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When we light the fire in our house after a while a loud banging noise can be heard from the pipes upstairs. The fire has a back boiler attached to it to heat the hot water but it has no pump. The banging can be so loud that I am concerned that the pipes will be damaged and eventually leak. The banging will happen regardless of whether our oil fired boiler is running or not.

Any advice on what is causing this (and how to cure it) would be greatly appreciated.

There is a fault in your system that is causing the water to boil which makes the noise you hear. This is very dangerous and must be rectified before using the back boiler again. There are several areas of danger here, so please do not disregard this advice.

Check that the header tank is full of water, and ensure the system is bled properly using any bleed screws near the cylinder. Unless you are sure you have remedied a fault please contact a professional heating engineer for a safe repair.

Thanks for the useful advice. I think I'll get hold of a plumber to check the system. I've checked the header tank and it is full of water. If it is not an airlock are there any other likely causes of the problem. Also is the easiest solution to permanently isolate the back boiler from the rest of the heating system? Does this cause any problems?

Grateful for the advice.

If we assume the system is full of water with no airlocks, I can think of two other problems which may cause the boiling. Firstly as the system is gravity fed, it is possible sludge has built up somewhere that has stopped the flow of water. Secondly, the boiler itself may be faulty which means the gravity flow cannot start hence the hot water cannot leave the boiler.

If you wish, the boiler can be disconnected and the remaining water allowed to boil off until dry.