How to Drain a Central Heating System Properly

Last updated on October 5th, 2024

Draining a central heating system can take a long time but is sometimes needed for replacing radiators and other heating jobs Knowing how to drain a radiator system (and refill it) can save you money and give you the skills to flush a central heating system.

I have been a full-time heating engineer for over a decade and have drained and flushed my fair share of heating systems.

Steps to Drain a Central Heating System

A central heating system should have at least one drain-off valve but not all do. If you don’t have one, you should get one fitted.

How to drain the system:

If you have a vented gravity heating system, you will need to turn the water off at the stopcock first.

1. Connect a drain down hose to a drain-off valve with a hose clip and run it to a drain outside
2. Open the drain-off valve with an adjustable spanner
3. Look for water coming out of the hose and going down the drain
4. Wait for the water to stop coming out
5. Vent the radiators by opening the bleed valves with a bleed key starting with the highest floor
6. Wait for the water to stop coming out of the hose
7. Vent the radiators on the next floor down
8. Wait for water to stop coming out of the hose
9. Repeat until all radiators on all floors are drained

Sometimes radiators on the ground floor will have their pipes going straight upstairs, typically because they have a concrete floor, These radiators won’t drain from a central drain-off valve. They will need to be drained individually.

How to Drain a System Without a Drain Valve

Draining a central heating system without a drain valve is sometimes needed.

Most combi boilers have a drain-off valve on the bottom which you can drain the system from, but you won’t be able to drain any radiators that are below the boiler from here.

On a vented gravity heating system, the feed and expansion tank (typically in the loft) can be bunged using central heating rubber bungs.

After draining the system, you should replace a radiator valve with a radiator drain-off valve for future draining.

Here are three ways to drain a system without a drain-off valve:

• Isolate a radiator and drain the system from the radiator valve
• Use a self-cutting drain-off valve
• Cut a heating pipe under the floor

Drain From a Radiator Valve

My preferred way to drain a system without a drain-off valve is to isolate and blank the radiator then drain the system from a standard valve, this can be from a TRV or lockshield.

You will need a 15mm nut, olive, and a small piece of copper pipe. You might need a 3/4″ nut and tail that come with new radiator valves also.

You can drain the radiator, remove it, or just blank it, but the end goal is the same which is to isolate an individual radiator and drain the system from a radiator valve.

Here’s how to isolate a radiator to drain the system:

1. Pick a small radiator downstairs (the lower the better)
2. Put some towels down under the radiator valve
3. Close both radiator valves
4. Place a Plumb Tub under one of the radiator valves
5. Open the bleed valve screw and catch the water
6. Wait until the water stops coming out (if it doesn’t stop, pick a different radiator)
7. Close the bleed screw
8. Crack the side nut (with an adjustable spanner) on the radiator valve that connects the valve to the radiator
9. Crack the nut on the bottom of the valve on the pipe to loosen it slightly
10. Unscrew the side nut fully and pull the valve away
11. Twist the valve towards you
12. Fit a 1/2″ or 3/4″ blank to the radiator nut to keep the water in the radiator (or drain the radiator)
13. Connect the small copper pipe to the radiator valve
14. Connect a long hose to the pipe and run it to the drain outside
15. Open the radiator valve and follow the steps above to drain the system

Self-Cutting Drain-Off Valve

You can also use a self-cutting drain-off valve to drain a heating system.

Here’s how:

1. Find the lowest radiator pipe (at least 15mm)
2. Connect the self-cutting drain valve clamp to the pipe and tighten it with a screwdriver
3. Screw in the brass valve with your hand then with some water pump pliers
4. Connect a drain down hose and run it to a drain
5. Open the drain-off valve and follow the steps above to drain the system

Cutting a Pipe Under the Floor

Sometimes cutting a pipe under the floor is needed to let the water out and onto the ground downstairs, just make sure there are no electric cables that could get soaked with the water.

You can use a push fit or compression coupler to connect the cut pipe again after you’ve finished draining the system.

1. Pull up a floorboard and locate a heating pipe
2. Cut the pipe slowly with a pipe slice cutter
3. Stop cutting as soon as the water starts coming out
4. Keep the pipe cutter and your hand on the pipe to stop any water from spraying everywhere
5. Wait for the water to stop coming
6. Cut the pipe fully and remove the pipe cutter
7. Follow the steps above to drain the system

Fit at least one drain-off valve when the system is drained so you won’t need to cut the pipe again in the future.

How to Drain a Radiator

Draining a radiator is sometimes useful for adding inhibitor and other chemicals, replacing the radiator, or accessing the valve properly for draining the system.

Draining a radiator can take a long time and get messy sometimes. Typically, the bigger and older the radiator the worse it is, and the smaller and newer the better.

Here’s how to drain a radiator:

1. Put some towels down under the radiator valves
2. Close both radiator valves
3. Place a Plumb Tub under one of the radiator valves
4. Open the bleed valve screw and catch the water
5. Wait until the water stops coming out (if it doesn’t stop, one of the valves must be faulty)
6. Crack the side nut on the valve connecting the valve to the radiator
7. Make sure the bleed screw is still open
8. Let the water drain into the Plumb Tub
9. Tighten the nut when the tub is full
10. Empty the tub
11. Repeat the process until the radiator is empty

How to Refill Central Heating System

Before refilling the heating system, ensure all the radiator bleed valves and drain-off valves are closed.

Gravity-fed Systems

Vented gravity systems will take a lot longer to refill than a pressurised system.

1. Turn the water supply back on
2. Leave it for half an hour or so to let the tank in the loft fill up with water
3. Bleed all the radiators starting with the lowest first

If the air stops coming out when you are bleeding the radiators, leave it for a while until the tank in the loft is full, giving you more pressure.

Follow the instructions below for how to bleed the radiators correctly.

Pressurised Systems

To refill your pressurised central heating system you must repressurise the boiler via the filling loop.

This is usually under the boiler on a combi boiler, or in the cylinder cupboard if you have a system or conventional boiler.

1. Open the filling loop valves to let the water in until you get to 1.5 bar on the pressure gauge
2. Bleed the radiators starting with the lowest radiators first
3. Hold a towel under the radiator bleed valve
4. Open the bleed valve screw by turning it anti-clockwise with a bleed key
5. Close the bleed screw when the water starts coming out
6. Keep repressurising the boiler when the pressure drops
7. Bleed all the radiators on the lowest floor
8. Move onto the next floor and bleed all the radiators
9. Keep repressurising the boiler and bleeding radiators until all radiators are bled on all floors

Knowing how to refill the central heating system properly is key.

You should always bleed the lowest radiators first. If you bleed upstairs first, the water will come down from the upstairs radiators when you bleed the lower radiators.

How to Clear an Airlock

After draining and refilling a heating system, you can get trapped air causing an airlock in the pipes. This can cause radiators to not get hot or sometimes the boiler not to fire up. You need to get rid of this air by bleeding it out. This can take all day (on certain systems) and be very frustrating.

Radiator Not Heating Up

Here’s how I clear an airlock in radiator pipes when a radiator is not heating up:

1. Turn the heating on
2. Close one valve (TRV or lockshield) on every radiator apart from the radiator(s) not getting hot
3. Wait for the airlocked radiator(s) to get hot
4. Bleed the radiator
5. Open all the radiator valves

Boiler Not Firing Up

If the boiler is not firing up or keeps cutting out, you need to get the air out which can take all day sometimes.

Here’s what I do:

1. Turn the heating on
2. Bleed the radiators
3. Bleed the bleed valve in the cylinder cupboard
4. Bleed the central heating pump
5. Reset the boiler
6. Repeat, repeat, repeat until the boiler stays fired up

Summary

Draining and filling a central heating system can be a very hard job, even for full-time heating engineers, especially on old or large heating systems. Lots of problems can pop up along the way and overcoming them can be very frustrating.

Feel free to ask any questions in the comments below, and I’ll do my best to help. If you found this post helpful, please consider sharing it.

FAQs

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

You only need to turn the water off to drain a central heating system if you have a gravity-fed system with a conventional boiler. With pressurised systems (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?

The time it takes to drain a central heating system depends on the system size, layout, and condition. It can take anywhere from 20 minutes to a few hours or more.

Where is the drain valve on a central heating system?

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