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Thread: S-plan with no valves?

  1. #1
    Automated Home Ninja
    Join Date
    Aug 2016

    Default S-plan with no valves?

    This is my first post, so please be gentle!

    I've just moved into a 1950s house, and I'm preparing for an evohome install. I'm trying to figure out what I've already got.

    The boiler (Potterton Suprema 30-100L, heat-only) is in an extension which is approximately 15 years old. The hot feed from the boiler splits into two within a few feet of leaving and then there are 2 pumps (one on each branch). These appear to be for DHW and CH.

    However, I can find no trace of any zone valves anywhere. Not near the boiler, not near the cylinder (airing cupboard), etc.

    I realise this isn't "normal", but is it even a thing? Should I look harder for valves or is it possible to not have them? Could the pumps somehow incorporate valves?


  2. #2
    Automated Home Legend
    Join Date
    Sep 2014


    I haven't seen this layout before, but I don't see why it couldn't work, even without zone valves.

    Normally you would have a single pump straight off the boiler which then either goes into a mid-point valve, or branches to two separate two port valves for CH and HW.

    However if the output of the boiler branches off to two separate pumps you don't really need zone valves - only the circuit(s) whose pump is running will flow. (Perhaps bar a small amount of thermosyphoning to the hot water cylinder with the HW pump off, but I suspect it would be minor)

    Either pump could run on its own or both could run at once. When both are running there wouldn't be much other than relative pump speeds to control how much "share" of the boiler output each circuit got, but it would work.

    In this configuration you should be able to use a 3x BDR91 (S plan) Evohome config - the boiler relay fires the boiler, the hot water relay runs the hot water pump and the central heating relay run the central heating loop pump.

    You can set hot water overrun on the hot water relay in the Evohome config but I don't know that you can do so for the central heating relay, so if you need pump overrun there too you might need to add an external timer for overrun, but that's not difficult.

    Have you managed to trace the wiring to the two pumps to figure out how they are currently supplied and controlled ? Does it seem to have any pump overrun for either pump at the moment ? Do you have an automatic bypass valve anywhere in the central heating loop or at the output of the boiler ?
    Last edited by DBMandrake; 6th August 2016 at 12:53 PM.

  3. #3
    Automated Home Ninja
    Join Date
    Aug 2016


    Thanks for you reply. Curiouser and curiouser... I can't find a wiring centre at all. There's a "box" on the wall in the airing cupboard which turns out to be a double-pole, double-throw relay. And it seems to me that both pumps are running whether the timer calls for DHW or for CH (although I'm only testing this by feeling them, and I haven't experimented with calling for DHW and then seeing if the radiators get hot). There is a pump overrun - that is, when I remove all demand, the pumps run for a minute or two. Although now I think about it, the boiler might still be going then too. I've not located a bypass circuit yet - although I've not looked very hard, if I'm honest.

    I've got a Honeywell recommended evohome chap coming round later in the week (well, two actually, to compare ideas and quotes), so I might just throw this all at him and see what he says.

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