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Thread: Boiler relay

  1. #1
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    Default Boiler relay

    I have a wood pellet boiler to provide the hot water for my house I am thinking about putting the evohome system in.
    The way the wood pellet boiler works is that it sends out power to the MV to when it as built up the temperature in the boiler and then it triggers the pump to come on.

    The question I have is if I use the power signal that the boiler sends out to the MV as the feed for the boiler relay would that cause any issues with it as it will be interment signal (there will be no power most of the time).

    I can not use the boiler relay signal as the live to the boiler as the boiler has to be controlled by it own circuit board.

  2. #2
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    Do you mean just for heating hot water for taps? Or the hot water that circulates in radiators?

    MV? Mixing valve?

    The relay takes a permanent 240V for powering the radio bit, then has volt free contacts that can typically replace a thermostat already in the system.

    If the boiler relies on being able to push water around the system to dissipate built up heat (unlike a gas boiler that simply switches off and cools down) does it not need a failsafe approach to this?

  3. #3
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    Mv = motorized valve

    The boiler has a bulit in buffer tank and when it reaches 70 degrees it stops heating the water.

    The problem I see is that when a room calls for heat the boiler realy will act as a theromsat and then switch the pump on. I need the realy to want untill the boiler has warm water in the buffer tank before it pumps. When the boiler has warm water in the buffer tank then it sends power to the Mv to open. So my question is could I use that power that goes to the Mv to power the boiler relay, rembering that it will always not be live.

  4. #4
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    The relay will need permanent 240V on the L+N terminals to be able to receive commands from the controller. If you take the "I'm hot now" feed from the boiler to the MV via the relay's volt-free A and B contacts then the MV will only be energised when both demand (EvoHome asks for heat) and supply (boiler has heat) conditions are met. If the pump is powered via the auxiliary switch contact on the MV then that should keep working as well.

    So is there no thermostat in whatever the pump circulates water through at the moment? I'm assuming this is a hot water cylinder, but still unsure if you mean a central heating circuit? If it is for the HW cylinder then you could probably leave that as is, and just use EvoHome for the space heating. If it has a tank stat then I'd have thought that would already be wired in similarly.
    Last edited by Little Tinker; 28th November 2016 at 09:04 PM.

  5. #5
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    This is for hot water and central heating

    Yes there is a theromsat one upstairs and one down stairs

  6. #6
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    Thanks for the response

  7. #7
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    Quote Originally Posted by Cwdm View Post
    This is for hot water and central heating

    Yes there is a theromsat one upstairs and one down stairs
    Is there not therefore some linkage between those thermostats and the boiler to ensure the behaviour that you describe, so that the pump only comes on if there is heat in the boiler to satisfy the thermostat request? If they are both room thermostats then presumably each goes to a 2-port zone valve, and if you're putting EvoHome HR92 devices on all the radiators you can use the relay to energise / open both valves in parallel while relying on the HR92 to manage flow. A schematic or deeper description of all the electrical and plumbing bits that you have might help.

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