Can somebody clarift TRV relationship to boiler and room stat?

gt568

Been reading the 24on or short burst heating thread and I have a couple of questions regarding my system. I've got an old non-condensing boiler, rads with trv's (not all though), and a room stat in the hall.

Now if I set the room stat to 20c, what should I set the trv's to? When the stat reaches 20 it shuts off the boiler (I think), but what difference will the trv's play in this? If I set them to 5 will it heat the house faster (as the rad will be hotter)? How do the trv's know when to shut off?

MarkyMarkD

The TRVs don't affect the temperature of the rads - they just switch them on and off (albeit gradually).

The only way to warm the house quicker is to increase the boiler temperature so that the water flowing through the rads is hotter.

I don't understand the point of having TRVs anywhere except EVERYwhere. With a partial system, you will always end up with the un-TRV'd rooms at stupid temperatures.

We have TRVs on all radiators and no room stat at all. So then all the rooms get to the right temperature and the relevant radiator switches off.

The TRVs switch off when the temperature of the air above the valve is at the level you've selected.

ic

Ideally the room stat should be in the coldest place that takes the longest to heat, and won't be affected by a nearby source of heat (cooker, fireplace, etc) - normally the hall. Set the stat to the temperature you want in that room, then adjust the trvs in the other rooms so that they switch off at the temperature you want in that room.

It can take a few days to get this correct, but essentially you need to go into a room and keep tweaking the trvs. Turn the up or down a nudge until the room seems right all the time.

In my house I have my stat in the dining room - this room is fairly big and is always the coldest as the stairs lead off it to upstairs - so the heat just whistles off up there. I've set that room to 19oC, and left the trv on full on that radiator (because the stat will switch the entire system off and on based on the temperature in that room).

I've then got the bedrooms and bathroom on "3", and the living room on "4", and the landing on "2". I've found this is comfortable all the time when the system is running. On a particularly cold night I might just need to nudge the stat up half a degree, or if I'm in the living room, just switch on the gas fire there.

I also have my boiler turned up to just under max - may as well have the place heat up quickly! After all, when it reaches the right temperature, the stat will switch the boiler off.

Cardew

To start from the beginning, the water temperature in your CH system is set on your boiler, say, 70C. That and that alone determines water temperature.

When your CH system is switched on, the boiler will fire until the water in the system(pipes and radiators) reaches 70C.

Your pump circulates water through the radiators. The radiators dissipate the heat and the water in the system cools down and the boiler fires to keep the temperature at 70C.

The room thermostat will switch off the central heating once its set temperature is reached.

TRVs set the level of heating in a room; not the speed at which it heats the room. So the water temperature in the radiator(when the TRV is open) will be exactly the same regardless of the setting of the TRV.

The principle of operation of TRVs is simply a 'bellows' that operates a plunger that shuts off the water to the radiator. The setting on the TRV only determines the air temperature of the room at which the plunger shuts off water to the radiator. For instance on my Honeywell TRVs(VT117E) the setting 1/2/3/4/5 correspond to 16/18/20/22/24C.

The important point to note is that if your room is at, say, 14C and you switch on your CH the room will reach 16C in exactly the same time regardless of what setting(1 or 5) you have on your TRV as the temperature of the water in the radiator will be the same.

The final temperature of room is determined by your TRVs provided the temperature of the room in which the room thermostat is situated hasn't been reached.

So if your room thermostat is in your living room and set to 20C, you will not be able to get above 20C even if you turn your TRVs in the same room to maximum.(assuming both are calibrated correctly)

Put your room thermostat set to 20C in a cold draughty hall and your heating will in effect be running all the time when so set by the timer.(the boiler will keep firing up and shutting down as the water reaches 70C)

So it is a combination of the room thermostat and TRVs that you need to experiment with to get the best solution for your house.

gt568

Right sorry I seem to be missing something.

Surely in my system the TRV's are redundant unless I want a room colder than what is set on the stat. When the stat reaches 20c it will shut down the boiler, and thus the rads.

Assuming I'm not to fussed about exact temps in rooms, then is the best thing to do is whack the trv's to 5, set my stat to 20, and wait for it to heat up?

Also what should I have the boiler set to? Mine goes from low, 1, 2, 3, 4, high?

gt568

TRVs set the level of heating in a room; not the speed at which it heats the room. So the water temperature in the radiator(when the TRV is open) will be exactly the same regardless of the setting of the TRV.

So why do they seem colder on 3 than 5? Is it because they keep shutting off at a lower temp?

Cardew

gt568 wrote: »

Right sorry I seem to be missing something.

Surely in my system the TRV's are redundant unless I want a room colder than what is set on the stat. When the stat reaches 20c it will shut down the boiler, and thus the rads.

Assuming I'm not to fussed about exact temps in rooms, then is the best thing to do is whack the trv's to 5, set my stat to 20, and wait for it to heat up?

Also what should I have the boiler set to? Mine goes from low, 1, 2, 3, 4, high?

In the room where you have the room thermostat situated you are correct that the TRV is redundant - unless you want the room cooler, say 18C, than that set on the room thermostat(at say 20C.)

However that doesn't apply to the other rooms with TRVs. To give a simple example:
Put the Room thermostat at 20C and shut off the radiator(s) in that room. The boiler and pump will keep running and all of the other rooms will reach the temperature set by their TRVs

Whilst I have a room thermostat in a large hall that I keep much cooler than my living rooms, I tend follow MarkyMarkD's practice of controlling the termperature of all the rooms by TRVs. My room thermostat is effectively redundant - not supposed to be best practice as the pump is always running and boiler firing up and shutting down a lot.

You boiler temperature will have a most efficient setting - see handbook - normally close to maximum. Some people don't like this as the radiators get very hot for young kids.

Finally, yes your radiators appear hotter on 5 because the hot water remains in them for longer and is not cooling down as it will on a setting of 2, but in the initial warming up the radiators will be the same temperature. If you boil a kettle it will heat up at the same rate, but if you keep switching it on every few minutes it will be on average hotter than one you keep switching on every 30 minutes.

gt568

Okay, so do you leave your heating on for long periods then (24/7 or all daytime) or do you only use it when you are in (couple of hours here and there)?

Also if I turn up the temperature on the boiler, the hot water out of the tap becomes unbearable, any way round this?

Lastly

Put the Room thermostat at 20C and shut off the radiator(s) in that room. The boiler and pump will keep running and all of the other rooms will reach the temperature set by their TRVs

Whilst I have a room thermostat in a large hall that I keep much cooler than my living rooms, I tend follow MarkyMarkD's practice of controlling the termperature of all the rooms by TRVs. My room thermostat is effectively redundant - not supposed to be best practice as the pump is always running and boiler firing up and shutting down a lot.

Does this not mean your boiler will be going like mad to get the hall up to 20c or whatever it is set at?

Cardew

gt568 wrote: »

Okay, so do you leave your heating on for long periods then (24/7 or all day) or do you only use it when you are in?

Also if I turn up the temperature on the boiler, the hot water out of the tap becomes unbearable, any way round this?

My use depends on the weather outside.

I turn it off when out of the house - obviously.

I have CH timed to go off about an hour before I go to bed and come on 20 mins before the first person gets up.

The TRVs in bedrooms and rooms not in use are turned down(effectively off) when not in use.

The heating is timed to switch off in the morning, but is overidden on an 'as required' basis. On cold winter days this might mean it on all day and only off at night.

Does this not mean your boiler will be going like mad to get the hall up to 20c or whatever it is set at?

No once the water gets up to 70C the boiler will shut off.

Also like you suggested above "Surely in my system the TRV's are redundant unless I want a room colder than what is set on the stat." That is exactly what I have - i.e. my TRV in the hall is set way below the temperature set on the room thermostst.

Cardew

MarkyMarkD wrote: »

I don't understand the point of having TRVs anywhere except EVERYwhere. With a partial system, you will always end up with the un-TRV'd rooms at stupid temperatures.

We have TRVs on all radiators and no room stat at all. So then all the rooms get to the right temperature and the relevant radiator switches off.

Just a note of caution on this point.

Some systems require radiators in your home, usually the bathroom, to not have a TRV. In these systems there needs to be one radiator always open to allow water to flow into it if all the others are turned off, to avoid damage to the pump or boiler.

You can incorporate a by-pass valve into the system(as obviously the system MarkyMarkD has) which means that you can have TRVs on all radiators as this valve will allow water flow.

The point being that if you have a radiator(s) without a TRV, it is probably missing for a purpose and you should check with a plumber before taking it on yourself to get a TRV fitted.

gt568

The TRVs in bedrooms and rooms not in use are turned down(effectively off) when not in use.

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?

Quote:
Does this not mean your boiler will be going like mad to get the hall up to 20c or whatever it is set at?
No once the water gets up to 70C the boiler will shut off.

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.