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Thread: Evhome - wiring for Stored Hot Water and Zoned Heating

  1. #1
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    Default Evohome - wiring for Stored Hot Water and Zoned Heating

    I have had Evhome for just over a year and love it.

    The system is S-plan with a Worchester Bosch Greenstar 18Ri (which was pumped frost protection and overrun) but we have afternoon “Bangs” when the heating pump stops and so I will replace the automatic bypass valve as I suspect it is stuck - system is due for a good flush, clean and inhibitor. Anyway, whilst thinking about it, and on reading things here, I suspect the system is not quite right. All our radiators have HR92’s, yet there is a CH zone valve. I suspect we need the “Stored Hot Water and Zoned Heating” setup on Figure 4 on page 43 of the Evohome Installation Guide however whilst there is the schematic, there is no wiring diagram and I seem to be failing to find one Googling.

    So, wiring the BDR91 to the boiler I am OK with - that's in the installation guide. A radiator calls for heat and a wireless signal will fire the boiler. The question is really what about hot water? I have the wireless hot water kit, so how does it fire the boiler? Does it use the boiler switched live from the junction box or wireless to the BDR91 attached to the boiler? If wireless, does that mean the zone valve attached to my second BDR91 is just a valve and that the internal switch is no longer used?

    Hope the questions are not too silly.

    David
    Last edited by dave1966uk; 29th October 2020 at 05:20 PM.

  2. #2
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    If you have a hot water zone valve and a heating zone valve (eg an S-Plan system) there are two basic wiring and configuration options available.

    Option 1 - two BDR91's - one connected to each zone valve, these are bound in the hot water configuration as heating and hot water relays. The orange wires from both zone valves are used to fire the boiler when the switches inside the zone valves close. No direct connection from Evohome to the boiler.

    So when either zone valve opens, (hot water demand or heating demand) the switch inside the zone valve closes and fires the boiler. (Both switches are wired in parallel)

    Option 2 - three BDR91's - the same as above except the orange wires from the zone valves are not used. Instead the third BDR91 is connected directly to the boiler and is bound as "boiler control" in the Evohome system, in addition to the other two relays being bound in the hot water config as before.

    This is the system I have. There are a couple of advantages to using 3x BDR91 - one is that you can make use of the hot water overrun feature which doesn't work if you only have two relays. It also gives more wiring flexibility for some non standard configurations, for example if you have a custom pump overrun timer. But the two relay system can work fine as well.
    Last edited by DBMandrake; 29th October 2020 at 11:24 AM.

  3. #3
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    Quote Originally Posted by DBMandrake View Post
    If you have a hot water zone valve and a heating zone valve (eg an S-Plan system) there are two basic wiring and configuration options available.

    Option 1 - two BDR91's - one connected to each zone valve, these are bound in the hot water configuration as heating and hot water relays. The orange wires from both zone valves are used to fire the boiler when the switches inside the zone valves close. No direct connection from Evohome to the boiler.

    So when either zone valve opens, (hot water demand or heating demand) the switch inside the zone valve closes and fires the boiler. (Both switches are wired in parallel)

    Option 2 - three BDR91's - the same as above except the orange wires from the zone valves are not used. Instead the third BDR91 is connected directly to the boiler and is bound as "boiler control" in the Evohome system, in addition to the other two relays being bound in the hot water config as before.

    This is the system I have. There are a couple of advantages to using 3x BDR91 - one is that you can make use of the hot water overrun feature which doesn't work if you only have two relays. It also gives more wiring flexibility for some non standard configurations, for example if you have a custom pump overrun timer. But the two relay system can work fine as well.
    Am I right assuming that if I have a system boiler I will still need a BDR91 with a temperature sensor fixed to the system tank to control that side of it or when it’s a system boiler is the water temperature picked up by the boiler itself and thus the second hot water BDR is connected to an appropriate point in the boiler? I am not familiar with system boiler systems, my current system is the old fashioned copper cylinder fed from a tank in the loft. I have just replaced the fan in my old boiler and it has made me start looking - again!

  4. #4
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    Quote Originally Posted by G4RHL View Post
    Am I right assuming that if I have a system boiler I will still need a BDR91 with a temperature sensor fixed to the system tank to control that side of it or when it’s a system boiler is the water temperature picked up by the boiler itself and thus the second hot water BDR is connected to an appropriate point in the boiler? I am not familiar with system boiler systems, my current system is the old fashioned copper cylinder fed from a tank in the loft. I have just replaced the fan in my old boiler and it has made me start looking - again!
    It'll pretty much stay the same as your current system however - You'll have no pump externally, and the boiler will require a permanent Live (If your current one doesn't already!). The BDR91s stay the same for the Heating and HW.

  5. #5
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    Quote Originally Posted by mtmcgavock View Post
    It'll pretty much stay the same as your current system however - You'll have no pump externally, and the boiler will require a permanent Live (If your current one doesn't already!). The BDR91s stay the same for the Heating and HW.
    Thanks. I guessed that may be the case.

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