it runs fine on central heating for about an hour, then cuts out. if u run the hot water then reset the same cycle will continue again. i have changed the thermister and am now think its the dhw hi limit stat. what resistance should i get from this hi limit stat when testing? could it be something else??
A lot of answers can be found with temperature sensors on the flow and return pipes. Sometimes definitive answers cannot be given without doing this sort of testing.
As Nosdda has said check your pump is working correctly in the boiler(the rest of his answer is off as the swiftflow is a combi boiler)as they can sometimes stick or even slow down from sludge build up which will cause boiler to overheat and lock out. Its quite an old boiler now so do all your rads work properly with and even heat all over or do you have cool/cold areas on the bottom.
Is your boiler pump running okay? can be a faulty DHW flow valve as well. If your system incorporates motorised valves, then you have 2 valves. One for hot water and one for central heating . Check that thesr valves are opening when you need hot water or central heating, and check the electric contacts on them.
