Potterton Performa 24, CH ignition goes on and off in a loop

[Clint000]

Hi all,
Firstly, great forum, normally I get the info I need from other user's posts, but I guess the time to ask myself has come!

l have a Potterton Combi Boiler (Performa 24). The DHW is working well, when I open the hot water tap the boiler stays on as normal. However I have noticed that during the last week, when switching the CH on, it works okay until the radiators reach a decent temperature, and afterwards all it does is igniting on and off in a loop.

When the CH circuit reaches hot temperature and the CH is activated, the boiler ignites, the flame stays on for a few seconds (~6-8s) and then the flame switches off; 2 seconds after the boiler makes another attempt to switch on, but the same happens and it carries on with the on/off loop.

I do not know much about boilers, but I read on these forums that this behaviour could be due to a faulty microswitch. I have then checked both microswitches (CH which is single pole and DHW which is double pole) on the diverter valve with a multimeter and they seem to work ok. Just for double checking, I have also bypassed the CH microswitch with a wire and the boiler still falls in the on/off loop.


Since I do not know how bad this can be for the boiler, I am not using it at the moment so it's a bit cold in the house!

I would really appreciate if someone could point me in the right direction.

Many thanks in advance,
Henry

 

[bengasman]

As these are know for wonky boards, that would be my first suspicion. Could be a number of other things though; it needs testing.

 

[Clint000]

Hi Ben,
thanks for your answer.
What testing could I do to discard other things, before going ahead and replacing the PCB?

 

[bengasman]

First thing I'd do on these would be to test polarity and earth impedance.
After that just start checking the sequence and see where it keels over.
I wouldn't recommend to diy this as it is well possible you do more harm than good.

 

[Clint000]

Would an electrics/PCB fault produce the fault only on the CH when hot?
The boiler works fine with either DHW or CH at cold temp, so I would have thought that the PCB was working okay and it was an external sensing element or malfunction (as the PCB does not get hot) which is triggering the fault over temperature. Am I missing something here?

I am myself an electronics engineer, so had a quick look at the electric connections and they all seem to be fine (measured resistance from PCB to electricity plug and the reading is zero ohm, plus all live and neutral connections are as expected).

Thanks for your help.

 

[bengasman]

Would an electrics/PCB fault produce the fault only on the CH when hot?
The boiler works fine with either DHW or CH at cold temp, so I would have thought that the PCB was working okay and it was an external sensing element or malfunction (as the PCB does not get hot) which is triggering the fault over temperature. Am I missing something here?

No. I did say reason for pcb suspect is that they are known to be wonky but can have other causes.

I am myself an electronics engineer, so had a quick look at the electric connections and they all seem to be fine (measured resistance from PCB to electricity plug and the reading is zero ohm, plus all live and neutral connections are as expected).

I meant the earth loop impedance, not the lead resistance.

 

[Clint000]

Yep, measured that as well and loop impedance reding is 1.18ohm.
Perhaps a bit higher than normal but it is the same reading as some months ago, when the boiler was working fine.

 

[gas112]

The heating sensor being faulty can cause these symtons left hand side in a dry pocket

 

[Clint000]

Thanks gas112.

Measured the resistance of the temp sensor and reads around 18kohm at very cold temp. Manual says it should read 11k at cold temp, which it does when measuring at ~ ambient temp so I would have thought it is ok (also resistance decreases with an increase in temperature, as expected).

The manual also says that when the CH water temperature reaches the set temperature, the boiler stops for 3 minutes before attempting to start again. I have checked this and it does indeed wait the three minutes to restart (I checked this when switched on, from cold temp). The problem is when it restarts after the 3min that gets into the on / off loop.

If the temp sensor was faulty (i.e. measuring higher temp than the real temp), the boiler would stop & wait for 3min before restarting again. The issue I have is that it ignites on /off every 10sec or so.

 

[bengasman]

If ch and dhw thermistors are the same, swap them to verify.

 

[Clint000]

Not sure whether this boiler has different temp sensors for CH and DHW. See images below, I can only see the temp sensor which is used for both CH & DHW, and the overheat thermostat.

I must say that have measured the thermostat resistance and the reading is 0 ohm (manual says it should be above 1.5ohm at cold) , and it does not change when getting hot (I have sunk it in boiling water and the resistance was still the same, and it has not triggered either). This is suspicious, however if the overheat thermostat was faulty, the boiler would not work for DHW either as it is shared between CH and DHW (and so far DHW works okay).

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• Clint000
• 5 Mar 2011
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• Clint000
• 5 Mar 2011
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• Clint000
• 5 Mar 2011
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[bengasman]

Compare the part numbers of the spares list in the manual.

 

[Clint000]

Do you mean the part numbers from overheat thermostat and temp sensor (keys 24 & 25)?
They are different, plus they physically look different as well so I do not think they can be swapped.

 

[joshjames]

this could also be an ignition fault though by the sound of it

you can measure sensor resistance when hot as well as some only start to fail when they get hot

 

[bengasman]

No, the order numbers. Usually a list near the end of the manual.