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Thread: Retro fitted Evohome with Megaflow

  1. #1
    Automated Home Lurker
    Join Date
    Jun 2019

    Default Retro fitted Evohome with Megaflow


    I have had evhome fitted into an existing system that originally had 2 zone valves for heating, and 1 zone valve for a megaflow hot water cylinder.

    Having some issues with hot water, and having had a gas / heating engineer out who is an expert on Megaflow, but not familiar with Evohome, we have some questions in case someone can advise / feedback. Many thanks!

    I have an unvented Megaflow install that used to be run from a Honeywell wired control fixed to the wall.
    Earlier this year I had the Evohome controller introduced to the system to run both my heating and hot water.

    About 3 weeks ago when I turned off my heating for the summer, I was NO longer getting hot water on the scheduled run from the Evohome controller. The only way I managed to “fix” this, was that I clicked the hot water zone valve open…

    I called out a gas engineer who is familiar with Megaflow installations and when investigating he found that:
    - The hot water zone valve is not wired through to the control stat to comply with G3
    - Does the evo controller comply with G3? And if yes how, and what is the wiring please?
    - He also told me I should not override the hot water zone valve for safety – just FYI – I now know that.

    The engineer now made me run the hot water from an immersion heater because the Megaflow system set up is not complying with G3, hence I need the above resolved. Any ideas?

    In the Evohome set up, is the temperature sensor provided for the hot water cylinder kit basically the only sensor for when the hot water should no longer call for heat? Is that the ONLY control mechanism that shuts down the boilers via wireless comms? What if that wireless link fails?

    In the same system set up, the Evohome / house automation engineer who fitted the Evohome a few months back, had simply bypassed the 2 zone valves that used to control the heating zones. He basically said, that as I now have the radiator groups controlled with the digital thermostats fitted onto the rads, the Evohome controller no longer needs to open or shut the individual or both zone values for the heating system when the system calls for heat.

    Is this appropriate, should the Evohome regardless just open / close the heating zone valves when 1 or more rads call for heat?

    Thanks for your help / feedback in advance.

  2. #2
    Automated Home Ninja
    Join Date
    Aug 2016


    In reverse order:

    3) Yes. In fact, this is Honeywell's preferred setup. If you have HR92 TRVs on all radiators, you don't need zone valves. What would they add?

    (Side note: make sure you have a bypass path, regardless of whether you have zone valves or not.)

    2) Yes and no. The sensor is the only sensor, yes. But it's not the only control mechanism. This is what's going on in question 1. You should have the zone valve wired through the BRD91 AND through the safety thermostat in series. If the wireless comms or the controller fails, etc. then the safety thermostat should eventually cut in and close the valve. And if your boiler is controlled by the orange wire from the valve, then the boiler will shut off too. If it's not - say it's controlled by a dedicated BDR or OpenTherm, then it could continue to run. Another reason for a bypass path.

    1) I'm not an expert here, by any means. My understanding is as above, that the safety thermostat should be in series with the BDR91 so that it can cut off power to the valve. I believe this makes is comply with the regulations, but you shouldn't take my word for it. As for the controller being G3 compliant, I don't even know what that would mean! Overriding the hot water should be safe in a functioning system because the controller will simply turn it off again within a few minutes.

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