Williamson wrote:

Lieutenant Scott wrote:
On Sun, 04 Mar 2012 16:55:27 -0000, Harry Bloomfield
wrote:

A resistor in the room thermostat housing

But it's wildly inaccurate. And there seems to be no adjustment for them.

It's a very cheap way to improve the responsiveness of the system. I
once disconnected one, and found out that re-connecting it it made a
noticeable improvement to my comfort in the room with no noticeable
increase in heating costs.

It was a single small room with a blown air calor gas powered heating
system, but the same principle applies to a central heating system as
used in my house.

I think the resistor in this one is making too much heat. It never switches the heating on for very long, and I don't think it's good for a central heating system to be cycled on and off repeatedly. I much prefer it to come on for an hour at a time.

--
http://petersparrots.com
http://petersphotos.com

If you want a stable relationship....buy a horse.

On Sun, 04 Mar 2012 17:54:22 -0000, Tim Streater wrote:

In article op.wann58raytk5n5@i7-940, "Lieutenant Scott"
wrote:

I know about hysteresis in electronics, but you've lost me with the gears :-)

Imagine the chain has a lot of slack.

Ahhhh, Sturmey Archer gears.

--
http://petersparrots.com
http://petersphotos.com

If you think people aren't creative, watch them try to re-fold a roadmap.

On Sunday, March 4, 2012 3:21:19 PM UTC, DerekG wrote:

Can someone explain the use of a heater INSIDE a mechanical (bimetal
strip) room thermostat? (It usually just seems to be a resistor placed
near the bi-metallic strip).

The thermostat is meant to measure the air temperature. The heater causes room air to pass through the thermostat housing by natural convection. There is otherwise no reason why the room air should be in contact with the thermostat element, the element would react to a change in room temperature at some indeterminable time. It introduces an application error, but the error is predictable. The only other way of getting an air flow through the thermostat housing would involve a fan.

The other reason is that the heater switches off the heating before the heating reaches the thermostat set-point but, because of the residual heat in the radiators and pipes, the room temperature will continue to rise slightly after the switch off.

It can be effective apparently, but most temperature sensors have been heading in the thermistor direction for a while. You very rarely see the heater connected because it requires the use of another neutral wire.

On Sun, 04 Mar 2012 16:59:58 -0000, "Lieutenant Scott"
wrote:

On Sun, 04 Mar 2012 16:38:14 -0000, Graham. wrote:

On Sun, 04 Mar 2012 14:42:26 -0000, "Lieutenant Scott"
wrote:

Can someone explain the use of a heater INSIDE a mechanical (bimetal strip) room thermostat? (It usually just seems to be a resistor placed near the bi-metallic strip).

All it seems to do for me is make the thermostat very inaccurate. If the temperature is only a bit lower outside than inside, the temperature inside is what I set the stat to. As the temperature outside drops further, the inside temperature strays further and further from what I set it to (in the colder direction). This is presumably because whenever the heating is on, the stat thinks it's warmer than it really is, because of the internal heater. If I disconnect this internal heater, the stats functions properly.

I've replaced the offending pile of crap with a digital one anyway, but I wanted to know why they design them like this.


It reduces the hysteresis of the system by forcing it to shut off the
heat earlier than it would do naturally.

Hysteresis is a bit like mechanical backlash when a gear chain goes
into reverse.

I know about hysteresis in electronics, but you've lost me with the gears :-)

I first came across the concept when studying electronic servicing
specifically magnetic recording, the B-H curve, hysteresis loops and
all that crap.


It wasn't until I noticed a similar effect on mechanical systems like
the slow motion vernier tuning arrangement on my home-made
transmitters that it sank in.

You turn the dial one way and the frequency changes smoothly. Turn it
the other way and nothing much happens for a small amount of rotation
because the gears aren't tightly meshed, then smooth operation returns
albeit with a calibration error compared with the other direction.

I found the mechanical backlash analogy very useful in understanding
the magnetic effect and wished the lecturer had used it.

--
Graham.
%Profound_observation%

On Sun, 04 Mar 2012 18:12:28 -0000, Graham. wrote:

On Sun, 04 Mar 2012 16:59:58 -0000, "Lieutenant Scott"
wrote:

On Sun, 04 Mar 2012 16:38:14 -0000, Graham. wrote:

On Sun, 04 Mar 2012 14:42:26 -0000, "Lieutenant Scott"
wrote:

Can someone explain the use of a heater INSIDE a mechanical (bimetal strip) room thermostat? (It usually just seems to be a resistor placed near the bi-metallic strip).

All it seems to do for me is make the thermostat very inaccurate. If the temperature is only a bit lower outside than inside, the temperature inside is what I set the stat to. As the temperature outside drops further, the inside temperature strays further and further from what I set it to (in the colder direction). This is presumably because whenever the heating is on, the stat thinks it's warmer than it really is, because of the internal heater. If I disconnect this internal heater, the stats functions properly.

I've replaced the offending pile of crap with a digital one anyway, but I wanted to know why they design them like this.


It reduces the hysteresis of the system by forcing it to shut off the
heat earlier than it would do naturally.

Hysteresis is a bit like mechanical backlash when a gear chain goes
into reverse.

I know about hysteresis in electronics, but you've lost me with the gears :-)

I first came across the concept when studying electronic servicing
specifically magnetic recording, the B-H curve, hysteresis loops and
all that crap.


It wasn't until I noticed a similar effect on mechanical systems like
the slow motion vernier tuning arrangement on my home-made
transmitters that it sank in.

You turn the dial one way and the frequency changes smoothly. Turn it
the other way and nothing much happens for a small amount of rotation
because the gears aren't tightly meshed, then smooth operation returns
albeit with a calibration error compared with the other direction.

I found the mechanical backlash analogy very useful in understanding
the magnetic effect and wished the lecturer had used it.

For some reason I understand electricity better than mechanics!

--
http://petersparrots.com
http://petersphotos.com

&
_____(_)____
/\/\/\/\/\/\/\
/\/\/\/\/\/\/\/\
/\/\/\/\/\/\/\/\/\
/\/\/\/\/\/\/\/\/\/\
/\/\/\/\/\/\/\/\/\/\/\
/\/\/\/\/\/\/\/\/\/\/\/\
/\/\/\/\/\/\/\/\/\/\/\/\/\
/\/\/\/\/\/\/\/\/\/\/\/\/\/\
/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\
^^^^^^^^^^^^\^^^^/^^^^^^^^^^^^
__.-)__(-,__
./' \_\_/_/ `\.
/ | //\ | \
/ \ | |/| | / \
/ |\ | |/| | /| \
/ /| \ | |/| | / |\ \
( ( | \| |/| |/ | ) )
\ \| Y |/| Y |/ /
\ | o| |/| |- | /
`\ | | `^` | | /'
`| o|=[Ll=|- |'
| / \ |
~~|` \ `|~~
| | |
| | |
| | |
| | |
| | |
| | |
| | |
| | |
|____|____|
/ / \ \
/ / \ \
`---' `---`

The resistor ensures a "sampling airflow" through the thermostat.
Forcing the thermostat to sense room *air* temperature, rather than a
cold wall due to unheated neighboring room.

A towel rail (uh-oh) with separate wall room thermostat needs the
resistor connecting, because it is in effect a high latency water
heating solution - rather than instant room impact electric fan heater.

Lieutenant Scott wrote:
On Sun, 04 Mar 2012 17:42:30 -0000, John Williamson
wrote:

Lieutenant Scott wrote:
On Sun, 04 Mar 2012 16:55:27 -0000, Harry Bloomfield
wrote:

A resistor in the room thermostat housing

But it's wildly inaccurate. And there seems to be no adjustment for
them.

It's a very cheap way to improve the responsiveness of the system. I
once disconnected one, and found out that re-connecting it it made a
noticeable improvement to my comfort in the room with no noticeable
increase in heating costs.

It was a single small room with a blown air calor gas powered heating
system, but the same principle applies to a central heating system as
used in my house.

I think the resistor in this one is making too much heat. It never
switches the heating on for very long, and I don't think it's good for
a central heating system to be cycled on and off repeatedly. I much
prefer it to come on for an hour at a time.

What's the duty cycle? Mine is about 50% at 5C outside temperature,
approaching 100% at -5C.

If my central heating stays on for an hour, the house gains quite a few
degrees in temperature, as it's fairly well insulated. This excess heat
then has to be lost before the heating will come on again. If I set it
to be comfortable when the heating turns off, then I'm too cool before
the heating turns on again, and vice versa. I don't think it makes much
difference to the reliability or system life if it runs on a ten minute
cycle rather than a 60 minute one, though I could be wrong. It certainly
makes a difference to my comfort, which is why I have central heating to
start with, otherwise I'd use gas heaters in each room.

--
Tciao for Now!

John.

Lieutenant Scott wrote

Can someone explain the use of a heater INSIDE a mechanical (bimetal strip) room thermostat?

Yeah, its one of those things that seems mad until you realise why its done like that.

It basically reduces the dead zone/hysteresis in the controller.

(It usually just seems to be a resistor
placed near the bi-metallic strip).

Yeah, thats all it is.

All it seems to do for me is make the thermostat very inaccurate.

It actually makes it more accurate, surprisingly enough.

There's quite a bit on it on the net.

If the temperature is only a bit lower outside than inside, the
temperature inside is what I set the stat to. As the temperature
outside drops further, the inside temperature strays further and
further from what I set it to (in the colder direction). This is
presumably because whenever the heating is on, the stat thinks it's warmer than it really is, because of the internal
heater. If I
disconnect this internal heater, the stats functions properly.

Yes, but you will find that the dead zone/hysteresis increases.

It wouldnt be there at an extra cost if there wasnt a reason for it.

I've replaced the offending pile of crap with a digital one anyway,
but I wanted to know why they design them like this.

Yeah, so did I when I first came across it.

Lieutenant Scott wrote
DerekG wrote
Lieutenant Scott wrote

Can someone explain the use of a heater INSIDE a mechanical (bimetal strip) room thermostat? (It usually just seems
to be a resistor placed near the bi-metallic strip).

All it seems to do for me is make the thermostat very inaccurate.

I've replaced the offending pile of crap with a digital one anyway, but I wanted to know why they design them like
this.

It's an anticipator, and should be set so as to introduce exactly the
right amount of heat inside the thermostat casing so that it doesn't
have to wait for the heat to percolate from a radiator across the
other side of the room, thereby tailoring the response so that it
stabilises sooner.

It's a bit like trying to throw a Yo-Yo away whilst still holding
onto the string. And yes it's a crap plumbers idea.

I've found that the convection of the room air from the heat source
(a radiator) heats the room very evenly anyway. The stat switches
off when the room reaches the required temperature without that
bloody heating thingy!

Yes, but you will find if you measure it properly that the heated
thermostat does produce less difference between the highs and
lows in the room.

As you said, it just doesn't work as designed - it's guessing!

Nope. Its trivial to prove that it does reduce the dead zone/hysteresis.

All that happens is when the heating is needed to be on more, it thinks
it's going to be warmer than it is, so it doesn't switch it on enough.

Fraid not. If that was the case, you would see a bigger swing in the room temps and you dont in fact see that.

They wouldnt include an extra cost heater in thermostats if there wasnt a reason for them.

"Lieutenant Scott" wrote in message
newsp.wanqbqizytk5n5@i7-940...
On Sun, 04 Mar 2012 17:42:30 -0000, John Williamson
wrote:

Lieutenant Scott wrote:
On Sun, 04 Mar 2012 16:55:27 -0000, Harry Bloomfield
wrote:

A resistor in the room thermostat housing

But it's wildly inaccurate. And there seems to be no adjustment for
them.

It's a very cheap way to improve the responsiveness of the system. I
once disconnected one, and found out that re-connecting it it made a
noticeable improvement to my comfort in the room with no noticeable
increase in heating costs.

It was a single small room with a blown air calor gas powered heating
system, but the same principle applies to a central heating system as
used in my house.

I think the resistor in this one is making too much heat. It never
switches the heating on for very long, and I don't think it's good for a
central heating system to be cycled on and off repeatedly. I much prefer
it to come on for an hour at a time.

Thing is, if it's on for an hour at a time you'll get shedloads of
overshoot. With the resistor it switches more frequently so you get a more
even temp i.e, smaller hysteresis about the set point.

Lieutenant Scott wrote
Harry Bloomfield wrote
DerekG wrote
Lieutenant Scott wrote

Can someone explain the use of a heater INSIDE a mechanical
(bimetal strip) room thermostat? (It usually just seems to be a
resistor placed near the bi-metallic strip).

All it seems to do for me is make the thermostat very inaccurate.

I've replaced the offending pile of crap with a digital one
anyway, but I wanted to know why they design them like this.

It's an anticipator, and should be set so as to introduce exactly
the right amount of heat inside the thermostat casing so that it
doesn't have to wait for the heat to percolate from a radiator
across the other side of the room, thereby tailoring the response
so that it stabilises sooner.

It's a bit like trying to throw a Yo-Yo away whilst still holding
onto the string. And yes it's a crap plumbers idea.

A plumber could not come up up with such a clever idea :')

It was a widely used idea to prevent over reaction of the heating
system, due to the wide hysteresis of a mechanical thermostat in the
days before modern electronic stats. The stat switches on, which then
powers a resistor, which warms the bi-metal sensing element, so the
stat gets both the warmth of the heating system and the warmth from
the resistor.

But it's wildly inaccurate.

No it isnt, its more accurate when installed properly.

And there seems to be no adjustment for them.

Doesnt need any, it always reduces the hysteresis.

Lieutenant Scott wrote
Tim Watts wrote
A.Lee wrote
Lieutenant Scott wrote

Can someone explain the use of a heater INSIDE a mechanical
(bimetal strip) room thermostat? (It usually just seems to be a
resistor placed near the bi-metallic strip).

It is to make the thermostat/heating more efficient by allowing the
stat to 'predict' a rise/fall in temperature.
If the stat is in good condition, it does work better than a plain
on/off stat, where the temperature can fall/rise a lot more than one
supplied with a neutral.

It wil also save you money by turning off/on the boiler more
efficiently.

Or another way to look at it is it narrows the thermostat's hysteresis, ie makes the off-on temperature closer to the
on-off point.

The trouble is it makes the on-off point drift depending on how much the heating is used.

No it doesnt, because the heater is off when the load is off.

And the set point would be allowed for in manufacture anyway even if it did.

Lieutenant Scott wrote
John Williamson wrote
Lieutenant Scott wrote
Harry Bloomfield wrote

A resistor in the room thermostat housing

But it's wildly inaccurate. And there seems to be no adjustment for them.

It's a very cheap way to improve the responsiveness of the system. I
once disconnected one, and found out that re-connecting it it made a
noticeable improvement to my comfort in the room with no noticeable
increase in heating costs.

It was a single small room with a blown air calor gas powered heating
system, but the same principle applies to a central heating system as
used in my house.

I think the resistor in this one is making too much heat. It never
switches the heating on for very long, and I don't think it's good
for a central heating system to be cycled on and off repeatedly. I much prefer it to come on for an hour at a time.

That will inevitably mean a higher hysteresis.

Onetap presented the following explanation :
The thermostat is meant to measure the air temperature. The heater causes
room air to pass through the thermostat housing by natural convection. There
is otherwise no reason why the room air should be in contact with the
thermostat element, the element would react to a change in room temperature
at some indeterminable time. It introduces an application error, but the
error is predictable. The only other way of getting an air flow through the
thermostat housing would involve a fan.

Wrong! There will always be a natural flow of air because they have
vents top and bottom. The resistor is simply there to reduce
hysteresis.

The other reason is that the heater switches off the heating before the
heating reaches the thermostat set-point but, because of the residual heat in
the radiators and pipes, the room temperature will continue to rise slightly
after the switch off.

Correct.

It can be effective apparently, but most temperature sensors have been
heading in the thermistor direction for a while. You very rarely see the
heater connected because it requires the use of another neutral wire.

There is just no need for a resistor to heat up the sensor, because the
sensor is not electro-mechanical.

A thermistor / electronic system can be programmed to operate on and
off within a fraction of a degree, so some hysteresis has to be
deliberately programmed in.

Most modern stats now are battery operated and wireless, it just would
not be possible to waste so much current on a heater resistor, when the
same result could be better achieved in the stat's software.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk

On Sun, 04 Mar 2012 19:41:06 -0000, John Williamson wrote:

Lieutenant Scott wrote:
On Sun, 04 Mar 2012 17:42:30 -0000, John Williamson
wrote:

Lieutenant Scott wrote:
On Sun, 04 Mar 2012 16:55:27 -0000, Harry Bloomfield
wrote:

A resistor in the room thermostat housing

But it's wildly inaccurate. And there seems to be no adjustment for
them.

It's a very cheap way to improve the responsiveness of the system. I
once disconnected one, and found out that re-connecting it it made a
noticeable improvement to my comfort in the room with no noticeable
increase in heating costs.

It was a single small room with a blown air calor gas powered heating
system, but the same principle applies to a central heating system as
used in my house.

I think the resistor in this one is making too much heat. It never
switches the heating on for very long, and I don't think it's good for
a central heating system to be cycled on and off repeatedly. I much
prefer it to come on for an hour at a time.

What's the duty cycle? Mine is about 50% at 5C outside temperature,
approaching 100% at -5C.

Similar to that. It's actually my garage - the house already has a digital one as the bi-metal ones can't handle the inductive load of the motorized valve - they arc.

If my central heating stays on for an hour, the house gains quite a few
degrees in temperature, as it's fairly well insulated. This excess heat
then has to be lost before the heating will come on again. If I set it
to be comfortable when the heating turns off, then I'm too cool before
the heating turns on again, and vice versa. I don't think it makes much
difference to the reliability or system life if it runs on a ten minute
cycle rather than a 60 minute one, though I could be wrong. It certainly
makes a difference to my comfort, which is why I have central heating to
start with, otherwise I'd use gas heaters in each room.

With the digital stat, my house goes up and down by 1C.

--
http://petersparrots.com
http://petersphotos.com

gods don't smite people anymore because people of many different religions now living in the same town.
No god wants to accidentally smite the wrong person and get sued by another god.
-- David James

On Sun, 04 Mar 2012 20:08:29 -0000, Rod Speed wrote:

Lieutenant Scott wrote

Can someone explain the use of a heater INSIDE a mechanical (bimetal strip) room thermostat?

Yeah, its one of those things that seems mad until you realise why its done like that.

It basically reduces the dead zone/hysteresis in the controller.

(It usually just seems to be a resistor
placed near the bi-metallic strip).

Yeah, thats all it is.

All it seems to do for me is make the thermostat very inaccurate.

It actually makes it more accurate, surprisingly enough.

Doesn't seem to work in anyt of the 5 I tried. It's just guessing at the room temperature with that resistor in there.

There's quite a bit on it on the net.

If the temperature is only a bit lower outside than inside, the
temperature inside is what I set the stat to. As the temperature
outside drops further, the inside temperature strays further and
further from what I set it to (in the colder direction). This is
presumably because whenever the heating is on, the stat thinks it's warmer than it really is, because of the internal
heater. If I
disconnect this internal heater, the stats functions properly.

Yes, but you will find that the dead zone/hysteresis increases.

I prefer a larger hysteresis (I'm not that fussy about temperature and wouldn't even notice the house changing temperature by less than about 4C) than the temperature being just plain wrong.

It wouldnt be there at an extra cost if there wasnt a reason for it.

I've replaced the offending pile of crap with a digital one anyway,
but I wanted to know why they design them like this.

Yeah, so did I when I first came across it.

It's more the arcing problem with inductive loads that ****ed me off.

--
http://petersparrots.com
http://petersphotos.com

I think car alarms should be set of explode after two minutes.
That way, we either take out a car thief, or deprive a noise-polluting jerk of his wheels.

On Sun, 04 Mar 2012 20:34:40 -0000, Rod Speed wrote:

Lieutenant Scott wrote
DerekG wrote
Lieutenant Scott wrote

Can someone explain the use of a heater INSIDE a mechanical (bimetal strip) room thermostat? (It usually just seems
to be a resistor placed near the bi-metallic strip).

All it seems to do for me is make the thermostat very inaccurate.

I've replaced the offending pile of crap with a digital one anyway, but I wanted to know why they design them like
this.

It's an anticipator, and should be set so as to introduce exactly the
right amount of heat inside the thermostat casing so that it doesn't
have to wait for the heat to percolate from a radiator across the
other side of the room, thereby tailoring the response so that it
stabilises sooner.

It's a bit like trying to throw a Yo-Yo away whilst still holding
onto the string. And yes it's a crap plumbers idea.

I've found that the convection of the room air from the heat source
(a radiator) heats the room very evenly anyway. The stat switches
off when the room reaches the required temperature without that
bloody heating thingy!

Yes, but you will find if you measure it properly that the heated
thermostat does produce less difference between the highs and
lows in the room.

As you said, it just doesn't work as designed - it's guessing!

Nope. Its trivial to prove that it does reduce the dead zone/hysteresis.

A hysteresis of 2 or 3C is not a problem.

All that happens is when the heating is needed to be on more, it thinks
it's going to be warmer than it is, so it doesn't switch it on enough.

Fraid not. If that was the case, you would see a bigger swing in the room temps and you dont in fact see that.

They wouldnt include an extra cost heater in thermostats if there wasnt a reason for them.

Well the one I have in the garage is currently keeping it at the temperature set on the dial (the heater is on 20% of the time). The outside temperature is about 10C. When it's 0C outside, the garage is about 4C lower than what's set on the dial (the heater is on 80% of the time). With the resistor disconnected, the temperature is what it says on the dial under both external conditions. The hysteresis is about 2-3C.

--
http://petersparrots.com
http://petersphotos.com

Joey's teacher sent a note home to his Mother saying, "Joey seems to be a very bright boy, but spends too much of his time thinking about sex and girls."
The Mother wrote back the next day, "If you find a solution, please advise. I have the same problem with his Father."

On Sun, 04 Mar 2012 20:41:03 -0000, brass monkey wrote:


"Lieutenant Scott" wrote in message
newsp.wanqbqizytk5n5@i7-940...
On Sun, 04 Mar 2012 17:42:30 -0000, John Williamson
wrote:

Lieutenant Scott wrote:
On Sun, 04 Mar 2012 16:55:27 -0000, Harry Bloomfield
wrote:

A resistor in the room thermostat housing

But it's wildly inaccurate. And there seems to be no adjustment for
them.

It's a very cheap way to improve the responsiveness of the system. I
once disconnected one, and found out that re-connecting it it made a
noticeable improvement to my comfort in the room with no noticeable
increase in heating costs.

It was a single small room with a blown air calor gas powered heating
system, but the same principle applies to a central heating system as
used in my house.

I think the resistor in this one is making too much heat. It never
switches the heating on for very long, and I don't think it's good for a
central heating system to be cycled on and off repeatedly. I much prefer
it to come on for an hour at a time.

Thing is, if it's on for an hour at a time you'll get shedloads of
overshoot. With the resistor it switches more frequently so you get a more
even temp i.e, smaller hysteresis about the set point.

Trouble is when the heater is required to be on more than a 50% duty cycle, I find it's cutting off too much. It thinks the room has warmed up when it hasn't.

--
http://petersparrots.com
http://petersphotos.com

Once you've seen one shopping centre, you've seen a mall.

On Sun, 04 Mar 2012 20:48:51 -0000, Rod Speed wrote:

Lieutenant Scott wrote
Harry Bloomfield wrote
DerekG wrote
Lieutenant Scott wrote

Can someone explain the use of a heater INSIDE a mechanical
(bimetal strip) room thermostat? (It usually just seems to be a
resistor placed near the bi-metallic strip).

All it seems to do for me is make the thermostat very inaccurate.

I've replaced the offending pile of crap with a digital one
anyway, but I wanted to know why they design them like this.

It's an anticipator, and should be set so as to introduce exactly
the right amount of heat inside the thermostat casing so that it
doesn't have to wait for the heat to percolate from a radiator
across the other side of the room, thereby tailoring the response
so that it stabilises sooner.

It's a bit like trying to throw a Yo-Yo away whilst still holding
onto the string. And yes it's a crap plumbers idea.

A plumber could not come up up with such a clever idea :')

It was a widely used idea to prevent over reaction of the heating
system, due to the wide hysteresis of a mechanical thermostat in the
days before modern electronic stats. The stat switches on, which then
powers a resistor, which warms the bi-metal sensing element, so the
stat gets both the warmth of the heating system and the warmth from
the resistor.

But it's wildly inaccurate.

No it isnt, its more accurate when installed properly.

And just how would I change the installation? There is no adjustment to make!

And there seems to be no adjustment for them.

Doesnt need any, it always reduces the hysteresis.

--
http://petersparrots.com
http://petersphotos.com

Why do sailors have tattoos on their backs?
So their shipmates will have something to read.

On Sun, 04 Mar 2012 20:55:33 -0000, Rod Speed wrote:

Lieutenant Scott wrote
Tim Watts wrote
A.Lee wrote
Lieutenant Scott wrote

Can someone explain the use of a heater INSIDE a mechanical
(bimetal strip) room thermostat? (It usually just seems to be a
resistor placed near the bi-metallic strip).

It is to make the thermostat/heating more efficient by allowing the
stat to 'predict' a rise/fall in temperature.
If the stat is in good condition, it does work better than a plain
on/off stat, where the temperature can fall/rise a lot more than one
supplied with a neutral.

It wil also save you money by turning off/on the boiler more
efficiently.

Or another way to look at it is it narrows the thermostat's hysteresis, ie makes the off-on temperature closer to the
on-off point.

The trouble is it makes the on-off point drift depending on how much the heating is used.

No it doesnt, because the heater is off when the load is off.

And the set point would be allowed for in manufacture anyway even if it did.

If the heater is small compared to the size of thee room, the thermostat appears to think it's warming up faster than it really is.

--
http://petersparrots.com
http://petersphotos.com

The 2 most common elements in the universe are hydrogen and stupidity.

on 04/03/2012, Rod Speed supposed :
Yes, but you will find if you measure it properly that the heated
thermostat does produce less difference between the highs and
lows in the room.

As you said, it just doesn't work as designed - it's guessing!

Nope. Its trivial to prove that it does reduce the dead zone/hysteresis.

All that happens is when the heating is needed to be on more, it thinks
it's going to be warmer than it is, so it doesn't switch it on enough.

Fraid not. If that was the case, you would see a bigger swing in the room
temps and you dont in fact see that.

Out of nothing more than curiosity, I have checked back through my
house temperature logs. I don't make any particular point of storing
the logs,
my system just stores weather data and along with that data, the
house's temperature and humidity - every 5 minutes.

At the moment we are running on an electronic wireless thermostat.
Aside from times when an outside door has been open for a while, the
room temperature is held within 0.6 deg. C for the entire day.

Looking back through my log to a little over two years ago, when we had
a wired electro-mechanical stat., in the same circumstances, the
temperature appeared to remain within 0.8 deg C.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk

Lieutenant Scott wrote
Rod Speed wrote
Lieutenant Scott wrote

Can someone explain the use of a heater INSIDE a mechanical (bimetal strip) room thermostat?

Yeah, its one of those things that seems mad until you realise why its done like that.

It basically reduces the dead zone/hysteresis in the controller.

(It usually just seems to be a resistor
placed near the bi-metallic strip).

Yeah, thats all it is.

All it seems to do for me is make the thermostat very inaccurate.

It actually makes it more accurate, surprisingly enough.

Doesn't seem to work in anyt of the 5 I tried.

Then you have either got it wired up wrong, or arent
actually measuring the hysteresis you are getting with
the heater connected and with it not connected.

It's just guessing at the room temperature with that resistor in there.

Nope, because it turns on when the heater isnt on, and only turns
the heater on when the load is turned on, so the load is turned off
earlier than it would otherwise be, reducing the hysteresis.

There's quite a bit on it on the net.

If the temperature is only a bit lower outside than inside, the
temperature inside is what I set the stat to. As the temperature
outside drops further, the inside temperature strays further and
further from what I set it to (in the colder direction). This is
presumably because whenever the heating is on, the stat thinks it's
warmer than it really is, because of the internal heater. If I
disconnect this internal heater, the stats functions properly.

Yes, but you will find that the dead zone/hysteresis increases.

I prefer a larger hysteresis

OK, but thats what that part of the design is there for, to reduce that.

(I'm not that fussy about temperature and wouldn't even notice the house changing temperature by less than about 4C)

Most do notice that much variation.

than the temperature being just plain wrong.

The temperature isnt just plain wrong. The load is switched on when
the thermostat heater isnt being powered, so the set point isnt affected.

It wouldnt be there at an extra cost if there wasnt a reason for it.

I've replaced the offending pile of crap with a digital one anyway, but I wanted to know why they design them like
this.

Yeah, so did I when I first came across it.

It's more the arcing problem with inductive loads that ****ed me off.

Thats only seen with thermostats that arent meant
to be used with inductive loads are used for those.

It was a widely used idea to prevent over reaction of the heating
system, due to the wide hysteresis of a mechanical thermostat in the
days before modern electronic stats. The stat switches on, which then
powers a resistor, which warms the bi-metal sensing element, so the stat
gets both the warmth of the heating system and the warmth from the
resistor.


That is my understanding of how they work

Lieutenant Scott wrote
Rod Speed wrote
Lieutenant Scott wrote
Tim Watts wrote
A.Lee wrote
Lieutenant Scott wrote

Can someone explain the use of a heater INSIDE a mechanical
(bimetal strip) room thermostat? (It usually just seems to be a
resistor placed near the bi-metallic strip).

It is to make the thermostat/heating more efficient by allowing the stat to 'predict' a rise/fall in temperature.
If the stat is in good condition, it does work better than a plain on/off stat, where the temperature can
fall/rise a lot more than one supplied with a neutral.

It wil also save you money by turning off/on the boiler more efficiently.

Or another way to look at it is it narrows the thermostat's hysteresis, ie makes the off-on temperature closer to
the on-off point.

The trouble is it makes the on-off point drift depending on how much the heating is used.

No it doesnt, because the heater is off when the load is off.

And the set point would be allowed for in manufacture anyway even if it did.

If the heater is small compared to the size of thee room, the
thermostat appears to think it's warming up faster than it really is.

Thats true regardless of the size of the heater and is why that type
of thermostat does produce a lower hysteresis than with no heater
for the bimetallic strip in the thermostat.

They wouldnt have included the heater if it didnt do something useful.

Yes, modern electronic thermostats are better again but thats a different
thing to why there is a heater in a bi metallic thermostat for a reason.

Harry Bloomfield wrote
Rod Speed wrote

Yes, but you will find if you measure it properly that the heated thermostat does produce less difference between the
highs and lows in the room.

As you said, it just doesn't work as designed - it's guessing!

Nope. Its trivial to prove that it does reduce the dead zone/hysteresis.

All that happens is when the heating is needed to be on more, it thinks it's going to be warmer than it is, so it
doesn't switch it on enough.

Fraid not. If that was the case, you would see a bigger swing in the room temps and you dont in fact see that.

Out of nothing more than curiosity, I have checked back through my
house temperature logs. I don't make any particular point of storing
the logs,my system just stores weather data and along with that data, the house's temperature and humidity - every 5
minutes.

At the moment we are running on an electronic wireless thermostat.
Aside from times when an outside door has been open for a while, the room temperature is held within 0.6 deg. C for
the entire day.

Looking back through my log to a little over two years ago, when we
had a wired electro-mechanical stat., in the same circumstances, the
temperature appeared to remain within 0.8 deg C.

You should see the biggest difference with the lowest outside temps.

And you dont have the figures for a bi metallic thermostat with no heater.

If you did, it would have a higher range again.