Hot then cold water on a combi boiler
The most common hot water problems are hot then cold water from a combi boiler. This is normally caused by a blocked plate heat exchanger. If your boiler is faulty you should call an engineer to look into it for you. Other causes include low or high water pressure, an interruption of your power supply, or a piping issue.
What is the most common combi boiler hot water problem?
Why is my water running hot then cold?
What is a plate heat exchanger?
What is happening with hot then cold water?
Hot then cold water: How to fix.
What is the most common combi boiler hot water problem?
The most common problem on older combi boilers is the water running hot then cold. A heat
Why is my water running hot then cold?
Hot then cold water from a combi boiler is a sign the plate heat exchanger is blocked. The boiler attempts to heat the water up but due to blockages it overheats and turns off. When it cools down again it follows the same cycle over and over.
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.
Will I get a fault code?
The boiler doesn’t overheat to the point of developing a fault code when the plate heat exchanger is blocked. You will just get very poor hot water.
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.
Summary
The best course of action if you feel your hot water is cycling hot then cold is to contact a gas-safe engineer to replace the plate heat exchanger.