Hi

I've got an intermittent fault with a 5 year old combi boiler. Every
3-4 months or so the heating will pack up for a couple of days and then
sort itself out.

What happens is that combi seems to stop responding to the water system
by not delivering hot water through the central heating. The hot
water taps work fine, the boiler fires up just as normal. But as far
as the central heating goes, the boiler fires up every 10 minutes or so
(perhaps less frequently), gives about 20 seconds of clicking and
whirring (as usual) and then switches off. The obvious effect is that
the radiators therefore don't get hot!!

The problem is that each time this happens, we give it 24 hours or so,
then despair (and freeze), call a plumber and by the time they turn up
(or schedule to turn up) the heating sorts itself out.

Any DIY suggestions for what I could do to investigate? I know the
boiler itself is out of bounds, but do you think draining the system /
cleaning the water in the radiators might help? Perhaps its a blockage
in the system somewhere which means that there aren't enough pressure
changes in the system to prompt the combi into action? I don't know
much about this stuff, so any pointers gratefully received, either to
do myself or to get a plumber in to at least try something
constructive!

Thanks!

Matt