Baxi Instant Combi 105e no CH

10 Feb 2015
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United Kingdom
I was wondering whether anyone can help me out here.
My Baxi Instant Combi 105e develops a fault nearly every year which is fixed by me replacing the DHW and CH diaphragms (simple).
This year this hasn't solved my problem...
When the DHW is used this knocks off the CH (as it should) but then the CH does not resume after the DHW is stopped. The DHW light remains on the burner goes out and it sounds like the pump is running (feels hot).
Any help appreciated.
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It may be that it is reheating the store but not registering the final temperature is reached.

With boiler totally cold measure resistance of each sensor.

Thanks but sensors OK.
CH works fine when the boiler is cold and DHW pre-heat is switched off.
However as soon as the DHW is used the CH problem starts.
I think that proves my assessment of the likely cause is correct.

What is an "OK" sensor to you?

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Going by the manual the 70deg/80deg lights should flash if there is a problem with the DHW/CH sensors. This isn't happening.

Seems like a simple task to replace these so I will get a couple ordered.

Thanks Tony
I asked you to measure the resistance!

Not to look for flashing lights!

Top tip dont trust indicator lights to work, SHMBO seized the engine on the car i bought for her, had ran out of oil, obviously it was MY fault as i never checked it and she said the light never came on!
Friend of mine did not bother to check oil and ran it dry on the motorway. New engine was £1700.

In case he forgot anything he covered the control panel of boiler with yellow reminded stick ons!

He committed suicide a year later.

At least I don't have to drive him out so he can get drunk anymore!

First of all does the boiler actually recognise that a dhw demand has finished, ie does the dhw light go out after you have finished drawing dhw, if not check that the dhw valve spindle has withdrawn fully and is not making the micro switch, the spindle can sometimes get rusty and seize or you may find the two differential tubes have blocked up.
Is the boiler still in preheat mode, recognised by a flashing dhw light, this would mean that the store has not reached temperature and if you have replaced both sensors the reason will be either the pressure differential valve is not seating properly and sending some heat around either the ch circuit, although only one rad may get warm as it is not fully in ch mode, the heat is being pushed around the bypass, or the diverter is sticking.
To test either theory when the boiler is stuck on the dhw mode does the left hand 22mm flow pipe get hot at least 18" beyond the jig which means heat is passing into the rad circuit and not satisfying the store, or behind the calorifier is the bypass pipe, is this getting hot in dhw mode.
This is not the easiest fault to prove as it can be either valve, the diverter valve can be replaced using a kit and you just unscrew it from the body and replace the guts, don't worry about the bottom part of the valve I've never had this fail and to get to you will have to remove the whole valve. The pressure differential valve guts can also be removed whilst leaving the body in place as this is also a mare to remove and rewasher unless you have the correct tools and have done one before.
Finally I presume you are actually getting good dhw temperature when you draw hot water, the boiler uses both the calorifier and store to preheat the water if the calorifier is restricted the store will take forever to reach temp, but you would have noticed poor hot water during the last few weeks ( which to be blatantly honest can also be attributed to either of the valves described above)
Hey if it was easy you'd all be doing it.
Thanks Wongfeifox for the detailed information.
Took the differential valve apart (again) and cleaned it thoroughly. Replaced the head nut through which the spindle passes. Gave the spring a bit of a pull to make sure it would retract the spindle fully. Seems to have done the trick.
Thanks again.

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