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Thread: Convert Combi to unvented cylinder setup with Evohome

  1. #1
    Automated Home Jr Member
    Join Date
    Dec 2015

    Default Convert Combi to unvented cylinder setup with Evohome

    Hi, I self installed the evohome system (thanks Evohome shop) on my Combi Boiler system with 12 radiator zones last December, it has been excellent and I found it easier to install than I expected.

    I have never been happy with the DHW flowrate from the Combi (its a 2006 35kW Worcester Bosch CDI combi), I am now thinking of converting the set up to an unvented cylinder with same boiler for two reasons:

    a) Get better flowrates to shower
    b) Fit solar heating to cylinder in the future.

    With the unvented cylinder , I will add a two port zone valve from the combi heating central circuit + the Evohome heating kit .

    Is my understanding of the control correct/possible:
    a) If the hotwater sensor temperature is below setpoint the controller sends signal to new Hot Water Kit BDR91 to open the inlet heat zone valve to Cylinder , and b) The controller will also send heat demand signal to the original BD91 relay that controls the boiler. I should not need any new control wiring into the boiler.

    b) Do I have to combine two of the radiators to single zone , to allow the addition of the hotwater kit on cylinder as a zone.

    Thanks for your help

  2. #2
    Automated Home Legend
    Join Date
    Sep 2014


    Hot water is its own zone - you won't lose one of your 12 heating zones.

    Yes the hot water relay will only come on when hot water is scheduled to be on and the hot water cylinder is below the desired temperature. The Boiler demand relay will also come on to fire the boiler.

    When the hot water temperature is reached the boiler relay will go off but the hot water zone valve relay can be configured to stay on a short time to help remove excess heat from the boiler heat exchanger when it shuts down. (This would only be effective if you have pump overrun of course)

    Make sure you either have HR92's on all radiators, or fit a heating zone valve (controlled by a third BDR91) in the flow to the central heating loop otherwise hot water demand while the central heating was off would cause those uncontrolled radiators to heat up.
    Last edited by DBMandrake; 6th August 2016 at 11:37 AM.

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