Combi boiler mystery issue

[CrappyCombi]

Hello all,
New here so please forgive my ignorance. I have a Main 24 HE combi boiler and it’s playing silly bugg*rs. When given power it runs constantly, providing both heating and DHW. We have to turn it off at the selector switch when we don’t want heating/hot water otherwise it just burns gas.

Have read through the manual and following the trouble shooting I have:

• replaced the timer clock, no joy
• replaced the PCB, no cigar
• removed, cleaned and tested the thermistor, no luck

The only other thing the troubleshooting says to do is clean the DHW heat exchange but I can’t see that making a difference, can you? Maybe I’ll try it if I get time over the weekend.

Has anyone experienced this fault before? Or have any clues what could be causing the issue?

Any thoughts welcome,
Thanks

 

[Deleted member 120897]

Just had chance to take another look - the micro switch on the front of the diverter assembly works as I previously described. But there is also a microswitch on the top that only retracts when power is removed (either turned off at plug or off at selector switch). The whole time the selector is on DHW or CH/DHW this microswitch pin is extended and the boiler runs.

It certainly does point towards the diverter valve.

 

[CrappyCombi]

Thanks Chris!

 

[king of pipes]

If your getting heat down the heating pipe work when the hot water is on then this does point to a faulty diverter . Kop

 

[CrappyCombi]

If your getting heat down the heating pipe work when the hot water is on then this does point to a faulty diverter . Kop

Thanks KoP,
That was the original symptom - now we just get heating whenever the boiler is on, whether we’re running hot water or not. I will have the diverter seen to and report back.
CC

 

[CrappyCombi]

Hi both, so I took the diverter assembly out and replaced the two diaphragms. I couldn’t for the life of me open the front and back of the main assembly - even with a pair of grips, WD40 and a hammer! So I left those unchanged. They contain pins and springs so I don’t know if that’s likely to have caused a failure - if so I’ll have to buy a whole new assembly as I just can’t open them.

But to my mind, they wouldn’t be causing the issue.

So if I assume I have ruled out the diverter assembly as being the cause, What else could it be?
Thanks again

I have managed to replace all the parts to the bottom of the picture, but could not replace the ones at the top.

 

[CrappyCombi]

Any idea what the switch on top is for? The one at the end is for DHW and behaves as I’d expect when the hot tap is opened. However, The one on the top extends whenever given power and retracts whenever power is switched off - but other than that does nothing which seems wrong (and pointless). I didn’t take the photo, I just found it online, but it’s the same unit.

Could this issue be caused by the bits I couldn’t replace? If so I’ll happily buy a new unit!

 

[Deleted member 120897]

Any idea what the switch on top is for? The one at the end is for DHW and behaves as I’d expect when the hot tap is opened. However, The one on the top extends whenever given power and retracts whenever power is switched off - but other than that does nothing which seems wrong (and pointless). I didn’t take the photo, I just found it online, but it’s the same unit.

Could this issue be caused by the bits I couldn’t replace? If so I’ll happily buy a new unit!

Yes, see attached

 

[CrappyCombi]

Yes, see attached

Thanks for that Chris. That makes sense!

So, given that there is nothing electrical in the diverter assembly, the only input to operate the microswitches is water pressure. Which to me means one of two things:
A seal in the diverter assembly is leaking, operating the switch when it shouldn’t be.

Or, water is being sent to the diverter when it shouldn’t be?

The first option doesn’t seem that likely to me given the leak would be slowish and I’m seeing the switch immediately operate and heating coming on full and fast.

So when heating is turned on, by the timer for example, what valve does it open to send water to the diverter valve? This is what I’m suspecting is at fault.

 

[Deleted member 120897]

I don’t think a valve opens, depending on demand would depend on which part of the diverter moves. In your case I think it’s a signal from room thermostat and a sensor, which I think I mentioned previously

 

[CrappyCombi]

I don’t think a valve opens, depending on demand would depend on which part of the diverter moves. In your case I think it’s a signal from room thermostat and a sensor, which I think I mentioned previously

Sorry I didn’t explain that clearly: back when it was working well, I would flick a little knob on the timer to switch the heating on/off/via the timer dial. That sends an electrical signal to the PCB which then must send some water through the diverter valve (since the diverter valve has no electrical inputs itself, it’s behaviour must be governed by what water is flowing, and from where). So what is it that the PCB does (under normal working conditions) to draw or push water through the diverter when CH is turned on?

Hope that makes sense!
Thanks, Greg

 

[Deleted member 120897]

Sorry I didn’t explain that clearly: back when it was working well, I would flick a little knob on the timer to switch the heating on/off/via the timer dial. That sends an electrical signal to the PCB which then must send some water through the diverter valve (since the diverter valve has no electrical inputs itself, it’s behaviour must be governed by what water is flowing, and from where). So what is it that the PCB does (under normal working conditions) to draw or push water through the diverter when CH is turned on?

Hope that makes sense!
Thanks, Greg

Try reading this - the pump and flow switch