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Plumberpro, Plumber
Category: Plumbing
Satisfied Customers: 634
Experience:  Heating and plumbing answers. 20 years experience
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Glow Worm Boiler Flexicom 24hx (open vent) operates OK until

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Glow Worm Boiler Flexicom 24hx (open vent) operates OK until it turns off. It then goes to F25. Previously it was operating OK except when it reached set temp it ran over by 3 degrees & then returned to set temp. I replaced the thermistors. The pump is an Alpha 2L
Hi , the may be air in the system blocking the flow rate enough to cause an over heat . try the pump at Max speed,
Also pump should be wired direct into the boiler to enable the pump to overun clearing the heat from the boiler ,if the temperature rises too high.
Customer: replied 2 years ago.

The pump is at max speed & the bypass is fully open, All the rads are turned off. There is a radiator used as a bypass and also an automatic bypass. There are 2 air vents. The pump overrun has been reset to 20mins.

Where is the bypass located ? How many metres in the circuit? Could it be a blockage?( sludge ). Doubles check any gate valves are fully open and not snapped half open .checking the temp of the return compared to flow is a good idea . should be around 10 degree difference.Feel the pipework throughout for major temp differences, this might indicate air pockets or do dge blockages .

Look for manual air release valve by the cy

Under or above the boiler

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Customer: replied 2 years ago.

The boiler heats all the rads and heats the hotwater cylinder OK. Previously when there was a flow problem the boiler shut down immediately after firing and a flow problem was indicated. Now it shows F25 after the boiler has completed the heating. The temperature over run is because the gas valve is not shutting off when the temp is reached. All the pipe work gets hot, both flow & return, and the bypass is within 6ft of the boiler

If you have changed the thermistors , pump and are sure the system is free from sludge ,scale and air then you are only left with the PCB . replace the PCB as it appears to have developed a fault .an expensive part unfortunately , but if you live in ?London let me know as there is a place that do reconditioned for half the price