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Discuss Pump on return is there a problem? in the Central Heating Forum area at UKPlumbersForums.co.uk.

  1. mgw

    mgw New Member

    I know years ago before the TRV the pump went on the return, but new house to me, all TRV's on radiators except for bathroom which does not really count as that is thermo syphon, not worried about boiler as thermo syphon domestic hot water with no motorised valve, but am worried about pump, will it cavitate as the TRV's close and damage it, with pump on supply easy a by-pass valve will allow it to continue to circulate, but will a by-pass work on the return.

    The system was in a mess, two pumps one for flat switched on with a thermostat, and one for main house, simply on a 13A plug, there was a time switch/programmer which said it allowed heating and DHW once or twice a day and DHW once or twice a day, plus continuous and off. However in real terms all it did was turn boiler on/off there was no connection to any pump.

    The area around the boiler in the flat under the house had a flat thermostat, and a wireless receiver for another thermostat, but no thermostat linked to it.

    On trying to use the system found if either pump used on it's own, it would reverse circulate water in other system, also radiators get hot on top floor with thermo syphon even when pumps switched off.

    Turning off the maintenance valves on the pumps did stop the circulation, so local plumber was to fit two motorised valves, since the motorised valves have micro switches built in, easy enough to wire so unless open pump will not run.

    So bought Nest Gen 3 and wired up ready for when motorised valves are fitted, also paired with 4 MiHome Energenie TRV heads in hall, dinning room and living room, at moment only using for DHW so all OK.

    However two things came to mind, one is what if all TRV heads close while pump is running, and two since linked why does the TRV not show same target temperature as Nest?

    As yet plumber not returned to fit valves, so if some thing needed I will not need a second supply of inhibitor, so now is the time, but most pipes hidden behind plaster board, will not only pipes, whole fuse box hidden just enough room to put hand through ceiling and turn off power.

    So no option but have pumps on return, only two supplies, there are three returns, plus I assume fill pipe.

    Looked at modulating pumps instead of three speed, but they need a minimum pressure, so seems can't be used on return, thought about some vac/pressure switch but would need latching and reset.

    I can't be only one with pumps on return and all TRV's fitted, so how is it done? I am not doing plumbing, however I am doing wiring, all set out ready, there will be a slight problem in that if flat calls for heat without either DHW or main house calling for heat, it will not turn on boiler, just circulate water, but not worth fitting relays as flat rarely used. And also no thermostat on DHW cistern, just limit time to 1/2 hour every other day, that's enough to have hot water to wash hands in summer, shower is electric, oil only for central heating and hand wash, OK maybe odd bath, but easy enough to press boost.

    So all comments welcome.
     
  2. Rob Foster

    Rob Foster Top Contributor!!

    Pump cavitation is for big boys to worry about in power stations submarines etc...your pump will give up after a while anyway. Are you an engineer or a knowledgabke diyer...it helps us help you I suppose and put up a diagram please ...Rob Foster
    centralheatking
     
  3. rpm

    rpm Trusted Plumber Top Contributor!!

    C'mon Rob it really doesn't matter what size of pump, you don't want cavitation happening.
     
    • Like Like x 1
  4. Rob Foster

    Rob Foster Top Contributor!!

    agree but its much more serious at the industrial scale
    centralheatking
     
  5. rpm

    rpm Trusted Plumber Top Contributor!!

    I'm sure that the op doesn't want to live with the noise and replace it every what 2yrs?
    Sometimes I wonder if we read the same post. :D
     
    • Funny Funny x 1
  6. mgw

    mgw New Member

    Pipework not sure but think it is something like this Pipework.jpg the boiler and pumps are in a flat below the main house so would not hear them, only two wires between main house and flat so fitted Nest once motorised valves are fitted will be wired like this C_Plan_My_HouseC.jpg which is not perfect but good enough, I was first trained as a mechanic and then slowly moved to be an electrical engineer which included working for SLD pumps so yes aware of damage cavitation can cause, however as to if a real problem with central heating I don't know.

    If the Nest switches off at same time or before the TRV's then it should be OK, but how would I know, I can set Nest one degree lower than the thermostat in the hall which should mean that thermostat is always open, but how accurate are TRV heads? Far better if there is some relief valve which will allow water into the pump, maybe all it needs is a lock shield set so there is always a bleed of coolant, but this is all above my knowledge, hence the question, it may be I don't need any thing? But time to fit anything is before new inhibitor is added, plumber did not turn up today, so have a little more time before I need to know.
     
  7. Rob Foster

    Rob Foster Top Contributor!!

    if all the trvs are on the correct side with a bypass and the pump is set at a suitable pace it should work fine, all the electronic crap hive etc is intermediate tech and way over priced in my opinion..reason why its fitted is to allow installers extra profit with little labour input.
    And people with mobile phones can pretend to be in charge In fact a properly balanced heating system by and large might not need trv but thats my opinion but as ever will keep selling the stuff because its high profit....loads of money !
    centralheatking
     
    • Funny Funny x 2
    • Like Like x 1
  8. mgw

    mgw New Member

    Reason for Nest was lack of wiring between flat and main house, with Nest two wires with 12 volt DC is only connection between main house and flat below. Also selected Nest as volt free contacts and will link to my existing TRV heads.

    But the wiring of Nest is not the problem, it is if I need some protection for the pumps? being on return not sure a by-pass valve will work?
     
  9. rpm

    rpm Trusted Plumber Top Contributor!!

    Captain obvious or what! :D
     
  10. rpm

    rpm Trusted Plumber Top Contributor!!

    After 92 views is there nobody able to help and keep the thread on track?
     
  11. mgw

    mgw New Member

    Thank you, and I hope some one can tell me if I should worry or not?
     
    • Like Like x 1
  12. Craig Watson

    Craig Watson Plumber GSR

    Pumps on the return are fine in heating systems, pumps are located on the return side of combis and system boilers and as the water is cooler it's probably better for the pump. Only thing you have to watch is pump location on open vented systems. And if all your radiators have trvs on them then you should really have a bypass to prevent damage to the pump if they all shut off
     
  13. mgw

    mgw New Member

    Question I suppose with pump on return, is where should that by-pass actually by-pass? Pipework-question.jpg
    The point I am making is the by-pass needs to prevent a depression rather than release pressure, so what setting and where? Flat radiators easy enough just open the kitchen TRV fully, no thermo syphon problem in flat, main house however a different story.
     
  14. Craig Watson

    Craig Watson Plumber GSR

    The bypass can go anywhere on the flow pipe and then tee into the return after the zone valve but before the pump. And as for setting, turn the valve to it highest resistance, then with your heating running slowly open the valve until you feel water lass through it, then give it a quater turn back, and that should be fine.

    You could also change one of the trvs to a lockshied so one radiators remains permanently open. But this will only work if your system doesn't have pump overrun.
     
  15. Rob Foster

    Rob Foster Top Contributor!!

    The open radiator is usually a bathroom radiator. centralheatking
     
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