I have a newly installed Mains Pressure Hot Water Cylinder. When I turn on the hot water only; after about 15 minutes some radiators begin to heat.

The plumber says this is backwash and I should turn the hot water off after 15 minures to solve the problem.

I think in a new, modern system, this problem should not occur and the plimber is trying to fob me off!

Does anybody know if this is a 'feature' or a 'problem'? If it is a problem, then what is causing it and how can it be fixed?

There may be a language barrier here. I am having trouble understanding your system. A ''Mains Pressure Hot Water Cylinder'' is not a common American term. No system that I am familiar with would cause the radiators to heat when you used hot water at a sink. If I understand the problem, I think the system is installed wrong. Please describe your system and the problem a little more.

America and Ireland, "Two nations seperated by a common language". I think the system may also be known as an 'Unvented Storage Water Cylinder'. Does this make more sense?

I still need more information. I am picturing a system where water is fed into the cylinder by water from the water main. 15 minutes after you turn the heat on to the cylinder, you start getting heat out of the radiators. Do you also have a pump that recalculates water through the radiators? Should it be controlled by a thermostat? In not, can you shut the pump off, or the water to the radiators?

There are several different ways heating systems can be made to work. Surely yours doesn't have water continuously flowing from the mains through the radiators and then down the drain or something.