how radiator thermostat valves work with room thermostats

Discussion in 'Plumbing and Central Heating' started by riffraff, 16 Aug 2010.

  1. riffraff

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    Hi All (again),

    Last winter we were tortured with constantly adjusting the two thermostats we have for central heating.
    One is for upstairs and the the other for downstairs.

    Trouble was that (as expected i suppose) with the doors shut some rooms were either too hot or too cold compared with were the thermostats are.

    My father in law said i should get individual valve thermostats for each radiator but i cant figure out in my head how that would work with the room thermostats because they ultimately control whether or not the central heating comes on.

    Say i install the thermostat valves and set each room radiator to 22 degrees, what do i set the room thermostats to? If i set those to 22 as well then there's every chance that it'll knock off the heating when it detects 22 meaning the radiator valves wont be able to much. If i set the room thermostats to something really high to avoid that...like 30 degrees then even though the room radiators will be shutoff at 22 degrees the heating will still be burning away to try and maintain the 30 degree setting for the room thermostats.

    The other reaon I'm asking is because i was thinking of replacing the analog room thermostats with digital (for quicker responses to temperatures) but i dont want to spend the money if installing radiator thermostats would mean i dont need to.

    I guess i dont understand how the system is supposed work.
    Any comments?

    Thanks in advance.
     
  2. kevplumb

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    you dont put a trv on the rad in the room with the stat ;)
     
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  3. croydoncorgi

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    The basic idea of room stat(s) isto control the BOILER so that it provides heat until ALL the building is warmed up. With no TRVs, in theory the flows through the rads should be balanced and the rads themselves sized accurately, so that all the rooms warm up nicely (with the doors shut) and then the boiler shuts down until everything has cooled down and needs to be warmed up again. And yes, there WILL be a cycle of warmer - hot - cooler - cold - warmer, etc. in some or all of the rooms. Correct positioning of the room stats is critical to avoid cold rooms when the heating cuts off too early or everything getting too hot because the room with the stat is under-heated and lags behind the rest.

    If you have TRVs on all the rads, they will (supposedly) close when each room reaches its individual setpoint. The trick then is to ensure that the roomstat stops heating boiler as soon as there's no demand from anywhere and restarts it as soon as ONE of the rooms requires heat. Almost as difficult as balancing the old way! TRVs can get confused eg. by having curtains draped over them and sometimes just don't work very well anyway.

    If you currently have NO TRVs, a better investment might be a boiler capable of modulation with a weather compensator or room compensating controller.
     
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  4. D_Hailsham

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    Reply edited 24/8/10 - see below

    So your house is zoned with two motorized valves, one controlling upstairs and the other downstairs?

    Where are the two thermostats located - in the hall and on the landing? The problem with that arrangement is that the upstairs stat will receive heat from the rad on the landing and from the rad in the hall (heat rises).

    That's why you do not have a TRV on the rad in the same area as the wall thermostat. If you set stat to 20C and TRV to 22V, the boiler will cut off before the TRV has reached temperature. If you set stat to 22C and TRV to 20C, the thermostat will never reach 22C, so the boiler will run continuously.

    For TRVs to work properly, (a) the system must be properly balanced, and (b) the area with the room stat must be the last to reach temperature, so the TRVs have a chance of working. This usually means having the room stat in the coldest area. Although some advocate having it in the living area on the assumption that will be the hottest area of the house (21C) and the other rooms will be at a lower temperature.

    If you install wireless thermostats you can easily experiment with the location - before fitting the TRVs, of course.

    If you find that the room stat turns the boiler off before the TRVs have operated, you should close down, a bit at a time, the wheel valve on the radiator without a TRV. This will reduce the output from that rad so the area takes longer to heat up.
     
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