How to Drain a Radiator, and When the Whole System Needs It

To drain a single radiator you do not need to empty the whole heating system. With the heating off and the radiator cool, close the valves at both ends, put a tray and towels underneath, then loosen the union nut on one valve and open the bleed screw so the radiator empties into the tray. A full system drain-down is only needed when a valve will not shut off, pipework is being altered, or the system water itself needs replacing, and it has more steps on the refill.

Why You Might Need to Drain a Radiator

• Taking a radiator off the wall to decorate or re-plaster behind it.
• Replacing a radiator with a new one, or a different size.
• Replacing the valve tail or resealing a joint that is leaking. See our guide to a leaking radiator.

You do not need to drain anything to bleed a radiator, adjust a TRV or balance the system.

What You Need

• Two adjustable spanners, one to hold the valve body and one to turn the nut.
• A lockshield key or small spanner, and a radiator bleed key.
• A shallow tray that fits under the valve, plus a bucket to empty it into.
• Old towels and dust sheets. System water is often black and it stains carpets and boards permanently.

A typical radiator holds several litres of water, and larger or double-panel radiators hold considerably more, so plan to empty the tray several times.

Isolating and Draining One Radiator

1. Heating off, radiator cool. Give it time to cool completely.
2. Close the control valve. Turn the TRV to its off setting or a manual valve fully clockwise.
3. Close the lockshield. Remove the cap and turn the spindle clockwise, counting the turns and writing the number down. You need it to restore the setting later. Our guide to lockshield valves explains why.
4. Loosen the union nut. On the lockshield side, hold the valve body still with one spanner and slowly loosen the nut joining the valve to the radiator with the other. Water will start to run into the tray.
5. Open the bleed screw. Opening it lets air in at the top so the radiator drains steadily rather than glugging.
6. Control the flow. Nip the nut back up whenever the tray is nearly full, empty it, and carry on.

Once it stops running, loosen the union nut on the other side too. Lift the radiator off its brackets and tip it to drain the last of the water, keeping the open tails pointing up. If it is going back on, plug or tape the tails so the sludge left inside does not drip on the way out of the room.

If water keeps running after the radiator has emptied, one of the valves is passing, meaning it no longer shuts off fully. Tighten the union nut back up and stop there. You will need to drain the system or have the pipe frozen to go further.

Putting It Back and Refilling

1. Hang the radiator, line the tails up with the valves and tighten both union nuts, holding the valve bodies still.
2. Close the bleed screw, then open the control valve fully.
3. Open the lockshield by the same number of turns you counted, not fully.
4. Bleed the radiator as it fills, then check both joints for weeping.
5. On a sealed system, check the boiler pressure gauge and top it up through the filling loop. Our guide to repressurising the system explains how.

When the Whole System Needs Draining

A full drain-down is needed when:

• A radiator's valves will not isolate it and the pipe cannot be frozen.
• Pipework is being moved, extended or replaced, or radiators are being added.
• The system water is badly contaminated and is being cleaned and treated, for example alongside a power flush.

The system is emptied through a drain-off cock at a low point, usually a small valve on a pipe near the floor, with a hose run outside. On an open-vented system the feed and expansion tank in the loft has to be stopped from refilling first. On a sealed system the pressure falls to zero as it empties. Radiators are opened at the bleed screws, highest first, to let air in so the water drains fully.

Refilling after a full drain-down

This is where most of the risk sits. Water has to be reintroduced slowly, every radiator and high point bled, the pump cleared of air, and the pressure set correctly. Airlocks in upstairs circuits are common and can leave whole rooms cold. The fresh water also needs a correct dose of corrosion inhibitor. The British Standard for heating system water treatment, BS 7593, expects this, and boiler manufacturers' installation instructions generally make proper water treatment a condition of their warranty.

What Can Go Wrong

• Seized or corroded union nuts. Forcing them can twist the pipe or snap the valve tail inside the radiator.
• Microbore pipe. The thin pipe used in many 1970s and 1980s homes kinks easily when a radiator is pulled out from the wall.
• Drain-off cocks that will not reseal. Old drain-offs often drip once they have been opened.
• Black water everywhere. Very dark water is sludge. It is also a signal that the rest of the system is carrying it, which is worth dealing with while it is drained. Our guide to the signs a system needs a power flush explains what to look for, and a magnetic filter helps keep it clean afterwards.

Do It Yourself, or Book an Engineer?

Draining a single radiator whose valves shut off properly is a manageable job if you are careful and prepared for the mess. A full drain-down, a valve that will not isolate, or a radiator being replaced with a different size is better handed to an engineer, because the refill, the inhibitor and the rebalancing are what decide whether the system works properly afterwards.

We replace radiators and valves across London through our radiator installation service, and the radiator replacement cost guide explains what affects the price. Book a free survey and we will give you a fixed quote before any work starts.