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Andrew Smith
Andrew Smith, Engineer
Category: UK Plumbing
Satisfied Customers: 3591
Experience:  Domestic Gas, Commercial Gas, LPG, Unvented Hot Water, Part P Electrical, Commercial Catering
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The boiler is a Vokera compact he 29 about 6 years old. The

Customer Question

Hi, the boiler is a Vokera compact he 29 about 6 years old.
The fault is that after the central heating has been operating for around 30 mins it shuts down with the red LED on constantly suggesting an overheat problem and a faulty thermister, switching off at the selector switch, then back on it will go through the same thing but not always, it may go for several hours with no problem.
I have changed the CH thermister and checked for continuity to the pcb and checked the pcb plug and socket for poor connections/corrosion but all is OK, the high temp limit switch is OK, flow through the system is OK i.e. the rads warm up rapidly, difference between flow and return is OK at approx. 12 deg, fan and pump overun as they should when the boiler comes up to temp and the flames extinguish.
With the CH set to max. temperature 80 deg I have monitored the temperature at the outlet from the heat exchanger adjacent to the thermister and as the boiler cycles up and down this never goes over 78deg, when it locks out the temperature again never goes over 78 deg.
Does this suggest a heat induced fault with the PCB ? Are the pcb's subject to dry soldered joints.
Thanks in anticipation.
Bill
Submitted: 1 year ago.
Category: UK Plumbing
Expert:  Andrew Smith replied 1 year ago.

Perhaps your overheat stat is tripping too low?

Customer: replied 1 year ago.
Hi Andrew.No, as I mentioned in my post the overheat stat or, as Vokera call it the high temp limit switch, or limit thermostat, appears to be OK, if it trips, the red LED should flash, (with this boiler problem the red LED is on constant when the fault occurs ), as a belt and braces test I monitored the resistance of the o/h stat switch contacts from cold to max temp and it was a constant 0 .1 ohm which is almost a perfect contact.The instant the constant red LED comes on, if the the mode selector switch is immediately is turned to off then back to heating and hot water, it fires up as normal, it will then go through its normal operating cycles, without problem, for half an hour or more until again it locks out with the constantly lit red LED.I also monitored the new CH NTC sensor resistance, at the PCB plug, from cold to hot i.e. 20 deg to 78 deg measured at the heat exchanger outlet pipe close to the CH NTC sensor, again these measurements were spot on, i.e. approx 12K at 20 deg dropping to approx. 1.5K at 78 deg, the readings checked at every 10 deg rise up to 78 deg corresponded to the expected readings from the NTC resistance graph, the readings were steady throughout i.e. no glitches or drop outs.I took the opportunity today to remove the PCB and check for such as dry joints, heat damage, water damage, damaged or corroded connector pins, damaged or corroded or spread female connector sockets all appeared OK.Head scratching time it appears.Any more suggestions ?Cheers, Bill
Expert:  Andrew Smith replied 1 year ago.

Isn't the red light an indication of flame failure?

Does it always fail at the same temperature or could it be once the boiler has been on a set time, perhaps a problem with the flame detection probe once it is hot?

Customer: replied 1 year ago.
Hi, no, the constant red is, according to the Vokera manual, an indication of primary NTC open / short circuit, but my tests of the circuit show neither from NTC to the PCB, I've also done a wiggle test of all wiring whilst the boiler is running with no problems.Though difficult to be there every time it happens, it appears to occur when the CH has been running for 30 mins +.From cold it fires up OK, modulates OK as it runs up to the max boiler CH setting, the flame then shuts down, as it should, the pump then runs for the 3 min anticycle sequence, as it should, then when the boiler temp drops, it fires up no problem and this normal cycle goes on till at some stage after a normal max boiler temp shut down the light comes on.The re start ignition sequence is then locked out until the mode switch is turned to off then back to CH, it then will instantly behave for the next half hour or more.I can't see it being the flame detection probe because (a) the flame has already been extinguished normally at the max boiler temp and (b) the next ignition sequence can't be instigated until the fault is cleared operating the mode switch, as above.Cheers
Expert:  Andrew Smith replied 1 year ago.

Yes, I can see what you mean.

But without it being a constant fault it is hard unless I am there.

None routine faults can be harder to diagnose, can you monitor and let me know?

Currently I would be guessing at the PCB until we can narrow it down more.

Customer: replied 1 year ago.
Hi again, yes I'll try to monitor it more closely but there is not much else I can think of to monitor.I also think it may a PCB component that is breaking down due to heat soak, however if it was this, I would normally expect to be unable to clear the fault until X time has elapsed and the component had cooled down, this can be cleared in a split second.
Expert:  Andrew Smith replied 1 year ago.

Yes, that is why I am thinking of a sticking relay on the PCB rather than a failing component

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