DIY Doctor UK DIY Forums

Hello all. I have a combi boiler that will heat the rediators fine but the hot water is running cold. If it's on hot water only then it doesn't fire up when I run a tap.

I had a read of the service manual and took the front off, and found that the rod on the DHW flow valve isn't pressing its microswitch. If I manually press the microswitch then it does fire up, then the other microswitch gets pressed by its rod. But the water still flows cold.

I was going to replace the DHW flow valve, but I'm wondering if it might be a waste of money ordering it. Is it possible that something else is jammed (3-way valve?), which prevents it from moving?

Is it possible to diagnose without stripping it down? I was hoping to get the parts before doing the work in one hit, but if it needs investigating first then not a problem.

I'm a newbie to boilers, in case it isn't obvious! Many thanks in advance of responses.

may sound daft.....but is the summer / winter switch on either setting or left in the mid posistion

Summer / winter switch? Do you mean the selector for HW/CH-Off-HW? That one's in the HW position.

I've since opened up the DHW flow valve. The rubber diaphragm looks partly perished, but I can't see any holes in it. I greased everything with silicone grease and reassembled, but it's still the same. The microswitch on the DHW valve doesn't get pressed by its rod when a tap is turned on.

If I press the microswitch, the boiler fires up and the two pipes to and from the heat exchanger get hot. The hot tap is flowing, but it's cold.

It's still possible that the diaphragm is either porous or too stiff to operate. But I can't imagine how I would still have no hot water even if I manually press the microswitch.

Any help on any diagnostic tests I can do to find out what the fault is?

change the diaphram, which boiler make model?

Sorted now. I've awarded myself 50 idiot points.

I'd managed to reverse the in and out pipes while plumbing in the kitchen. The cold tap was cold, and the hot tap was flowing through the boiler, but backwards. So it's hardly surprising it wasn't working.

Works lovely now it goes the right way! Luckily I had an accessible corner where I was able to cross them over (thank god for those crossover joints!).

Doh!