Last updated on September 23rd, 2023
When your Main boiler pressure drops too low the boiler will stop working which means you will have no heating or hot water.
To repressurise a Main boiler, you must add more water (pressure) to your heating system from the cold water supply via a filling loop.
I have been repairing boilers and heating systems for over a decade, and have come across every boiler pressure problem.
Here I’ll give you my best advice for repressurising a Main boiler based on my experiences over the years.
What Should the Pressure be on a Main Boiler?
The pressure on a Main boiler should be between 1 and 1.5 bar when the central heating is off and the radiators are cold.
The pressure rises when the heating is on, so you will not get the proper reading until it has cooled down.
If it is below 1 bar, you should top the pressure up. If you have to keep topping the pressure up, you have a problem that needs to be fixed.
Having the Main boiler pressure too high (above 1.5 when the system is cold), means you will need to let some pressure (water) out of the system.
You can do this via a drain-off valve. A drain-off valve can be on the boiler, a radiator valve or anywhere on the central heating pipework.
Or, you can bleed a radiator from the bleed valve into a bucket. You will need to open the bleed valve with a bleed key and let the water out and into a bucket until the pressure has dropped back down.
How to Repressurise a Main Boiler
If your Main boiler pressure drops below 1 bar you will need to increase the pressure.
To increase the pressure on a Main boiler you must:
- Open one valve fully (only if there are two valves)
- Open the other valve slowly until you can hear the water
- When the pressure has reached 1 to 1.5 bar close both valves
- Reset the boiler (if needed)
- Turn the heating on
Do not open both filling loop valves fully straight away as the water pressure in the house might be too much for the boiler and cause the Main boiler pressure to shoot up too much.
https://www.youtube.com/embed/8ngD6RJk_Js?feature=oembedHow to repressurise a Main boiler
A filling loop will be built in on the bottom of your boiler on most modern and small combi boilers, but it could also be on the pipework.
Usually, this is below the boiler but there could be an external flexible filling loop anywhere on the system, sometimes hidden away in cupboards or behind boxing.
There should be one of these types of boiler filling loops under your Main boiler:
- Two black or blue lever-type handles: These will need turning a quarter turn each to open and close.
- Easy-Fill green handle: This will simply need to be pulled and then released to stop filling.
- Two knobs: These will need turning anti-clockwise like taps to open, and fully clockwise to close.
Main Boiler Valve Positions
All the Main boiler valve positions below are fully open.
- Black Valves: All 3 black handle valves in the photo are open which they are meant to be at all times. They are (from left to right) the heating flow, cold water inlet, heating return valve.
- Blue Valves: The blue handle valves are the filling loop valves which are fully open in the photo and are increasing the Main boiler pressure. They should be closed unless repressurising the boiler.
Main Boiler Losing Pressure Overnight
If your Main boiler is losing pressure overnight you will have a leak somewhere on your heating system, or the pressure relief valve is letting the water out.
To find out which it is, you can tie a carrier bag to end the end of the copper blow-off pipe outside.
If your Main boiler has lost pressure overnight again, you should check the bag to see if it’s wet and if it is, that means the pressure relief valve is letting the water out.
This is most likely an expansion vessel problem, if so, you will need to recharge it or replace the expansion vessel if the diaphragm inside is split.
If the bag is dry after your Main boiler loses pressure overnight you must have a leak on the central heating system which will need to be found and fixed.
A leak could be anywhere on the full system, boiler, radiators, or pipework under floors.
How Often Should You Need to Repressurise a Main Boiler?
If you need to repressurise your Main boiler often you have a problem that you need to get fixed.
A combi boiler is a sealed pressurised system so you shouldn’t need to repressurise it at all.
If the pressure drops, it means the water is getting out which should not be happening.
If you need to repressurise your Main boiler once or twice a year I wouldn’t worry about it as it’s such a small leak that it’s barely noticeable.
Main boilers are sometimes undersized when fitted and are not large enough to cope with a large house’s central heating system.
This means the expansion vessel on the Main boiler might not be big enough and can cause problems with the pressure.
Getting the best combi boiler for a large house is very important, but adding an external expansion vessel that is large enough for the central heating system is also an option rather than upgrading to a bigger boiler.
How to Release Pressure on a Main Boiler
Knowing how to release pressure on a Main boiler might save you from some boiler problems.
If you repressurise a boiler too much and cause the pressure to go too high, you will need to release the pressure.
There are a few ways to release the pressure on a Main boiler.
Here are my go-to ways when out on boiler repair jobs:
- Open a drain-off valve on the Main boiler or central heating system with a hose attached and let the pressure drop.
- Bleed a radiator from a bleed valve and let the water squirt out into a bucket. This can take a while for the pressure to drop.
- Open the pressure relief valve on the Main boiler by turning the red plastic head.
- Crack a nut on the Main boiler heating pipes or a radiator valve and catch the water in a Plumb Tub or similar.
Main Boiler Guides
It’s good to know for sure what the problem is when your Main boiler is not firing up but the first thing you need to check is the pressure gauge.
If the pressure gauge is reading zero (or close to zero) your boiler needs to be repressurised to get it working.
Repressurising a Main boiler is an easy job. It can save you from having to pay a plumber to do such an easy task.
If you have to keep repressurising your Main boiler you should get that problem found and fixed as soon as possible.
Feel free to leave feedback or ask me any questions in the comment section below and I’ll try my best to help.
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