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Boiler lockout on hot water demand (potterton combi 100) - ch working fine

Boiler lockout on hot water demand (Potterton Combi 100) - CH working fine


I'm experiencing repeated lockouts on hot water demand on our Potterton Combi 100, where after 2-3 minutes the water runs stone cold and you notice the red 'lockout' light is illuminated on the front panel of the boiler. This means we can't have a shower unless myself or my wife is on-hand to press the reset button on the boiler :-(

After pressing the reset button and turning on the hot tap, water takes 30-60 seconds to heat up again and may run OK for 2-3 mins before another lockout. Occasionally before the lockout occurs you can hear the odd 'bang' associated with heat expansion.

I have also noticed that the CH return pipe near the bottom of the boiler is scorching hot when the boiler is on for hot water demand (even when the CH has not been on for hours). Heat does pass down the CH flow pipe 10-20 seconds after the hot water demand has stopped and the boiler has turned off as you would expect).

I would be really grateful if someone could let me know what the likely cause of this is.

Many thanks in advance!


UPDATE - 28/01/2015

Many thanks to Full Flame, Gas Aces and TMDB Gas Services for your advice!

Full Flame: I can confirm that the boiler is staying lit for 2-3 seconds after hot water demand has been turned off so your hunch seems right on the money!

I've also noticed that when I'm running hot water in the kitchen sink and say the toilet is flushed, the boiler switches itself off. This never used to happen in the 14 years we have lived here; this has only happened since an engineer 'supposedly' replaced the wax diverted valve 2 weeks ago. He did report that we have an issue with our Grohetherm shower mixer as it appears to be allowing cold water down the hot water pipe when hot water turned off at the boiler and kitchen tap opened for hot water.

On a subsequent engineer visit, they decided to change the pump - not that they thought it was causing/contributing to the lockout issue, he happened to have one on his van - only to find that the impeller was split in half; the outer disc was lodged into the housing the pump bolts onto with the other part still attached to the pump with its fins virtually worn away.

I'm currently running Sentinel X400 & X200 now we have a fully functional pump to clean up the CH pipe work/rads and also to de scale the boiler; I had hoped that this might cure the lockout but sadly not.

Anyway, thanks once again for your great advice - much appreciated!!!


UPDATE - 29/01/2015

Thanks for your comments Full Flame!

HW flow switch and diverter valve arriving tomorrow.

On running the hot water this AM, the temperature was tepid and boiler did not lockout; with hot tap still on I found the CH pipes were v hot despite CH being off for 2+ hours.

Also, boiler now turning off when the HW demand stops unlike the 2-3 seconds overrun we had.


3 Answers from MyBuilder Heating Engineers

Best Answer

That's the flow switch not swithing off quick enough
Turn the heating off then run the hot tap look at the flame in the boiler
when you turn the hot tap off the flame should go out straight away
if it stays lit for a few seconds more the you need a new flow switch
Flow switch is about £70 plus 1 hour labour

i hope you're on a service contract and didn't have to pay for these parts that you didn't need for your boiler
like i said before you need a flow switch assembly
too many so called boiler engineers just guess at parts and haven't got a clue what they're doing.
They wouldn't last 5 minutes on their own in the real world

Remember I Have Just Told you how to fix your boiler, The problem is the FLOW Assembly get someone in that knows what they're doing!


Answered 29th Jan 2015


It can be very difficult to make an accurate diagnosis without seeing the installation. The information you have provided indicates that the diverter valve within the appliance is possibly sticking slightly and causing the hot water intended for you plate heat exchanger to circulate around the CH circuit. Given the higher gas rate for a hot water demand it tends to heat the water alot more than when demanding CH and it sounds like it is causing an overheat lockout situation. I would have the diverter valve inspected and when any remedial work is carried out consider asking for the Thermistors (temperature sensors) to be checked.



Answered 26th Jan 2015

Plate heat exchanger is clogged up with iron oxide from radiators.
Advise new/cleaned plate to plate heat exchanger plus at least MagnaClean filter if you don't want to spend £350+ for proper system flush.


Answered 26th Jan 2015

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