Wiring a room thermostat downstream of a CH controller

[JohnW2]

Thanks all. Yes, there is a neutral available at the CH Controller, and I'll be fitting the thermostat just beneath that on the wall, so it shouldn't be too much of a fankle (Scottish term, sorry) to get 3-core between the two. I've been enjoying this discussion - learned a lot, and I don't get to say "Hysteresis" as often as I'd llke to.

You're welcome.

"Fankle" - I rather like that word

Kind Regards, John

 

[AndyPRK]

Don't forget the position of the thermostat is important to give good control. Usually in the downstairs hall

 

[Taylortwocities]

And not near a radiator, especially one that has a TRV fitted.

 

[ericmark]

My system with a thermostat with 0.5 degrees between on and off normally runs for an hour at a time.

But mine is not condensating.

The idea with a condendating boiler is as the TRV closes so either more water is forced through each remaining radiator or the by-pass valve lifts. Either way it means the return water gets hotter and hotter and the flame reduces as a result so in the heart of Winter the boiler will never switch off it will just run was a variable output.

As the spring approaches it will reach a stage where the boiler can't turn the flame down low enough so it switches off. Those with anti-cycle software wait a short time then fire up again. If after a short time it needs to switch off again it will extend the time before trying again and if the water does not return hot within the sample time then it will reduce the time before retry.

By placing a thermostat where it gets the morning sun one can inhibit the boiler from starting where likely it's going to be a warm day. Other than that there is little reason why one would put a thermostat on a modern condensing boiler with anti-cycle software.

There are a few major problems.
One is incorrect installation where the return water is for some reason always returned too cool where for example too many towel rails or fitting a hall radiator without a thermostat has upset the system.
Also not setting lock shield valves.
Setting too many off times on the timer can also mess up the system as each time it fires up it can depending on software reduce the restart time as cool water is returned.
Without studying the software carefully knowing what boiler will allow for multi operations of the time clock and which will not is impossible. Fitting a electric thermostat could cause the same problems and mess up the boilers software.

I am not saying it will mess up the software in this case but the poster needs to be aware some systems are just not designed to have thermostats.

In the main pre condensate type there is no problem with a thermostat. Post condensate type is needs careful reading of the documentation to know.

Not all boilers have anti-cycle software and some do continue to pump the water around all the time. So in some cases a thermostat can help.

But one has to be aware boilers today do have complex systems built in and one can't just do something because that's how we have done it for years.

 

[JohnW2]

My system with a thermostat with 0.5 degrees between on and off normally runs for an hour at a time.

If that is the case, then your TRVs must presumably be doing virtually all the work of controlling temperature, and the thermostat is pretty redundant. As you go on to say ....

By placing a thermostat where it gets the morning sun one can inhibit the boiler from starting where likely it's going to be a warm day. Other than that there is little reason why one would put a thermostat on a modern condensing boiler with anti-cycle software.

... and,in fact, I would think that much the same would be true with a non-modern non-condensing boiler (like mine ) if one is getting reasonably effective temperature control from TRVs, wouldn't it?

Kind Regards, John

 

[EFLImpudence]

By placing a thermostat where it gets the morning sun one can inhibit the boiler from starting where likely it's going to be a warm day.

I would say the converse is the more desirable.

I.e. to have the heating automatically fire on the unexpected cold days.

I know some people turn off the central heating during the Summer but I do not understand why.
If the weather is warm it won't come on anyway but if not ...

I want my heating to come on when it's cold; be it January or July.

 

[JohnW2]

I know some people turn off the central heating during the Summer but I do not understand why. ... If the weather is warm it won't come on anyway but if not ... I want my heating to come on when it's cold; be it January or July.

That's true if one has a thermostat. However, if one doesn't and is totally reliant on TRVs, then the system presumably has to fire up periodically, even during a heatwave, in order for the boiler to 'discover' (from water return temp) that the TRVs were all shut, or nearly shut. Maybe that's one reason for having a thermostat, even when there are TRVs everywhere.

Kind Regards, John

 

[AndyPRK]

In sure you really don't need heating in July!

Maybe August thus year but I was happy to turn it in manually fir 3 hours

 

[EFLImpudence]

Maybe that's one reason for having a thermostat, even when there are TRVs everywhere.

Yes, it is; there should be a thermostat.

 

[EFLImpudence]

Maybe August thus year but I was happy to turn it in manually fir 3 hours

... but my way you wouldn't have had to do that.

I have had this conversation with several people and will never understand the thinking behind it.

 

[JohnW2]

... but my way you wouldn't have had to do that. ... I have had this conversation with several people and will never understand the thinking behind it.

What temperature do you leave the thermostat set at during the summer? If you left it as the temperature you wanted to be maintained by heating during the winter, it would usually come on quite a lot in summer. That would be fine if on really wanted to maintain the same minimum temperature throughout the year but the reality is that most of us accept and tolerate at least short periods during the summer during which the temperature is less than we would heat to during the winter.

Kind Regards, John

 

[EFLImpudence]

I have it set at 20 in the Winter (now) and turn it down to 17 or so in the Summer.

I think it may be slightly inaccurate so actually a bit lower and a couple of degrees between off and on.

 

[JohnW2]

I have it set at 20 in the Winter (now) and turn it down to 17 or so in the Summer. ... I think it may be slightly inaccurate so actually a bit lower and a couple of degrees between off and on.

That presumably results in the heating coming on, at least briefly, at least at the start and end of some days in even a 'normal' summer? Is that what you want to happen?

Kind Regards, John

 

[EFLImpudence]

That doesn't seem to happen very often but if it does then it is cold.

Conversely - you are saying that you like it colder in the Summer than Winter.

My place isn't as large as yours.

 

[EFLImpudence]

P.S. The heating is not running at the moment.