Probably the HW flow sensor, Try turning on 2 or more taps, sometimes works, and will also rule out the switch itself. Different boiler, use different sensing, some are via a diaphram, which operates a pin, which pushes on a microswitch. If that type, once the front cover is off, you would be able to see the movement, or not. (probably with a plastic sheild over it, But you would be able to twist it to see the pin movement, (on the end of the 3 way valve) Some use more electronic means, test meter needed. If 2 or more worked, possibly diaphram in the 3 way valve, depends how you feel about it, can get them cheap from ebay.
SD, March 2012
