On Sun, 04 Mar 2012 23:39:40 -0000, Rod Speed wrote:
Lieutenant Scott wrote
Rod Speed wrote
Lieutenant Scott wrote
Rod Speed wrote
Lieutenant Scott wrote
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.
Yeah, thats all it is.
It actually makes it more accurate, surprisingly enough.
Doesn't seem to work in anyt of the 5 I tried.
Then you have either got it wired up wrong, or arent
actually measuring the hysteresis you are getting with
the heater connected and with it not connected.
There's nothing that can be wired up wrong really. It's only got a neutral, a live, and a switched live. The live
and neutral come from the mains, and the switched live goes to an electric fanheater which does not blow at the
thermostat.
Turns out you dont care about the hysteresis. The designers do.
It's just guessing at the room temperature with that resistor in there.
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?
Yes, but you will find that the dead zone/hysteresis increases.
I prefer a larger hysteresis
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.
(I'm not that fussy about temperature and wouldn't even notice the house changing temperature by less than about 4C)
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?
than the temperature being just plain wrong.
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. 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.
Also true of decent proportional control too.
What is one of those?
Yeah, so did I when I first came across it.
It's more the arcing problem with inductive loads that ****ed me off.
Thats only seen with thermostats that arent meant
to be used with inductive loads are used for those.
Well I've used just about every one I can find and none apart from digital ones work with inductive loads. They all
actually state the
maximum load with resistive and inductive, so they are designed to work.
You've got a higher inductive load than you think if they wont work with your load.
It's only a motorized valve. The pump and boiler are switched by the contacts in the motorized valve, that current doesn't go through the thermostat contacts.
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