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Thread: Automatic bypass Valve plan, your thoughts

  1. #21
    Automated Home Guru
    Join Date
    Nov 2017
    The Borderlands


    That's correct.

    In case it's of interest to anyone I'll copy my full exchange with W-B below, in chronological order.

    Me: A question about auto bypass valves when a modulating pump is used. I have a 32/50 Danesmoor boiler and a Grundfos UPS2 pump, which is used in modulating mode. Every radiator is fitted with an Evohome HR92 wireless TRV and these are managed by an Evohome controller. The boiler manual says that an auto bypass valve must be fitted (and one is) but, on reflection, it seems wrong to fit a bypass valve with a modulating pump because the pump sensor will not know when the bypass valve has opened and will therefore not control the pump speed correctly. The Evohome controller will only cause the boiler to fire when an HR92 is open and there is a heat demand so there is no risk of the pump running when all TRVs are closed. It therefore seems that a bypass valve is unnecessary in this system and will prevent the pump from working correctly. I would appreciate your comments and advice.
    W-B: in response they sent the link I mentioned earlier (
    Me: Thanks for the reply, but I'm struggling to see how this provides significant cooling flow through the boiler because the pump and boiler are wired in parallel and both the pump and the boiler call for heat will therefore stop at the same time. Please can you explain.
    W-B: It is correct that this boiler does not have a pump overrun, however there is still a risk that should the boiler fire for a demand and all the TRVs be closed, that it would require a circuit to flow around sufficiently.
    The function you describe that the Evohome provides would negate this as an issue, however it would still be our recommendation to have one in case this were to fail in any way.
    Me: Thanks again, but this prompts one final question.
    The boiler demand and pump are wired in parallel (and controlled by the zone valves, which are controlled by the Evohome system) so the boiler firing without water circulation could only occur if either the pump or a zone valve failed; I do not see how it's possible to provide a bypass in this situation so the boiler would protect itself by locking-out. Do you agree, or have I missed something?
    W-B: Ultimately this is just a recommendation to prevent a potential problem, if the pump is not running there will be no circulation however if the pump is running but circulation is restricted elsewhere this helps to stop the boiler overheating and potentially needing manual intervention to reset.

    My view is that the only time the boiler might fire without circulation being available is in a fault condition, and that the boiler will then protect itself. The bypass valve is now screwed-down.

    Thanks for making me think about this.

  2. #22
    Automated Home Jr Member
    Join Date
    Oct 2020


    An update

    Put the ABV in place put a reducing tee 22mm down to 2 x 15mm out, one side going to immersion output from H/W motorised valve, other 15mm out going to the return from the immersion heating coil. A gate valve was installed on each 15mm output. I found that when the output was directed to the return there was still excessive noise in the system despite adjusting the ABV but once switched to sending output threw tank heating coil a nice quiet system. When you think about it using the heating coil in the tank is only like having a radiator which is always open to flow, plus you get the benefit of not wasting any excess heat produced in the system. But it was not installed in the very coldest part of winter so before i call it a complete success i will wait till next winter at its coldest to see how things go with very cold pipes etc. I am not advocating to others this is the way to go just saying it worked for me, i hope.

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