On Mon, 05 Mar 2012 03:20:14 -0000, Rod Speed wrote:

Lieutenant Scott wrote
Rod Speed wrote
Lieutenant Scott wrote
Rod Speed wrote
Nope, because it turns on when the heater isnt on, and only turns
the heater on when the load is turned on, so the load is turned off
earlier than it would otherwise be, reducing the hysteresis.

But the resistor doesn't know how powerful the main heater is,

It doesnt need to.

so it could warm the bi-metal strip at at a different rate to the
room being warmed up. Now if the resistor was adjustable....

Doesnt need to be.

So explain why mine is rubbish?

Maybe its just a lousy design.

All five of them? Different makes?

OK, but thats what that part of the design is there for, to reduce that.

Couldn't they have made a better bi-metal strip?

They did, by including the heater.

I meant the actual strip. Thus negating the need for a workaround.

It isnt even possible and the heater isnt a workaround, its a significant
design feature which does what its designed to do, reduce the hysteresis.

It means the thermostat is guessing at the temperature, as the actual sensor is picking up artificial heat from the resistor. I would not call that a good design. It may well have been the best the technology could come up with before digital ones, but it's still rubbish, and from my experience of five of them, they make things worse.

If you dont like how it performs, it makes more sense to go digital and
get all sorts of other advantages like time of day and day of week stuff
too and automatic proportional mode with loads that can do that.

Yes I now have done.

Most do notice that much variation.

What on earth do these folk do when they go outside?

Thats a different matter to what they prefer when inside.

Yes, I vary what I wear when outside. I prefer not to do that inside.

Are you cold blooded like a lizard?

Yeah, I am. I have always preferred rooms warmer than anyone else, and
dont mind hot weather when everyone else is bitching about how hot it is.

I have to remember to turn the cooler on when visitors show
up, I dont even think about turning it on till it gets over 31C

And have a top on over the T shirt before anyone else does when out too.

I do not wear two layers at any time of year. I know someone else who feels the cold and not the heat like you - they said they have a vitamin D deficiency and anaemia.

The temperature isnt just plain wrong. The load is switched on when
the thermostat heater isnt being powered, so the set point isnt affected.

The point at which the the load comes on is fine. But the resistor heats up the bi-metal strip before the load has
warmed the room up much, so it's turning it off too soon.

The only effect of that is that it comes on again sooner.

But if the heating is required to be on an 90% duty cycle to keep the
room at the requested temperature, the thermostat never achieves this.

Sure, but most do size the heater properly and prefer the lower hysteresis.

So for the sake of a crap thermostat, the answer is to have a heater capable of heating the room with a 50% duty cycle? Oh dear.

Obviously a decent modern digital thermostat handles that much better.

The insided of the thermostat are getting quite warm due to the resistor, but the room isn't.

Yes, a proper modern electronic thermostat is better again,
and allows other stuff like time of day and day of week variations
in the set point etc but thats a different matter entirely.

They still seem to be selling these bi-metal things, even though an electronic one doesn't cost much more now.

Yeah, you can get some bizarre stuff like that.

The best digital thermostats do cost quite a bit more than
the cheapest bi metallic thermostats from china tho.

The cheapest digital thermostat works perfectly.

Also true of decent proportional control too.

What is one of those?

Proportinal control adjusts the power output of the load rather than
just turning it off or on. So you can have 61%, 62% etc etc etc.

Corse not all loads can do that.

I suppose theoretically a central heating boiler could do this if you could interface the room stat with the water temp stat.

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