Heater INSIDE thermostat?

[Lieutenant Scott]

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

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.

wanted to know why they design them like this.

For some reason I understand electricity better than mechanics!

[js.b1]

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.

[John Williamson]

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.

[brass monkey]

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.

[Harry Bloomfield]

Onetap presented the following explanation :

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.

Correct.

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.

[Lieutenant Scott]

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.

[Rick]

That is my understanding of how they work

[Andrew Gabriel]

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

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.

wanted to know why they design them like this.

The contacts need to have an over-centre toggle action for positive switching, or they would open and close slowly and arc. The over-centre toggle action requires hysteresis to provide the two different switching points for opening and closing the contacts.

[The Other Mike]

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

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.

wanted to know why they design them like this.

Your new digital one has very similar functionality in the software.

[Lieutenant Scott]

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

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.

wanted to know why they design them like this.

Maybe that's what mine were all missing (but I did try FIVE different makes and all of them arced).

[Lieutenant Scott]

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

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.

wanted to know why they design them like this.

No, all it does is switch the load on at 0.5C below what I set it to, and switch it off at 0.5C above what I set it to.

[Brian Gaff]

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

[Graham.]

Oops, sent to early

and this is what contributes most of all to the hysteresis IMHO

[Graham.]

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

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.

wanted to know why they design them like this.

Often by incorporating a magnet in the contact arrangement.

[Lieutenant Scott]

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

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.

wanted to know why they design them like this.

Maybe mine don't have magnets, hence the arcing.

[Mathew Newton]

Err, yes it is.

What do you think it is?

Mathew

[Onetap]

TBH I don't know and don't much care so I won't be researching it. It is = about as relevant for me as acetylene car headlights.

I made some suggestions above as to how you might reduce hysterisis. Appl= ying heat directly to the thermostat element seems counterintuitive, since = you'd expect the element temperature to have little correspondence to the r= oom temperature. I'd imagine someone in the 20s did a lot of fiddling and m= ade something that was at the leading edge of technology, when temperature = control involved the amount of coal on the fire.

The heater would cause some convection through the housing, but I don't k= now how significnt the convection would be. I've had experience of local ho= t/cold spots in offices and think it would be the major factor.

They also made pneumatic thermostats, mostly for AC, and mostly in the US= (Roberts?) since they had more need of AC. They worked by having some vane= moving to obstruct a nozzle. In that case the compressed air would cause a= diabatic cooling within the thermostat housing, but I doubt that was intent= ional. I was involved mostly in replacing the pneumatics with BMS and elect= rically actuated valves. I think I've still got one of the pneumatic thermo= stats in the shed.

[Mathew Newton]

which the heater reduces hysterisis. How do you think it does it?

Oh come on. It describes it quite clearly and I even quoted it for you:

the] thermostat is subject to or reaches the critical circuit-opening tempe= rature sooner than it otherwise would'

How much simpler do you need it? At first I thought you simply didn't understand, which is fair enough, but now I'm beginning to think you don't even *want* to understand.

Mathew

[Onetap]

Yeah, we know what it's meant to do; the question is, how does it do it? If you heat the element, it will open earlier because it's hotter, but how does that help you get a more accurate measurement of the air temperature?

[Mathew Newton]

it? If you heat the element, it will open earlier because it's hotter, but = how does that help you get a more accurate measurement of the air temperatu= re?

It's not about getting a more accurate measurement of the air temperature, it is about reducing hysteresis and thereby reducing overshoot.

Mathew