Baxi Combi 80 Eco - constant cycling

16 Mar 2010
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United Kingdom
Hi, I'm after a little advice regarding my Baxi Combi 80 Eco fault(s)

I've read the forums, googled and found "similar" faults, however nothing jumps out as identical, so I'm posting in the hope that some kind soul(s) will be able to put their finger on the likely cause or provide troubleshooting advice on what to check and in what order! Please accept my apologies if the answers have been posted before - I've done my best to search before posting! I have an electronics/mechanical background and understand the way in which the boiler functions, the various control and safety mechanisms, basic firing sequence, etc, however I do not profess to be a heating/boiler engineer, and clearly it goes without saying I won't be touching anything deemed unlawful/dangerous without being on the gas-safe register.

Here goes:

I noticed recently (following the recent cold weather) that the boiler was firing, running for between 10 and 60 seconds and then going off for 10-60 seconds, and constantly repeating this cycle.
The time between on and off and duration appears to be very variable, however I would say typically quite short 10-60 seconds bursts are quite an accurate description.

On closer inspection I noted that there was water dripping (slowly) from the underside of the boiler. I isolated the boiler, had the cover off and noticed immediately that the DHW valve pin was leaking drops of water, and appeared to be stuck closed - ie turning on a hot tap did not push the pin out - Unfortunately I live in quite a hard water area, and we get a lot of limescale; i'm therefore not suprised to note that there was a fine build up of lime/water marking on the case where the water had been running/dripping. I'm unsure how long this had been going on, however my guess would be for several weeks unnoticed!!

Needless to say I purchased a replacement diapraghm kit, cleaned the DHW valve assembly, re-assembled carefully and fixed this fault - the pin now comes out when hot tap is opened, retracts when tap is closed, and no more leaking!

Unfortunately the cycling problem outlined first still persists - more detailed behaviour as follows:

1) If the boiler selector switch is in the off position (0), nothing happens which is entirely as expected - the boiler is effectively "off"

2) If the boiler selector switch is moved to the right (HW only position), then the boiler fires, and starts to cycle on and off as described - note that this is WITHOUT turning a tap on!! The DHW valve pin remains in as expected as there is no water flow from hot taps. If I open the hot tap whilst the boiler is running, then the DHW valve pin comes out and the HW outlet pipe starts to warm - water from the tap typically manages to get luke warm before the boiler goes off (part of this cycling behaviour described above). I have not checked extensively and will monitor more closely tomorrow, however I believe that the CH flow remains cool, I hope this means that the 3 way ch/dhw valve is behaving normally??

3) If the selector switch is moved to the left (HW & CH position) then the boiler fires and does its cycling routine as mentioned - NOTE THAT the Boiler Timer is on the OFF position - so CH should not be coming on!!! Anyway - CH flow gets warm/hot as expected and given a little time doing this cycling behaviour radiators do start to get warm around the house indicating (I hope!) that the pump is running, and water is flowing around the system.

From the above, and reading various threads with similar faults I put a few scenarios forward, and my thinking behind them - hopefully you can point me in the right direction with these:

1) what would be causing the boiler to fire and run when in HW mode, without actually turning a tap on? I thought perhaps the DHW microswitch may be stuck in the "on" state - however doesnt this then mean that if I DO turn a tap on, the boiler should simply run normally and provide hot water without going off and on repeatedly?

2) I thought about a blocked heatX plate or return filter/etc - this would explain the boiler going off after a short burn as the heat would build and trip the overheat sensor however surely this would not be prompting the boiler to cycle continously without HW demand? I am not having to reset the overheat sensor by the way.

3) Although I have replaced the DHW diaphragm I did not go any further and remove the whole 3 way valve - is it possible that the diverter valve is stuck/sticking and causing the boiler to think HW is being demanded when it isnt? surely this would not be possibly anyway as the DHW valve pin is not out and microswitch not actuated? furthermore this would not be a cause of the boiler going "off" if HW was to be demanded by opening a tap surely?

4) I thought about the thermistor/overheat sensors, however I cannot see how these would cause the boiler to fire without cause in the first instant without a tap open!

5) The final consideration I had is with the PCB - it is clear that water has been leaking near the PCB from the DHW valve for some time (although not directly onto the PCB due to the plastic shrouding). I guess it is possible that the PCB has failed. I have read that the relays on these boards can be problematic - from my perspective thats not suprising - many PCB's are manufactured with relays that aren't up to the job and fail after moderate use with contacts stuck open/closed! Is it possible that a sticky relay could cause the boiler to keep initiating the firing sequence ? This still wouldnt necessarily explain the boiler going off if I have a tap open?

Ultimately I guess I could have more than 1 fault which happen to make an awkward fault.

Please can someone help me out of this predicament by providing thoughts on the above. Apologies for all that reading! Please ask any questions you see fit, and I will do my best to provide answers.

Kind regards,

As you can see im going round in circles
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Anyone at all? ;(

I am in two minds about purchasing a PCB and having a friend (gas fitter) fit it - I appreciate gas pressures have to be tested & possibly adjusted via the two pots on the pcb so this is not something I can do myself.

Is this a wise move? or a waste of money without further fault finding?

Kind regards,
Have I gone about things wrongly?! Maybe I posted too much too read?!

I find it hard to believe that the symptoms I have posted are completely alien to everyone, so I can only assume I must be doing something wrong asking for help in this manner?

Someone must have a clue and be willing to help! Please!

Kind regards,
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Just get an engineer out. Lost the will to live after the 2nd line.
No offence but get someone on site. B/g on call assist usually good bet or an independent engineer who knows his/hers way around a combi. If independent make sure they know fault finding work and not just installer stuff
Hi, i know i'm probably too late with this information to be of help to you, but maybe it's of use to someone else, as i have/had a similar fault.
First change the overheat thermostat pt no 5115349, and the Temperature sensor, only a few quid and easy to fit (this should solve the switching on and off problem). Then you will need to remove and dismantle the whole Diverter valve, clean it out and be sure it's working correctly. Let me know how you get on. Good Luck.

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