Maximum delta temperature. Thermistor fault or system is too restrictive(sludge,too many trv's,pump struggling,partial blockage etc).
Also been told that burner seal can also cause f26. Glowworm have issued a new burner seal kit with nyloc nuts that don't loosen accidentally and a graphite seal now instead of rubber.
I'd probably start with looking at the pump and the system design. Then maybe change the pump and update the case seal. If still did f26 then possibly thermistor issues-replace.
GasMan, July 2009
