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Honeywell CM907 problems

Discussion in 'Plumbing and Central Heating' started by heatcrazy, 6 Apr 2021.

  1. heatcrazy

    heatcrazy

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    Moved into a house with 2 x CM907 thermostats: 1 in hallway downstairs 1 in hallway upstairs.
    Got a couple of issues we can't get to grips with which result in inconsistent heating:
    downstairs thermostat clicks on many more times than the one upstairs and today noticed that the target temperature was set at 19 c, room temperature showed 19.5 c but the heating came on.
    Upstairs in evening, temp set to 21.5 c - some days its absolutely boiling and other days its quite cool (even if weather is similar) also if both upstairs & downstairs are set to the same temperature, the rooms downstairs are considerably warmer than those upstairs.
    Any help appreciated!
     
  2. stem

    stem

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    Your description of their location isn't clear. When you say...
    ...is that effectively the same space? (ie top and bottom of the same stairs) If so, they won't work as one will be in a zone heated by a radiator that it has no control over.

    The room thermostats should each be connected to distinct and separately plumbed heating zones. This means that they would be located in an area that is only heated by a radiator that they have control of.

    Usually there will be a thermostat in the hallway downstairs controlling the heating in the living areas, and a separate thermostat for the sleeping areas, usually in the master bedroom. Ideally the rooms in which the room thermostats are located should be the last in their respective zones to warm up. Sometimes the radiator where they are located is downsized slightly to make sure that happens. That way, the other rooms reach the temperature set on their respective TRV's (if fitted) first before the room thermostat turns off the entire zone.

    If the downstairs and upstairs hallways are isolated from each other and meet the above criteria there shouldn't be a problem. In which case here are a couple of suggestions:

    1. Sounds daft, but I've seen this. Check to make sure that they are not wired the wrong way around. You can do this by switching both room thermostats to their minimum setting and allowing all of the radiators to go cold. You do not mention any TRV's but if they are present, for the test, set them all to their maximum setting. Now turn the downstairs thermostat to maximum and check that the downstairs radiators (or living areas) only heat up including the one in the area that the room thermostat is located.

    Repeat the process with the upstairs thermostat to maximum and downstairs set to minimum, check that the upstairs radiators (or sleeping areas) only heat up including the one in the area that the room thermostat is located.

    2. In areas where the room thermostats are located, the radiators should not have a TRV fitted. Otherwise one device can interfere with the operation of the other. If this is the case the TRV's in the areas where the room thermostats are located should be left set to maximum so that they do not stop the room thermostat from operating properly.
     
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  4. heatcrazy

    heatcrazy

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    Thanks for the info, the thermostats are basically in the same space which from what you've said is wrong. All the radiators have TRV's fitted. I will have a look and see if I can get the one upstairs moved to the main bedroom at the weekend. Thanks again!
     
  5. DP

    DP

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    I assume the TRV on the radiator is fully open where the thermostat is fitted
     
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  7. stem

    stem

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    Sounds like a bad install. At least from a controls point of view. Maybe the plumbing is OK, but I would want to check to make sure that the radiators are plumbed up as two separate heating zones, each controlled by its own motorised zone valve. Otherwise it will never work, no matter what you do with the controls.

    As @DP setting the TRV to maximum in the room where the thermostat is will allow the room thermostat to take sole control.
     
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