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If your combi boiler is losing pressure or if you have recently carried out some maintenance to your central heating system—e.g. bleeding a radiator or fixing a leak—then you may need to refill your central heating system. A combi boiler heats water directly from the mains, and you require the right pressure to enable it to run efficiently.
If the level gets too low, your boiler will shut down, and you won’t have hot water or heating. Re-filling it returns the pressure to normal and gets your system running. Here, we show you how to re-fill your combi boiler safely and efficiently in your home.
What Does a 'Pressurised System' Mean?
A pressurised central heating system like a combi boiler requires just the right amount of water pressure to work properly. If you look at your boiler, you’ll see a small dial—this is a pressure gauge. This indicates the pressure in the system. When everything is turned off, it should read between 1 and 1.5 bar.
Your boiler’s pressure has a bearing on how effectively your heating works. It needs to be in the right range—neither high nor low.
- Low pressure (below 1 bar) – The system may not be able to circulate warm water properly, and your radiators stay cold.
- Over-pressure (over 2.5 bar) – This may stress your boiler and result in a break or a leak.
The ideal pressure level would be between 1.0 and 1.5 bar when idle. Having it in these ranges maintains your heating in a safe and efficient manner.
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How to Check Your Boiler Pressure?
- Find the Pressure Gauge – Stand in front of your boiler. You’ll see a dial with a pointer.
- Check Needle Position – The best range will have a green mark on it (1–1.5 bar).
- If it’s too low – You’ll need to recharge the system by refilling it with water.
- If it’s Too High – A professional will be required to let water out to bring it back to a safe level.
Regular monitoring of your pressure gauge will keep you from having sudden heat problems.
Why Refill a Combi Boiler?
Your boiler can lose pressure with time, and when it does, it needs to be refilled. The following are some reasons why pressure can fall:
- Leaks – A small leak in a pipe, a radiator, or a valve can slowly reduce pressure.
- Bleeding radiators – Draining air from radiators improves heating, but it does decrease system pressure.
- System maintenance – You can be required to fill with water in case you have replaced a radiator or have performed repair work.
If your boiler pressure has dropped below what it ought to be, your heating system will not function properly. Re-pressurising, or refilling, restores it to a suitable level so that your home remains warm and comfortable.
Preparing to Refill the Heating System
It’s a good idea to do a few quick checks prior to refilling your central heating system. This makes the operation easier and prevents possible problems.
Tools You’ll Need:
- Radiator bleed key – To let out locked-in air.
- Old cloth or towel – To soak up spills.
- Adjustable grips – to tighten or loosen valves as needed.
- Filling loop key (if necessary) – This can be required to unlock the filling loop.
Draining Your Central Heating System
It’s a good idea to understand how draining works before refilling. Draining gets rid of stale water, sludge, or stuck air in your system.
How to Drain the System:
- Switch Off Heating and Water Supply – Switch off your boiler and find the mains water supply to your heating. It may be near your boiler or in an airing cupboard. Turn it off to stop the water from refilling.
- Find the Drain-Off Valve – This will be at the bottom of a radiator in a basement.
- Attach a Hose – Attach a hosepipe to a drain-off valve. It can be led outside, into a bath, or into a bucket to collect water.
- Open the Valve – Use a spanner or adjustable grips to loosen it. The water will start to flow out. Let it drain for about 10–15 minutes.
- Flush a Radiator (Optional) – If you do have a sludge build-up, remove the radiator from the wall and hose it out with fresh water outside.
Once you have drained it fully, you can then start refilling it.
How to Refill Your Combi Boiler?
Let’s go through refilling your combi boiler in a step-by-step manner. It’s simple—just make sure to do it with caution to avoid issues. And if you’re facing frequent boiler troubles, check out this guide on common boiler problems and how to fix them.
Step 1: Switch Off the Boiler
Start by turning off your boiler. Give it time, a half hour to cool. This will make it easier to work with, as you don’t want to have a hot system to deal with.
Step 2: Identify the Filling Loop
It’s a short, flexible pipe with two valves — usually located under or near the boiler. It connects your boiler to the water supply.
- Built-in filling loop: Some new boilers come with a built-in filling loop — refer to your instructions if you’re unsure.
- External fill loop: You will have to manually connect a hose in case your boiler does not come with one.
Step 3: Attach Filling Loop (if required)
If you have an external filling loop:
- Connect one end of the hose to the water inlet valve of the boiler.
- Connect the opposite side to the mains cold water supply.
- Make sure both ends are secure before moving ahead.
Step 4: Gradually Open the Valves
There will be small valves or taps at both ends of the filling loop. Turn them slowly open. You will be able to hear water entering into the system. Don’t do it hastily — filling with water too rapidly can produce surges in pressure.
Step 5: Monitor the Pressure Gauge
Your boiler comes equipped with a pressure gauge, normally located at the front. Look at the needle. You want it to be between 1 and 1.5 bar. Cut off when it reaches that.
Step 6: Close the Valves
Once you have the correct pressure, close both valves in the fill loop. This keeps water from entering into the system anymore.
Step 7: Remove the Filling Loop (if required)
If your boiler has a detachable filling loop, remove it once it has been refilled. This will prevent you from accidentally putting in too much water later.
Step 8: Switch the Boiler Back On
Switch your boiler back on and let it run for a few minutes. Check the pressure again. If it’s a bit low, open the valves for a second or two to fill it to the top.
Step 9: Bleed Your Radiators
Once you’ve refilled, you may have air in your radiators. You can bleed them. This is how to bleed your radiators:
- Get a radiator key: This will be needed to open the small valve at the top of your radiator.
- Let out the air: Put in the key and turn it slowly in a clockwise direction. You’ll notice a hissing noise — that’s escaping trapped air.
- Close the valve: Turn the key to close the valve immediately as soon as the water starts to dribble out.
Start with the radiator that’s located furthest from the boiler and move through them.
How to Refill a Pressurised Heating System?
Step 1 – Close the Drain-Off Valve
Ensure that there is no water leaking from the hosepipe. Remove it when it ceases to flow and close off the drain-off valve by turning it clockwise with adjustable grips.
Step 2 – Close All Bleed Valves
Go around all of your radiators and close off their bleed valves tight. This prevents leaks when water circulates back into the system.
Step 3 – Let Water Into the System
Find your filling loop (usually under the boiler) or the mains water supply for your heating system. Slowly open the valve to let water in. Watch the pressure gauge on your boiler—it should reach 1 bar. Once it does, turn the water off. Do not let the pressure go above 1 bar at this stage.
Step 4 – Bleed the First Radiator & Repressurise
Find your filling loop (which will be somewhere under your boiler) or your mains water supply to your heating system. Slowly open the valve to let water in. Watch your boiler’s pressure gauge—it will reach 1 bar. Switch off the water as soon as it does. Don’t let it go beyond 1 bar at this stage.
Step 5 – Bleed and Repressurise the First Radiator
Start with a radiator that’s at ground level or one that’s furthest from the boiler. Use a radiator bleed key to release the trapped air. Next, read the pressure gauge. If it has dropped below 1 bar, open the filling loop again to get it back to 1 bar.
Step 6 – Repeat for All Radiators
Bleed each radiator, removing air and then refilling with pressure as needed. If you live in a house, start with ground-floor radiators and move upwards. If you live in a flat, start with the radiator that’s furthest from the boiler.
Step 7: Look for Leaks
Once you have bled radiators and re-pressurised the system, check pipes, joints, and radiators for leaks. Fix problems before you turn on the heating.
Step 8 – Add Inhibitor (Optional but Strongly Recommended)
To maintain your radiators and pipes in good condition and to stop sludge and rust from forming, you can use a chemical inhibitor in your central heating system.
Step 9 – Switch On Heating
Switch on the boiler and allow the radiators to warm up. The pressure gauge will be around 1.5 bar when in operation and 1 bar when off. If radiators do not get warm, there may be air left in them—switch off and bleed the radiators again and repressurise.
Your central heating system has been refilled and it’s now functional.
How Long to Refill a Central Heating System?
Refilling a central heating system can take anywhere from 30 minutes to one hour. The time can vary with:
- System size – Larger houses with more radiators require longer.
- Amount of air in the system – Air in a system can be a slowing factor.
- How effectively you bleed the radiators – Correctly releasing air shortens the procedure.
Once refilled, it’s a good idea to leave the heat on for a time frame of 30 minutes to ensure the radiators heat up evenly. If they stay cold, you may have to bleed them again.
Common Refilling Central Heating System Problems
When refilling your heating system, you might run into a few problems:
- Pressure not rising: This means that the fill loop isn’t connected properly, or that the valves aren’t fully opened. Inspect them both.
- Pressure rising too fast: Don’t fill it up too much — use between 1 to 1.5 bar. If it reaches over 2 bars, you may have to release some water from a radiator.
- Water leaking: If you spot water leaking from radiators or pipes, do not refill and get these fixed first.
- Boiler not turning back on: Low pressure or airlocks may sometimes stop a boiler from working. Bleed radiators and try again.
- Noise from radiators or pipes: Gurgling sounds usually mean there’s air in them. Bleed radiators until you get water, and not air.
How to Avoid Airlocks?
Airlocks can block water from circulating properly. Here’s how to prevent them:
- Refill Slowly – Have water enter into the system slowly to avoid big pockets of air.
- Bleed Radiators in Order – Start with the radiator that’s furthest from the boiler and work your way to the nearest.
- Monitor the Pressure Regularly – Monitor the boiler’s pressure gauge during refilling.
- Run Heating – Switch on the heating and have it work for at least 30 minutes to remove stuck air.
- Listen for Weird Sounds – If you do hear gurgling in radiators or pipes, bleed them again.
What If Your Boiler Keeps Losing Pressure?
If your boiler is losing pressure and you have refilled it, but the problem persists, it could indicate a more severe problem. Here is what to check:
- Leaks: Check for any visible leaks on radiators, pipes, and also around the boiler.
- Faulty Pressure Relief Valve: The valve releases the extra pressure, but in a faulty state, it discharges more water than necessary.
- Faulty Expansion Vessel: Permits pressure to fluctuate — if defective, it would lead to sudden loss of pressure.
It is always better to have a gas-safe engineer attend and fix and test the system in such a case.
Tips for Maintaining Your Central Heating System
Check boiler pressure: It should be between 1 and 1.5 bar. If it’s below that, you will have to top it up.
- Bleed your radiators: This needs to be carried out every few months to remove stuck air and ensure even heating.
- Check for leaks: Check radiators and pipes for water. Small leaks may be responsible for low-pressure drops.
- Arrange for a yearly service: A professional will visit your boiler each year to spot possible issues early.
Keep the surrounding space around your boiler clear: Ensure there’s adequate ventilation and no obstructions in close proximity. A little effort goes a long way in avoiding costly repairs.
When to Call a Professional
While you can do simple maintenance, there are problems that require professional assistance. Call a heating engineer if:
- Pressure drops too often – This could be a sign of a hidden leak.
- Your boiler produces loud noises – Banging or gurgling noises can be a sign that there’s a problem.
- Even after bleeding them, the radiators stay cold.
- You notice a smell of gas or water escaping from the boiler.
- The boiler shuts off unexpectedly.
A professional can diagnose and fix issues safely, preventing bigger problems later.
The Bottom Line
Refilling a central heating system with a combi boiler is easy. If your pressure gauge shows below 1 bar, refill it with the filling loop slowly to a level between 1 to 1.5 bar. Turn off the boiler before refilling and close the valves once you have finished.
If it’s falling or goes too high, it means there’s a fault or a leak. You can then have a call-out from a gas-safe engineer. Having your boiler at the right pressure allows it to heat your home efficiently and prevents costly breakdowns.
Frequently Asked Questions
Yes, you can. You can do it safely by turning off the boiler, using the filling loop, and closing it when you reach a pressure of between 1 to 1.5 bar.
To decrease boiler pressure, you can:
- Bleed the radiators: Turn a radiator key to slowly open the valve and you’ll get a hiss. Turn it back when you have a flow of water.
- Drain water from the system: In case bleeding does not work, you can let out some water by utilising the drain-off valve.
You shouldn’t have to keep refilling your boiler so often. If it keeps dropping, there may be a leak or another issue. If so, check for leaks or call a professional.
Yes, you can do so. This proves useful in case you wish to repair a radiator or a small leak. You just perform the draining steps, but close the valve once you have drained enough water.
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