Hot water affecting CH

[Hamuel]

When the hot water goes on, the radiators cool down considerably. I don't have a combi boiler. I gather I need something called a gate valve. Can anyone help?

[plumbbob]

Chances are the DHW cylinder does not have a lock shield valve fitted and all the hot water is circulating this system rather than the radiators.

This valve is usually sited near the inlet to the cylinder coil and is a gate valve with a red topped wheel.

[Hamuel]

Many thanks!