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Extra thermostatic controls

Postby bristolman » Sat Nov 22, 2008 1:59 pm

I have a Baxi back boiler providing central heating only (hot water from an Aga). There is no room thermostat just TRVs on the living room radiators and a timer to control the on/off periods. I want to add a room thermostat using the existing timer and more TRVs. At present the timer switches on the boiler and the pump and the boiler is then controlled by the boiler thermostat and the pump stays on until the timer switches off. Should the new room thermostat switch both the boiler and the pump on and off or just the boiler? Also I would like advice on whether to leave the hall radiator without a TRV since that is where I will site the room thermostat. Some advice suggests having TRVs on all the radiators and an automatic bypass valve but I've read that you should not have a room stat and a TRV in the same area. Thank you.
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Postby plumbbob » Sat Nov 22, 2008 4:11 pm

It is usual practice to fit the thermostat in series with the timer, so the stat will control both the boiler and the pump. Obviously, it is pointless running one without the other.

As for the TRV in the hall, it must not be used (just remove the top). The TRV may shut down and prevent the room stat from ever reaching the set temp.
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Postby DEEARR2 » Sat Nov 22, 2008 11:54 pm

I would suggest you control the boiler by the room stat as this will prevent any chance of the boiler going to overheat. The location of your room stat is important and it shoud be positioned in an area that will represent the comfort temp required throughout the house. If fitted in the hall, without a TRV, then the hall temp will determine the house comfort. I have found through experience that the room stat is better fitted in the area where people are at rest and seated.
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Postby sparx » Sun Nov 23, 2008 12:51 pm

Hi, first yes it would be a good plan to fit all TRV's except in area with room stat which should be set slightly higher than temp required in rest of the house, if set lower it will cut off boiler before other rooms are warm enough.
It should cut off both boiler and pump otherwise as at present you will be circulating cold water when boiler off. Check boiler does not require pump over run timer.
your hall rad provides 'bypass' circuit so no other required, it's just somewhere for pump to circulat to when all TRV's are shut.
Main function of 'stat is to prevent boiler cycling on/off when house is warm and if outside door opened to let in cold air will 'kick' boiler in quicker,

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