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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
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