how to drain a central heating system

How to Drain a Central Heating System

Knowing how to drain a central heating system (and refill) will save you money, and then you should be able to flush your own heating system if needed.

I have been flushing and draining central heating systems for over ten years. I have come across every problem going, and I will share the process for my ways to drain a central heating system and also refilling and adding chemicals.

How to Drain Central Heating System

If you have a gravity fed system that does not have a pressure gauge, you will need to isolate the water supply to the heating system. You can do this by bunging the feed and expansion tank in the loft, or turning off the mains water at the stopcock. Or, if you’re lucky, you might find a gate valve to turn off the feed.

The easiest way to drain a central heating system is to connect a long hose to a drain off valve and run it outside to a drain. A proper rubber drain down hose is best, but a standard garden hose should do the job.

You need to push the hose on, then tighten on a thumb turn hose clip or cable tie, and run it to a drain outside. Then open the drain off valve, with a radiator valve tool, adjustable spanner or grips, and look for water coming out and going down the drain.

radiator drain hose
Radiator drain down hose connected to drain valve with thumb turn hose clip.

Vent the Radiators

When the water stops coming out of the hose and the pressure gauge is on 0 (if you have a gauge), you should start venting all the radiators with a radiator bleed key, starting with the highest radiators first.

Venting means to open the bleed valve on the radiator, like bleeding, except you are letting air in and water out. Venting is the opposite to bleeding. You should hear hissing when you vent the radiators as the water is draining out.

So, you should vent all the radiators on the top floor first and wait until the water stops coming out of the hose outside. When it stops, you should move down to the next floor and repeat until you’ve drained all radiators on all floors.

If you do not vent the top radiators first, then water will come out of the bleed valves on the lower radiators when you try to vent them. You should also vent the tallest radiator on each floor first, as not doing this can cause the same problem.

How to Drain Central Heating System Without Drain Valve

Learning how to drain a central heating system without a drain valve is a lot harder to do. If your drain valve is not letting water out, or maybe you don’t have one at all (not uncommon), then here is my preferred method.

Remove a Small Radiator

You will need a 15 mm nut, olive, and a small piece (50 mm or more) of copper pipe. If both radiator valves have a 3/4″ thread on them, then you will also need a 3/4″ nut and tail that come with new radiator valves. You might need two people to lift the radiator off, unless it’s very small.

  • Find the smallest radiator on the ground floor. If there are no small ones, then pick the best positioned
  • Close both radiator valves. Turn any TRVs to zero or off, and turn lockshields anti-clockwise as far they’ll turn
  • Put some old towels underneath the radiator valves and get a small tub (a Plumb Tub is perfect) to catch any water
  • Open the bleed valve and catch the water that squirts out until it stops coming out
  • Undo the two nuts (anti-clockwise) on the radiator valves that are connecting the valves to the radiator, not the bottom nuts
  • Pull radiator valves away from radiator and put your thumbs over tails on the radiator to stop the water coming out
  • Lift the radiator off the wall brackets and carry it outside to empty the radiator water out
  • Connect the small copper pipe to one of the radiator valves with the nut (or nuts) and olive
  • Connect a long hose and run it to the drain outside, and then follow the how to drain a central heating system process above

Before refilling the central heating system, you should replace one of the radiator valves with a new one that has a drain off valve built in. This will make draining in the future a lot easier.

How to Refill Central Heating System

If you have drained it, you will definitely need to know how to refill your central heating system.

Before refilling the system, make sure that all the radiator bleed valves and any drain off valves are closed.

Unpressurised Gravity Systems

Gravity systems will take a lot longer to refill than a pressurised system. You will need to turn the water supply back on and let the tank in the loft fill with water. The tank will then fill the system automatically, you will need to bleed all the radiators, but you won’t have to fill the pressure up.

If the air stops coming out when you are bleeding the radiators, you should just leave it for a while until the tank in the loft is full, this will give you more pressure. Then follow the instructions below for how to bleed the radiators and system correctly.

Pressurised Systems

To refill your pressurised (with an expansion vessel and pressure gauge) central heating system, the first step is to find the filling loop, this is normally under the boiler on a combi boiler, or in the cylinder cupboard if you have a system boiler.

You should open the filling loop valves to let the water in and fill the system to between 1 and 1.5 bar on the pressure gauge.

Bleeding the Radiators

Then you can start bleeding the lowest radiator on your central heating system. When the radiator is fully bled of air, the water will start coming out of the bleed valve on to your towel.

Close the bleed valve with the bleed key, then move on to the next radiator. Work your way around all the radiators on the same floor.

One Floor at a Time

Next, move up to the next floor up and bleed all the radiators on that floor. When the air stops coming out, you will need to top the pressure up again (unless it’s gravity). Keep doing this until all the radiators have been fully bled.

Knowing how to refill the central heating system properly is key. If you have 3 floors, you should bleed the ground floor first, then the 1st floor, then the 2nd floor.

You should always bleed the lowest radiators first because if you bleed the upstairs first, the water will come out then when you bleed the lower radiators, and you will have to bleed the upstairs radiators again.

How to Clear an Airlock in Central Heating System

Here’s how to clear an airlock in a central heating system for when you might have some radiators that are not getting hot or if your boiler keeps cutting out, typically after draining and refilling a central heating system.

If you are having airlock problems where the boiler keeps cutting out, you need to focus on getting the air out as much as possible. This can take all day and is very frustrating. Keep bleeding the radiators,

On a gravity system, there should also be a bleed point in the hot water cylinder cupboard. This is the main one to focus on. Get a towel wrapped around as good as possible to catch any water and keep bleeding the air out until the boiler stays on and heats all the radiators.

Most of the time, an airlock will usually mean one or more radiators are not getting any heat. To solve this problem, with the heating on, turn one radiator valve off on every radiator except from the ones that are not getting hot.

This will force all the pressure from the pump to that radiator and force the airlock along and out. Once the radiator heats up, you should bleed any air out and then open all the other radiator valves.


frequently Asked questions

Do you need to turn the water off to drain a central heating system?

Only if you have a gravity fed central heating system with a conventional boiler. Combi boilers and system boilers you do not need to turn the water off.

How long does it take to drain a central heating system?

It depends on the system size, layout, and condition – Anywhere from 20 minutes to a few hours or more.

Where is the drain valve on a central heating system?

Not all central heating systems have a drain off valve. The most common place to find a drain off would be on a radiator valve. It could also be anywhere on the heating pipes, sometimes under the boiler or under a radiator. Some boilers also have drain off valves on them.

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