It's probably caused by the earth leakage trip at your mains control switch [ fuse box ] detecting a minor amount of leakage caused by some moisture build up in the heater during the summer when it was turned off. The solution is to get the heater elements on so that this moisture is driven off by their own heat. This can be done by a temporary bypass of the earth leakage trip unit . After a short heating period the earth leakage detector can be reinstated. A fault inside the heater can also cause the trip to operate. For spare parts for your heater contact 01 200 423070
