room stat or trv's controlling boiler or both?


Postby flossy » Sat Mar 20, 2010 11:43 pm

Just moved and heating system is vokera combi boiler, drayton timer, rads with trv's, and room stat in hallway, our question is do the trv's govern the boilers on off cycles or the room stat or a combination of both, our main concern is with the room stat at 19c our groung floor rooms do not seem warm enough so we turn up the stat to 22c and this is better but we are concerned about high energy bills. Having a very similar system at our previous address gave no such concerns but it had no room stat, and relied on its trv's. With this in mind and knowing the call for heat was due to the trv's we decided to put the room stat at our new address on full setting 30c and rely on the rads trv's this gave a better outcome, is this ok to do, we notice that the boiler still runs its pump doing it this way even when the flame is off (no call for heat) where as relying on the room stat switches the pump off when the stat is not calling for heat is this correct. Is what follows correct then:
If boiler had no room stat it would cycle its flame on and off keep pump on and rely on call for heat from trv,s.
What is the best way to use this system, sorry for length of query but its a difficult one to put into words.
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Postby htg engineer » Sun Mar 21, 2010 4:58 pm

On a gravity HW system then yes the room stat directly controls the pump, but not on a combi as if the pump stopped when the boiler was still hot , this could damage components. The boiler will have a pump over-run, so if you turn the heating off the pump will still run until the boiler has cooled a little (some boilers also use an over-run on the fan to cool the boiler).

Current regulations require a room stat, and TRV's are advisable. The room in which the room stat is located should not have TRV's and the stat should be sited away from radiator, doors and windows etc.

The room stat will control the CH, as in when the room reaches temperature there'll be no demand for heat but if not sited properly the rest of the house might not be warm enough, TRV's control the temperature of each room but the boiler will still cycle as long as the heat is being lost elsewhere (other radiator and pipework).

I'd advise turning room stat up to 22 - 25 and forgetting about it (as many people do) then use the boiler stat to determine the temperature of the radiators and the TRV's to control each room.


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Postby revinkevin43 » Mon Mar 29, 2010 2:30 pm

Thanx for reply htg engineer, now that we are going to try your method to run our heating we have one final concern, we have set boiler at 65 and room stat at 25 as you said, most rad trv's are either on 4 or five, with these settings the room stat will probably never switch off on cold days so will the boiler be constantly using gas ie: flame on at all times during our heating periods 6am til 8am and 5pm til 8pm with the odd boost or will the rads with their trv's leave the pump running but flame cycling on and off as per our old system at old address, is the boiler monitoring the return temp of the water from the rads and then triggering on and off of the flame, sorry if this sounds a bit thick but heating aint my game, thanks for your time so far in this matter.
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Postby htg engineer » Tue Mar 30, 2010 6:19 pm

The TRV's will not shut the pump down, the pump will continue to run - there's thermistors/sensors in the boiler, they are in dry pockets or directly into the CH water - as each TRV shuts down the boiler will not stay on as long, so as each TRV shuts down the less the boiler will come on to retain the temperature.

"with these settings the room stat will probably never switch off"
if there's not many 'air changes' (people opening and closing doors and windows) - the room stat may still shut the boiler down.


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