Honeywell CM907 with Underfloor Heating

Discussion in 'Plumbers' Talk' started by fred812, Sep 23, 2014.

  1. fred812

    fred812 Screwfix Select

    Hi All, would appreciate your thoughts on this.

    Extension on the house is heated by underfloor heating (2 zones) controlled by 2 x Honeywell CM907 programmable thermostats. Main house heating (radiators) and hot water feed to tank is separately controlled but from the same new Worcester/Bosch 30CDI gas boiler. Zone valves and thermal actuators control all this. This was all professionally installed.

    But been left to configure the CM907's. Its a bit confusing because the config for cycles per hour mentions gas boiler (6 cph), thermal actuator (12 cph), zone valve (6 cph) which are all part of the system. Also I notice that the minimum on time is 1 minute. Checking the spec of the thermal actuators (22C) I notice that their opening time is 90s (start) - 3 min (end), which means that when at the minimum on time of 1 minute no hot water is going to enter the underfloor system.

    I thought I would ask here because have read through many posts on the TPI thermostats and its a bit of a minefield, also the Honeywell advice doesn't appear to be that hot either.

    Anyone any thoughts on how this should be configured please.

    Thanks for any help.
    Fred
     
  2. sam spade

    sam spade Active Member

    What is each CM907 controlling?
    What controls the boiler?
     
  3. fred812

    fred812 Screwfix Select

    The CM907's control the thermal actuators on the two underfloor heating zones. The boiler comes on as soon as there is a demand from the CM907 but I think the flow goes down the bypass until the actuators start to open.
     
  4. sam spade

    sam spade Active Member

    1. Does the signal for the boiler to turn on come directly from the CM907, or from the actuator?

    2. Which make/model actuators?

    3. Are there zone valves on the U/F? If so, what controls them?

    4. Why did the installers not commission the controls?

    5. Did they provide a wiring diagram? If so, can you post it?
     
  5. fred812

    fred812 Screwfix Select

    In answer:-
    1. Don't know but the boiler switches on immediately there is a demand on the CM907 even though the TA lags by a minute or so.
    2. 22C (Watts)
    3. Yes, dunno but the zone valve opens as soon as there is a demand on the CM907
    4. I''m intending to have a word with them about that.
    5. Sorry, they didn't.

    I should add that by a demand on the CM907 I mean putting it in manual mode with a very high (26C) set temp to get outside the proportional band.

    Despite the above bear in mind my question is really about the fact that with thermal actuators on TPI stats (as the CM907 is) Honeywell state 12 cycles per hour with a minimum on time of 1 minute. The Watts website for the actuators states that the actuators start to open at 90s and complete at 3 minutes. So at the minimum off time of one minute the flow will never go down the u/f heating circuit.

    Thanks for replying.
     
  6. sam spade

    sam spade Active Member

    I was trying to get some idea of the system's control logic in case it affected the settings of the CM907. From what you say it appears to be like this:

    CM907 - zone valve - actuator - boiler.

    This would require a four wire connection to the actuator. Two wires to control the actuator, via the zone valve; the other two wires connect to an auxiliary switch, which control the boiler.

    As for the CM907 parameters, the minimum on time can be set, in one minute steps, up to 5 minutes.

    I know Honeywell recommend 12 cycles/hr for thermal actuators, but don't understand why. In some ways it doesn't matter as the control algorithm means that the setting is just a starting point. The system works out how often it needs to turn on and for how long.
     
  7. fred812

    fred812 Screwfix Select

    Thanks Sam, agreed on the 12 cph, I don't get that either given that relates to a 5 minute cycle duration which equally means that given the thermal actuators take 3 minutes to fully open, that once the ambient temp is within 1C (67% of the proportional bandwidth if set at 1.5C) of the set temp that they never fully open.
     

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