Pump over-run and bypass circuit

Discussion in 'Plumbers' Talk' started by mrjohnskelton, Dec 1, 2017.

  1. mrjohnskelton

    mrjohnskelton New Member

    Hi All-

    I'm having problems with my CH system. I'm getting very long pump over-run (forever!), with the boiler not cooling down any quicker than you think it would if it was unpumped.

    Based on other Q&As here, I've done loads of investigating and narrowed it down to two possible symptoms - most likely first:
    • Faulty microswitch on one (possibly two) of the valves (I've got three valves: 1xHW and 1 each for upstairs and downstairs HTG circuits)
    • Faulty pump over-run thermostat on the boiler
    At one point the boiler fired up when I'd got the controller in my hand and unconnected to it's wall plate, so I'm pretty sure it's the microswitches. Next step is to see if I can replace these or need to buy whole new snap-on heads.

    So that was all by way of explanation (and to double check, just in case I'm on completely the wrong lines). Having done this investigation I'm puzzled by the pipework - this is my 'real' question...

    The pipe layout is a pump which is pumping away from the boiler and immediately into the three valves (2x HTG, 1xHW). There's no bypass circuit/between pump and valves, and no 'obvious' bypass before the pump. (There is a bathroom ladder radiator with no TRV, which is in the right place in the house to be between boiler and pump - but why make it an unpumped bypass/dump? Also, it doesn't seem to stay any more warm than other radiators when over-run is happening.)

    So it seems like when over-run is happening, the pump is pushing water against three closed valves.
    • Boiler is a Potterton Profile 100e
    • I think the system was installed when the house was built around 1992.
    • As far as I can tell all thermostats and controller are working fine.
    Any thoughts/suggestions?
    • Am I missing something in looking for the bypass?
    • Would the bypass be before or after the pump (after surely!)
    • Any way to tell if the ladder definitely is the bypass?
    • Am I totally missing something about how these systems work (I am pretty 'handy' but strictly amateur)
    Many thanks in anticipation - John
  2. Dave does Gas

    Dave does Gas Screwfix Select

    From your description John it sounds as though you have what is known as an S Plan system, which has to have a bypass. Given the age of the instalation I would surmise that the bypass is in 15mm with a gate valve fitted in the middle with the wheel removed. It should be the fist tee after the pump and before the valves going back into the return.

    So given that you have a fully pumped system you should have had the pump wired through the boilers terminal block this ensures that the pump overun works.

    What you can check if you know how to access the electrical section is that the change over switch in the control box hasn't been inadvertently reset to "gravity".

    Other than that your second diagnostic statement is most probably the cause, may just be the overrun stat which also double as the hot water stat on a gravity system hence the switch over on commissioning.

    I would advise though if you do need the stat changing it would be best done by a Gas Safe fitter as you need to open up the burner compartment to access the stats probe. This needs to be checked upon refitting for POC leakage.
  3. mrjohnskelton

    mrjohnskelton New Member

    Thanks so much for the reply and extra pointers Dave.

    You're right, I think it is an S plan (sorry, should have said - retro-figured out after spending ages carefully working out the actual wiring plan and comparing it to the Honeywell guide - standard S plan, but has re-ordered the terminal block numbers). I think it's all wired correctly - the only difference is that the grey and orange wires from valve microswitches have all been used the other way around (grey instead of orange, orange instead of grey) but I don't think that makes a difference as they 'just' go across the microswitches. (Unless I'm wrong on that?)

    I'll take a look for the bypass.

    The gravity/pumped switch in the boiler is set to pumped - I dared to go as far as removing the white cover but no further!

    Other than than, it sounds like it's time to call for a Gas Safe Fitter.

    I'll post back if anything else comes up but otherwise it sounds like you've sorted it for me Dave - thanks again :D
  4. Mike83

    Mike83 Screwfix Select

    If the programmer was off the wall and the boiler still fired this would indicate a fault with one of the valves. The switch could be sticking or the valve not fully closing.
    Changing the complete valve head is usually best.

    The pump overrun sounds like it’s doing it’s job. The lack of a by-pass and possible faulty microswitch will only keep it on for way longer than normal.

    The grey and orange don’t matter. Anyway is fine.

    The bypass would usually be fitted after the pump but before the 3 valves.

    If the system has always worked fine without the by-pass I would guess a microswitch is the main issue as it’s been this way for 25 years.
  5. mrjohnskelton

    mrjohnskelton New Member

    Mike83 - thanks for your response and the clarification on wiring. I think the valves are closing OK, so of the two it does sound like the microswitches.

    OK - picture round :) (at the end of the post)

    I looked again for a bypass - annotated picture attached - there's definitely no funky gadget or 15mm pipework (or any other pipework) between pump and the valves. Sounds like that's not the end of the world - just takes the system longer to cool down.

    I'll go for microswitches first - need to see if I can replace just those, the actuator unit or the whole thing.

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