F23 fault when boiler fires

Thank you. Interestingly the underfloor heating is on constantly maintaining 18 degrees on the thermostat and that doesn’t trigger the fault. It seems to be when the heating clicks on either on the schedule or when it’s been off for a while on the thermostat. It always works fine after a single reset. Presumably because it generally fires for a couple of cycles so the system has started warming up?
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Sorry I don’t follow you? Old boiler and pump generally worked fine until the old boiler died. So we didn’t have a lot of choice about needing a new boiler!
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The heat exchanger in your boiler has a very high resistance to flow, if you were to run the boiler flat out it would take ( over half the output of the pump) just to get the water through it leaving little to push the water through the pipework for 13 rads, take it your old boiler was not a condensing boiler ?
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No. Both boilers are condensing on a sealed system. Last one was a bit less powerful (30kw) but was installed before an underfloor heating system and large extension was built.
The pump is too small , you need something like a 25/80 or another 15/60 in series with the pump you have now.
Doesn’t the 15 refer to the diameter of the inlet? You can’t put a 25/80 with pipes that are 15 can you?
Yes, it does. Hopefully not all of your pipes are 15mm. They should be 22 or 28 at your boiler and pump. If they are all 15mm though, that will go a long way towards explaining why you're getting problems

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