How To Drain A Central Heating System
A hose on the drain-off valve and a few open bleed screws are all it takes to empty the heating safely before a repair.
The short answer
To drain a central heating system, switch off the boiler and let the water cool, then clip a hose onto the drain-off valve at the lowest point and run it outside. Open the drain-off with a spanner, then open the radiator bleed valves from the top floor down to let air in. Once the flow stops, close everything and remove the hose.
Draining the heating is the first job before replacing a radiator, moving one or fixing a leak in the pipework. It takes a while, but it means you can cut into pipes and undo valves without water pouring onto the floor. It's also a good chance to flush out some of the sludge and limescale that builds up in older systems.
The photo further down was taken during a radiator replacement at Richard's own house, and it shows the basic setup. A hose goes onto the drain-off valve and runs straight out through the nearest door.
Sealed Or Open Vented System
There's one check to make before you start. A combi or system boiler runs on a sealed system with a pressure gauge on the front, and switching the boiler off is enough before draining.
An older open-vented system has a small feed and expansion tank in the loft that tops up the water automatically. If you don't stop that supply, fresh water carries on flowing in as fast as you drain it out. Turn off the stopcock to the tank, or tie the float arm up to a length of wood laid across the top so the valve stays shut.
What You'll Need
- A hose long enough to reach outside to a drain
- A jubilee clip and screwdriver to hold the hose on
- An adjustable spanner or drain valve key
- A radiator bleed key
- Old towels to catch drips and spills
How To Drain A Central Heating System
Work through these steps in order, and keep a towel under the drain-off to catch the drips as the hose goes on and comes off. Here's how the hose was set up for that radiator swap at Richard's house.
- Switch off the boiler. Turn the boiler off at the controls and at the fused spur so it can't fire up while the system is empty. Give the water time to cool, as draining hot water through a hose risks scalds.
- Find the drain-off valve. It's a small brass valve with a hose connector and a square head, fitted at the lowest point of the system. You'll find one on the pipework near a ground floor radiator or at the bottom of a radiator valve.
- Attach the hose. Push the hose onto the drain-off connector and tighten a jubilee clip over it. If it isn't clipped on firmly, it can slip off once the water starts flowing and empty dirty, black heating water across the floor.
- Run the hose outside. Lead the hose out through a door or window to a drain. The photo above shows it run out through an open door. If your hose doesn't reach, buy a longer one before you start rather than trying to drain into buckets.
- Open the drain-off valve. Turn the square head anticlockwise with an adjustable spanner or drain valve key. A quarter or half turn is enough to get the water moving, and you'll hear it running through the hose.
- Open the bleed valves. Start with the radiators on the top floor and work down, opening each bleed valve with the radiator key. Letting air in speeds up the flow, and you'll hear it being drawn into the radiators as the water level falls.
- Wait for the flow to stop. The time depends on how many radiators you have and the size of the system, so pop outside now and then to check the hose is still running. Once it stops, the system is empty.
- Close up and remove the hose. Close the bleed valves, then tighten the drain-off valve. Water will still be sitting in the hose, so lift the end and walk it straight outside before you take it off.
Refilling The System Afterwards
Once your repair is done, check the drain-off and every bleed valve are closed. On an open-vented system, turn the supply back on to the loft tank and it'll refill on its own. On a sealed system, open the filling loop under the boiler and let the pressure build to the figure in your manual, which our guide to repressurising a boiler covers step by step.
Bleed each radiator as it fills, starting downstairs, until water rather than air comes out. On a sealed system the pressure drops each time you bleed, so top it up again before you fire the boiler. Run the heating, check every joint you've worked on for drips, then bleed once more when it's up to temperature.
Adding Inhibitor And Protecting The Boiler
Draining a system washes out most of the corrosion inhibitor, so add a fresh dose when you refill. On a sealed system it goes in through a radiator bleed point or a dosing port on the filter, and on an open-vented system you pour it into the loft tank.
If you haven't got one fitted yet, now is a good time to add a magnetic filter for central heating systems, as it catches the sludge that drains and refills stir up. A pressure gauge that keeps dropping after refilling points to a joint that isn't quite tight, and our guide on why a boiler keeps losing pressure helps you find it.
What People Also Ask Us
Do I need to drain the whole system to change a radiator?
You don't always need to drain the whole system to change a radiator. If both valves shut off fully, you can isolate the radiator and drain just that one into a tray. If a valve won't close, is leaking or needs replacing, or you're altering pipework, the whole system has to be drained first.
Where is the drain-off valve on a central heating system?
The drain-off valve sits at the lowest point of the heating system, which is on the ground floor in most homes. Look on the pipework near a downstairs radiator, at the bottom of a radiator valve, or near the boiler. It's a small brass valve with a ridged hose connector and a square head you turn with a spanner.
Should I add inhibitor after draining central heating?
Yes, add a corrosion inhibitor every time you drain and refill the heating, as draining flushes most of the old inhibitor out. Fresh inhibitor slows rust and sludge forming inside radiators, pipes and the boiler. Follow the dose on the bottle for the number of radiators you have and add it while you refill.
Why does my heating make gurgling noises after draining?
Gurgling after draining and refilling comes from air still trapped in the radiators and pipes. Bleed each radiator again with the heating running until only water comes out, then check the boiler pressure and top it up if it's fallen. A few rounds of bleeding over the first couple of days clears most of it.
What's the biggest mistake to avoid when draining a heating system?
Leaving the hose loose on the drain-off. Without a jubilee clip holding it, the hose can pop off under the flow and send black heating water across your floor. The other classic on an open-vented system is forgetting the loft tank, which keeps topping the system up as fast as you drain it.
Why open the radiator bleed valves from the top floor down?
Water can only drain as fast as air gets in to replace it. Opening the top floor bleed valves first lets air in at the highest points of the system, so the water runs out of the drain-off more freely, and you'll hear the air being drawn into the radiators as the level drops.
How do I know when the system is fully drained?
When the hose outside stops running, the system is empty. Bigger systems with more radiators take longer, so don't assume it's finished until you've checked the end of the hose. Then close the bleed valves, tighten the drain-off and lift the hose end before pulling it off, as there's still water sitting in it.
Can I drain into buckets if my hose won't reach outside?
Buy a longer hose instead. Ferrying buckets of dirty heating water through the house is slow and messy, so run a hose from the drain-off valve out through a door or window to a drain. Let the water cool first as well, because draining hot water through a hose risks scalds.
Richard has spent years renovating properties and turning them into rentals and Airbnbs. Like a lot of people he started out as a beginner, and these days there isn't much DIY he won't take on.