How do you verify that a battery-powered digital thermostat is working
like it should? From what I've read, it seems that it is supposed to
electrically bridge the connection between R wire and the W wire, when
it turns on heating. I detached a thermostat from the HVAC system and
tested the connection between the two terminals using a multimeter *when
the room temperature was well below the heating set-point*; the contacts
on the thermostat corresponding to the R & W wires didn't seem to allow
current to flow through. Given that it's an electronic device and may
not respond to the small DC voltage from the multimeter the same way it
may to 24 VAC, I can't tell whether the thermostat is good or bad.

Any ideas?

Put it back on the wall. Run the temp way up. Go see if
there is a 24 volt signal at W.

--
Christopher A. Young
Learn more about Jesus
www.lds.org
..


"DGF" wrote in message
...
How do you verify that a battery-powered digital thermostat
is working
like it should? From what I've read, it seems that it is
supposed to
electrically bridge the connection between R wire and the W
wire, when
it turns on heating. I detached a thermostat from the HVAC
system and
tested the connection between the two terminals using a
multimeter *when
the room temperature was well below the heating set-point*;
the contacts
on the thermostat corresponding to the R & W wires didn't
seem to allow
current to flow through. Given that it's an electronic
device and may
not respond to the small DC voltage from the multimeter the
same way it
may to 24 VAC, I can't tell whether the thermostat is good
or bad.

Any ideas?

On Fri, 29 Jan 2010 20:01:01 -0500, DGF wrote:

How do you verify that a battery-powered digital thermostat is working
like it should? From what I've read, it seems that it is supposed to
electrically bridge the connection between R wire and the W wire, when
it turns on heating. I detached a thermostat from the HVAC system and
tested the connection between the two terminals using a multimeter *when
the room temperature was well below the heating set-point*; the contacts
on the thermostat corresponding to the R & W wires didn't seem to allow
current to flow through. Given that it's an electronic device and may
not respond to the small DC voltage from the multimeter the same way it
may to 24 VAC, I can't tell whether the thermostat is good or bad.

Any ideas?

Like making a test circuit that provides the necessary 24 volts?

On Jan 29, 9:16*pm, PeterD wrote:
On Fri, 29 Jan 2010 20:01:01 -0500, DGF wrote:
How do you verify that a battery-powered digital thermostat is working
like it should? From what I've read, it seems that it is supposed to
electrically bridge the connection between R wire and the W wire, when
it turns on heating. I detached a thermostat from the HVAC system and
tested the connection between the two terminals using a multimeter *when
the room temperature was well below the heating set-point*; the contacts
on the thermostat corresponding to the R & W wires didn't seem to allow
current to flow through. Given that it's an electronic device and may
not respond to the small DC voltage from the multimeter the same way it
may to 24 VAC, I can't tell whether the thermostat is good or bad.

Any ideas?

Like making a test circuit that provides the necessary 24 volts?

I think he's right. Most T-stats have "power stealing" circuitry,
meaning they depend on the 24VAC from the HVAC system.

On Jan 30, 6:06*am, Mikepier wrote:
On Jan 29, 9:16*pm, PeterD wrote:

On Fri, 29 Jan 2010 20:01:01 -0500, DGF wrote:
How do you verify that a battery-powered digital thermostat is working
like it should? From what I've read, it seems that it is supposed to
electrically bridge the connection between R wire and the W wire, when
it turns on heating. I detached a thermostat from the HVAC system and
tested the connection between the two terminals using a multimeter *when
the room temperature was well below the heating set-point*; the contacts
on the thermostat corresponding to the R & W wires didn't seem to allow
current to flow through. Given that it's an electronic device and may
not respond to the small DC voltage from the multimeter the same way it
may to 24 VAC, I can't tell whether the thermostat is good or bad.

Any ideas?

Like making a test circuit that provides the necessary 24 volts?

I think he's right. Most T-stats have "power stealing" circuitry,
meaning they depend on the 24VAC from the HVAC system.


Hook it back up. With the room temp above the heat setpoint you
should have 24VAC between the wires that turn on the furnace. Set
the thermostat above the room temp and the voltage should drop to
close to zero, indicating the circuit is closed.

On Jan 29, 7:01*pm, DGF wrote:
How do you verify that a battery-powered digital thermostat is working
like it should? From what I've read, it seems that it is supposed to
electrically bridge the connection between R wire and the W wire, when
it turns on heating. I detached a thermostat from the HVAC system and
tested the connection between the two terminals using a multimeter *when
the room temperature was well below the heating set-point*; the contacts
on the thermostat corresponding to the R & W wires didn't seem to allow
current to flow through. Given that it's an electronic device and may
not respond to the small DC voltage from the multimeter the same way it
may to 24 VAC, I can't tell whether the thermostat is good or bad.

Any ideas?

Hook it up like its intended to be, and with power off, jump it or
power it wrong and it will be trash.

In article ,
DGF wrote:
How do you verify that a battery-powered digital thermostat is working
like it should? From what I've read, it seems that it is supposed to
electrically bridge the connection between R wire and the W wire, when
it turns on heating. I detached a thermostat from the HVAC system and
tested the connection between the two terminals using a multimeter *when
the room temperature was well below the heating set-point*; the contacts
on the thermostat corresponding to the R & W wires didn't seem to allow
current to flow through. Given that it's an electronic device and may
not respond to the small DC voltage from the multimeter the same way it
may to 24 VAC, I can't tell whether the thermostat is good or bad.

Can you hear it operate? Mine has a sort of electrically operated switch
rather than solid state or relay type to reduce battery consumption and
are clearly audible when they make or break. It also only operates when in
its housing so to test you'd need to bring out wires from that to connect
to a DVM.

--
*I feel like I'm diagonally parked in a parallel universe*

Dave Plowman London SW
To e-mail, change noise into sound.

In article
,
Mikepier wrote:
I think he's right. Most T-stats have "power stealing" circuitry,
meaning they depend on the 24VAC from the HVAC system.

In the UK the switching volts can be mains. So these universal
programmable stats seem to have some form of electrically operated switch
which is isolated from the actual device - rather in the same way as a
relay. But because they only take current from the battery when they alter
status are more suited to this sort of app.

--
*Virtual reality is its own reward *

Dave Plowman London SW
To e-mail, change noise into sound.

On Sat, 30 Jan 2010 03:06:51 -0800, Mikepier wrote:

On Jan 29, 9:16*pm, PeterD wrote:
On Fri, 29 Jan 2010 20:01:01 -0500, DGF wrote:
How do you verify that a battery-powered digital thermostat is working
like it should? From what I've read, it seems that it is supposed to
electrically bridge the connection between R wire and the W wire, when
it turns on heating. I detached a thermostat from the HVAC system and
tested the connection between the two terminals using a multimeter
*when the room temperature was well below the heating set-point*; the
contacts on the thermostat corresponding to the R & W wires didn't seem
to allow current to flow through. Given that it's an electronic device
and may not respond to the small DC voltage from the multimeter the
same way it may to 24 VAC, I can't tell whether the thermostat is good
or bad.

Any ideas?

Like making a test circuit that provides the necessary 24 volts?

I think he's right. Most T-stats have "power stealing" circuitry, meaning
they depend on the 24VAC from the HVAC system.



That is correct thermostats electronic or mechanical always depend on the
power source in the HVAC system they control for power and are never self
powered.

The battery is just there for power interruptions to keep the thermostat
from resetting to a different temperature nothing more.

To test it you will have to provide the 24v source for it to operate from
otherwise nothing will happen.

Gnack

Gnack, you are mistaken:: a lot of thermostats ARE battery powered and
work independantly of the voltage they are controlling. Some can even
control millivolt systems for wall/floor furnaces. True, some are power
stealing, and some must have a common wire connected, and use batteries
to keep the program if the power is interrupted, but don't say ALL are
that way, because it isn't so. .

In article ,
Gnack Nol wrote:
That is correct thermostats electronic or mechanical always depend on the
power source in the HVAC system they control for power and are never self
powered.

In the UK that's simply not true. There are many different makes that
operate entirely on battery power. And can be used to control a mains *or*
low voltage circuit. I'd be very surprised if the same models weren't on
sale in the US. Something like these:-

http://www.tlc-direct.co.uk/Main_Ind...m_3/index.html

which are a very good way of upgrading an older system. Better modern
ones tend not to have a room stat but a simple temperature sensor with the
house temp being set via the boiler electronics.

I know US systems can be different from UK ones - but the same principles
must apply. And much needed in these days of high energy prices.

--
*I have a degree in liberal arts -- do you want fries with that

Dave Plowman London SW
To e-mail, change noise into sound.

In article , Mikepier wrote:
On Jan 29, 9:16=A0pm, PeterD wrote:
On Fri, 29 Jan 2010 20:01:01 -0500, DGF wrote:
How do you verify that a battery-powered digital thermostat is working
like it should? From what I've read, it seems that it is supposed to
electrically bridge the connection between R wire and the W wire, when
it turns on heating. I detached a thermostat from the HVAC system and
tested the connection between the two terminals using a multimeter *when
the room temperature was well below the heating set-point*; the contacts
on the thermostat corresponding to the R & W wires didn't seem to allow
current to flow through. Given that it's an electronic device and may
not respond to the small DC voltage from the multimeter the same way it
may to 24 VAC, I can't tell whether the thermostat is good or bad.

Any ideas?

Like making a test circuit that provides the necessary 24 volts?

I think he's right. Most T-stats have "power stealing" circuitry,
meaning they depend on the 24VAC from the HVAC system.

Most thermostats have a relay. One has to know the circuit to troubleshoot
it. Some of the best thermostats have power stealing circuitry
and will opperate without batteries, but most of the cheap
ones just have a small relay, and contacts could have some resistance
over time.

greg

In article ,
GregS wrote:
Most thermostats have a relay. One has to know the circuit to
troubleshoot it. Some of the best thermostats have power stealing
circuitry and will opperate without batteries, but most of the cheap
ones just have a small relay, and contacts could have some resistance
over time.

Battery operated thermostats don't have a conventional relay - ie one
which takes power all the time it's made. That would run down the battery
in short order. They use some form of 'motorised' switch, which only takes
power when it changes state. On mine you can hear it operate.

--
*A dog's not just for Christmas, it's alright on a Friday night too*

Dave Plowman London SW
To e-mail, change noise into sound.

(GregS) wrote in
:

In article
,
Mikepier wrote:
On Jan 29, 9:16=A0pm, PeterD wrote:
On Fri, 29 Jan 2010 20:01:01 -0500, DGF
wrote:
How do you verify that a battery-powered digital thermostat is
working like it should? From what I've read, it seems that it is
supposed to electrically bridge the connection between R wire and
the W wire, when it turns on heating. I detached a thermostat from
the HVAC system and tested the connection between the two terminals
using a multimeter *when the room temperature was well below the
heating set-point*; the contacts on the thermostat corresponding to
the R & W wires didn't seem to allow current to flow through. Given
that it's an electronic device and may not respond to the small DC
voltage from the multimeter the same way it may to 24 VAC, I can't
tell whether the thermostat is good or bad.

Any ideas?

Like making a test circuit that provides the necessary 24 volts?

I think he's right. Most T-stats have "power stealing" circuitry,
meaning they depend on the 24VAC from the HVAC system.

Most thermostats have a relay. One has to know the circuit to
troubleshoot it. Some of the best thermostats have power stealing
circuitry and will opperate without batteries, but most of the cheap
ones just have a small relay, and contacts could have some resistance
over time.

greg



I believe the batteries power the clock.
there's probably a diode to allow the 24VAC to power the clock until the
mains drop off,and then the batteries take over.But the batteries don't
power the relay that controls the system.

--
Jim Yanik
jyanik
at
localnet
dot com

In article , "Dave Plowman (News)" wrote:
In article ,
GregS wrote:
Most thermostats have a relay. One has to know the circuit to
troubleshoot it. Some of the best thermostats have power stealing
circuitry and will opperate without batteries, but most of the cheap
ones just have a small relay, and contacts could have some resistance
over time.

Battery operated thermostats don't have a conventional relay - ie one
which takes power all the time it's made. That would run down the battery
in short order. They use some form of 'motorised' switch, which only takes
power when it changes state. On mine you can hear it operate.


I always assumed a relay, and yes they are loud. On a soild state unit I could also
hear a click, but I think that was from current flow. If units use a switch, I hope
they have a safety overheat breaker.

greg

(GregS) wrote in
:

In article , "Dave Plowman (News)"
wrote:
In article ,
GregS wrote:
Most thermostats have a relay. One has to know the circuit to
troubleshoot it. Some of the best thermostats have power stealing
circuitry and will opperate without batteries, but most of the cheap
ones just have a small relay, and contacts could have some
resistance over time.

Battery operated thermostats don't have a conventional relay - ie one
which takes power all the time it's made. That would run down the
battery in short order. They use some form of 'motorised' switch,
which only takes power when it changes state. On mine you can hear it
operate.


I always assumed a relay, and yes they are loud. On a soild state unit
I could also hear a click, but I think that was from current flow. If
units use a switch, I hope they have a safety overheat breaker.

greg


the relay would be in the AC unit itself.The TS just switches the low
voltage to the relay.

--
Jim Yanik
jyanik
at
localnet
dot com

In article ,
GregS wrote:
Battery operated thermostats don't have a conventional relay - ie one
which takes power all the time it's made. That would run down the
battery in short order. They use some form of 'motorised' switch, which
only takes power when it changes state. On mine you can hear it operate.


I always assumed a relay, and yes they are loud. On a soild state unit I
could also hear a click, but I think that was from current flow. If
units use a switch, I hope they have a safety overheat breaker.

Certainly in the UK it's a form of mechanical switch. It can carry mains
or low voltage AC or DC - not really possible with a solid state device.
My guess is it may be piezo operated. But that's only a guess. Mine is
rather louder than a relay.

--
*Just remember...if the world didn't suck, we'd all fall off*

Dave Plowman London SW
To e-mail, change noise into sound.

On Feb 1, 7:08*am, "Dave Plowman (News)" wrote:
In article ,
* *GregS wrote:

Battery operated thermostats don't have a conventional relay - ie one
which takes power all the time it's made. That would run down the
battery in short order. They use some form of 'motorised' switch, which
only takes power when it changes state. On mine you can hear it operate.

I always assumed a relay, and yes they are loud. On a soild state unit I
could also hear a click, but I think that was from current flow. If
units use a switch, I hope they have a safety overheat breaker.

Certainly in the UK it's a form of mechanical switch. It can carry mains
or low voltage AC or DC - not really possible with a solid state device.
My guess is it may be piezo operated. But that's only a guess. Mine is
rather louder than a relay.

--
*Just remember...if the world didn't suck, we'd all fall off*

* * Dave Plowman * * * * * * * * London SW
* * * * * * * * * To e-mail, change noise into sound.

There are solid state relays that will supply fairly high current at
240vac with a 24vdc switching voltage. They are very common on
automated machinery, I'm not sure if they would be used in a
thermostat, the ones we use are pretty spendy. You can get ones that
are latched, and will only need a signal to change state. Some of the
cheaper digital thermostats that I have installed in rental properties
never make a sound. I've never really had to back engineer one
though, if it fails and I don't have voltage coming through, then I
toss it and get another. Of coarse, if you have a higher end
thermostat then that may not be the thriftiest way to go.

When I test...
-I test to make sure I have a 24vdc supply at the thermostat, by
checking for potential across the wires incoming wires with the
thermostat off. This doesn't work all of the time, if there is a
broken wire or a burnt out relay or fuse, or a bad ground.
-Then check the voltage drop again after the thermostat is supposedly
engaged
-If that's good, check the voltage of the thermostat leads at the
furnace or boiler or whatever. This can be dangerous if you don't
know what you are doing. Usually there is a point on the outside of
the furnace where you can tie in to test the wires.

The rest depends on the end piece of equipment that the thermostat
runs. On many furnaces now, the controller will flash a code if there
is a detectable failure, kind of like the computer in a modern car.
So that's something to consider as well.

-J

In article
,
Sansui Samari wrote:
Certainly in the UK it's a form of mechanical switch. It can carry
mains or low voltage AC or DC - not really possible with a solid state
device. My guess is it may be piezo operated. But that's only a guess.
Mine is rather louder than a relay.


There are solid state relays that will supply fairly high current at
240vac with a 24vdc switching voltage. They are very common on
automated machinery, I'm not sure if they would be used in a
thermostat, the ones we use are pretty spendy.

Thing is in the UK it was pretty common to have mains to a mechanical
thermostat. Which also supplied an accelerator coil round the bi-metallic
strip to improve accuracy. So aftermarket types can all handle this. On my
last system here everything electrical was mains - gas valve, pump, three
way valve, room stat and water stat. A very simple system which worked
well for years. I've replaced the boiler with a condensing type which has
electronics so has to have a low volt supply for that. But uses sensors
instead of thermostats for house and water temperature which is now set
via the programmer, rather than locally.

Aftermarket thermostats will most often be fitted to older systems here -
so have to cope with either AC mains, or low volt AC or DC. Which I'd
guess rules out a solid state switch.

But even a solid state switch will take some current when made - does it
not? So not ideal for battery operation.

The programmable thermostat I retro-fitted to the old system - allowed you
to set different temperatures for parts of the day and days of the week -
used four AAs for everything - no power from the line - and they lasted
about 2 years. They are a very worthwhile addition to an older system.

--
*Also too, never, ever use repetitive redundancies *

Dave Plowman London SW
To e-mail, change noise into sound.

On Tue, 02 Feb 2010 09:30:13 +0000 (GMT), "Dave Plowman (News)"
wrote:


The programmable thermostat I retro-fitted to the old system - allowed you
to set different temperatures for parts of the day and days of the week -
used four AAs for everything - no power from the line - and they lasted
about 2 years. They are a very worthwhile addition to an older system.

Sounds a bit like my Danfoss TP75. It uses an Omron latching relay type
G6CK-2117P which works off a 20mS 3V pulse:-
http://www.omron.com/ecb/products/pry/121/g6c_2.html


--
Geo

In article ,
Geo wrote:
The programmable thermostat I retro-fitted to the old system - allowed you
to set different temperatures for parts of the day and days of the week -
used four AAs for everything - no power from the line - and they lasted
about 2 years. They are a very worthwhile addition to an older system.

Sounds a bit like my Danfoss TP75. It uses an Omron latching relay type
G6CK-2117P which works off a 20mS 3V pulse:-
http://www.omron.com/ecb/products/pry/121/g6c_2.html

Interesting - but nothing like those on my board. I no longer have it so
can't take pics. The description isn't that clear - does it take (near)
zero current when made or just a reduced amount? For long battery life it
would need to be zero. Nor did the noise it made when the 'switch'
operated sound anything like a relay.

--
*I don't suffer from insanity, I enjoy every minute of it *

Dave Plowman London SW
To e-mail, change noise into sound.

On Tue, 02 Feb 2010 14:54:02 +0000 (GMT), "Dave Plowman (News)"
wrote:

In article ,
Geo wrote:
Sounds a bit like my Danfoss TP75. It uses an Omron latching relay type
G6CK-2117P which works off a 20mS 3V pulse:-
http://www.omron.com/ecb/products/pry/121/g6c_2.html

Interesting - but nothing like those on my board. I no longer have it so
can't take pics. The description isn't that clear - does it take (near)
zero current when made or just a reduced amount? For long battery life it
would need to be zero. Nor did the noise it made when the 'switch'
operated sound anything like a relay.

No current at all. Imagine a balanced metal arm pivoted in centre and a coil at
each end with some remanent magnetism. A current pulse through either coil is
sufficient to overcome the small static magnetic pull and trip the bistable to
the other condition. The Danfoss WP75H uses a much larger Gruner 703H to switch
up to 25A for an immersion heater.

This article has a short explanation and some driving circuits:-
http://www.electronicsweekly.com/Articles/2008/11/21/44977/circuits-drive-single-coil-latching-relays.htm

--
Geo

In article ,
Geo wrote:
Interesting - but nothing like those on my board. I no longer have it
so can't take pics. The description isn't that clear - does it take
(near) zero current when made or just a reduced amount? For long
battery life it would need to be zero. Nor did the noise it made when
the 'switch' operated sound anything like a relay.

No current at all. Imagine a balanced metal arm pivoted in centre and a
coil at each end with some remanent magnetism. A current pulse through
either coil is sufficient to overcome the small static magnetic pull and
trip the bistable to the other condition. The Danfoss WP75H uses a much
larger Gruner 703H to switch up to 25A for an immersion heater.

This article has a short explanation and some driving circuits:-
http://www.electronicsweekly.com/Articles/2008/11/21/44977/circuits-drive-single-coil-latching-relays.htm

Excellent - thanks for that. I can think of a few uses I could put them to.

--
*Why don't you ever see the headline "Psychic Wins Lottery"?

Dave Plowman London SW
To e-mail, change noise into sound.