A combi heat exchanger leaking is quite an unusual fault but is possible especially in older boilers or those that are heavily corroded.
What is a plate heat exchanger?
A plate heat exchanger is a way for the boiler to heat your clean hot water without it coming into contact with the dirty water in the boiler.
There are two sets of channels separated by plates. One set contains clean tap water, the other dirty central heating water. The heat is passed through the tin metal but the two sets of water never mix.
How can combi boiler heat exchangers leak?
Combi heat exchangers can leak in 2 ways which causes two different faults..
Leaking heat exchanger from failed seals.
The first is from the seals connecting it to the boiler. This would show as water dripping (or spraying) from the boiler case.
Leaking inside the heat exchanger
The second is water passing from the higher pressure cold water mains side of the heat exchanger to the lower pressure boiler side of the heat exchanger. This shows itself by the pressure gradually rising for no obvious reason.
If the filling loop has been removed and capped so you are sure the combi boiler is not filling from the mains the heat exchanger is the only remaining cause.
Combi boilers have a heat exchanger that takes the heat from the boiler and transfers it to the cold water coming from the water mains. Modern heat exchangers are made from very thin stainless steel in the form of flat plates.
If the steel fails either through corrosion or manufacturing faults the mains water can mix with central heating water. The hole that is caused by the corrosion/fault is normally very small so the pressure rise is generally over several hours or even days.
If a combi heat exchanger is leaking they can’t be repaired so have to be replaced.
This isn’t a DIY job so should only be done by a registered boiler engineer.
Almost every brand or model of combi boiler has a different plate heat exchanger that isn’t compatible with other brands.