Baxi 105e tripping when running hot water

[martyb1965]

My boiler cuts out whilst usually running a bath and leaves the green 50 (thermometer logo) light on constantly. If I leave it for 30 seconds and reset the dial, it comes on again.
I read the manual and it indicates either the overheat sensor or the pcb. I replaced the sensor because it was the cheapest option but it has just happened again. Before buying a new pcb, is there any other thing I can check?

 

[baxpoti]

Your DHW diaphragm needs replacing!

The green light indicates mains power to the boiler.

 

[Agile]

Whilst I would check the DHW diaphragm first, I would not necessarily expect that would be the only possible fault.

If he leaves the hot water running and only turns the HW temperature knob and that brings it back on then I would be suspecting the PCB was faulty.

Tony

 

[martyb1965]

Thanks for the feedback. I thought the green light indicated the fault. It is the left-hand knob that I have to turn to "0" before turning it back to CH and HW that resets it and not the temperature knob.

 

[Agile]

Are you running CH when using the DHW?

You should be testing to see if it gives DHW with the heating OFF ! If not then its definately the diverter. If so then its likely to something else but not necessarily the PCB either!

Tony

 

[martyb1965]

Yes, the CH is on as well. It is such an intermittent fault that we can't leave the CH off as it can be 3 weeks before it trips again. It only goes off during PROLONGED hot water demand such as the bath.
It never happened before winter arrived when the selector was set to DHW only by the way.

 

[Agile]

But it may not have developed until the winter.

All I can say is that it would aid diagnosis if you switched off the heating for a while and see how the DHW behaves.

There is probably an autoreset over heat stat fitted but I dont remember if the display shows any fault indication if that has operated and in any case it would usually take a few minutes to reset.

Tony

 

[martyb1965]

If the diaphragm HAS gone, surely the DHW wouldn't come on at all? The plunger operates the microswitch when the tap is turned on so it must be working? We can go for weeks using the boiler with no problems with hot water demand. It's just the times in the evening when the CH has been on for a couple of hours and we decide to run the bath. It doesn't reset straight away, but leaving it for a minute before switching it back on works. This is why I originally thought about a dodgy thermal overload sensor being the problem. In my mind it is something to do with temperature and sensors...but then again I am not an expert.

 

[martyb1965]

I remember that that this DID once trip whilst I was diagnosing and the selector WAS just set to DHW and not CH. Hope that helps!

 

[baxpoti]

Does the tap light stay on?

 

[martyb1965]

No, only the power led.

 

[baxpoti]

Your DHW diaphragm needs replacing!

 

[martyb1965]

It did it again tonight. I took the front panel off and looked at the insides whilst it was tripping. At no time did the diaphragm indicate failure. The electricity cut out to the circuit FIRST whilst the DHW three way valve plunger was STILL activating the microswitch. The lights flicker and dim leaving the power LED on. It didn't reset immediately after but I finally got it going again after a minute and turned the delector to DHW only. I was able to run the rest of the bath water and it is still OK now.
What difference will occur if I change the diaphragm? I appreciate your assistance on this one by the way but I think that this occurrence might change your mind?
Many thanks.

 

[baxpoti]

What colour wires do you have on your DHW microswitch?

 

[martyb1965]

There are two wires, both green. I'm looking at the larger of the 2 diaphragms (facing towards the front of the boiler) on the valve assembly as it's this one which pushes out on DHW demand. The top (smaller one facing up) has two red wires which operate when there is a demand for CH. Hope this helps.