Last updated on September 23rd, 2023
Having your boiler pressure too low causes the boiler to stop working which means you will have no heating or hot water.
To repressurise a 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 increasing combi boiler pressure based on my experiences over the years.
What Should the Pressure be on a Combi Boiler?
The pressure on a combi 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 up the boiler pressure. If you have to keep topping the pressure up, you have a problem that needs to be fixed.
If the boiler pressure rises too much (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 Combi Boiler
If your combi boiler pressure drops below 1 bar you will need to increase the pressure.
To increase the pressure on an combi 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 Potterton boiler pressure to shoot up too much.
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.
This is usually 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 filling loops under your combi boiler:
- Two black or blue lever-type handles: These will need turning a quarter turn each to open and close
- Easy-Fill green or blue 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. These are on older Baxi boilers.
Combi Boiler Losing Pressure Overnight
If your combi boiler is losing pressure overnight and you have to top it up regularly, you must 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 combi 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 pressure 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 combi 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 Combi Boiler?
If you need to repressurise your combi 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 combi boiler once or twice a year I wouldn’t worry about it as it’s such a small leak that it’s barely noticeable.
Combi 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 combi boiler might not be big enough and can cause problems with the pressure.
Getting the best size combi boiler for a big 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 Combi Boiler
Knowing how to release pressure on a combi boiler might save you from some boiler problems. If you repressurise a combi 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 combi boiler. Here are my go-to ways when out on combi boiler repair jobs –
- Open a drain-off valve on the boiler or central heating system with a hose attached and let the pressure drop
- Bleed a radiator 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 combi boiler by turning the red plastic head
- Crack a nut on the combi boiler heating pipes or a radiator valve and catch the water in a Plumb Tub or similar
It’s good to know for sure what the problem is when your boiler is not working 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 combi 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 combi boiler you should get that problem found and fixed as soon as possible.
Feel free to ask me any questions in the comment section below and I’ll do my best to help.
How to Repressurise a Boiler FAQs
Can you repressurise a boiler by yourself?
Yes. You can repressurise a boiler by yourself, and you should definitely learn how to repressurise your own boiler. Some boilers can be tricky to repressurise but most are very simple.
What causes a combi boiler to lose pressure?
There are many causes for a combi boiler to lose pressure. The most common cause of a combi boiler losing pressure is a leak, this could be on the boiler or anywhere on the central heating system.
Another cause could be the expansion vessel has lost its air and needs to be recharged or replaced.
The pressure relief valve could also be letting by. Or, the filling loop on the combi boiler could be slightly open or damaged.
Do I need to turn my boiler off to repressurise?
No, you do need to turn your boiler off to repressurise.
You should make sure the heating is turned off before repressurising the boiler as it will sometimes fire up as soon as it has reached the minimum working pressure.
This can cause the pressure gauge to move around so you don’t get an accurate reading but it isn’t a big deal.
Why won’t my boiler pressure go up?
If your boiler pressure won’t go up it’s usually because of two main reasons:
1. The filling loop is broken or blocked – This will need to be repaired or replaced.
2. The house mains water pressure is too low – If the house’s water pressure is only 0.5 bar then the boiler pressure can not be raised above 0.5 bar.