DIY Doctor UK DIY Forums

Background
The symptoms:
1. CH works
2. HW does not. Used to work but hasn't since a few weeks ago and I have opened all hot water taps and can hear/feel water running through the boiler, but the secondary heat exchanger never gets hot.
3. Actuating lever that registers a call for hot water does not move so the microswitch is never activated
4. When I push the microswitch manually and run the taps, the water still runs cold but the radiators get hot.

Based on the above, I believe the diaphragm needs to be replaced.

2 questions:
1. Do you think my diagnosis is correct or at least a good starting point?
2. If I am to replace the diaphragm, do I need to drain the boiler first?
-The manual says to shut off boiler which includes: close off the CH valves, close DHW cold water supply valve, close gas inlet valve, then open taps, then remove the housing and replace the diaghragm.

It won't take much longer to do, but if I can get away with not draining it, then I will. Seems like I would have water spilling out when I get to the diaphragm if I don't drain the boiler.

Thanks in advance.

You need to drain the boiler - no need to turn off gas supply. Turn off cold water supply.

Yes it will be the diaphragm which has split or stretched - will probably require a new fulcrum arm too as they normally snap.

Be careful when removing and replacing pump the screws are different lengths - if you put them back the wrong way you can puncture the pipe behind.

Clean out ports too as will probably have a build up of limescale.

Many thanks htg for the prompt response and including tips and what to look out for!