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]

You have a combi boiler, so hot water is only on demand unless there’s a preheat setting. Would have been cheaper for you to get an engineer out, which is what I suggest, not just slinging parts at it and hoping. Removing certain parts on boilers is only for qualified competent people. If the heating is on constantly, I’d be looking at controls.

 

[CrappyCombi]

You have a combi boiler, so hot water is only on demand unless there’s a preheat setting. Would have been cheaper for you to get an engineer out, which is what I suggest, not just slinging parts at it and hoping. Removing certain parts on boilers is only for qualified competent people. If the heating is on constantly, I’d be looking at controls.

Sorry I worded some of that poorly. It’s the CH that is on constantly with hot water on demand.
I have had an engineer out who spent about 2 hours looking at it. It was he who replaced the timer with no effect and then said the PCB was likely the issue. I fitted the replacement PCB myself because it was no different from switching out a motherboard. But nothing seems to improve the situation.
With regards to controls, there is no thermostat - only an inbuilt clock/timer. This has been replaced and the wiring tested. The Thermistor has been tested at various temperatures and gives reasonable readings. So I’m just not sure what it could be.
I’m more than happy to get engineers out but, as the two who I’ve spoken to don’t know what the problem is I thought I’d try here for suggestions.
Thanks

 

[Deleted member 120897]

Can you show the wiring at the pcb? I‘d say a more than 1 link has been left in. Do you have any motorised valves fitted? Are you sure there‘s no thermostat?

 

[CrappyCombi]

Can you show the wiring at the pcb? I‘d say a more than 1 link has been left in. Do you have any motorised valves fitted? Are you sure there‘s no thermostat?

Hopefully these pictures are clear but let me know if you want to see anything in more detail. Plugs connected on M1, M2, M3 (timer), M4, M5, FA1, FA2. CM1 and CM2 and left unconnected.
No thermostat, just a timer, thermistor, flue and safety thermostat.
Can’t see any motorised valves either.
Thanks in advance.

 

[Deleted member 120897]

View attachment 81481

Thanks for these, wasn’t sure which type of boiler you had. Also, I wasn’t thinking straight, check the microswitch on the diverter assembly. To the rear on the photo I’ve left attached - the big brass unit.

 

[CrappyCombi]

Thanks for these, wasn’t sure which type of boiler you had. Also, I wasn’t thinking straight, check the microswitch on the diverter assembly. To the rear on the photo I’ve left attached - the big brass unit.

Sorry I’ve been out all day so no chance to reply. Micro switch on the diverter seems to be working fine. Pin extends when hot tap turned on and retracts when tap turned off. Which makes sense, I guess, since I’ve not had any issue with DHW: if it were stuck in then I’d have no hot water, if it was stuck out I’d have no CH.
Could it be the diverter diaphragm?

Thanks

 

[Deleted member 120897]

Sorry I’ve been out all day so no chance to reply. Micro switch on the diverter seems to be working fine. Pin extends when hot tap turned on and retracts when tap turned off. Which makes sense, I guess, since I’ve not had any issue with DHW: if it were stuck in then I’d have no hot water, if it was stuck out I’d have no CH.
Could it be the diverter diaphragm?

Thanks

Wouldn’t have thought the diaphragm. Thought there was a rear switch, bit I could be thinking pump proving switch, so likely the flow switch for CH, see attached

 

[king of pipes]

Check whether this link needs to be removed ?? I'm sure Shaun Corbs, Murdoch could confirm this ? the boiler is getting a feed from somewhere has it always been like this ? . Kop

 

[CrappyCombi]

Wouldn’t have thought the diaphragm. Thought there was a rear switch, bit I could be thinking pump proving switch, so likely the flow switch for CH, see attached

No switch on the back of the diverter assembly as far as I can tell. The reason I suggested the diverter valve is that my wife just reminded me, at the start of our current troubles, the radiators were getting warm with the hot water running.
Then the problem got much worse with the boiler on all the time. If the diverter valve is stuck, could the boiler think there is hot water running constantly? Which it heats up and diverts to the CH?

With regards to your attachment, the boiler does seem to reach temperature and the burner goes out, but not for as long as three minutes I don’t think - I think it was reigniting after about 30 seconds but I will check again.
Thanks

 

[CrappyCombi]

Check whether this link needs to be removed ?? I'm sure Shaun Corbs, Murdoch could confirm this ? the boiler is getting a feed from somewhere has it always been like this ? . Kop

Hi KOP,
We’ve lived at the property for 4 years and not had any issues before. This started about a month or so ago - we contacted half a dozen engineers but none of them would come out. It wasn’t pressing because we could just turn heating on and off manually. But turning the heating on every time you need DHW is an issue with the weather turning hotter. I don’t mind the heating being on half an hour while I shower in the winter but that’s not so good in June.

Finally got an engineer out last week and he replaced the timer, then suggested we get the PCB replaced for £260. I found a new one online for £80 and fitted it myself but exactly the same behaviour continues.

As I just said to Chris, my wife reminded me that at the start of the year we noticed the radiators warm up when we ran the hot water, but quickly after this the problem got worse.

Thanks

Edit: just looked through the manual and that link seems to be for external timers/thermostats/frost protection. All I have on my boiler is an integral timer - the one that came as standard with the boiler.

 

[Deleted member 120897]

Check whether this link needs to be removed ?? I'm sure Shaun Corbs, Murdoch could confirm this ? the boiler is getting a feed from somewhere has it always been like this ? . Kop

No room stat mate, so likely needed to bring CH on.

 

[Deleted member 120897]

@CrappyCombi - are you saying you need the heating on to get hot water? If yes, I think that’s different to what you said previously. It certainly does point towards the diverter valve.

 

[CrappyCombi]

Sorry, I wasn’t very explicit in my original message. But yes that’s what I’m saying.
We leave the selector switch in the off position so no energy is used. If we switch to the DHW setting or CH/DHW the result is the same - boiler fires up, CH comes on and we have access to hot water.
So whenever we want a bath, shower, to do the washing up we have to switch the selector switch to anything but 0 and live with the heating. Same if we want the heating on.
Fortunately the boiler is in the kitchen next to the only bathroom so it’s not as inconvenient as it sounds. But all that heat will be a massive waste in the spring/summer.

Thanks

 

[CrappyCombi]

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.

 

[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