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.

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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.