How to refill a central heating system efficiently

If you've done some maintenance work on your central heating, such as replacing a radiator, changing a valve, flushing out sludge or removing an airlock, you might have needed to drain all of the water out of your system in order to carry it out. Of course, once you've drained your system and done what you needed to do, you'll then have to refill your system back up again so that your heating system can function properly.

How you actually refill your system depends on the type of heating system you've got in your home. The process for refilling a gravity-fed system with a feed and expansion tank is slightly different to a pressurised system. In this article we'll look at how you can refill both types of central heating system to equip you with the knowledge and confidence to carry out this task yourself. 

Table of Contents

1. Draining your system
2. Refilling a pressurised system
3. Refilling a gravity-fed system

Draining your system

Knowing how to drain your heating system correctly is important and is closely linked to refilling it back up, so we'll briefly recap the draining process first.

• In order to drain your system, you'll need to switch off your central heating and turn off your mains water supply, which will stop the system from refilling with water. The mains water supply for your heating could be on your boiler, in an airing cupboard or somewhere else. Locate it and turn it off.
• To actually drain out the water, locate the drain-off valve (or valves) for your radiators. In a two-storey house, the drain-off valve will be on one of the downstairs radiators.
• Take a piece of hosepipe and attach it to the drain-off valve. The hosepipe will need to be long enough to go outside, into a bath or into a bowl so that the old, dirty water can be collected and discarded.
• Using your adjustable grips or spanner, unscrew the bleed valve and the water will start to flow out. You can leave this process to it's own devices for 10 - 15 minutes to ensure that all the water has flowed out.
• If you wanted to flush out a radiator, you can do it at this point by removing the rad from the wall, taking it outside and flushing it through with water. This is a useful thing to do if you know that radiator sludge has settled at the bottom of your rads.

Refilling a pressurised system

Tools you'll need

• Radiator bleed key
• Old towel or cloth
• Adjustable grips
• Filling loop key (optional)

Step 1 - Close the drain-off valve

Make sure that the hosepipe you connected to your drain-off valve no longer has water trickling out of it. Once you've established this then you can then remove the hosepipe and can close your drain-off valve You can do this using your adjustable grips and by turning the valve clockwise.

Step 2 - Close all the bleed valves

Closing all the bleed valves on all the radiators is very important, otherwise you may have a lot of leaks everywhere when you turn the water back on. Go around each of your radiators and close the bleed valves.

Step 3 - Start letting water into your system

Locate your filling loop, or your system's mains water supply, and start letting water back into your system. Locate your pressure gauge, which will likely be on the front of your combi boiler or possibly in your airing cupboard. Let water into your system until the pressure gauge reads 1 bar. Once your system reaches 1 bar of pressure, turn off the water. Do not allow the pressure to go above 1 bar.

Step 4 - Bleed air out of one radiators and re-pressurise

Letting water back in a repressurising your system is not the end of the process as there will be air trapped in your radiators. You need to go to a downstairs radiator (or a radiator furthest from the boiler) and bleed the air out of it using your radiator bleed key. After bleeding the first radiator, check the pressure gauge and you will notice it has dropped below 1 bar. Allow more water into the system until the pressure gauge reaches 1 bar again. For further assistance with this step, you can read our guide on pressurising a combi boiler.

Step 5 - Repeat for all radiators

Go to each radiator in turn, bleed the air and then allow the enough water into the system to repressurise it to 1 bar. If you're in a two-storey house, start with the downstairs radiators and then move upstairs. If you're in an apartment, start at the radiator furthest away from the boiler. It's very important to check and repressurise after bleeding each radiator.

Step 6 - Check for leaks

As a precautionary measure once you've bled all radiators and fully repressurised your heating system, have a look around all pipes and radiators and check that there are no leaks.

Step 7 - Inhibit the system

At this stage, it's a good idea to add some chemical inhibitor into your system to slow down the rate of internal corrosion andhelp to prevent a build-up of radiator sludge. Read more about this in our complete guide to chemical inhibitor.

Step 8 - Switch on your heating

Switch on your heating system and check that all radiators are getting hot equally. The pressure on your gauge should read about 1.5 bar when the heating is running and about 1 bar when it's off. If any of your radiators are not getting hot, it might be that there's some air still trapped in them, so switch off your system and go back to step 4.

Refilling a gravity-fed system

Tools you'll need

• Radiator bleed key
• Old towel or cloth
• Adjustable grips

Step 1 - Leave the drain-off valve open

If you previously drained your radiator, you will likely have attached a hosepipe to the drain-off valve, opened it up and allowed the water to escape outside or into a large container. Leave the drain-off valve open and the hosepipe connected for now.

Step 2 - Close the bleed valves

To ensure you don't get water leaking out of each radiator when you let water back into the system, go round each radiator closing the bleed valves with your bleed key.

Step 3 - Turn on your feed valve

With your gravity-fed, feed and expansion heating system, you will likely have a feed valve located in an airing cupboard (or possibly somewhere else). You will probably have turned it off before you drained your system previously. Locate the feed valve and turn it on (anticlockwise). This will push out any air and water stuck in your system.

Step 4 - Check your drain-off hosepipe

Turning on the feed valve will push air and some water out of the hosepipe you've got connected to the drain-off valve. Leave the air and water to come out of the hosepipe for about 10 mins to ensure the air is out of the system.

Step 5 - Close the drain-off valve

Once the air and water has stopped trickling out of the hosepipe, you can take your adjustable grips and close your drain-off valve (clockwise) and remove the hosepipe.

Step 6 - Bleed all your radiators

You'll now need to bleed out all of the air from your radiators. If you're in a two-storey house, start bleeding the radiators on the ground floor and move your way up. If in an apartment, start with the radiator the furthest from the boiler.

Step 7 - Add inhibitor to your system

Once all the air is out of your radiators, you can now add chemical inhibitor to your heating system to protect it from internal corrosion and radiator sludge.

Step 8 - Turn on your heating

Turn on your heating system and check that all radiators are getting hot. You should check there are no leaks anywhere at this stage as well. If a radiator isn't fully heating up, swithc off your heating and go back to step 6 to bleed out any air.

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