How to Drain a Radiator Without Draining the System

You almost never need to drain the whole heating system to deal with one radiator. If both of its valves shut off properly, that radiator can be emptied on its own in under an hour, with the boiler, the other radiators and the hot water left alone. The whole job depends on those two valves actually closing, so most of this guide is about checking that before you undo anything.

Why you would drain a single radiator

Usually to take it off the wall for decorating, to change a valve that is weeping from its body or tail, or to replace a radiator that has rusted through. Autumn is when most people find out they need to: the first proper heating cycle warms joints that have sat cold since spring, and a valve that was dry in August starts weeping in October. If that is why you are here, radiator leaking from the valve explains which part of the valve is usually at fault, because some weeps can be dealt with without draining anything.

What you need

• Two adjustable spanners, or one spanner and a grip wrench to hold the valve body still.
• A radiator bleed key.
• A shallow tray that fits under the valve: a roasting tin or a wallpaper tray works. A bucket will not fit under most tails.
• A jug to empty the tray, and old towels. The water is black with sludge and stains carpet permanently.
• The manual cap for the thermostatic valve, often called the decorator's cap. If you do not have one, read the next section before you go further.

Before you start: make sure both valves really shut

Every radiator has two valves at the bottom. One is the valve you turn to set the temperature, usually a thermostatic valve with a numbered head. The other is the lockshield, a plain valve under a plastic cap, which was set when the system was balanced and is not meant to be touched day to day. Radiator valve types has pictures of each.

The thermostatic head turned to zero or the snowflake is not a shut-off. That setting is frost protection. It only closes the valve until the room gets cold, and on a worn valve the pin underneath may not seal at all. To shut it properly, unscrew the head and fit the manual cap in its place, then turn the cap fully clockwise.

Count the lockshield turns. Pull off the plastic cap and close the lockshield with a spanner or small grip, clockwise, counting quarter turns until it stops. Write the number on masking tape and stick it to the pipe. When the radiator goes back on, you reopen it the same number of turns, or that radiator will take more than its share of hot water and starve rooms further along the circuit. Lockshield valve explains the setting in more detail.

Now test the seal before you drain. Open the bleed screw a quarter turn with a cloth under it. On a sealed system, a short hiss and then a trickle that stops is what you want. If water keeps coming steadily after a minute, one of the valves is passing and you should not open the union nut. Close the bleed screw and skip to when the valves will not close.

How to drain the radiator, step by step

1. Heating off, and cold. Switch the heating off at the programmer and give the radiator time to go cold. System water can run hot enough to scald, and it comes out under pressure the moment the union moves. Check with the back of your hand.
2. Both valves shut, as above: manual cap on the thermostatic end, lockshield closed and counted.
3. Tray and towels down under whichever valve gives you most room.
4. Hold the valve body and slacken the union. The union is the large nut joining the valve to the radiator tail. Put one spanner on the valve body to stop it turning, and use the other to undo the union anticlockwise, looking at it from the radiator side. Hold the valve still: if it twists, you strain the pipe under the floor, and that joint is the one you will not see leaking.
5. Crack it, do not remove it. A turn or so is enough. Water will start to run from the joint into the tray. Control the flow with the nut: tighten it slightly when the tray is getting full, empty the tray, then loosen it again.
6. Open the bleed screw. Once the flow slows, open the bleed screw at the top so air can get in behind the water. The radiator drains much faster with air in it. Keep emptying the tray.
7. Wait for it to stop dripping. Some water always stays in the bottom of the panel. If you are lifting the radiator off, undo the other union too and tip it towards the tray before you carry it anywhere, with two people.
8. Protect the open valves with a bag and tape, so grit stays out and a slightly passing valve does not drip on the floor.

When the valves will not close

This is where most single-radiator jobs go wrong. Old valves seize open, lockshields strip their spindles, and thermostatic pins stick. A valve that is still passing water means the union cannot come off without the system emptying through it.

What you find	What it means	What to do
The thermostatic valve has no cap, and the head will not turn it off	The head cannot close the valve, or the pin is stuck	Buy a manual cap for that make of valve. Do not rely on the head at zero.
The lockshield turns and turns without stopping	The spindle is stripped	The valve cannot isolate. It needs replacing, which means draining the circuit or freezing the pipe.
The bleed screw keeps running after a minute	One valve is passing	Close the bleed screw. Do not undo the union.
Water weeps from a spindle as you turn it	The gland packing is worn	Stop turning. A gland that has been moved after years still often weeps. See radiator valve leaks.

If a valve will not close, you have two options. You can drain the whole heating circuit, which is a longer job with its own steps and is covered in how to drain down your system. Or an engineer can freeze the pipe on each side of the valve with a pipe freezing kit, which forms a plug of ice inside the pipe so the valve can be changed while the rest of the system stays full. That is what we do when a customer wants one valve changed without the whole system being drained and refilled.

Sealed and open-vented systems behave differently

On a sealed system, the kind with a pressure gauge on or under the boiler, the pressure will drop a little when you drain a radiator, because some water has left. That is expected. Once the radiator is back on and refilled, you top the pressure up through the filling loop, back to 1.0 to 1.5 bar when cold for most boilers, and close and disconnect the loop afterwards. How to repressurise a boiler goes through it.

On an open-vented system, fed from a small tank in the loft, that tank refills whatever you drain. A valve passing even slightly means water keeps coming, so be sure both valves hold before you open anything.

Putting it back and refilling

1. Refit the radiator and tighten both unions, holding the valve bodies still as before. If a union weeps once refilled, a quarter turn more usually stops it. If it still weeps, the joint needs remaking rather than more force.
2. Reopen the lockshield by the number of turns you wrote down, then open the thermostatic valve and refit its head.
3. Bleed until water, not air, comes out, then top up a sealed system and disconnect the filling loop.
4. Run the heating and check every joint you touched with dry kitchen paper, hot and again the next morning.

If the radiator stays cold at the bottom afterwards, the lockshield was probably not reopened far enough. Radiator not heating up works through the other causes, and how to bleed a radiator covers air left behind after refilling. If you took the radiator off for decorating and a joint starts weeping days later, leaks after a radiator is taken off to decorate explains why the old olive is usually the cause.

What not to do yourself

• Anything inside the boiler casing. That is gas work for a Gas Safe registered engineer.
• A towel rail with an electric element, unless the element is switched off and isolated at its spur first.
• A valve or union you have to force. Old brass snaps, and a snapped valve on a full system cannot be closed again.

When it is quicker to call someone

If a valve will not shut off, a union has started weeping and more tightening is not stopping it, or the pressure keeps falling in the days after you refill, it is time to hand it over. Pressure that drops again and again once the radiator is back on is water leaving the system somewhere, and that is not always at the radiator you touched.

We change weeping radiator valves and radiators across London, freezing the pipe where that saves draining the system, and we remake the joints rather than tighten them until they hold for a week. We charge £150 per hour, agreed before we attend, with no call-out fee. If the leak turns out to be somewhere under the floor rather than at the valve, we find it before anything comes up, and it is no find, no fee. See radiator and valve leak repair, or check the pricing page before you call.