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Posted: Mon Nov 29, 2010 10:32am

Post Subject: Central Heating

I have a water and radiator system heated by a Rayburn oil fired boiler The radiators remain cold but the small expasion tank with a single pipe going into the bottom gets hot and blows hot water out through the radiator cap. We have tried to bleed all the radiators but cannot get the water to circulate through the radiators. Is there a schematic diagram of pipework for this type of heating system? What is the purpose of this expansion tank. Should this system also have a header tank? The Boat is kept near Bristol. Any ideas?


Posted: Mon Nov 29, 2010 11:57am

Post Subject: Central Heating

Dear Josephine, It all depends upon the size of the pipes, where they run along the boat and where the stove is. If the pipes are like domestic water pipe (15mm dia.) then there should be an electric circulating which needs to run all the time the Rayburn is turned on. This circulates the water so the heat from the boiler is dissipated through the radiators. Without a pump the water in the Rayburn will boil and force hot water and steam out of whatever is the vent. In your case the expansion tank. In this case pretty much any simple domestic central heating diagram will do. If the pipes are much larger and one runs at gunwale level or higher then it is probably a gravity system which relies upon water rising in the boiler when it is heated to push water around the system by convection. This is known as a gravity system and again any OLD book about domestic heating should give a diagram. There is also a diagram in Waterways World's The New Narrowboat Builders Book of about 10 years ago. The new version may omit it. If you email me I think I have some generic diagrams I can send you ( The other reason for this could be that someone has changed the antifreeze in the central heating and did not mix it with the water BEFORE putting it the system. In this case drain it all down, mix it and refill. Hot antifreeze is denser than cold water so it just sits in the boiler and will not circulate by convection. Even with a pump you often get hot pipes and cold radiators. The expansion tank is just that. A tank that allows room for the water in the system to expand when it gets hot and perhaps top up very, very minor weeps. Tony Brooks

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