[The Medway Handyman]

What's the +/- tolerance on a bog standard wall thermostat?

I mean, if you set it to 20c will it trigger on at 19c & off at 21c?


--
Dave - The Medway Handyman
www.medwayhandyman.co.uk

[Roger Mills]

In an earlier contribution to this discussion,
The Medway Handyman wrote:

What's the +/- tolerance on a bog standard wall thermostat?

I mean, if you set it to 20c will it trigger on at 19c & off at 21c?

Are you sure that you mean tolerance - 'cos what you've described is
hysteresis. If by 'bog standard' you mean a mechanical by-metallic stat,
that will inevitably have a difference of several degrees between the switch
on and switch off points. As someone else has said, these often have a
built-in accelerator heater. This is a small resistor which is powered when
the stat is 'on' and warms up the innards so that the stat switches off
slightly before the target room temperature is reached. Of course, the rads
don't cool immediately, so the room goes on getting hot for a bit - so the
accelerator heater just helps to reduce overshoot.

In terms of absolute accuracy (which would be closer to my definition of
tolerance), I suspect that the relationship between the number shown on the
knob and the actual temperature at which it switches is probably not a very
close one! It's probably best to ignore the numbers, and set it to give a
'comfortable' temperature - or hang a mercury thermometer alongside it if
you want to calibrate it.

Now, digital stats are different altogether - and not *that* expensive.
--
Cheers,
Roger
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[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember "The Medway Handyman"
saying something like:

What's the +/- tolerance on a bog standard wall thermostat?

I mean, if you set it to 20c will it trigger on at 19c & off at 21c?

Do you mean accuracy? I'd be surprised if most domestic stats are any
tighter than +/- 5% of indicated figure, and 10% on older ones.

[John]

"Grimly Curmudgeon" wrote in message
...
We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember "The Medway Handyman"
saying something like:

What's the +/- tolerance on a bog standard wall thermostat?

I mean, if you set it to 20c will it trigger on at 19c & off at 21c?

Do you mean accuracy? I'd be surprised if most domestic stats are any
tighter than +/- 5% of indicated figure, and 10% on older ones.


I think the usual term is "Hysteresis" - the amount of slack in the system
between on and off.

Tolerance would be associated with 20c meaning 20c and not 18c.. - ie the
calibration of the dial or display relative to the real temperature

Hysteresis of 1 degree would imply on at 20c and off at 21c

[The Medway Handyman]

Roger Mills wrote:
In an earlier contribution to this discussion,
The Medway Handyman wrote:

What's the +/- tolerance on a bog standard wall thermostat?

I mean, if you set it to 20c will it trigger on at 19c & off at 21c?

Are you sure that you mean tolerance - 'cos what you've described is
hysteresis.

I mean hysteresis now I know the correct term :-)

If by 'bog standard' you mean a mechanical by-metallic
stat, that will inevitably have a difference of several degrees
between the switch on and switch off points.

Thats what I was after knowing - thanks.

As someone else has
said, these often have a built-in accelerator heater. This is a small
resistor which is powered when the stat is 'on' and warms up the
innards so that the stat switches off slightly before the target room
temperature is reached. Of course, the rads don't cool immediately,
so the room goes on getting hot for a bit - so the accelerator heater
just helps to reduce overshoot.

Didn't know that.


--
Dave - The Medway Handyman
www.medwayhandyman.co.uk

[[email protected]]

The Medway Handyman wrote:
Roger Mills wrote:
In an earlier contribution to this discussion,
The Medway Handyman wrote:

What's the +/- tolerance on a bog standard wall thermostat?

I mean, if you set it to 20c will it trigger on at 19c & off at 21c?

Are you sure that you mean tolerance - 'cos what you've described is
hysteresis.

I mean hysteresis now I know the correct term :-)

If by 'bog standard' you mean a mechanical by-metallic
stat, that will inevitably have a difference of several degrees
between the switch on and switch off points.

Thats what I was after knowing - thanks.

As someone else has
said, these often have a built-in accelerator heater. This is a small
resistor which is powered when the stat is 'on' and warms up the
innards so that the stat switches off slightly before the target room
temperature is reached. Of course, the rads don't cool immediately,
so the room goes on getting hot for a bit - so the accelerator heater
just helps to reduce overshoot.

Didn't know that.


Unfortuately its not correct. Hysteresis in bimetals is typically
around half a degree. The natural hysteresis of bimetals is indeed a
few degrees, but the accelerator resistor warms the stat mecha
slightly after it switches to reduce this down to half a degree or
better in use. The acelerator resistor has nothing to do with
preventing overshoot.

BTW you cant tell the hysteresis by turning the dial, as that will
only show you the hysteresis of the bimetal mecha, which is then
modified by the accelerator R in use to give much smaller hysteresis.

Bimetals never go into proportional mode.


NT

[Roger Mills]

In an earlier contribution to this discussion,
wrote:

The Medway Handyman wrote:
Roger Mills wrote:
In an earlier contribution to this discussion,
The Medway Handyman wrote:

What's the +/- tolerance on a bog standard wall thermostat?

I mean, if you set it to 20c will it trigger on at 19c & off at
21c?

Are you sure that you mean tolerance - 'cos what you've described is
hysteresis.

I mean hysteresis now I know the correct term :-)

If by 'bog standard' you mean a mechanical by-metallic
stat, that will inevitably have a difference of several degrees
between the switch on and switch off points.

Thats what I was after knowing - thanks.

As someone else has
said, these often have a built-in accelerator heater. This is a
small resistor which is powered when the stat is 'on' and warms up
the innards so that the stat switches off slightly before the
target room temperature is reached. Of course, the rads don't cool
immediately, so the room goes on getting hot for a bit - so the
accelerator heater just helps to reduce overshoot.

Didn't know that.


Unfortuately its not correct. Hysteresis in bimetals is typically
around half a degree. The natural hysteresis of bimetals is indeed a
few degrees, but the accelerator resistor warms the stat mecha
slightly after it switches to reduce this down to half a degree or
better in use. The acelerator resistor has nothing to do with
preventing overshoot.


I fear that we're getting into semantics! Yes, the accelerator resistor
reduces the hysteresis rather than the overshoot per se - but the effect is
very similar. By switching off *before* the set temperature is reached, the
likelihood of overshooting the set temperature is reduced.

We seem to be in agreement that - in the absence of an accelerator
resistor - a bi-metal stat will have a hysteresis of several degrees.
--
Cheers,
Roger
______
Email address maintained for newsgroup use only, and not regularly
monitored.. Messages sent to it may not be read for several weeks.
PLEASE REPLY TO NEWSGROUP!

[Roger Mills]

In an earlier contribution to this discussion,
wrote:

Roger Mills wrote:
In an earlier contribution to this discussion,

I fear that we're getting into semantics! Yes, the accelerator
resistor reduces the hysteresis rather than the overshoot per se -
but the effect is very similar. By switching off *before* the set
temperature is reached, the likelihood of overshooting the set
temperature is reduced.

But it doesnt do that. The hysteresis means it overshoots its set
point every time. Not by enough to cause a problem.


It doesn't do what? Are you saying that it doesn't switch off before the set
temperature is reached? Clearly it does! The innards will have reached the
set temperature because of the effect of the accelerator resistor, but the
surrounding air will be at a lower temperature - giving scope for the
thermal inertia in the rads to raise the room temperature a bit more without
overshooting the *true* set temperature.
--
Cheers,
Roger
______
Email address maintained for newsgroup use only, and not regularly
monitored.. Messages sent to it may not be read for several weeks.
PLEASE REPLY TO NEWSGROUP!