Worcester 240, Central Heating Overheats, Hot Water Works OK

I have a problem with my Worcester Bosch 240 boiler. The hot water works just fine. However, if I make a demand for central heating, the boiler fires up, and within about 20 seconds, the boiler pressure gauge rises rapidly to about half way, before the boiler overheats, and the reset switch pops up.

Is this something to do with the pump not circulating the water from the boiler, or is this likely to be something else?

Is there an easy way to test the pump without removing it?

Would expect it to overheat on HW as well, but to test pump, remove centre scrrew and poke a small screwdriver in. It'll be obvious the pump is turning, though they can block or the impeller can fall off.

Could be your diverter valve is stuck at the HW end so there's inadequate CH circulation.

I guessed maybe the flow of cold water through the boiler was providing sufficient cooling in hot water mode, preventing the overheat, but I was wrong. It is not the pump. The pump is working using the test that you suggested. I drop the bottom of the boiler containing the switches and circuit board and this reveals a circular valve with a spider of pipework coming off. I guess that is the diverter. It looks a pig of a job to replace, because all the pipework is twisted to fit into the boiler. The front relay to this valve is wet. There must be a slowly leaking joint somewhere. I think the relay may have had it. Can I remove the relay, without removing disconnecting the pipework to the valve? If so, how? Does it just pop or slide off? If I pull it will I damage it?

It turns out that this is not a relay or solenoid, but a couple of momentary microswitches. There is a plunger coming from the valve, which activates the switches. On my boiler, the switch housing has cracked, so the switches come away from the plunger. The switch housing is attached to the valve using a circlip. I have removed the circlip and I am supergluing the housing at the moment.

This appears to be the flow detector for hot water demand. I must have knocked it when I lowered the bottom housing. I still have the problem with central heating. Am I looking at the correct valve? Is the diverter valve controlled by the flow of hot water, ie the valve is always in the central heating position, unless hot water flow opens it? Or is there a second electronically switched valve hidden at the back of the boiler somewhere? Another symptom I am getting is that the radiator water does not circulate after hot water demand. Normally I get a couple of minutes run to cool down the boiler.