Can somebody clarift TRV relationship to boiler and room stat?

Cardew

gt568 wrote: »

From another point of view, does this have any bearing in the cost of my heating? In my house the stat is in the hall, so if I turn off the rad in bed 2 (trv to 0) will it make any difference in cost? Presumably as the boiler is firing anyway it wont?



So does this mean that you are just pumping hot water around your radiators? I thought the boiler had to be on (firing) for this to happen, is that not the case? Is the boiler pump seperate to the firing of the boiler? My pump only seems to come on when the boiler is firing, perhaps this is because I have an old boiler.

The point you are missing is that the boiler will stop 'firing' when the temperature of the water in the CH system reaches 70C(or whatever you set it to)

As your bedroom radiators are off there is less water being heated so the boiler will switch of and run for far less time, so you will save money.

gt568

Cardew wrote: »

The point you are missing is that the boiler will stop 'firing' when the temperature of the water in the CH system reaches 70C(or whatever you set it to)

As your bedroom radiators are off there is less water being heated so the boiler will switch of and run for far less time, so you will save money.

Okay I get that, but what circulates the water when the boiler isn't firing, or does the water just sit in the rad at 70c (or whatever)? See I thought the boiler would keep heating the house till the temperature set on the stat was reached.

Cardew

gt568 wrote: »

Okay I get that, but what circulates the water when the boiler isn't firing, or does the water just sit in the rad at 70c (or whatever)?

If the water could 'just sit' in the the rad at 70C it would have defied the laws of physics!;) Unfortunately the water temperature drops.

So when the water temperature in the system drops the boiler fires up again until to get the water back up to 70C. This of course is assuming that the temperature in your hall is below that set on your room thermostat.

So in essence in your system 3 things shut off your boiler.

1. Your timer.

2. Your room thermostat

3 The water in the system reaching its set temperature.

All the TRVs do is control the amount of water circulating in the system. If the temperature of every room with a TRV is above that set on the TRV there will be very little water circulating in the CH system as every radiator will not get any water running into it because the valve on the TRV is closed. Thus the boiler will fire up occasionally to heat the water in the pipes(provided the room thermostat is set above the temperature where it is situated.

See I thought the boiler would keep heating the house till the temperature set on the stat was reached.

That is indeed what it will try to do - but as soon as the water reaches 70C boiler shuts off - temp of water drops - boiler fires - water reaches 70C - boiler shuts off - etc etc.

gt568

Thanks I think I'm getting it, but what I can't grasp is, when the water reaches 70c and the boiler shuts off what, if anything, circulates the water in the rads? Is it not sat in the radiators cooling and not circulating?

MarkyMarkD

@Cardew - yes, we have a bypass and hence we are OK with TRVs on all radiators. I am sure that you are right that this leads to increased boiler cycling but that's not a major issue except in the marginal times of year - autumn/spring - when the heating only needs to be on "a bit" but the boiler is cycling on and off more than required. This is probably when a room stat would be useful - on a slightly warmer than usual spring day, we might need no heating at all from say mid-day but the system won't switch off.

We also don't operate our boiler at high temperature for two reasons - it makes the hot water inefficiently hot, which wastes more energy, and it leads the radiators to get too hot for touch. And although in theory the room temperature won't go higher with warmer radiators, I believe that it will if your TRVs are not very good - and ours are not very good - as the temperature will tend to overshoot in a hysteresis loop and do more so if the water temperature is hotter.

@gt568 - indeed, if the boiler is off, the water isn't circulating, and the only reason the house warms is that the water temperature in the rads falls as the heat is released into the house. That's the reason for NOT having a room stat controlling the boiler - you might have many rooms colder than you desire, because the room with the stat in is hot enough to switch off the boiler.

Essentially, any system is a compromise between maximum efficiency and maximum comfort. My system gives maximum comfort as every room is TRV temperature controlled, but doesn't maximise efficiency as I get excessive boiler cycling, particularly in spring and autumn. Cardew's system has better efficiency because of his room stat, but I believe he won't have comfort quite as optimised as my system.

Most people don't mind having one room a bit cold - typically the hall - where the room stat sits and which controls the boiler. But our hall is TERRIBLY cold and I am quite happy without the room stat in the circuit.

Cardew

The pump runs when the boiler is firing and this circulates the water. Don't forget that if all the radiators are shut off(because the TRV's are closed) there is little water in the system and it will still cool off and cause the boiler to fire again.

As I said earlier, the disadvantage of MarkyMarkD's system, and indeed the way I run my system, is that the boiler is firing up when not required to pump water into the radiators

Incidentally you should have a control(stat) on your Hot water tank that controls the temperature of your hot water for baths etc - it should be considerably lower than the CH water temperature.

gt568

Cardew wrote: »

Incidentally you should have a control(stat) on your Hot water tank that controls the temperature of your hot water for baths etc - it should be considerably lower than the CH water temperature.

That's what I thought, however there isn't one.

Cardew

MarkyMarkD wrote: »

@ Cardew's system has better efficiency because of his room stat, but I believe he won't have comfort quite as optimised as my system.

MMD,

For exactly the same reason as you, I, in effect, run my system in exactly the same manner as you do - my Room stat is always set higher than the ambient temperature and is 'not in play'; I put up with the additional cycling this causes.

I mentioned the bypass valve because somebody without a bypass valve might have read this thread and been tempted to fit a TRV to a non-TRV'd radiator.

Have you not got a stat on your HW tank to control the temperature of your hot water? Mine switches a valve that stops hot water from the boiler going to the tank. From EST site:

<H3>What is a cylinder thermostat?

A cylinder thermostat switches on and off the heat supply from the boiler to the hot-water cylinder. It works by sensing the temperature of the water inside the cylinder.

</H3>

gt568

I can't find one, and as such I'm a bit perplexed as to how to control the HW temp.

Cardew

gt568 wrote: »

I can't find one, and as such I'm a bit perplexed as to how to control the HW temp.

If you haven't got a cylinder thermostat your hot water will eventually reach the same temperature as your CH water temperature.

Mine is strapped to the bottom of my HW cylinder and I have set it to 60C.
When it reaches that temperature it shuts off the supply to the tank.

My CH water is quite a bit higher as my boiler is apparently more efficient at higher temperatures. In fact the handbook states that in Winter the temperature should be set at 5(77C) or 6(82C).

The added advantage of very hot water in the CH system is that very hot radiators mean the house warms up quicker.