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My CH/HW system is a standard Honeywell Y-plan. I had a problem with the 3 port valve recently, I could only get CH if the HW was on. I replaced the valve and that fault has gone away, but it seems that it was masking another little oddity.

If I call for HW without the CH on, the boiler (a fairly new Potterton condensing boiler) fires up. I can hear the fan start and then the muted whoomp. Problem is, it usually only runs for a couple of minutes before shutting down. It's not the tank 'stat because 1: twiddling it up doesn't restart the boiler,
2: twiddling it down WILL stop the boiler if it is running and
3: I attached a lamp to the power feed to the boiler and pump from the connector strip (the ORANGE wire from the valve) and when the boiler shuts down unexpectedly, that lamp stays lit, so the system is still calling for hot water
So presumably the boiler is shutting itself down.

The flow and return legs of the tank coil get hot, and if the CH is on, the boiler stays on exactly as it's expected to. And the HW does get hot under those circumstances

I'm thinking possibly that the water in the primary circuit is getitng too hot and an internal thermostat is shutting the boiler off, but if water is circulating in the CH, that is keeping the primary circuit temperature down.

Does this make sense? And if so, what could be causing it? The pump seems OK and it's turned up to 3.

RoyG wrote:If I call for HW without the CH on, the boiler (a fairly new Potterton condensing boiler) fires up. I can hear the fan start and then the muted whoomp. Problem is, it usually only runs for a couple of minutes before shutting down.

Why do you believe the boiler should stay on for a longer period when only heating the hot water? Are you not getting hot water then?

The capacity of the boiler is far in excess of the ability of the tank coil to transfer the energy to the cylinder water so the boiler will only fire for, well, only seconds, probably. As the cylinder reaches temperature, the "off" period will become even longer and longer.

If the boiler fired for a longer period, the water in the primary circuit would simply begin to boil.

[quote]Why do you believe the boiler should stay on for a longer period when only heating the hot water? Are you not getting hot water then?

The capacity of the boiler is far in excess of the ability of the tank coil to transfer the energy to the cylinder water so the boiler will only fire for, well, only seconds, probably. As the cylinder reaches temperature, the "off" period will become even longer and longer[/quote]

I guess I'd got hold of the idea that the boiler should bring the HW up to temperature in one go rather than bit by bit. I hadn't realised that doing it in short bursts was normal. I didn't remember my old boiler doing it. Come to that, I hadn't noticed the new one doing it until recently

I'll relax now. Thanks