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Thread: Pump overrun

  1. #1
    Automated Home Jr Member
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    Default Pump overrun

    Iím not sure if i have lost my pump overrun feature, i had the hot water on today and boiler was running at 78C, Evo HWstat reached temp and boiler cut off, and then the pump switched off about 10 seconds later. I would have thought at such a high temp i would have seen it stay on much longer like it did pre evo.

    I had pump overrun switched on at the evo controller for 2 minutes, and the BDR did stay on with the valve open, but the pump being powered by the boiler was off, so pointless.

    Just thinking as i have disconnected the orange and grey cables from the valves in the junction box, have i now stopped the overrun? Looking at Honeywells pump overrun wiring the Orange cables are connected to the switched live on the boiler. but Fig 9 in the evo book says "if a wireless boiler relay is used the Grey/Orange cables are not required"

    S Plan design, with Opentherm controlling the boiler and BDR runnin ghot water valve.

    Many thanks

  2. #2
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    There's no "pump overrun" setting in Evohome, you might be thinking of hot water overrun - what that does is when the hot water temperature is reached it will turn off the boiler demand immediately but leave the hot water zone valve open for the additional time that you specify. It will not have anything to do with pump overrun, and it requires that your system already has some pump overrun to be effective.

    Evohome doesn't control pump overrun - that's entirely up to your existing boiler and possibly wiring centre if you have an external pump. So we'd need to know how your pump is powered, whether it's built into the boiler or external, whether there is any external timer involved etc.

    On many boilers the pump is controlled by the boiler and the overrun is not a timer but is based on the measured flow temperature where the pump runs on until the flow temperature is below a certain temperature.

  3. #3
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    Boiler Make and Model?

  4. #4
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    Viessman 100w system boiler. the External Pump overrun worked fine before Evo was fitted, but has i have removed the orange cables coming from the valves as per the instructions, and looking at wiring diagrams these would go to the switched live of the boiler. Does that make sense?
    Last edited by Norfolkboy; 25th November 2016 at 09:37 PM.

  5. #5
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    When you removed the grey and orange wires on the junction box did you wire the relay in at the same points? If so then whatever was driving the pump over run ought to still be getting the signals it used to get.

    If you wired the relay in somewhere else then maybe something lost the signal it previously got.

    Did the orange wires perhaps used to signal the overrun controller to fire boiler and pump, and turn boiler off before pump? But new wiring from relay is going direct to boiler and pump?

  6. #6
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    Pump overrun on a system boiler should be controlled directly by the boiler either on time or temperature. So some else has been disconnected that shouldn't have been.

  7. #7
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    think i may have solved the issues, looking at techincal manual fo rmy boiler it says:


    OpenTherm signal -Pump off delay - 20 seconds

    If its running on the standard room thermostat (like it used to) then the pump off delay is 4 minutes.
    I dont undertand why Opentherm control would be so short, in practise it can fire the boiler to hotter than it use to go with DHW demand (78c) and then switch off
    Last edited by Norfolkboy; 29th November 2016 at 01:53 PM.

  8. #8
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    That's got to be a configurable parameter.

  9. #9
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    Quote Originally Posted by bruce_miranda View Post
    That's got to be a configurable parameter.

    Email has been sent to Viessman

  10. #10
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    Quote Originally Posted by Norfolkboy View Post
    Email has been sent to Viessman
    Interested to hear their feedback as I have the same setup as you

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