How to Safely Drain a Household Radiator Without Flooding the Floor

To drain a single radiator safely, switch the heating off and let it go completely cold, close both valves and write down how many turns you closed the lockshield, then open the bleed screw to let air in while you release the water at the valve union into a container. Take it slowly, because what comes out of a central heating radiator is usually black, oily and very good at staining carpet.

Draining one radiator is not the same as draining the system

People use the two phrases interchangeably and they are very different jobs. Isolating and emptying one radiator so you can take it off the wall, decorate behind it or swap a valve is straightforward household DIY. Draining the entire system through the drain-off cock is a bigger job that ends with refilling, adding inhibitor and rebalancing everything. This guide covers the single radiator version, which is what most people actually need.

What you need before you start

• Two adjustable spanners, so you can hold the valve body while you turn the nut
• A radiator bleed key that fits your bleed screw
• A shallow tray that fits under the valve, plus a bucket to decant into
• Old towels and a plastic sheet or bin bags for the floor
• Rubber gloves, because the water contains iron oxide sludge and inhibitor

Step by step

1. Turn the heating off and wait. Not warm, cold. Hot system water will scald you and it is under pressure in a sealed system. Give it an hour at least, longer for a big cast panel.
2. Close the thermostatic valve. Turn the TRV head fully clockwise to the off position or zero mark. On old TRVs the pin can stick, so if you are removing the radiator entirely, fit a decorator's cap in place of the head to be certain it is shut.
3. Close the lockshield and count. Pull off the plastic cap on the other end, put your spanner on the spindle and turn it clockwise until it stops, counting the turns and part turns as you go. Write the number down. That number is your balance setting and putting it back exactly restores the flow rate through this radiator when you reopen it.
4. Protect the floor. Sheet under the valve end, towels around it, tray in position. The stain does not wash out.
5. Crack the union nut. That is the larger nut joining the valve to the radiator tail. Hold the valve body still with the second spanner so you do not twist the pipe, then loosen the nut a quarter turn until water weeps out. Do not undo it fully yet.
6. Open the bleed screw. Once water is flowing, put the bleed key in the top corner valve and open it a little. This lets air in behind the water so it drains properly instead of glugging. If you open the bleed screw before the bottom is open, you achieve nothing.
7. Drain in stages. Let the tray fill, close the union nut to stop the flow, empty it into the bucket, then reopen. Repeat until the flow stops. A single panel holds several litres.
8. Finish the job. If the radiator is coming off, undo both union nuts fully, close the bleed screw so the residue stays inside, then lift it off the brackets keeping it level. Get help, there is always water left in the bottom.

Refilling and getting the pressure back

Reversing the process matters as much as the drain. Reconnect the union nuts and nip them up firmly but not brutally, since the seal comes from the olive or fibre washer rather than raw force. With the bleed screw closed, open the TRV end first and let the radiator fill, then open the lockshield by exactly the number of turns you wrote down.

Bleed the radiator until water appears at the screw, then close it. Check both unions with a dry tissue rather than your eye, because a weep the size of a pinhead shows on paper long before you can see it.

On a sealed system with a combi or system boiler the pressure gauge will have dropped. Top up using the filling loop until it reads roughly 1.0 to 1.5 bar cold, then close both filling loop valves properly. Run the heating, then bleed once more once it has cooled, since fresh water carries dissolved air that comes out on the first heat cycle.

Common mistakes worth avoiding

• Not counting the lockshield turns. Reopen it fully and that radiator hogs flow from the rest of the house, so distant rooms run cool.
• Undoing the union nut fully first go. You get a surge you cannot stop, and the tray is never big enough.
• Twisting the pipework. Always counter-hold the valve body. Joints behind the skirting are easy to disturb and expensive to fix.
• Forgetting inhibitor. Every litre you throw away takes corrosion inhibitor with it, so top the system up afterwards if you have drained a lot.

When to stop and call someone

If the TRV pin is seized open and the radiator keeps filling with both valves shut, or the valve body itself is weeping rather than the union, stop there. The same goes for old imperial pipework where nuts round off on sight. Draining a radiator is well within normal DIY, but a snapped valve on a full system becomes an emergency in about ten seconds.