Hot water cylinder siphoning air when hot and cold open

10 Feb 2023
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United Kingdom

I have a problem.

I have a system boiler + hot water cylinder in the loft and need to install a shower on the floor below (immediately below the cylinder) The cold water is balanced with the hot. The pipe runs from the cylinder to the shower are pretty short.

Usually, turning off the hot and cold water is easy. I can isolate the cylinder via a valve (not sure of its proper title), or turn off the mains. Either way, the hot and cold run dry quickly. Presumably suction keeps the water in the tank.

But not with this shower, which is currently two capped pipes. If I uncap the cold, the water runs dry quite quickly. Same for the hot. All fine so far.

But, if I uncap both at the same time, noisy stuff happens, which sounds like air being siphoned up into the system through the hot water pipe, and then all hell breaks loose as warm water suddenly comes pouring out of the cold tap.

If someone could possibly explain what is happening I’d be very grateful. Obviously I could drain the entire cylinder and will do so if I have to, but I’d prefer not to, and if there’s a fault, I’d like to know about it.

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