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Have called out BG multiple times. DHW sensor changed, new PCB and clean of DHW heat exchanger. Have Ideal boiler and keep getting L9. It happens overnight when the boiler tries to preheat hot water and doesn’t switch off. D comes on, but no taps or open(or leaking) and boiler overheats because there is nowhere for the hot water to go. It then trips and house is freezing in the morning, so reset boiler, D comes on again and doesn’t go off. Trips again. So we turn it off at wall and turn on again and c comes on and heating starts working-and works normally along with hot water. It seems to happen when the boiler goes off as the central heating thermostat switches it off at designated temperature. Unless it comes on again because room temperature is too low, it then repeats L9 as soon as it goes into preheat as it doesn’t switch off! BG say may need power flush but heating fine and we have magnetic and scale filters on system. Can anyone help? Or has anyone come across this before? Cannot switch off preheat🙁
 
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garygvl

Gas Engineer
What pressure is the pressure gauge showing. Is the boiler the highest point of the system. BG should be able to sort this out easily. If your heating is working ok you don’t need a flush. They need to look at the plate heat exchanger again. You say it was cleaned not replaced
 
Yes thank you for replying. They’ve been out 5 times, first time couldn’t find anything, the last time he cleaned the heat exchange with coke but it still keeps happening! They’ve now ordered a pump in case it’s sticking but like I said, heating is ok. He said it may be pump as there’s more resistance if just circulating around boiler so has to work harder. All seems ok providing the boiler is warm and doesn’t need to go into preheat mode. The rads are hot apart from a cool spot on one and there was a bit of silt in the boiler so was told there may be blockage. But when I get home or overnight, and house is freezing, because of L9, when I press reset, it just stays on d until it overheats!
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Pressure is normal. Boiler is highest point of system.
 

garygvl

Gas Engineer
when you run hot water does the boiler stay on (steady blue light on the display or does it start flashing after a while) and does the pressure guage bounce about at all. pump overrun is controlled by the pcb and will be around 4 mins on heating and 30 secs on hot water depending on which version of pcb is fitted. if you turn off the heating and and wait for boiler to go to zero on display then run a tap, watch the blue light it will flash as boiler goes thru its ignition sequence then go to steady when burner is lit. does it stay on while tap is running. after you turn tap off burner will stay on with steady blue light until plate heat exchanger is up to temp. when blue light goes out boiler will display just d on the display and you will hear the pump still running if overrun is working. after about 30 secs pump will stop and display will go back to zero
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What ideal boiler is it ? , guessing Logic
why would you guess L9 on a logic
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ps love the fact that BG clean plate heat exchangers with coke, always carry a bottle with me just in case
 
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when you run hot water does the boiler stay on (steady blue light on the display or does it start flashing after a while) and does the pressure guage bounce about at all. pump overrun is controlled by the pcb and will be around 4 mins on heating and 30 secs on hot water depending on which version of pcb is fitted. if you turn off the heating and and wait for boiler to go to zero on display then run a tap, watch the blue light it will flash as boiler goes thru its ignition sequence then go to steady when burner is lit. does it stay on while tap is running. after you turn tap off burner will stay on with steady blue light until plate heat exchanger is up to temp. when blue light goes out boiler will display just d on the display and you will hear the pump still running if overrun is working. after about 30 secs pump will stop and display will go back to zero
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why would you guess L9 on a logic
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ps love the fact that BG clean plate heat exchangers with coke, always carry a bottle with me just in case
when you run hot water does the boiler stay on (steady blue light on the display or does it start flashing after a while) and does the pressure guage bounce about at all. pump overrun is controlled by the pcb and will be around 4 mins on heating and 30 secs on hot water depending on which version of pcb is fitted. if you turn off the heating and and wait for boiler to go to zero on display then run a tap, watch the blue light it will flash as boiler goes thru its ignition sequence then go to steady when burner is lit. does it stay on while tap is running. after you turn tap off burner will stay on with steady blue light until plate heat exchanger is up to temp. when blue light goes out boiler will display just d on the display and you will hear the pump still running if overrun is working. after about 30 secs pump will stop and display will go back to zero
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why would you guess L9 on a logic
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ps love the fact that BG clean plate heat exchangers with coke, always carry a bottle with me just in case
Thank you! Just did that. Pressure gauge did not move at all, and light flashed then stayed steady. When turned tap off, light stays on for a while with d and then display goes to zero. When c comes on for heating pressure gauge rises slightly but doesn’t jump. The water seems to work ok if the heating is on-problem seems to be on preheat . BG guy finds it frustrating that error code seems to happen randomly in middle of night or when we’re not home. I think it’s because the boiler is cold and it’s preheating but not switching off.
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Will ask him why they don’t just change the heat exchanger. He said it has to be Diet Coke apparently 😁
 

garygvl

Gas Engineer
there are two parts to a plate heat exchanger. one where the domestic water passes thru and is heated and the other where the water from the heating system is passed thru to as the name implies exchange its heat into the domestic hot water. some plate heat exchangers suffer from scaling so something like coke which is slightly acidic can remove the scale to aid water flow thru the plate. however if the primary side has flow problems due to system debris blocking the plate no way will coke solve this problem
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also changing the hot water sensor will not solve this issue. the L9 issue is triggered by the main heat exchanger casting sensor which is there to stop overheat flue gases going into the ( plastic/composite this term always does my head in) flue and causing issues.
 
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So it’s possible that a power flush will sort it out? If it is debris, why does the hot water and heating work fine at other times? Water is hot, no fluctuation in temperature and like I said, radiators are done apart from a cool spot on one. Had to reset again this morning but seems to be fine now, although I can hear pump working quite noisily! Btw, thanks for the info, don’t think the engineers who come round enjoy being bombarded with questions about how boilers work!
 

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