How to Drain a Central Heating System or a Single Radiator
Step-by-step guide to draining down a sealed or open-vented central heating system, or taking off just one radiator, plus how to refill, bleed and repressurise it afterwards without airlocks or leaks.
To drain a central heating system, switch the boiler off and let everything cool, isolate the water feed (close the filling loop on a sealed system, or shut off the feed to the loft header tank on an open-vented one), open every radiator valve fully, attach a hose to the drain-off valve at the lowest point and run it to an outside drain, then open the drain-off and the bleed valves, starting upstairs. To drain one radiator, close both its valves, counting the turns on the lockshield, and undo the union nut into a tray.
Key Takeaways
- Always drain a cold system with the boiler switched off, including at the fused spur.
- Check whether your system is sealed (pressure gauge, no loft tank) or open-vented (small header tank in the loft) before you start.
- To remove one radiator you rarely need to drain the whole system. Close its two valves instead.
- When refilling, bleed from the bottom of the house upwards and add inhibitor to protect the system.
- Draining down an old system can disturb joints and valves, so check everything for weeps after refilling.
To drain a central heating system, turn the boiler off and let it cool, shut off the water that feeds the system, open all the radiator valves, run a hose from the drain-off valve at the lowest point to an outside drain and open it, then open the radiator bleed valves from the top floor down to let air in. If you only need to take off one radiator, you don't have to drain the lot: close the two radiator valves and drain that radiator into a tray. Below are the steps for both, and how to refill and repressurise afterwards.
Why you might need to drain the system
Autumn is when most people find out their heating needs work, and a lot of those jobs start with a drain-down. Common reasons include:
- Replacing a radiator, a radiator valve or a section of pipe.
- Fixing a leak on a joint or valve that can't be isolated.
- Cleaning out sludge, either as a simple drain and refill or before a power flush.
- Moving or adding radiators during building work.
- Leaving a property unheated over winter, where draining reduces the risk of frozen and burst pipes.
If your aim is to get rid of sludge because radiators are cold at the bottom, a drain and refill on its own rarely shifts it. Read our guide on the signs your radiators need a power flush before you decide.
Sealed or open-vented?
How you isolate and refill depends on which type of system you have.
| Sealed system | Open-vented system |
|---|---|
| Combi or system boiler with a pressure gauge | Usually a regular boiler with a hot water cylinder |
| No small header tank in the loft for the heating | Small feed and expansion tank in the loft, as well as the larger cold water tank |
| Filled from the mains through a filling loop | Filled from the header tank through a float valve |
| Isolate by making sure the filling loop is closed | Isolate by shutting off the supply to the header tank's float valve, or tying up the float arm |
How to drain the whole system
What you'll need
A hose long enough to reach outside, a jubilee clip, a radiator key, an adjustable spanner or drain-off key, old towels and something to catch drips. The water that comes out is usually black or brown and stains, so protect carpets.
Switch the boiler off and let it cool
Turn it off at the controls and at the fused spur, and give the system a couple of hours to go cold. Heating water can be very hot, and draining a hot system risks scalds.
Isolate the water feed
On a sealed system, check both valves on the filling loop are closed. On an open-vented system, turn off the isolating valve on the pipe to the loft header tank, or tie the float arm up with a piece of wood across the tank.
Open every radiator valve
Turn thermostatic valves to maximum and open any wheelhead valves fully so water can drain from every radiator. Leave lockshield valves as they are.
Attach the hose to the drain-off valve
Find the drain-off at the lowest point, clip the hose on firmly and run it to an outside drain that is lower than the valve. Heating water containing inhibitor should go to a foul drain, such as a gully that takes kitchen waste, not to a surface water drain or the garden.
Open the drain-off valve
Turn the square head slowly with a spanner. Water should start flowing. If the valve is stiff, don't force it, because old drain-offs can snap or start leaking from the spindle.
Open the bleed valves, top floor first
As the level drops, open the bleed valves on upstairs radiators to let air in, then work down through the house. The flow at the hose will speed up as air gets in.
Wait for it to stop
When the hose stops running, the system is drained as far as it can be from that point. Some water will remain in pipes and radiators below the drain-off.
How to drain a single radiator
To take off one radiator for decorating, replacement or a valve repair, you usually only need to isolate that radiator.
Turn the heating off
Let the radiator go cold.
Close both valves
Turn the thermostatic valve to off, or the wheelhead fully clockwise. Remove the cap on the lockshield and close it with a spanner, counting the turns so you can reopen it by the same amount later. That keeps the system balanced.
Loosen one union nut
Put a tray and towels under the valve and slowly loosen the nut between the valve and the radiator. Open the bleed valve a little to let the water run out faster.
Lift the radiator off
Once it has drained, undo the other union, block the tails with cloth and lift the radiator off its brackets, keeping it upright so the last of the dirty water doesn't pour out.
If either valve won't shut off completely, water will keep coming. In that case close the union again and either drain the system down or get a plumber to fit a new valve. A weeping valve is often the same fault behind a leaking radiator.
Refilling and repressurising
- Close the drain-off valve and every bleed valve, and check all unions you undid are tight.
- On an open-vented system, restore the feed to the header tank and let it fill. On a sealed system, open the filling loop slowly and watch the gauge.
- Bleed the radiators, starting downstairs and working up. Keep an eye on the gauge on a sealed system, because bleeding lowers the pressure.
- Bring a sealed system to the pressure given on the boiler, typically 1 to 1.5 bar cold, then close the filling loop fully. Our guide on how to repressurise a boiler covers this step in detail.
- Add inhibitor, following the dose on the bottle for the size of the system.
- Switch the boiler back on, run the heating and bleed again once the system is hot and settled.
- Check every joint you disturbed, and the drain-off, for drips over the next day.
Some air almost always stays trapped after a refill. Gurgling, radiators that stay cold at the top or a boiler that locks out on start-up are signs it needs more bleeding. If a radiator won't warm up at all, see our guide to a radiator not heating up.
When to leave it to a heating engineer
Draining a system is manageable for a competent DIYer, but it is worth getting help if the drain-off valve is seized, the system has no obvious drain point, the pipework is old and you are worried about disturbing joints, or the job involves the boiler, the cylinder or the gas supply. Old systems that have sat untouched for years sometimes start weeping at valves after being drained and refilled, so it can make sense to combine a drain-down with any repair work that's due.
We regularly drain down systems as part of radiator replacement, radiator repairs and power flushing. Our labour is £95 an hour, the £60 call-out is waived when the job runs over 2 hours, and we confirm the price before starting.
Need a radiator changed or a system drained?
Tell us how many radiators you have and what the drain-down is for. We'll give you the options and confirm the price before any work starts.
📞 Call 020 7870 3200Or read about our radiator replacement and central heating repairs.
Frequently Asked Questions
How long does it take to drain a central heating system?
Draining itself usually takes somewhere between half an hour and a couple of hours, depending on the size of the house, the number of radiators and how freely the drain-off valve flows. Allow extra time for letting the system cool first and for refilling and bleeding afterwards.
Do I need to drain the whole system to replace one radiator?
Usually not. Turn off the thermostatic or wheelhead valve and the lockshield valve at each end of the radiator, then undo the unions and drain just that radiator into a tray. You only need a full drain-down if one of those valves won't close properly or is being replaced.
Where is the drain-off valve on a central heating system?
It is normally at the lowest point of the pipework, often on the pipe at the bottom of a downstairs radiator, near the boiler or by the hot water cylinder. It looks like a small brass valve with a ribbed spout for a hose and a square head that you turn with a spanner or drain key.
Can I drain a combi boiler system myself?
You can drain the radiator circuit yourself using the drain-off valve and radiator bleed valves, with the boiler switched off. Do not open any drain points inside the boiler casing. When refilling, bring the pressure up to the level on the boiler's gauge, typically 1 to 1.5 bar cold, and close the filling loop fully.
Do I need to add inhibitor after draining?
Yes. Draining removes the inhibitor that protects the radiators and boiler from corrosion. Add a correctly dosed inhibitor when you refill, through a radiator bleed point on a sealed system or through the header tank on an open-vented one.
Sources and further reading
Source links checked 27 September 2026. Scheme rules and manufacturer instructions can change; use the current official guidance for your account or exact appliance.