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Timers, Thermostats, MZCV, Boilers...How does it all hang together

Discussion in 'Plumbing and Central Heating' started by jdoherty76, 23 Jan 2017.

  1. jdoherty76

    jdoherty76

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    Can someone please give me a brief overview of how timers, thermostats and boilers all interact in an CH system. For example, my set up is as follows:


    I have a gravity feed CH system being serviced by a Baxi Solo HE Condensing boiler

    I have a Horstmann Channelplus H27XL2 channel timer

    I have a wireless Horstmann HRT4-ZW and thermostat sender and ASR-ZW receiver

    At the boiler I have 2 motorized zone control valves and a thermostat on my hot water cylinder


    When the timer comes on how does this interact with the thermostat? Can I assume that it determines the setting of the room verses the temperature required. If heat is required it informs the boiler to activate. Once room temperature has been reached then cease heating the boiler.

    What I’m trying to understand is the timer just a basic on/off switch with no bearing on boiler activation?

    If the thermostat is wireless and this talks to a receiver how does the receiver send that signal to the boiler to switch on/off depending on what the thermostat is set at

    How do motorized zone control valves actually work i.e. what do they do and do they have any impact on boiler activation on/off?

    Does the thermostat on the hot water cylinder instruct the boiler to activate/deactivate when water has reached the desired temperature? It’s currently set at 60 degrees



    Thanks JD
     
  2. stem

    stem

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    The timer or programmer has overall control of the system and overrides the thermostats, if the programmer is set to 'off' nothing will operate. You have a two channel programmer. One channel for hot water and one for central heating, so you can control each of them separately. [Your type of heating system with 2 motorised valves is called an 'S Plan']

    The room thermostat (or thermostat receiver in the case of a wireless thermostat) receives an electrical supply from the timeswitch or programmer when the programmer is set to 'on'. The thermostat then switches the heating on and off to maintain the selected temperature. So if the thermostat is set to 20 degrees and the room temperature is less than 20 degrees the heating will be switched on, when it reaches 20 it will be switched off. When the room temperature drops below 20 it will switch it on again.

    No. When heat is required, it opens up the motorised valve for the central heating. When the valve is fully opened it has a little switch inside which in turn switches the boiler on

    When the room reaches the set temperature the thermostat switches off and the motorised valve returns to the closed position and releases its internal switch so the boiler goes off.

    It is just an on / off switch, or two switches. one for heating & one for hot water, but it can start a chain of events that will operate the boiler if heat is required.

    [Combi boilers because they don't have a stored hot water system to control sometimes have a programmable thermostat that carries out both time and temperature control functions in one unit.]

    There is a receiver that is wired into the system in place of a wired room thermostat and carries out the switching functions. The separate, remote wireless thermostat then tells the receiver via radio signal, when to switch on and off.

    They actually switch the boiler on and off. Because there is one boiler and pump, the motorised valves are responsible for directing the water from the boiler to either the radiators or hot water cylinder as required by their respective thermostats. When either motorised valve is open its integral switch starts the boiler.

    Yes it works exactly the same as the central heating. The programmer (or timeswitch) supplies electricity to the hot water cylinder thermostat, which if the water is below 60 degrees will then power the motorised valve for the hot water system, which will open and start the boiler.

    To summarise:

    The programmer switches the heating and hot water 'on' and 'off' as required.

    Only when the programmer is set 'on' can the thermostat(s) maintain the selected temperature by opening and closing the motorised valve(s).

    The motorised valves direct the water from the boiler to the heating, or hot water cylinder as required by the thermostats. When a valve is open it switches the boiler 'on'.
     
    Last edited: 23 Jan 2017
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  3. jdoherty76

    jdoherty76

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    Absolutely fantastic reply Stem and makes it all more understandable. Thank you very much as I greatly appreciate the explanation. I had an issue at the weekend whereby the boiler wouldn't switch off. Though the thermostat wasn't calling for heat and the timer was actually off the radiators were roasting. I couldn't understand it. I believe it was a stuck MZCV which appears to have corrected itself. is this possible?

    If so the next job would be to determine which MZCV actually services the CH as opposed to the HW.

    Do you need to drain the whole system to change one of these?

    once again Stem I can't thank you enough.. Thanks again..
     
  4. ianmcd

    ianmcd

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    your CH motorised valve is sticking intermittently, post a pic of the motorised valves, some you can change the actuator head without having to drain down but not all of them.
     
  5. jdoherty76

    jdoherty76

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    is it possible that this was the problem. Cheers Ian I'll do that when I get home... These valves are at the side of the boiler which is in the loft, no rads above this height, though 2 up the stairs. Hoping if it comes to it maybe I'd get away with shutting the header tank off, closing the rads off at both ends and only dropping sufficient water to do the change over. Thanks again...
     
  6. stem

    stem

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    Just as a curve ball, sometimes systems have a frost thermostat fitted, particularly if the boiler or pipework is in an unheated outbuilding or garage. These override everything and turn the heating 'on' when the temperature drops to prevent the system freezing. Normally, if installed properly it is linked to a separate pipe thermostat that stops the radiators getting hot and just keeps them warm.

    Very few domestic installations have them fitted, but I mention it just in case...
     
  7. jdoherty76

    jdoherty76

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    no Frost thermostat fitted thanks. here are the pictures of the MCZV attached...
     
  8. jdoherty76

    jdoherty76

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    as requested please find pictures of the MZCV
     

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  9. ianmcd

    ianmcd

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    The good news is they both have removable heads and can be replaced without draining but a good chance it is just the synchron motor in the CH valve that needs changing
     
  10. stem

    stem

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    If that's the case it is unlikely to be the motor, but more likely to be the microswitch or mechanism sticking. The valve wouldn't open at all if the motor had failed. So, the spring would keep it in the closed position and it wouldn't mate with the microswitch at all meaning the boiler wouldn't switch on.
     
  11. ianmcd

    ianmcd

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    If you read the complete post @stem he says the thermostat was not calling for heat and all the radiators were roasting, if it was a microswitch sticking the boiler the boiler would keep false firing but wouldnt heat the radiators, I have seen Synchron motors sticking in the open position before
     
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  12. stem

    stem

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    OK agreed. I had forgotten the bit about the rads being hot, and focused on just the boiler staying on. The valve indeed must be open for the rads to get hot. so I stand corrected. My apologies and thank you for pointing it out.

    Personally I've never known a motor to stick, I've only ever found the mechanism to jam & when I've taken the motor out it has always spun freely. Although it must be possible if you have come across it and you will probably have had more experience than me. Perhaps the OP will let us know when he's fixed it.
     
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  13. ianmcd

    ianmcd

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    Hi Stem
    A lot more common for just the end switch to stick, but have seen the whole valve staying open, sometimes it is the mechanism sometimes its the synchron,I find the Honeywell ones that the OP has are usually the most reliable.
     
  14. jdoherty76

    jdoherty76

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    Thanks Ian/Stem what exactly does the "synchron motor in the CH valve that needs changing" mean to a layman? funny enough I've not had the problem since, personally I think it was one of the MZCV that had stuck. Also when you say the head can be replaced without draining is this a simple process?

    Thanks again for your time and patience JD
     
  15. stem

    stem

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    The motorised valve is geared device, which is wound open by a synchron motor. Once the valve is open the motor stalls at the end of its drive but remains powered and so holds the valve open. When power is removed a strong spring pulls the valve back to its closed position. [at this point you can hear a whirring noise as the motor is spun quickly in reverse by the spring] The motors that I have replaced have failed electrically and do not operate so the valve remains permanently closed.

    Synchron Motor.jpg

    To change it you will need to remove the cover from the powerhead and there are two small screws that hold it in place. Once they are removed the motor is lifted out of position and replaced with the new one. The wires can be connected either way around. It's a cheaper solution than a new powerhead, but make sure it is just the motor that is faulty, otherwise you may still end up replacing the whole head. Bear in mind too that if the valve is elderly, then a new powerhead will probably be a better solution in the long term.

    Yes, it's easy. Remove the retaining screws and pull the powerhead assembly gently off the spindle, you will have to move the lever to the 'Man' position to free it. The wiring will also have to be changed over. Note carefully which wire goes where before removing the original powerhead.

    You can find instructions here. Yours looks to be the 'new style' head but the instructions tell you how to check.
     
    Last edited: 31 Jan 2017
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