Combi boiler issues

There is another heat exchanger, AIUI, to heat the water- there's the closed circuit for the rads, and the HW exchanger gets connected to it instead of the rads by the diverter, AIUI.

When my old combi was doing this it was the early sign of a HW flow switch failing- the burner presumably wasn't being modulated correctly.

I have the identical problem as the OP with mine and have been unable to persuade anyone that it is a real problem. I suspect the "gas man" is unable to see what's wrong. Both the secondary heat exchanger and the diverter valve have been replaced.

Could it once again be sludge. In my experience, power flushing is an expensive but imprecise thing. It depends on how old the system is of course. I never did quite understand how sludge in the heating, which appears to be working can affect the hot water, but then I don't use gas any more, and my pocket thanks me for that. I guess when my first storage unit dies, I'll be cursing that instead!

Brian

It could be any part that controls the HW heating but not CH, eg the cylinder thermostat.

NT

Thanks. Is the flow switch responsible for any modulating though or is it a simple on/off switch?

I can see how that could cause the water to oscillate between hot and cold but not how it could cause a steady(ish) flow of warm water.

Tim

Hmm, where would I find the cylinder stat in a combi boiler?

Tim

In all honesty, I don't know, but yes, I'd agree with your train of thought. I don't know if it is just a simple switch.

On its little cylinder?

And where on a Greenstar boiler would this be? It would be helpful if you could substitute facts for wild guesses.

Even if it did have a little cylinder I don?t see how a duff cylinder stat would cause random swings between warm, hot and cold.

Tim

combis typically have a very small HW cylinder. I never said all do.

iffy connection

NT

In mine the problem was the DHW heat exchanger blocking. I've had two combis but I'd never chose to fit one now except perhaps in a very small house or flat. Give me a proper hot water cylinder any day. Too many things to go wrong (heat exchanger, valves, control system).