i have an old falcon boiler which works fine normally but in this cold weather the heating comes on but the rads get luke warm then it swithes over to stanby. I have no thermistat fitted just whats on the boiler . How do i fix this?
Without knowing the boiler, it could have a built in frost thermostat which would act as you describe, it would cut in just long enough to prevent the boiler from freezing.
