Blockage in hot water circuit?

My /guess/ is that it's a piece of grit fouling the pump impeller. Ergo, when the pump stops, the grit is at the pump. Or some other intermittent fault with the pump.

Owain

If it's broke after you've fixed it, you haven't fixed it.

I'd suggest that you check the wiring of the valve and ensure that it's the right motorized valve head; there are 2 different Honeywell types, at least. It may be that the 3-way valve is being closed by the thermostat, but that the boiler isn't being turned off as it should be. One of the 2 outlets of the valve should always be open, so this doesn't quite explain the problem, unless you've got the heating valved off.

The Honeywell wiring diagrams are available on their web-site.

garysz presented the following explanation :

Which will probably be the actual cause of 'the blockage'.

If the valve is not wired and the actuator properly located on the valve, then that might be 'the blockage'. Turn it on to provide hot water, then lift the actuator off the valve spindle, then try turning the valve by hand to the correct position. You should be able to hear when the water is flowing through and determine which circuit is being supplied with hot water by the pipes warming up.

I didn't :-)

The pump does not stop.

The 3 way valve was professional installed and works fine.

Interestingly the problem does not occur when both central heating and hot water are on - presumably less pressure throught a large circuit.

garysz formulated the question :

The problem IS the valve.