How to drain a central heating system, the easy way
By Lee Devlin, Director, Homecure Plumbers Last updated 13 October 2026
To drain a central heating system, turn the boiler off and let it cool, fix a hose to the drain-off valve at the lowest point and run it outside, then open the valve and the radiator bleed valves from the top of the house down. To refill, close everything, top up the water, bleed the radiators and add inhibitor. You might need to drain down to replace a radiator or valve, fix a leak or before a power flush.
Sealed or open-vented system?
Check which you have first, as the refill is different.
Sealed system (combi and system boilers): there is a pressure gauge on or under the boiler and a filling loop. There are no tanks in the loft for the heating.
Open-vented system (conventional boilers): there is a small feed and expansion tank in the loft that tops up the heating, usually next to the large cold water tank.
What you need
A garden hose long enough to reach outside or to a drain
A radiator key
An adjustable spanner, or a drain valve key
Towels and a small bowl for drips
Central heating inhibitor for the refill
How to drain the system
Switch off the boiler and heating controls, and isolate the electrical supply at the fused spur or wall switch, so the pump can’t run and the boiler can’t fire while the system is empty. Let the radiators cool completely.
Open-vented systems only: stop the feed and expansion tank in the loft refilling, either by closing its supply valve or by tying the float arm up.
Find the drain-off valve. It is a small valve with a hose fitting, usually at the lowest point of the system, on a downstairs radiator or on the pipework near the boiler.
Push the hose on firmly and run it outside to a drain. It must run downhill all the way.
Open the drain-off valve with a spanner or drain valve key. Water will start to run out.
Open the bleed valve on the radiators upstairs, then downstairs. Letting air in makes the system drain faster and more completely.
When the water stops, the system is drained. Close the drain-off valve before you take the hose off.
A drain-off valve on the pipe below a radiator. Push the hose onto the ridged outlet, then open the valve with a spanner or drain valve key.
Dirty or black water is a sign of sludge in the system. That is worth dealing with before it blocks the pump, valves or radiators.
Expert comment
“If you open a drain-off valve and nothing comes out, nine times out of ten the washer has stuck solid to the seat or the pipe is blocked with sludge. Don’t be tempted to poke around inside the valve nozzle with a screw or pliers to free it up. If that blockage suddenly clears or the washer snaps, you can unleash a dirty flood with no easy way to stop it. A seized or blocked drain valve is strictly a job for a heating engineer who has the proper kit to clear or replace it safely.”
Expert comment by Lee Devlin, Director, Homecure Plumbers
How to refill
Close the drain-off valve tightly and make sure every radiator bleed valve is fully shut.
Sealed system: open the filling loop slowly until the pressure gauge reads around 1.5 bar, then shut it off. See how to repressurise a boiler.
Open-vented system: turn the supply back on to the feed and expansion tank in the loft, or untie the float arm, and let the tank fill up.
Add your central heating inhibitor. On a sealed system, dose it through a radiator bleed point or via the magnetic filter. On an open-vented system, pour it straight into the loft feed tank.
Start bleeding the radiators, working from downstairs up to the top floor. Listen for air hissing and close each valve as soon as a steady stream of water appears.
Keep an eye on the pressure (sealed systems): as you bleed the air out, the boiler pressure will drop quickly. Don’t wait until you’ve bled the whole house. Go back to the boiler every couple of radiators and top the filling loop back up to 1.5 bar, so you don’t draw more air into the system.
Once all the radiators are bled and the pressure is sitting steady between 1 and 1.5 bar, turn the heating on and let it come up to temperature.
Check for any cold spots across the tops of the radiators, and give them a final quick bleed once the system has cooled down.
The bleed valve sits at the top corner of the radiator. Open it with a radiator key until water comes out, then close it.
Call us instead if
the drain-off valve will not open, or will not close fully afterwards
the boiler shows a fault code after refilling, or the pressure keeps dropping
you find a leak once the system is refilled
you would rather not do it yourself
Our central heating engineers can drain and refill the system for you, and our leak detection team can trace a leak that keeps the pressure dropping. Repairs from £95 + VAT for the first hour, including diagnosis (or a fix if it can be done within the first hour with no parts needed), no callout charge. Prices apply during office hours; parking or the congestion charge may be added.
