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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!
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