I have one of those multipoint boilers that do the hot water and central heating but no hot water tank iykwim? Every so often, usually when I am on my own (so notice it more) the boiler will fire up into action, for a few seconds and then go off again. No one has been drawing water from it and the central heating is not on. Any idea why this happens? My husband thinks its some sort of self test thing but I don't see why this would be.
My DH (who is not a plumber / heating engineer but he does know a lot about boilers) says:
That's usually something about the tuning of the flow sensor. Basically, when you run the hot tap, these boilers detect it - often using a rubber diaphram which has a switch attached to it. when the diaphram changes size, the switch triggers. It's possible that a nearby appliance such as a washing machine, when it finishes filling and abruptly turns the water off, is triggering the flow sensor. It's hard to trigger the same behavior with a normal tap. Possible fix would be to identify the washing machine (if it's hers), and turn down the inlet slightly, so that the rate turned off is lower. That might slightly slow down the washing cycle. In theory she could have the flow sensor adjusted, but I'd hesitate to fiddle about in a boiler which is working.