What faults cause your Boiler Leaking?


A leaking boiler is never a good sign, especially before the cold season starts. A small leak underneath the boiler can cause a lot of damage, and in some cases, the faulty parts cost so much that is not even worth the repair, it’s better to replace the boiler.

For our gas engineers at MML Plumbing, it’s a day to day job to go out for a leaking boiler or boiler breakdown. Most of the cases it’s a small leak, so we can fix the fault right on the site, but in some unfortunate case, the leak will damage some electrical parts, what we need to order.

So don’t forget, if you have any leak at the boiler, the first thing what you should do is to switch off the power at the main fuse box. The second steps you need to take is to close all the pipe isolation valves turning by hand, or with a screwdriver (you can find these underneath the boiler). To close the valve turn it clockwise.

Meanwhile, you waiting for the gas engineer to arrive, at least you are safe.


Here is a couple of faults why your boiler can leak:


Your pressure is too high

If your pressure is too low, even if you don’t have any leak or fault at your boiler, the boiler won’t work. The fault most of the cases are in the heating system, the radiator thermostatic valves it starts to leak. The ideal pressure for a boiler to work is 1.2-1.5 Bar.

Don’t get scared if the pressure will go above and stays at 2 bar, that is not a boiler fault, that is normal. When you heat up the water in your heating system, the water will expand and this will cause pressure to rise.


Is your system pressure is too high? You can diagnose this boiler fault by checking the pressure gauge. The pressure should be 1.5 bar or need to stay in the colour coded green area.

If the pressure is high or low it will be in the red zone or under 0.5 bar and above 3 bar.

If the pressure is too high, this can cause a leak at the boiler, what can cause damage and boiler breakdown. Normally you can see the leak coming out trough the PRV at the discharge pipe.


A few things you can check yourself, might you find the fault.

Check the filling loop is closed. Sometimes the valve letting by, or the isolation valve left open by mistake. The filling loop is connected to the cold water inlet and central heating return pipe.


You can also lose pressure when you bleed the radiators. You let out the air from the radiators, but the pressure will also drop. You need to top up the pressure to 1.2- 1.5 bar.


Pipe corrosion

Corrosion in your heating system or in the boiler can cause a small leak and you end up with a huge flood and few faulty components and at the end a boiler breakdown.

Older boilers often break down because of corroded parts inside the boiler. Again as we said previously, you close all the isolation valves and you need to call a gas engineer who.


The seals are damaged

After a few years the boiler seals will become hardened, break down and the boiler it starts to leak.

If you don’t deal with the issue asap, you can end up with a bigger boiler fault than just a leak. Your PCB can burn out, cost you hundreds of pounds.

In any of these cases, call a gas engineer from MML Plumbing ltd. We can help you out in any boiler breakdown or boiler faults.



You see leaks coming out at the overflow pipe

This leak’s easy to spot. You only have to go outside, in the garden where you might find the boiler pipes and check the small white condensate pipe if it’s leaking. Sometimes this pipe has black isolation.

The leak from this pipe can cause by high pressure in the system, and the boiler PRV letting trough the excess water. If your pipe is leaking, then you need a qualified gas engineer who can fix this fault for you.

Heat exchanger and Pump leaking

If your pump is leaking it is probably because the rubber seal is damaged or hardened. You can see the leak coming trough under the boiler on the right, sometimes the left side of the boiler.


Another, and the most expensive leak you can have, is the heat exchanger faulty rubber seal. If the heat exchanger is faulty then you need to replace it. Sometimes it’s just the rubber seals what need to be replaced, which are the cheapest option.

You can’t diagnose this issue by yourself, you need to call a gas engineer. Don’t try to remove the case of the boiler, if the seal is damaged, this needs to be replaced too.

If you service your boiler regularly, every year, you can avoid the boiler faults and breakdowns what will cause by a small leak and you will end up spending hundreds of pounds for parts or a new boiler.

A gas engineer from MML Plumbing can help you out to avoid high expenses, regarding your boiler if you call us on time, and you regularly maintaining your boiler.



Don’t try to fix any boiler fault or leak yourself, because sometimes you can cause more problems, it’s better to call a gas safe registered engineer from MML Plumbing.



We can help you out and give you advice in boiler installation, boiler breakdown and fault findings covering Barnet, North, East, West London.



Make a booking: [email protected]

Give us a call: 02083550840


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