Combi boiler misbehaving

[Sandytoe]

My potterton puma 80e will only fire when i turn the central heating on.
It won't fire up when I just turn a hot tap on.

So the only way to get hot water is to turn on the central heating, then once the boiler has fired, turn on a tap, and then turn the central heating off.

Any ideas on what might be wrong?

 

[ChrisR]

Then what happens - does the boiler stay on while the tap is on or does it ignore it?

There's a tap hot water flow switch left of centre inside with a microswitch on its rhs. Does it click when you turn the tap on? If not its the switch, if so it's the pcb. Probably!
From memory the dhw flow switch is £35 ish and the pcb 100 ish depending. I'd like to get my mitts on on one of the 80e boards misbehaving like this to know what goes wrong. It could just be a bad joint.

 

[Sandytoe]

The boiler stays on while the tap is on, also the switch does click when the tap is turned on
I had the PCB replaced a few months ago - so it 'should' be functioning normally.

Any ideas what might be wrong if it's the PCB?

 

[ChrisR]

When it's stone cold does operating the flow switch bring the boiler on?
Just wondering if the dhw temp sensor is bust so blr thinks water is hot already, and stays off.

" had the PCB replaced a few months ago - so it 'should' be functioning " but it's Potterton! SHould be gteed though hard to return boards unless they're coded by the supplier.

 

[Sandytoe]

When i turn the hot tap on, the boiler tries to fire, but doesn't succeed, and then the lockout LED comes on. If i press the reset button the same thing happens again.

 

[Sandytoe]

Any other ideas?