Tiresome CH question (sorry!)

[matthew.larkin]

I'm certain that this has been discussed before, but I've never quite understood the full logic of the answer, so please forgive me asking it again. The question is in relation to CH systems which have a thermostat in a room with TRVs on the radiators.

I'm aware that the general principle is that in the room with the thermostat the radiator should be allowed to operate at full capacity, so either the rad shouldn't have a TRV fitted, or if it is fitted then it should be fully open. This should logically mean that you would put the thermostat in the room of the house which is the hardest to heat, so that only when that room is satisfied should the heating turn off. Other TRVs (in easier to heat rooms) will therefore be set lower, and be satisfied at an earlier stage.

In my case, the thermostat is actually fitted in a well insulated room which is simple to heat. And to achieve a good balance of heat around the house (without changing the location of the thermostat) I set the TRV at a low level in that room, and higher in others. It works (insofar as achieving good temperatures).

There are two radiators in the house which do not have TRVs. One is in the utility room (where the combi is located) and the other directly above it. My understanding of the reason for the general principle above is so that excess heat from the system can always be dissipated through a radiator somewhere (am I wrong?), and I would have imagined that these two non-TRV radiators would provide that outlet. Can you explain the fundamentals please as I'm clearly not understanding something!?!

What can go wrong with my system, and how concerned should I be to relocate the thermostat to a "colder" room in the house?

(System is a WB Greenstar 30CDi).

Thanks!

Matt

[John Rumm]

The rad in the room with the stat does not need to run at full capacity, it is more important that it runs at a constant fixed (but selected) output. Hence it will need balancing (using one of its lockshield vavles) such that it does not allow that room to reach temperature too soon. What you don't want is to have a TMV that might shut it off altogether before the main stat is satisfied.

Doing the same trick with the manual valve would work better.

Its normal to have at least one rad with no TMV to act as a bypass. That way there is no possibility that the flow of the water in the heating circuit can be choked off completely should all the TMVs close.

The stat probably wants to be in the room the cools the fastest, rather than necessarily one that is hardest to heat. Halls are often a favourite since there are lots of air changes with people moving through them. Setting the rad output with the lockshield and removing the head of the TMV shoudl fix any likely problem on your system.

[Toby]

I think you mean TRV (Thermostatic Radiator Valve), rther than TMV (Thermostatic Mixing Valve) Just to avoid confusing the OP even more!!

Toby...

[matthew.larkin]

Ah, that's starting to make some sense. And it also highlights a problem I have with TRVs. As I understand them, they are *supposed* to allow you to have the rad turn off when the room is at temperature. But as they only usually have the numbers 1-6 on them, how do I know what is the *right* setting. Trial and error I suppose, though I suspect there are tools for heating engineers.

But in essence, you are saying that if I have a stat which desires a room temperature of 18degC, I should make sure that the TRV only shuts off when the temperature reaches higher than that.

In terms of whether my system is safe operating as I have it at the moment, I guess you're indicating that as there are two non-TRV radiators in the house, in the event of the stat remaining unsatisfied when the rad has shut down then they will be an appropriate outlet for the heat from the system.

Why?

OK, again that makes sense - where it is at the moment, the room may retain heat better than the living room, so the living room could be cool when the stat room is warm.

So I'd just restrict the flow of the rad in the stat room , whilst leaving the TRV fully open.

The idea about halls is an odd one. I've lived in many houses where the stat was in the hall, and I could never understand why this was a good idea. I find it really difficult to set the temperature of my inhabited rooms by reference to the temperature of the hallway, as by its nature it has more access to external (cold) air. I often found at my parents house that the lounge would be roasting as a consequence of people coming in and out of the house a lot. But if no-one was coming in or out, then the lounge would be cool. That's why we had the stat put in an inhabited room, so that if we were satisfied with the temperature there, to hell with the temperature of the hall!

Thanks for your help. I will implement!

Matt

[John Rumm]

Indeed I do, sorry fingers on autopilot.

[John Rumm]

Indeed. You may find that new ones come with a leaflet that translates the numbers into approximate temperatures. There are probably too many variables to print actual room temperatures on the valve with any degree of reliability.

Yes, if you have a TRV on the rad in the room with the main stat, then ideally you want the valve on the rad to never shut off the rad (i.e. you want the rad to cool only when the main stat shuts off the heating altogether) - so set it to its maximum. Unscrewing the valve head normally leave it permanently on "full on"

Yup - not only the heat, but also the flow of water. Some pumps may not take kindly to pumping against an obstructed flow for too long.

Setting the TRV will limit the maximum room temperature - possibly to something lower than the main stats "satisfied" temp. This will lead to inefficient operation as the heating will continue to run all the time, even when the house is already warm enough.

Reducing the output of the rad with the lockshield will not limit the room temperature - however it will reduce the power output of the rad resulting in slower heating of the room.

Yup - so the heating will run longer, tending to keep the living room up to temperature.

That is where the TRVs come to the rescue. You can obviously complicate the system with multiple zones, each with their own room stat if you want. Its a case of designing something that works well for your house and the way you use it. So there is no guarantee that the "normal" way is the right way in every case.

[Roger Mills]

Trial and error it is - adjust the TRV until the room is comfortable, moving it half a notch at a time and allowing a few hours between adjustments.

No - you shouldn't have a TRV at all - although it's better that way round than the other way. If the room stat cuts the whole system off when that room reaches 18degC, the TRV will never operate. But it's better still to remove the thermostatic head - then you *know* it can't operate.

If the TRV shuts the rad off *before* thew room reaches the room stat temperature, the room stat will never switch the system off - so it will rum all the time as though there were no room stat. Not good for your wealth!

Assuming what was meant was having manual valves in fixed position, this is better than a TRV in the room stat room because it is consistent, and there's no possibility of having two stats 'fighting'.

[Hamish Shufflebotham]

I set my thermostat to maximum (28.5C) and let all the TRVs control there own space. Works absolutely fine for me, and has done for the last 10 years.

[John Rumm]

It will work just fine, but cost you way more than it needs to since the boiler will be kept running all the time the house is already warm enough.

[Hamish Shufflebotham]

It has to anyway - gravity fed solid fuel boiler - 90000 btu / 16 rads / 3 tons anthracite per year - £780.

luverly 24 hr warmth.

[Roger Mills]

It *will* actually reduce the room temperature - but may be no bad thing. With a reduced rad output, the equilibrium temperature - in extremis - for which the rad output equals the heat loss, will be lower.

[John Rumm]

Indeed, but I was trying to keep it simple! ;-)