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Can mid position valve cause banging?

Postby Al G » Sat Mar 21, 2009 2:43 pm


I have a gas fired boiler (not combi), CH, indirect HW cylinder. A Grunfoss pump, ACL Lifestyle mid position valve.

The HW and CH both heat fine (rads regularly bled). HW is on timer to come on a couple hours morning and evening; CH normally left on continuous and controlled entirely by thermostat.

When the HW clicks on, there is often a big bang followed by banging around the pipes of the house, the pump stops and boiler cuts out. After a few seconds' silence, it all starts up again and the performance is not repeated. This just happens twice a day when the timer switches on the HW, and it does not always happen. I have yet to determine whether there is a pattern to this, eg. whether it corresponds to the pump already being on or off for CH when the HW switch operates.

Could there just be air that gets trapped somewhere that causes this?

Do I need a new mid position valve?

Or could it be something else?

I hope I've explained this adequately, happy to provide any more info. Any advice appreciated.

Al G
Al G
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Postby Steve the gas » Sun Mar 22, 2009 6:29 am

Hi Al,

I suggest you let a good RGI look at it.
It sounds like it could be 'explosive ignition' but not necessarily?

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Postby AdamsHeating » Mon Mar 23, 2009 10:03 pm

Is the boiler a potterton?

I've seen a few boiler that are positive pressure fanned and they have a tendency to have this explosive ignition due to a delay between the gas being let in and the ignition sequence.

There has been a HSE warning about a particular potterton, the prima I believe, but i can't remember exactly. It was to do with the delay on ignition.
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Postby Al G » Tue Mar 24, 2009 6:45 pm

Thank you for the replies.

The boiler is a Myson Apollo Fanfare S.

The bang occurs far less often than I imagined - I've waited for it to happen at the times I thought it did, to see what actually happens at the boiler, but only normal starting up has occurred. I have tried but been unsuccessful to reproduce conditions that induce the problem (nothing technical just starting and stopping HW/CH in different sequences). It has not in fact happened at all since I started this thread! I still think it was originating in the airing cupboard, but you are the experts!

If it happens any more I will try again to confirm where it is coming from, noting your concerns and if it seems it is indeed the boiler will get a RGI to it. I will otherwise just mention it at service. Hopefully whatever it was is no longer a problem and if so I apologise for taking your time.
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Postby christrinder » Wed Dec 22, 2010 9:31 pm

Hi - did you ever find a solution to this. My CH and HW is doing exactly what you describe everytime the Mid Position Value moves (i.e. when I switch either the HW or CH on/off). The banging and thuddering from the pipes is getting worse and worse each time. Any help would be gratefully appreciated. We have British Gas booked to come and look at it but we're having to wait a few weeks for the appointment. Thanks, Chris
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