Combi boiler not lighting up when running hot water taps

Not knowing too much about the inner workings of these, do I assume correctly that there is some kind of flow switch inside that fires up the burner when it detects the cold water tape running? Guessing there is also thermostat in there somewhere.
Over the past month, it's got steadily worse from whereby messing about turning the tape on/off got it to fire up to really not working at all.
CH side seems fine.
Any ideas?
Thanks, Rob.
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The Ferroli Modena does not have the potentially troublesome 3 way valve. It can also work in hot water mode if the pump fails. It is simple and reliable.
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On Thu, 15 Jan 2004 23:31:11 +0000, IMM wrote:

The OP has not said what make and model of combi he has. There are 3 or maybe 4 different ways the boiler might detect the water flow.
If the CH is working then the fault most likely rests with the flow detector (or PCB just maybe). The diverter valve is unlikely to alter whether the gas lights when a tap goes on.
Just for completeness here are some of the mechanisms that I know of.
1) Differential pressure on a venturi acts on a diaphragm which moves something. 2) Nozzle acts on a paddle connected to a microswitch.
3) Flow drives a small turbine to give out electrical pulses.
4) Differential pressure cause by the hydraulic resistance through the heat exchanger.
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I'll check later and repost - it's not my boiler.

There is a bank of lights on the combi itself showing pilot light, burner on etc. What sometimes happens when the tap turns on is that the lights all light up to the top (top is ignititoin) I think but then fall back down to just below "burner running" light.
If that helps :-)
Cheers, Rob.
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