Combi boiler heat exchanger problems
A combi boiler heat exchanger leaking is quite an unusual fault but is possible, especially in older boilers or those that are heavily corroded.
The two problems associated with heat exchangers are unexplained pressure rising and hot then cold hot water. Follow this guide for unexplained pressure rising. Click here for hot then cold water.
What is a plate heat exchanger?
How can combi boiler heat exchangers leak?
Leaking heat exchanger from failed seals.
Leaking inside the heat exchanger
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.
Can I do this myself?
This isn’t a DIY job so should only be done by a registered boiler engineer. It can be messy and you may have to remove the combustion case to access the part.
Almost every brand or model of combi boiler has a different plate heat exchanger that isn’t compatible with other brands.
Summary
Leaking plate heat exchangers are quite common especially in older more corroded boilers. Each boiler has its own unique heat ex and you need a qualified gas engineer to replace it.
One response to “Combi boiler heat exchanger leaking”
I had this fault and two engineers could work it out and just fitted extra expansion tanks and other stuff that did not fix it etc. Eventually I phone Grant and a very helpful guy said exactly what you have written. It makes me mad that I then had to tell a registered boiler engineer how to fix a fault, but we do seem to have a boiler engineer mafia in our area.