combi boiler off and on

Hi,
I have a combi boiler (biasi). The pilot light is on OK but the flame keeps switching off and on. This results in the hot water in shower/bath/kitchen tap running hot then lukewarm then hot again. The flame ignites then switches off after about 5 seconds then on again after another couple of seconds (this seems consistent with the cycle of water running hot then warm). It repeats this cycle constantly (basically until I've run the tap or shower until I turn it off) Any ideas would be very helpful!

Several problems may be causing this, the heat from the burner is not being transferred quickly enough to the DHW, so the boiler primary water reaches the limit & the controls turn off the flame, etc.

DHW heat exchanger may be scaled - has the water flow dropped off? Does the hot-cold-hot symptom reduce if more DHW is drawn-off by opening more than one tap?

Gas valve may be faulty and not modulating (that is if it's supposed to), reverting to ON-OFF action.

DHW sensor or DHW temp setting potentiometer may be faulty.

Control PCB may be faulty.


I suggest you have a qualified fitter service the boiler and address the issues.

How old is the boiler - has it always done it?
Does it do it on CH too?

The problem is caused by the APS not correctly responding to the airflow.

This will require a CORGI registered engineer as its a gas safety issue.

If you could get Danny at Ranock then I can give him any advice he requires.

Tony Glazier