Combi boiler problem

It's that time of year I suppose, although this one is about the hot
water, not the heating.
It's an Ariston Microgenus. It seems to have developed an intermittent
fault whereby sometimes when you turn on a tap the hot water just
doesn't get hot. While the hot tap is running there seems to be a lot
of churning coming from the boiler, which I assume is the pump
running, but neither the fan nor the burners come on. The "fume
sensor" LED comes on until you turn the tap off, whereupon the light
goes out again. This fume sensor LED seems to be a mystery from the
research I've done. The manual doesn't say what exactly it senses. And
I don't see how it can be sensing "fumes" because the burners aren't
Any immediate ideas as to what would cause this to happen
Reply to
Martin Pentreath
Has it got a pressurised container? If so, is the pressure high enough in the pressure vessel? I ask, as I had a similar problem with a Baxi a few months ago, I had drained all the water out to fit new rads, on refilling, the air pressure had all gone, so although the pressure gauge said it was fine, there was not enough air in it to allow the boiler to turn on for more than 10 seconds. It also made an awful lot of gurgling sounds as of there was a lot of air in the boiler. After a drain off of a few litres, then re-pressurise the vessel with a car pump, all was fine. Alan.
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