glow worm combi intermittant hot water

Hello again, an update from Nov 16,. to recap
glow worm 24 ci combi boiler ( about 4 years old ). Problem ; CH works fine but when using shower or hot tap the burner will fire for about 7 seconds then fade out for the same 7 seconds or so, and then repeat etc.
The DHW heat exchanger was a suspect ( still not off the hook) but when moving 2 radiators and flushing through recently found the water to be fairly clear.
Now found that if I turn on the hot tap all the way the burner will stay alight but there`s too much water coming through to be heated properly.
Does this point to a faulty water pressure sensor and are there any tests I can do to pinpoint the culprit. Is there a certain order of things to check in a case like this.
A hot shower for xmas would be great but the kids want presents ( doh),
Any help would be great thanks..

Sure - check the signal from the flow sensor...
I'm sure it's in the manual?

My guess is its either 1) flow detector (part No S57202) or 2) Hot Water heat exchanger.

I'd try the flow detector first as early ones had rotors that were inclined to stick, and they modified them. This fits with the symptom of just working under full water pressure. Its almost certainly a lot cheaper than a new plate to plate heat exchanger, so worth a try first.

I doubt its the system water pressure sensor or your CH would not work, and you'd get a fault code and lockout

Alfredo

Thanks for those replies,
to check the flow meter would this be feasable,
Turn off mains suppy to the boiler
remove wires from flow meter
connect multimeter on ohms settings to flow meter
turn on taps and notice any varying readings,

Thanks once again.

Not really!

If I remember its a turbine which has a rotating magnet which induces an AC voltage into the sensing coil.

You can look for an ac voltage.

The most common fault is that the turbine rotor sticks and does not rotate.

Tony

The manual is not terribly helpful..it says under fault finding

Domestic Water Flow ToTest: Use a new detector and connect it electrically in place of the faulty one. Blow through to simulate a demand for water.


As I said, not terribly helpful.

Alfredo