BillV wrote:
"David Hearn" wrote in message
...
We replaced our boiler in Feb (new cast iron Ideal 15kW boiler).
System is fully pumped with 2x zone valves (one for heating, one for
water). Pump is on lowest speed (in airing cupboard next to a bed,
anything higher and it makes a loud whirr which is too noisy).

Everything has been working great, the boiler is nice and quiet and
had no problems with it. Heating on 24/7 on programmer and
controlled by programmable thermostat. Water comes on twice a day,
morning and evening for about 1 hour. The problem is that now with
our warm weater the room stat is always above the temperature
required - so only water is being heated. When this happens the
boiler starts making banging noises. I understand this is probably
locallised boiling of the water due to the water flowing too slowly
to extract the heat out the exchanger fast enough.

I have some pipe thermometers on the flow + return of the boiler and
boiler is set to 2nd highest setting. I can't remember the exact
temperature, but they around 60-70 degrees.

If the flow is too slow when heating water only, I believe the
problem lies in one of the following areas:
1.) Zone valve not opening fully (I've moved the manual opening
lever and that moves fine, but I don't know if that directly moves
the valve, or not)
2.) 'Balancing' valve fitted after zone valve not opening fully
(I've tried opening it fully by turning it and it appears to open,
but makes no apparent difference).
3.) Pipework is blocked, sludged or furred up (system was flushed a
lot before boiler replacement, including removing each rad -
inhibitor present, and certainly has been for last year prior to
boiler replacement, ie all the time we've lived here). However, no
similar problems with heating circuit - both circuits T off after
the pump to their respective zone valves. Therefore blockage must be
only within heating circuit.

At present I've wedged the heating zone valve open, so whenever the
water is heated, the hall and bathroom rads come on (rest are TRV'd)
and this removes the banging noise. I don't like this solution as
it means the heating zone valve never moves, and therefore
potentially could jam. It's not a problem at present and is working
okay - but ideally I would like to get to the root of the problem.
I don't want to alter the pump speed as any faster and it gets too
noisy, besides, it works okay on heating only (or heating and water).

Any ideas?
Is there any kind of a bypass for the boiler, automatic or otherwise,
when all the TRVs shut?
e.g. non-trv bathroom rad before the valves?

On the heating circuit the hall radiator (where the room thermostat is) is
non-TRV. In the bathroom the TRV is set to max (when we arrived, it was the
only radiator with a TRV bizarrely!). So, that satisfys the bypass for the
heating circuit. At present, even in the hot weather we've been having, the
bathroom radiator gets hot, so I assume Max means no-thermostatic control
and fully open. No problems when running with heating only.

On the hot water circuit there is no bypass, but then there isn't a TRV so
that's not a problem. Problem occurs when running with hot water only.

The only issue I can see then is if the boiler fires whilst both valves are
shut/jammed. The wiring is such that when the boiler is on, at least 1
valve is open, so only the jamming situation is an issue. Is it normal to
have a bypass for jammed valves situations? Incidentally, there is a 15mm
pipe (whereas rest is 22mm) with manual valve (same as balancing valve after
HW zone valve) which bypasses both valves. I don't know what this is for
and has always been shut.

Thanks

David