On 18 Mar, 07:26, Dave Fawthrop
wrote:
I have just had a new Combi fitted to the old *single pipe* radiator
system. Two zones upstairs and downstairs controlled with valves. It
worked fine.

The downstairs valve was in the wrong place so I drained the system and
moved it. Crossed a thread and had to replace it with a different make.
Refilled, bled radiators, and adjusted CH pressure to 1-2 bar.

Now the problem! The system gives a *single* bang a few times a day, at
random. Random events are a pig to fault find. It sounds to me as if it
*could* be water hammer caused by the valve closing. It really does not
sound like a gas explosion, just the pipes reverberating.

What do people think? What is a good method of fault finding?

Google giveshttp://www.water-hammer.com/

A US site says that air chambers are no longer recommended.http://www.builderswebsource.com/tec...aterhammer.htm
High temperature sealed are filled bladders?
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Ignore US sites.
The problem will be due to the new valve shutting more suddenly then
the old one. Some makes are more prone to this than others. The
Honeywell rubber ball type "usually" are least prone to it but YMMV
A shock arrester might be useful but no guarantees