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Discuss Heating Bangs when switching off. in the Central Heating Forum area at

  1. Paul Cooksn

    Paul Cooksn Member


    I have an older style open vented heating system with a Glow worm 50/60 RF flued fan boiler. I have a hot water tank on the first floor and two header tanks in the loft, one large for the hot water and a smaller one for the heating.
    Last week I removed a radiator from the landing and capped off the pipes under the floor. Since doing this when the boiler switches off there is a loud bang and the whole heating system shakes. I thought it was an air lock but I have bled the system and all radiators are warm. It only does it when it's on heating, not when on hot water. Another point, when the boiler switches off water runs into the header tank for about 3 seconds.p from the expansion pipe.
    I've even tried connecting the pipes I cut off together just in case I had messed up the flow and return but it's still the same. All radiators have trv's apart from a heated towel rail in the bathroom and large radiator in the hall which is by the thermostat.

    Anybody have an ideas what it might be. I have also flushed the system and it is clean. It's worked fine for years until I removed the radiator.

    Many thanks.

  2. Vee

    Vee Plumber GSR

    Do you have an automatic bypass on the system. You may need it if the radiator you removed acted as manual bypass.
  3. Last Plumber

    Last Plumber Plumber GSR

    It sounds like a spring return motorised valve slamming shut.
    Vee may well be right with the bypass but you need to identify the cause first.
    If you have a motorised valve, check it when you turn the heating off to see if it’s that, which is making the bang.

    If not let us know and we’ll think of something else
  4. Paul Cooksn

    Paul Cooksn Member

    I don't think it has one. Where would it usually be located. i can take pictures if required.
  5. Last Plumber

    Last Plumber Plumber GSR

    It may not have one. I was going off the description you gave. Difficult on a forum.
    Normally, airing cupboard or near Boiler !

    If it hasn’t got one, maybe an auto bypass is the answer.

    Any chance of photos of Boiler and airing cupboard ?
  6. Paul Cooksn

    Paul Cooksn Member

    It has two motorised valves. I can hear the motors opening and closing when both the heating and hot water are in use. The bang is really hard to identify, it seems to be coming from around the pump but difficult to say for sure. Is the hot water being pushed up into the header tank part of the same problem.

    When I removed the radiator I capped the ends and have also tried connecting them together similar to how the radiator would function but no joy.

    Thanks for your advice.

  7. Last Plumber

    Last Plumber Plumber GSR

    Honeywell ( silver box top ).?
  8. Paul Cooksn

    Paul Cooksn Member


    The boiler is in the garage 15 meters from the airing cupboard. Since this problem the bleed valve no longer works and is full of water. If you undo the screw water overflows from the nipple.


  9. Last Plumber

    Last Plumber Plumber GSR

    Yes it needs a bypass.
    If you try holding the valve open whils someone turns heating off you’ll see if it stops the bang
  10. Paul Cooksn

    Paul Cooksn Member

    How would you do that, I'm not sure. Presume you would take the cover off but then what?
    Many thanks.
  11. Last Plumber

    Last Plumber Plumber GSR

    No. 230V in box.

    There’s a lever on side where wire exits.
    Auto - Manual
  12. Christopher buzzard

    Christopher buzzard GSR

    Opposite side to the label on the zone valve is a little lever. When your heating is on there should be no resistance on the lever and it should move left and right freely. Heating off and you should feel tension on the lever and when you move it is should move back to close position on a spring action. Switch your heating off and hold the lever open. If there's no bang once the pump has shut off then you def need a auto bypass. There is also a little nib on the valve where you can hook the lever onto so it won't return to close, if you are doing this alone.
    • Like Like x 2
  13. Last Plumber

    Last Plumber Plumber GSR

    Do what C Buzzard said.
    It’s what I was about to write.
  14. Paul Cooksn

    Paul Cooksn Member

    I'll try it in the morning and be in touch then (kids asleep). Thanks for your help and advice, much appreciated. Regards. Paul.
  15. Last Plumber

    Last Plumber Plumber GSR

    No worries, do let us know how you go on!
  16. king of pipes

    king of pipes Trusted Plumber GSR

    Vent your pump to it sound like a typical air locked system , good luck kop
  17. Paul Cooksn

    Paul Cooksn Member

    Hi I did what you said. When the heating is off I checked the tension on the lever and there isn't any. When it's on there is tension so I held the lever open and turned the heating off and it still bangs.

    Could it be an air lock.?

  18. Paul Cooksn

    Paul Cooksn Member

    I've bled the pump. The air vent above the pump is faulty and leaks water through the bleed nipple. I know it's supposed to be left undone but I can't as it will FLOOR the airing cupboard.
  19. Last Plumber

    Last Plumber Plumber GSR

    When the heating is on, there shoul be no tension as the valve is open. If it is calling for heat that is.
    i.e. programmer on, room stat up above room temp.

    Maybe you meant that ?

    If it still bangs with the valve left open, it needs looking at, I can’t guess without witnessing it.

    I don’t think it’s an air lock but that is my opinion.
    • Agree Agree x 1
  20. Paul Cooksn

    Paul Cooksn Member

    Sorry was checking the wrong one. You are right, no resistance when on. I held it open, turned off and no bang. Can still hear water going into header tank? Does this mean the valve is faulty.

  21. Paul Cooksn

    Paul Cooksn Member

    Are these bypass valves or air bleed valves. They are a few meters from the boiler.

  22. Last Plumber

    Last Plumber Plumber GSR

    It can be a faulty valve yes but in your case I would suspect that the lack of a system bypass is causing the valve to slam shut as the pressure increases on closing. (valve damage may have occurred as a result of not having a bypass).

    My advice:-
    Install a system bypass (22mm Auto) and see if it stops the bang. If not replace the valve. You need the bypass anyway.
    • Like Like x 1
  23. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Agree with the above but as your draining down I would replace the valves at the same time, if you can afford the slightly extra cost
    • Agree Agree x 1
  24. Last Plumber

    Last Plumber Plumber GSR

    Good point!
    Do that!
    Replace valve and install a Bypass.
    • Agree Agree x 1
  25. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Looks like auto air vents
    • Agree Agree x 2
  26. Last Plumber

    Last Plumber Plumber GSR

    Bit dark but I agree wi Shaun
    • Agree Agree x 1
  27. Paul Cooksn

    Paul Cooksn Member

    Great help guys thanks very much. One more question where should I fit the bypass valve. Close to the boiler or in the airing cupboard.

  28. Paul Cooksn

    Paul Cooksn Member

    They were hissing air when the system was refilling so guess they are air vents.
  29. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Airing cupboard

    I would fit it in between the port valve tees and straight onto the return
  30. stratplus

    stratplus Plumber GSR

    Wouldn’t the rad in the hall and heated towel rail act as a bypass. Neither of these have trv’s.
    So removing that landing rad shouldn’t have made a difference. He also joined the pipes from the rad he removed and it was still banging.
  31. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    If these are piped in before any port valves yes they would
  32. Last Plumber

    Last Plumber Plumber GSR

    Not if they’re after the valve and they most likely are.
    The bypass needs to be before the valve for when the valve shuts!
  33. Paul Cooksn

    Paul Cooksn Member


    I had a plumber attend yesterday and the valve was faulty, when taken off you could hear it making a grinding noise. He replaced the valve and the banging stopped but did say it probably needs a bypass valve fitting between the pump and valves then into the return. After a few hours the banging returned but it was much quieter. The plumber said the new valve is much stronger than the old one.

    The bypass is being fitted Thursday so I will let you know what happens.

    Cheers for all your advice.


    • Like Like x 1
  34. WYEO54

    WYEO54 New Member

    Hi Paul
    I would suggest that you check that the zone valves are actually positioned correctly for the differential pressure being created by the pump. The type of sound you have described indicates to me that a zone valve is slamming back when closing - this happens when a valve is positioned the wrong way to the flow - but looking at the photo they seem good to me but obviously I cant see the whole system layout design - so its difficult to be sure whats going on?
    Best of luck getting it sorted... Bill
  35. Paul Cooksn

    Paul Cooksn Member

    Hi All,

    I had the bypass fitted but there is still a slight bang when turning off. It still pushes water into the header tank as well. I also had fitted a new pump as the old one was nearly blocked. I have fitted a magnaclean on the return to the boiler and you wouldn't believe what muck came out of it in 24 hours.

    Should I just put the radiator back.

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