Heater INSIDE thermostat?

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

[ ... ]

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
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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.
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That though depends on the site of the stat, and how well designed the placement of the radiators were in the first place. Brian
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Brian Gaff wrote

Nope, you always get a benefit.
The only real downside is that the system switches more.
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and doesn't warm up from cold so fast.
Robert
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On Tue, 6 Mar 2012 07:56:10 -0800 (PST), RobertL

I don't follow. Stat calls for heat. Heater/boiler runs full pelt until setpoint is reached. Stat turns off heat.
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Graham. wrote

The earlier part of the thread is gone from the quoting now.
We were discussing why bi metallic strip mechanical thermostats have a heater for the bi metallic strip.
They do that to reduce the hysteresis, and they reduce it by heating the bi metallic strip with the heater when the load is on.
That means that they switch more often than a bi metallic strip thermostat with no heater would.
That means they switch more, and do take longer to get up to temperature when starting from cold, because they switch off before the set point is reached, because the heater heats the bi metallic strip.
They do however still get to the set point, they just dont stay on all the time until the setpoint is reached when starting from cold.
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Rod Speed was thinking very hard :

That is not what my graphs from when we had a mechanical stat show. They show an almost constant heating run, until the set point is reached, then several short rapid boiler runs, as heat is absorbed by the fabric. Once the fabric is up to temperature the graph settles down to a more leisurely pace.
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Harry Bloomfield wrote

Cant see how thats possible with a bi metallic strip thermostat with a heater.
Maybe the heater wasnt actually connected properly so there was no heater.

I'd expect more than several. Whats the fabric ?

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on 07/03/2012, Rod Speed supposed :

There definitely a heater resistor installed and it was working. I would guess the heat output of the resistor was rather well matched to what was needed.

Insulated cavity brick. It seems we are quite well insulated, because the internal temperature, even on a below freezing cold night, rarely falls more than a couple of degrees over night with heating off.
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Harry Bloomfield wrote

Yeah, likely.

That would certainly explain why it doesnt switch much when its up to temp.
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In message

Not relevant to this particular post but I've just dismantled (smashed up) my old thermostat. The resistor is located at the back and is coloured red, red, yellow which is 22Ok (I think) so it's not going to generate an awful lot of hot air.
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hugh wrote

Yep.
I dont believe that that is a bi metallic strip heater. Its there for something else.
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wrote:

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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Lieutenant Scott 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.

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

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

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.
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on 04/03/2012, Rod Speed supposed :

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.
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Harry Bloomfield wrote

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.
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on 04/03/2012, Rod Speed supposed :

Having just checked - there was no noticeable difference, once a settled temperature was achieved. There were much bigger temperature swings when a door was opened in cold conditions, but once back to a settled condition no obvious difference other than the more rapid firing of the heating.

Agreed! I would expect temperature swings of at a guess 5 deg C, with no resistor in place.
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