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Thread: Evohome & Electric Underfloor Heating

  1. #1
    Automated Home Jr Member
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    Default Evohome & Electric Underfloor Heating

    Hi all, first post on here but have had some good tips from this forum so I thought I'd pass along my experience in fettling. I've got the Evohome Wi-Fi multi-zone controller and have a 3 bedroom detached house. We only moved in back in November and it needed a new central heating system so we got a Weismann combi-boiler to feed 7 radiators and fitted HR92s to 6 of them leaving the bathroom towel radiator as the bypass.

    However, I wanted underfloor electric heating and wanted the Evohome to control it but everyone I spoke to said, "no mate, can't be done" so being an obtuse kind of bloke, I found a way of doing it.

    I purchased a BDR91 & Y87RF single zone kit and an underfloor heating kit and wired it thusly:

    • Mains feed from consumer unit to a switched fused spur.
    • Separate L&N feeds from the spur to both the BDR91 mains feed in and the Thermostat mains feed in. Terminated at both location - no onward feed.
    • Heat Load (power to the heat mat) neutral from the thermostat to the heat mat, live to port A of the BDR91
    • Live from port B on the BDR91 to the heat mat
    • Heat mat kit thermostat sensor to the thermostat sensor connections - this is your overheat protection cut out and first line of defence


    So what you have in effect is two relays in series, both with permanent live feeds. In operation it's quite simple to set up.

    • Heat mat kit thermostat set for 80-90% of max (or as hot as you'll ever have it)
    • Heat mat kit thermostat controls the power going through the BDR91 relay as on/off
    • Y87RF bound to the BDR91 controls the on/off of the actual mat power based on the local temperature of the Y87RF
    • Bind the Y87RF to the Wi-Fi controller and create a zone called Bathroom


    Bathroom gets too hot (controller gone mental or faulty DBR91) then the heating kit thermostat cuts out. Other than that, it's pretty much on all the time and it is the BDR91 that controls the power to the heat mat and that is triggered on/off by the Y87RF installed in the bathroom.

    Now I can do this despite the bathroom radiator being the by-pass one because it is in the same room so effectively it works in tandem with the heat mat and both come on if the bathroom zone calls for heat. Only the radiator comes on if the zone is set for off but another zone call for heat - this is because the bathroom is the bypass radiator. Annoying but almost everyone says you have to have one, I'm still 50/50 as to whether that is strictly speaking an actual fact.

    If you don't have a radiator in the bathroom then it won't work like mine does and there may be some quirks such as the boiler firing when the heat is called for and heating whatever room you have the bypass radiator in - you can figure out if that will be an issue on your own system.

    Comments? Questions?

    Happy to help.

  2. #2
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    Quote Originally Posted by Fireblade69 View Post
    Only the radiator comes on if the zone is set for off but another zone call for heat - this is because the bathroom is the bypass radiator. Annoying but almost everyone says you have to have one, I'm still 50/50 as to whether that is strictly speaking an actual fact.
    As long as you have an Automatic Bypass Valve installed and adjusted correctly you shouldn't need a bypass radiator on most systems. On my system the hallway radiator originally doubled as both a bypass radiator and an uncontrolled radiator for the wall stat to measure, however before installing the Evohome system I had an Automatic Bypass valve installed as it didn't previously have one.

    When I installed Evohome I changed the valve body on the hallway radiator to a TRV valve body and fitted an HR92 so there is no bypass radiator any more. Doing so without an ABV would cause problems - either boiler lockouts, over stressing the pump or possibly system leaks due to excessive pressure.

  3. #3
    Automated Home Jr Member
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    A lot of what I understand is based on exactly what you say, pump stress or over-pressure. What I have been hearing is contradictions as my boiler has a bypass but not everyone I've spoken to agree as to whether it is a natural bypass or an emergency bypass. Currently it is safely assumed to be an emergency one and treated as such but it would be nice to get a TRV & HR92 onto the bathroom rad. Thanks for the input, it is most welcome.

  4. #4
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    Quote Originally Posted by Fireblade69 View Post
    everyone I spoke to said, "no mate, can't be done" so being an obtuse kind of bloke, I found a way of doing it.
    It is not that it cannot be done (I did it over a year ago here on our all-electric heating system), it is just that none of the evohome equipment is designed for use with E-UFH (BDR91 is only rated to 5A resistive load, no floor protection probes available from Honeywell, no electric configuration option on the evohome Controller, etc, etc...) and the cross combination of evohome doing an electric system and wet system will cause the heating system boiler to fire and waste energy when the E-UFH is also heated. There is also the risk that if you run the boiler through a 'bypass circuit' continuously you will have potential to cause overheating and damage to the boiler with some modern low water content boilers (an open circuit radiator is a better route but still a waste of energy IMHO).

    Surely better to use a Y87RF with an RFG100 separate to the evohome system to prevent the boiler firing? You will be able to control from the same TCC App but as a different location.

    Honeywell, nor anyone selling evohome is going to give the green light for E-UFH, because when the house burns down or boiler fails because someone hasn't understood correctly the configuration patterns of evohome or how to do the installation correctly the seller would be the first person to 'blame' had they said 'Yes, E-UFH with evohome is piece of cake to do'.

  5. #5
    Automated Home Jr Member
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    Yep, and that was one of the options but I like the ability to centralise the system. As I mentioned, for me the bypass radiator combined with the heat mat and Y87RF essentially create a Zone as far as Evohome is concerned, it just happens to be a bypass radiator but it's controlled by the HR92. It is also only a small bathroom so the mat is within tolerance for the relay in the BDR91, forgot to mention that in the original post.

    It would be great if Honeywell added electric controllable zones that don't fire the boiler, that's the Holy Grail and one that lots of people are aiming for. Cost wise, electric in small areas is way, way more cheaper than wet using Evohome controllers.

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