I have a Vokera Maxin 28e combi boiler and it has started only providing intermittent hot water. This makes itself known mostly during a shower when there's hot water for a couple of minutes then cold for a couple, then hot, then cold, etc.

At the boiler, while this is happening, the DHW diverter pressure sensor appears OK because the microswitch is closed during all this.

I took the DHW heat exchanger out and gave it a clean but it seemed to be already pretty good - there were only a few little bits of stuff that washed out.

At the boiler basically the gas comes on, once the initial demand for hot water has caused the microswitch to close, and that's OK. The water does get pretty hot, maybe too hot, and then the gas goes off. After a while the gas will re-ignite but the water has run cold by this time.

Any idea what it might be?

Colin, July 2009

Is there a flue venturi pressure switch?. If there is, is this switching out when the boiler is running?, if it is, the venturi probably has a little bit of dirt in just the wrong place, (it would need to be spotless to operate properly, borderline ones cause the behaviour you describe). Sounds as if you have the manual and are taking it to bits anyway, but this isnt recommended for unqualified individuals. The free Vokera technical help line people are usually very helpful.

c, July 2009