Loud bang when boiler turns off

[Padzster]

Hi
I recently had a new Worcester boiler and Megaflo cylinder fitted. All worked fine for three months. Then one radiator was taken off for decoration. After the rad was replaced I started getting the banging problem. (Not sure if the rad being taken off and put back on is relevant).
Anyway, heating runs fine but when the heating is turned off or clicks off when the house hits temperature there is one single loud bang. At first I thought this was from the boiler, but now it seems like it's coming from pipes in the first floor landing - none of which had been touched with the install.
The plumber initially thought it was due to air in the system, but he is now stumped.
Considering we have an unvented system I have had to bleed the upper rads quite a bit but then I just assumed everything was "settling in".
The single loud bang is very loud and worrying. Everything else works perfectly, but my plumber has given up as he has no idea.
Any suggestions?

 

[kgreen84]

Did he check the pipework under the floorboards? When I changed a rad last year had the same. Turns out I'd dislodged the flow pipe from a clip and everytime the boiler shut off it was banging against a joist I think it was

 

[45yearsagasman]

I would be looking at the motorised zone valve.Possibly a spring return valve that is faulty and closing with a bang.Very difficult to diagnose when not on site.Are you sure its not coming from the boiler?
If a 2 port valve is fitted wrong way round this can cause noise when system on pump overrun.

 

[Padzster]

He didn't check the pipework under the floorboards as none of the rads or pipes on the first floor have been touched.

He's sure that the bang isn't from the boiler. I've stood right by the boiler with it turning off and the bang sounds like it's coming from elsewhere. Put my ear to the landing floor and it's very loud there.

 

[Agile]

Based on what you have said only the radiator valve settings have changed.

What setting is the lockshield on?

Has it been balanced or is that beyond the ability of whoever took it off?

There is more to heating than what most builders or decorators usually seem to know.

Tony

 

[Padzster]

The changes made to the house were as follows:
Ground floor: all rads as before. New boiler.
1st floor: all rads as before.
New loft conversion: megaflo installed, 1 new rad + 1 towel rail rad.

All worked fine with no banging after installation. Then the loft rads were removed for painting and then replaced. They didn't work due to air in the system. After much bleeding they then worked but then the banging when boiler off problem began. Still need to bleed the 2 loft rads weekly as they fill with air even though system in unvented (?!).

None of the other rads were changed to my knowledge.

 

[Agile]

What setting is the lockshield on?

Has it been balanced or is that beyond the ability of whoever took it off?

Tony

See questions above.

If boiler on ground floor what pressure is it set to cold?

Tony

 

[Padzster]

Apologies for lack of knowledge.
Lockshield on which rad? Lockshield is the valve that isn't the thermostatic one, right?
How do I find out about boiler set to cold pressure?
Rads haven't been balanced using proper equipment. So far plumber hasn't seemed particularly able!

 

[Boilerman2]

I'm with 45Yearsagasman - Check the 2 port valves have been fitted the right way round!!! I have seen this fault many many times!!
Honeywell & similar 2 port valves with a seating ball will make a single and VERY LOUD BANG when shutting if the valve is not correctly orientated!

 

[Padzster]

Thanks for all replies.

Would I be able to tell if it's the wrong way round myself or should I just get the useless plumber back?

 

[45yearsagasman]

2 port valves are marked A and B on the brass body.Water flows into port A and out of port B. These valves are normally fitted after the pump.

 

[Padzster]

Just took a look at the valve and I can't see the A or B, must be on the side I can't get to - couldn't see with a mirror either.
So I called the "plumber" to suggest this and he said that if this was the case then I wouldn't be getting any heating at all as it wouldn't be letting in the flow at all. Is he correct? He said he'll take a look at it "some time next week".
And in the meantime?.....

Here's a couple of pics of boiler and valves in case it helps:

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