I am going to put my hot water tank( indirect )in the loft next to a combi boiler i am worried about what height the vent pipe should be above the cold water tank as the cold water tank will only be about 12" above the cylinder tank and the water level will be about 36"at the top when full will the hot water in the expantion pipe when hot rise above the level of the cold water tank and cause a problem . :?: :?:
Do you mean conventional boiler? A combi is not normally connected to a cylinder.
Anyway. The height of the domestic expansion pipe doesn't really matter too much. Just put it as high as you can if you are stuck for height. The expansion volume when the cylinder is heated will be taken up by the storage tank, not the expansion pipe. Think of an expansion pipe as a vent pipe (for air) and a safety valve (for overheating).
More important is to use a 28mm feed pipe from the tank to the cylinder. Using a 22mm feed when the cylinder is fitted directly below the storage tank in the loft will almost certainly draw air into the pipes when the taps are run, especially the bath tap.
Another consideration is the height of your feed and expansion tank above the boiler assuming you are not going with a sealed system.
The pipework will need careful consideration if you have low head to avoid problems with the heating system. A sealed system should be your preffered option unless there are other considerations.
Why not just use a combi?
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