Wiring a room thermostat downstream of a CH controller

[Thorfinn]

Hi all,

To keep the peace with Wotsername, I have reluctantly agreed to add a room thermostat to our CH system.

The timer/controller switches hot water and heating (oil-fired boiler). We never use the hot water facility - electric immersion cheaper.

Newly purchased thermostat is a bimetal-strip Drayton, capable of three-wire or two-wire installation.

Can I take the heating load output wire from the controller and regard it as the Supply side of a two-wire installation of the 'stat??

Or is there a more elegant, three-wire solution, as I feel there should be?

I'm reasonably competent with things electrical but wanted to check this one with more knowledgeable people first.

Hope I've written this clearly -All thoughts welcome!

 

[kbdiy]

Sounds straightforward enough. CH 'on' terminal to stat, return from stat to trigger boiler & pump. Presumably your existing setup is a direct feed from CH 'on' to the boiler & pump?

Don't quite understand why you would be reluctant to fit a room stat which would allow you to have better control over your heating system and hence cheaper bills.

 

[EFLImpudence]

Can I take the heating load output wire from the controller and regard it as the Supply side of a two-wire installation of the 'stat??

Yes, the thermostat is just a switch.

Or is there a more elegant, three-wire solution, as I feel there should be?

Not really.

There may be two terminals in the boiler which are linked.
This link can be removed and the two thermostat wires substituted.

However, if you have a wiring centre it would be best to connect to that using the first method.


In this case Wotsername is right but don't tell her.

 

[Thorfinn]

Sounds straightforward enough. CH 'on' terminal to stat, return from stat to trigger boiler & pump. Presumably your existing setup is a direct feed from CH 'on' to the boiler & pump?

Don't quite understand why you would be reluctant to fit a room stat which would allow you to have better control over your heating system and hence cheaper bills.

Gentlemen, have you EVER known a lady turn a thermostat DOWN???

 

[ban-all-sheds]

Yup.

Crank up the temperature until she moans that it's too hot, then suggest that in that case she could take her clothes off.

 

[JohnW2]

Or is there a more elegant, three-wire solution, as I feel there should be?

Not really.

Is not the 'third wire' we're talking about going to be a neutral to allow the contained 'heater' (resistor) to improve the hysteresis of the stat? If that were the case, it would presumably be better to give it a neutral?

Kind Regards, John

 

[Thorfinn]

Or is there a more elegant, three-wire solution, as I feel there should be?

Not really.

Is not the 'third wire' we're talking about going to be a neutral to allow the contained 'heater' (resistor) to improve the hysteresis of the stat? If that were the case, it would presumably be better to give it a neutral?

Kind Regards, John

Yes, that's what I was getting at...I had assumed, prior to buying this 'stat, that it would be a simple two-pole switch. If the third pole is a Neutral, what is happening here...mains potential is dropped across a resistor, to provide hysteresis - but how?

I'm keen to understand this!

 

[JohnW2]

Yes, that's what I was getting at...I had assumed, prior to buying this 'stat, that it would be a simple two-pole switch. If the third pole is a Neutral, what is happening here...mains potential is dropped across a resistor, to provide hysteresis - but how? ... I'm keen to understand this!

I said 'improve hysteresis' (essentially reduce), rather than 'provide' it.

As I understand it, when there is a neutral available, a small 'heater' (usually just a resistor) warms up the bi-metallic strip to some extent, thereby causing the stat to 'switch off' the heating sooner than would be case if it had to wait for the air temperature in the room to rise high enough to switch it off. In that way, the tendency for there to be appreciable 'upward overshoots' in room temperature during the on/off cycles is reduced.

Kind Regards, John

 

[Thorfinn]

Many thanks John, that was what I thought might be going on but it's nice to have it confirmed. So it looks like I can extend the Ring-Main Neutral from the Controller to the Thermostat. UNLESS, of course, anyone sees an obvious and elementary flaw in that plan?

 

[JohnW2]

Many thanks John, that was what I thought might be going on but it's nice to have it confirmed. So it looks like I can extend the Ring-Main Neutral from the Controller to the Thermostat. UNLESS, of course, anyone sees an obvious and elementary flaw in that plan?

The neutral needs to come from the same circuit as the live which is being switched - presumably from the CH timer/controller (that supply probably protected by a fuse in a FCU). You must not just take a neutral from some other circuit (such as a sockets ring circuit). You should be able to achieve what is required by taking a 3-core+E cable or 4-core flex from the timer/controller to the stat.

Kind Regards, John

 

[ericmark]

I found with a old mechanical thermostat even with the heater connected there was around a one degree difference between on and off temperature and without the neutral getting nearer to two degrees. I swapped for a battery operated thermostat with timer built in the difference between on and off reduced to 0.5 degrees and instead of the clock switching central heating off the thermostat now changed temperature so rather than fully off swaps from 20 deg to 14 deg.

Down side two AAA batteries every year. Will last two years but when they fail no heat so swap every year.

At £25 for the Horstmann DRT2 Programmable Digital Room Thermostat it just does not make sense tiring to gain a neutral. The Siemens REV24 RF 7 Day Programmable Wireless Digital Room Thermostat at £80 is wireless. With either no need for frost stat and you will be unaware of the conditions outside.

 

[JohnW2]

At £25 for the Horstmann DRT2 Programmable Digital Room Thermostat it just does not make sense tiring to gain a neutral.

If there were an existing thermostat, which didn't have a neutral feed, I would be inclined to agree. However, since the OP is 'starting from scratch', and needs to install new cable, I'm sure that there must be a neutral available in the timer/controller, so he just needs to install 3-core (and earth) cable/flex to get a neutral to the new stat.

As for the hysteresis, I'm not sure that is is really necessary, and maybe not even desirable, that one should attempt to get it too small. If one gets the 'on' and 'off' temps less than 1 degree apart, the boiler (and/or pumps and motorised valves, according to the set up) is presumably going to be switching on and off pretty frequently, which is not necessarily 'good for it/them'!

Kind Regards, John

 

[ericmark]

It will clearly depend on the heating system. With my simple gas boiler with no TRV fitted down stairs where the thermostat is situated the 0.5 degree unit works fine.

With a system controlled by TRV's on every radiator and a heater with both condensating system and an anti cycle software built in then fitting any electric thermostat would be daft unless of course swapping the TRV heads for WiFi controlled models and using a central server to control the heating will possibly telemetry allowing mobile phone override.

Nearly every boiler has an option to fit a timer and frost stat and clearly those contacts can also be used for an electric thermostat. However unless one is fitting a thermostat in every room controlling each radiator when TRV are fitted using an electric thermostat rather defeats the whole idea of TRV's.

 

[Thorfinn]

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.

 

[JohnW2]

It will clearly depend on the heating system. With my simple gas boiler with no TRV fitted down stairs where the thermostat is situated the 0.5 degree unit works fine.

I have no doubt that it "works fine" in terms of temperature control. I was thinking of whether very frequent cycling of pumps, and boilers, MVs etc. is necessarily all that good for them.

Kind Regards, John