Baxi Instant 80HE problem

[MallyA]

Can anyone help me solve a problem on a Baxi Instant Condensing 80HE (same as a Potterton Performa 24i HE). The DHW works fine, but not the CH. The DHW neon remains lit constantly, regardless of whether DHW or CH is selected. When CH is selected, the first couple of radiators get warm (eventually), and none of the red neons show a fault.

Thanks

 

[baxpoti]

Disconnect the DHW micro-switch, clip slides out (green wires).
Put your C/H on

 

[MallyA]

Thanks for that. I've now established that the (flimsy) DHW micro-switch is faulty and I'll be replacing it tomorrow.

 

[baxpoti]

It's not normally the microswitch itself, but the diaphragm plunger sticking and operating the microswitch, does the plunger move in and out when you open a hot tap? It should.

 

[MallyA]

I replaced the microswitch, just to find that the original was OK. The reason I thought it was the switch was because I put a continuity tester across it and it was permanently on. However, when the wiring is disconnected from it, the switch functions normally.
With regard to the diaphram plunger, it seems to operate OK, moving out whenever a hot tap is switched on, and retracting again. I even operated the microswitch manually in time with the plunger, but to no avail.
I also tried disconnecting the DHW thermister to disable the DHW, and although the red LEDs reported the DHW fault, the CH side worked fine.
So it seems that, no matter whether the plunger is in or out, or the microswitch is off or on, there is always a call for HW.
Could this be a dodgy PCB, or have you any other ideas?
Many thanks.

 

[Agile]

That may serve as a reminder to never measure continuity with a component in circuit! You can end up damaging the PCB !!!!

If you have correctly described the fault ( which I wonder! ) then it would apparently be a PCB fault unless some of the Potty engineers know of any other likely cause.

All I can say is that if its really doing that then it would be very unusual. I usually find that kind of thing is caused by an electrical misconnection rather than the PCB itself. Unless anyone has tried a continuity measurement on the PCB and damaged it of course!

Tony

 

[baxpoti]

How old is the boiler?
Has anyone carried out any work on it just prior to this fault?
I noted the colour of the DHW microswitch wires in previous post as being green
what colour are yours?

 

[ChrisR]

Re Tony's warning - yes. I have an analogue meter which dumps 150mA for a continuity test, which is enough to blow all sorts.

(Fluke 100 tester is quite good. If it's across open contacts it indicates voltage, as soon as they close it beeps to indicate continuity)

Thought I'd measure the resistance of a tape head once - magnetised it beyond use!

 

[MallyA]

The boiler came as part of a house purchase and although it looks as good as new (even got the protective film on the case) the previous occupants never left any paperwork, and the CH has never worked.
In relation to the colour of the wires to the DHW switch, they are black & white, not green and green as stated. The functional flow diagram on p24 of the manual does refer to them as green whereas the illustrated wiring diagram on p40 shows them correctly as black and white, so presumably they have changed at some time.
I take the point about using a continuity tester. Mine is a Fluke, and I was reasonably confident that it wouldn't cause any damage (famous last words!) but to be absolutely safe I should have disconnected the plug from the PCB before checking.
Anyway, I've taken the plunge. Bought a new PCB, fitted it, and everything works fine.
Many thanks to you all for your words of wisdom, and keep up the good work.