False call for hot water

[tod]

Good day

I’ve had three heating engineers look at the symptoms that follow to no avail. One of them replaced the electrodes ( part 5110992 )

Boiler: Potterton Promax HE Plus (SL) (2008)
Hot water tank: Heatrae Sadia Megatech Solar 190 Indirect (2008)
Thermostat: Nest 3rd Gen (2017)

Symptoms: (this started in December 2018 with no obvious trigger) an example from this morning, the hot water schedule turns on at 07:00 at which point the boiler turned on and did it’s thing for about 20 minutes. No hot water was used for the next 90 minutes. The call for hot water came in again and the following took place for the next 15 minutes:
The boiler goes through its startup routine, flame comes on, 5 second later it cuts off. Rinse and repeat for 10x+ iterations before it finally stayed on for 2 minutes after which the call for hot water disappeared.
No lights come on the PCB at the time of cut off to suggest there’s anything wrong.
the behaviour isn’t unique to hot water but with hot water this false start routine happens almost always. With heating it happens about 10% of the time.
The PCB was replaced April 2017. No other work happened between that and the electrodes being replaced.
As I said I had three heating engineers looking at it no avail. A lot of shrugging and head scratching turns into a phone call to a colleague but nothing comes out of it.
one heating engineer checked the resistance on the PCB and stated that electrically it is in order. That same engineer suggested that it’s a faulty gas valve inside the boiler(?) that displays the symptoms when the boiler is cold. That sounded odd considering heating does not trigger such behaviour as much as hot water but I don’t know anymore.

Any help would be truly appreciated as hearing the boiler fail to stay on is driving me mad on a daily basis. Thank you.

 

[Chuck]

Any help would be truly appreciated as hearing the boiler fail to stay on is driving me mad on a daily basis. Thank you.

Check the simple things first.

Is the tank thermostat in the correct location and firmly strapped to the tank? If it gets loose, so the thermal contact is poor, or if it is set significantly higher than 60°C, you can get the symptoms you describe.

Measure the actual temperature of the hot water in the tank. (Use a thermometer and continously run a hot tap near the tank into a mug.)

 

[tod]

Will do that once I acquire a thermometer, thanks for the advice.
A couple of points to add: the symptoms aren’t limited to when the call for hot water is false. For example now the call for hot water is genuine as it was off all day and we used hot water but it’s been cycling through the startup routine for 20+ minutes before finally staying on.
Also, when the boiler cuts off the call for heat does not, ie the motorised valve stays open near the tank and the pump stays running.

 

[Brambles]

If the boiler is cutting out, whilst you still have an external call for heat, you really need a decent gas engineer ( who is familiar with the Promax HE ) to trouble shoot for you. It is not too difficult, they ( the Promax range )are very logical in how they perform.

Alternatively, try to get a fixed priced repair from Potterton.

I would doubt that this is a difficult fix, but it is not a DIY repair. Apologies but I cannot say more.

 

[Chuck]

A couple of points to add: the symptoms aren’t limited to when the call for hot water is false.

Sorry, I didn't read your original post carefully enough to notice this was not limited to the DHW. @Brambles has given you the only answer we can offer on this forum.

 

[tod]

Thank you both.
I do have heating breakdown cover with no excess so I could keep getting them out but it’s just a nuisance. Posts on DYINOT suggest it is a pretty common fault with this model however their boilers normally result in ignition lockout which mine never does. One evening it kept trying to light for over an hour.

My ranting aside, at what point does one justify getting a new boiler instead of trying to repair a 10+ year old Potterton? Clearly it’s still got life in it as it’s been doing this for a year now but it would be good to know if that’s it signalling me that it’s on its way out and I should send Santa a letter changing what I’ve wished for.

 

[Brambles]

I don’t think that your boiler is “on the way out” - but you do need to decent gas safe registered technician to resolve the issue for you.

The fact that the boiler does not lock out, narrows the issue down to the root cause(s) - non of which are a DIY fix.

 

[tod]

How can I gently steer the engineer in the right direction please? As I’ve said I’ve had three different engineers look at the symptoms live and none of them had any ideas thus my coming here.

 

[Brambles]

The best advice I can give you is to put your current service provider on notice to either fix it or if they cannot, to involve the boiler manufacturer ( Potterton).

Don’t give up, all the service providers have some first class people - you just need to escalate the problem to get them engaged to fix it.

 

[tod]

I thought I’d follow up in case someone else has similar symptoms and is struggling to get them addressed.
Firstly I will clarify the symptoms - the boiler cuts out be it for call for heat or hot water. With hot water it cuts out almost always and almost immediately whereas with heating it is less evident but does happen.
The cause was the water pump that was on its way out and did not circulate water fast enough. This only became apparent once it started pumping water intermittently rather than continuously. Changing the pump transformed the heating system completely and it became evident that it was on its way out for at least the last 3 years.
In the process of troubleshooting various heating engineers replaced the electrodes, the gas valve and the PCB.