fixmyplumbing

Why have I got hot then cold water?

Hot then cold water from a combi boiler is a sign the plate heat exchanger is blocked.

What is a plate heat exchanger?

The plate heat exchanger is a metal component that transfers the heat of the boiler to the hot water coming out the taps.

It is a series of thin metal channels welded together. Hot tap water and boiler water flow in separate channels beside each other without mixing.

Unfortunately the thin metal channels are easily blocked up by sludge and dirt in the boiler.

If the plate heat exchanger is blocked the heat can’t be transferred fast enough.

What is happening with hot then cold water?

The boiler doesn’t know that the plate heat exchanger is blocked so it operates as it normally would.

When the hot tap is turned on the boiler goes to full power and expects the heat it generates to be sent out from the boiler into the hot water.

The plate heat exchanger is blocked so the boiler can’t transfer the heat. It then gets too hot and turns itself off.

The plate heat exchanger slowly cools down as cold water is flowing through it. The boiler then starts to fire again but the blockage stops the transfer of heat so it overheats.

The boiler then repeats this cycle over and over again causing the hot then cold water problem.

Hot then cold water: How to fix.

The only effective way to fix this problem is to replace the plate heat exchanger. Each model of boiler has its own particular component that only works with that boiler.

To access the plate heat exchanger you have to open up the combustion case. This means a competent gas engineer is required to complete this job.