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?

Thanks

David

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