Thumping Noise - Please Help

[David1983]

Good Morning! I've noticed recently that when the boiler comes on to heat the hot water, if the three-way valve has been to heating, that there is a thumping in the pipes as the hot water starts flowing. I've got a header tank in the loft so would that be causing air to get into the system?

Thanks again, David

 

[David1983]

Can anyone help? The thump only happens when I've had the central heating on and, once it's off, when I turn the hot water on. There is what appears to be a bleed screw by the inlet to the hot water cylinder, have opened that until water comes out then re-tightened it leaving only the holes showing but no water seeping out.

I've also got a Grundfos Selectric 15-50 pump off one of the legs on my three-way valve, it is set on speed 3, but is this just for the radiator circuit and not the hot water? From what I've read I've got a pumped heating circuit and gravity hot water circuit so is there anything else on the hot water system I could bleed?

Please help as this thumping is quite loud and I'm worried in case it's causing any shock damage to pipe joints etc....

Thank you all and have a Happy Christmas! David

 

[Biguns]

If you have a 3 way valve it sounds like you have a fully pumped vented system. It could be a problem with the 3 way valve or a pump overrun switch. If the boiler fires up and the pump does not, this will cause banging as the hot/ boiling water vents to the F/e tank.

Try to re create the symptoms and see if the boiler fires and the pump does not. There is a large screw on the end of the pump. With a dish or bucket or towels under the pump, remove this screw and gently insert a screw driver. If you feel a vibration the pump is working. With the screw removed there will be a slight continuous trickle from the pump

See FAQ 14 here //www.diynot.com/forums/viewtopic.php?t=175736

 

[David1983]

Snug, thanks for telling me that, I tried it and it made no difference. However, I did turn the speed of the pump down from 3 to 2 which seems to have sorted it.

Any reason why it woudl thud when hotwater is selected on speed 3 but not thud when heating is selected on speed 3? i can see that the pipes into the hot water cylinder are larger than those into the radiators - 28mm to 15mm - could this be the cause?