Room Stats and TRVs

[rikki]

I am upgrading the controls on my CH system and want advice on where to put the room thermostats.

Background: I live in a large 3-floor victorian house. I am installing a 3 channel programmer so that I can heat the 4 bedrooms separately from the ground floor and DHW. The hall and stairs right up to the top floor are heated by a radiator on the ground floor. All radiators are fitted with Thermostatic Radiator Valves except the bathroom (first floor) which is where the room stat is at the moment.

Problem: The TRVs will shut off the rads in each room when that room gets up to the desired temp. I don't want a room stat to turn off before the rooms are warm enough, but I also don't want it calling for heat when all the TRVs are closed.

I know that room stats are digital (i.e. either calling for heat, or off) whilst TRVs are analogue (i.e. the warmer they get, the less flow they allow). Should I therefore have the upstairs channel as it is (bathroom rad with no TRV and the room stat in there) and put the door-stairs stat in the main living room whilst removing the TRV from the rad in there? Or does anyone know of a better solution?

 

[Chri5]

TRV's allow for room temp with door closed. Keeping the doors open tends to eman a TRV will never reach set temp and won't close.

So if you like most and have bedroom doors shut and reception rooms and kitchen open for easy of movement the natural place for the digi system stat is in the common areas.

Bathrooms don't have TRV's due to the CH design normally making the bathroom rad the safety rad (so if all TRVs closed and CH on, the CH has a rad to flow and return to / from).

If you want it's quite possible to have a series of wall stats that work together. So say you have 3 (one to each hall) then once any reaches the set temp the CH system closes down.

 

[Lectrician]

The hallway rad is usually fitted with lockshield valves, not TRV's, and the room stat located here.

A bathroom will get to temp far quicker than any other room usually, and then switch the entire house off.

I guess the bathroom rad has lockshields as it was designed as the system bypass, and I also guess that originally there was no room stat in there.

You can get programmable room stats, and one thing to beware with these is the learning type. you set the air temp you want, and the time you want it. The stat then 'learns' how long it takes to get to temp, and then turns the heating on early to compensate. I have had issues where a chap wanted his house at 26deg (yep, a little hot). The stat learned it took 6 hrs to get up to this temp due to room sizes and crappy rads/microbore pipework. The stat therefore started the heating at midnight, as the chap had set the stat for 6am thinking it would come on at 6am.

In my opinion, these stats are very clever, but too clever for their own good!

 

[ericmark]

Modern boilers tend to vary their output rather and go off and on and also have anti-cycle built in so the thermostat is really just to stop the boiler on warm days and not really bothered too much with house temp. So in window facing morning sun most likely best place so that when we are likely to have a warm day the central heating does not fire up. During the winter it will never turn off and the radiator valves will do their job but as spring arrives we don't want the central heating warming the house in the morning when latter in the day we will need to open windows to cool house down so if morning sun switches off central heating on warm days that's a good thing and it saves once we get to point where maybe we should be switching off for summer having to do it manual. Central heating is great but central control is not so good. Never worked out how storage radiators became central heating as they were all individual units must have been no trades description act then?

 

[rikki]

Very helpful. Thanks.

Yes, the stat was originally in the hall but when I fiited the TRV I thought it pointless keeping it there. Maybe I should revert to having the downstairs stat in the hallway and remove the TVR from the rad there and put it on the bathroom rad. I plan not to have the down-stairs circuit on in the morning. Then I can have the upstairs stat in the east-facing bedroom. I've fitted a by-pass valve so there shouldn't be a problem with not having a system by-pass radiator.

 

[Lectrician]

Modern boilers tend to vary their output rather and go off and on and also have anti-cycle built in so the thermostat is really just to stop the boiler on warm days and not really bothered too much with house temp

This is true, however, a room stat will still kill the heating completely if it switches, which it will if the bathroom heats up quicker than the rest of the house.

 

[sparkyspike]

How about a wireless room stat? This way you can find the optimum location through trial and error and then leave it there.

 

[rikki]

I've thought about wireless, but don't like (a) having to replace batteries, and (b) having yet another radio signal poluting the house.

 

[rikki]

Why does a room stat need three-core wire?

 

[rikki]

To answer my own question - by reading the numerous other posts on this very topic - the room stat needs a permanent live and neutral (terminals 1 and 2) to power the heat anticipator resistor, with a switched live on 3 to call for heat from the boiler.