Heating system work done - now pump runs continously

Discussion in 'Plumbers' Talk' started by Bargain Bucket, Jun 22, 2017.

  1. Bargain Bucket

    Bargain Bucket Active Member

    Glow Worm 24HXi condensing boiler with vented hot water cylinder and Hive 2 controller.

    This system has been working fine since we moved into the house as far as I am aware (although I've never specifically checked for the problem I'm about to describe) . I recently had work done to move the cylinder from an upstairs airing cupboard to a downstairs cupboard. A new cylinder was used along with a new 3 port valve (Y plan system I guess ?) and new cylinder stat. I've noticed now that during the scheduled hot water ON periods, the pump is running continuously even when the boiler is not burning gas. CH is not on right now (too damned hot for that). Plainly I will be getting the fitters back to investigate but I like to know how things work so that I don't get fed BS. In my mind, the pump should only run whilst there is heat demand from the cylinder stat i.e. when the boiler is burning gas, and maybe for a couple of minutes overrun when the boiler stops burning. Am I right? My pump keeps running until the scheduled period for HW has expired.

    Thanks in advance for any thoughts etc.
     
  2. sam spade

    sam spade Active Member

    The pump is wired to the boiler (possibly via the wiring centre) so the pump overrun will work. It sound as if the overrun thermostat is faulty, so the pump runs continuously after the boiler has gone off, instead of running for a few minutes.
     
  3. Glad its Friday

    Glad its Friday Active Member

    Or the pump has been incorrectly wired to a permanent live.
    The pump overrun on Glowworms is on a timer which can be adjusted. Perhaps been set to max (60 mins) factory setting normally 5 mins.

    If you're correct and the pump is running 24/7 then my guess is that the wiring is wrong - should be simple to put right.
     
  4. Bargain Bucket

    Bargain Bucket Active Member

    Thanks for the replies. The pump doesn't run 24/7. It only runs during the scheduled HW time (I havent checked what happens during the scheduled CH team) regardless of whether or not the boiler is actually trying to heat anything.
     
  5. Matt Hamilton

    Matt Hamilton New Member

    thats working correctly, if the HW is ON, then the pump wil continue to circulte the water and when the stat drops it will kick in the bolier
     
  6. sam spade

    sam spade Active Member

    The Ultracom hxi versions use a timer (GC 41-019-04); the hxi (GC 41-047-69) uses a thermostat.
     
  7. sam spade

    sam spade Active Member

    I have never heard of that happening on any correctly wired installation; it doesn't make sense.
     
  8. 14th edition

    14th edition Well-Known Member

    Sounds like pump has been wired to the permeant live from the valve rather than the switch line....?
     
    Dave does Gas likes this.
  9. Glad its Friday

    Glad its Friday Active Member

    Ummmmm, which is what I said 2 days ago!
     
  10. Glad its Friday

    Glad its Friday Active Member

    Yeap,what I said 2 days ago ....
     
  11. sam spade

    sam spade Active Member

    Both are incorrect; the pump must be connected to the pump terminals on the boiler. If not, pump overrun will not work.
     
  12. Mike83

    Mike83 Screwfix Select

    Your correct in the way you think the system may work.
    Maybe testing the heating to see if it works may help out a bit.
    I'm wondering if when the hot water is turned on at the programmer the boiler is switched on via the cylinder stat as normal.
    However I'm wondering if the wiring could have been messed up, meaning the pump gets a live directly from the programmer when the water is turned on,thus bypassing the boiler and negating any pump overrun.
    This would mean that if the water was on from 2pm until 4pm the pump would run for exactly 2 hours but the boiler would still turn on and off via the cylinder stat and boiler stat.
    Could this maybe be the scenario Bargain.
     
  13. Bargain Bucket

    Bargain Bucket Active Member

    Well I eventually solved this. Quite simple really. The cylinder stat was set at 80 degrees and the boiler was set at 70. So the boiler could never deliver the 80 degrees that the cylinder stat was demanding (the boiler would stop heating when it's temp hit 70), The pump would be running to circulate the 70 degree water to the cylinder that wanted 80 degree water. When I changed the cylinder stat to 65 and the boiler temp to 80, the problem was resolved.
     
  14. Maxxy

    Maxxy New Member

    Hi, yes I agree, I have a brand new Worcester Greenstar 15 ri and had a similar problem but believe it is not a problem as such, rather a design fault.
    If you run the boiler set at a low-ish temperature, then when in hot water only mode the pump overrun does not turn off.
    Turn the boiler temp up a bit and pump turns off as it should.
    I think this is because the boiler is expecting the water temp in it’s heat exchanger (when boiler temp is set low-ish, say 50 deg) to drop below this setting when the boiler turns off but the heat now stored in the hot water tank is being transported back to the boiler, keeping the water temp in the heat exchanger at a stable temp slightly too high for the pump overrun thermostat to turn off. If the boiler temp is set higher than the hot water thermostat then this effect cannot happen as the water in the hot water tank is not hot enough.
     
  15. sam spade

    sam spade Active Member

    It's not a design fault; it's basic physics.

    Heat flows from the higher temperature to the lower. So the boiler temperature must always be higher than the cylinder temperature. If you want water at 80C then the boiler water must be higher than 80, say 90C. Setting the boiler to 60C and the cylinder to 80C will inevitably result in the boiler running flat out continuously as the water in the cylinder can never exceed 60C.

    The reverse problem occurs if you have a TRV in the same room as the wall stat. If the TRV is set to a lower temperature that the wall stat, the room will never get up to the temperature set on the wall stat. So the boiler runs continuously.
     
  16. Maxxy

    Maxxy New Member


    Yes, I know the transfer of heat is not a design fault. I was referring to the pump overrun failing to turn off even though the boiler is not running due the transfer of heat back from the hot water cylinder. This happens when the boiler temperature is just high enough to satisfy the demand from the cylinder tank and therefore shuts off but the pump overrun does not turn the pump off because the heat transfer back must be telling the overrun stat that the water is not cooling. This does not happen if the boiler temperature is higher than the cylinder as it obviously does cool when it shuts off. All I’m saying is the overrun stat should be able to discriminate between a too high temperature that may damage the heat exchanger and a safe temperature that is maintained by heat transfer back. It appears that the overrun stat switching temp alters with the boiler temp stat but in my mind, should be fixed at whatever is thought to be too high. It only means that if the boiler stat is set only slightly higher than the hot water cylinder, then, when the boiler shuts off, after satisfying the cylinder demand, the pump runs continuously. This should not happen as no harm is being done - hence the design fault.
     
  17. Mike83

    Mike83 Screwfix Select

    If the boiler thermostat is set at 60 and the cylinder stat at 70 the cylinder stat will never be satisfied.
    The boiler will continue to run due to demand from the cylinder stat but with sporadic flame as system temp drops.
    At this stage the pump will continue to run from the switched live to the boiler not the pump overrun.
    The pump overrun only kicks in when there’s no call for heat/hw but the temp is still high.
     
  18. Mike83

    Mike83 Screwfix Select

    Also on some systems if there’s no auto bypass this can cause the pump overrun to run for a very long time.
     
  19. candoabitofmoststuff

    candoabitofmoststuff Screwfix Select

    I'm not a plumber, but I know my own system very well, and my understanding is that your statement is not quite right, though close...

    Ignoring any overrun facility that may exist from the boiler, I would expect the pump to be running when the boiler is "on"... that doesn't necessarily mean that it's "burning gas"... it may be on but the boiler itself has reached it's temperature and is cooling down a little before firing up again... In that situation I would expect the pump to keep pumping.

    That's not to say that you don't have a problem! And I repeat that I'm not a plumber!

    Good luck,

    Regards,

    Cando
     
  20. Maxxy

    Maxxy New Member

    Ok but in the scenario I have shown above the pump still runs when the temperatures are set as stated even when the boiler has shut down for a long period and the cyl stat is satisfied but only when the boiler temp is slightly or the same as the cyl stat. It’s just a quirk on my new Worcester Greenstar and not an issue as I set the boiler high enough. This is not about the boiler running continuously but the pump running continuously once the cyl demand is satisfied - but only in this single scenario. It is a Y plan with live rads but when in hw only and the cyl tank is hot, the pump fails to switch off once demand is met and boiler shut down even though the temp of water in the system is relatively low. Turn the boiler stat up a bit, so water in system is hotter! and pump turns off normally.
     

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