How to drain a radiator without draining the whole system

One radiator or the whole system?

Draining one radiator is the right approach for decorating behind it, replacing it with a similar size, or changing one of its valves once it is off the wall. Both valves hold the rest of the system's water back, so only the radiator itself empties.

Draining the whole system is a bigger job. It is needed for work on the main pipework, a new boiler, a system clean or when a radiator valve will not close. It means emptying every radiator, refilling, bleeding and redosing the system with corrosion inhibitor. On a sealed system it also means repressurising it correctly. If that is what your job needs, it is usually worth having a heating engineer do it.

What you need

Old radiators hold more water than people expect, and it is often black with sludge that stains carpets and grout. Prepare the room properly before you start.

• Two adjustable spanners, one to hold the valve body and one to turn the nut
• A radiator key
• A shallow tray or roasting tin that fits under the valve, and a bucket to empty it into
• Plenty of old towels and a dust sheet or plastic sheet
• A decorator's cap or manual cap for the thermostatic valve, if you have one
• Something to plug or cap the radiator outlets when you move it

Step 1: turn the heating off and close both valves

Switch the heating off and wait for the radiator to cool. On one side there will be the control valve, usually a thermostatic valve with a numbered head. On the other side there is a lockshield valve with a plain plastic cap.

Turn the thermostatic head to its lowest setting. Many heads still allow a little flow at that setting for frost protection, so the safer method is to unscrew the head and fit a manual cap, which closes the valve fully. At the lockshield end, pull the cap off and turn the spindle clockwise with a spanner until it stops, counting the turns as you go. Write the number down. You will reopen it by the same number of turns later so the radiator stays balanced with the rest of the house.

Step 2: release the water

Put towels and the tray under the valve you are going to open first. Hold the valve body steady with one spanner and use the other to loosen the union nut, which joins the valve to the radiator tail. Holding the valve stops you twisting the pipe underneath, which can crack a joint under the floor.

Open the nut only a little at first. Water will start to run. Now open the bleed valve at the top of the radiator a quarter turn; letting air in makes the water flow faster and more evenly. Control the flow with the union nut, empty the tray into the bucket as it fills and keep going until nothing more comes out.

If water keeps flowing well after the radiator should be empty, one of the valves is not holding. Tighten the union nut again straight away and stop. The valve needs attention before the radiator can come off.

Step 3: take the radiator off the wall

Once it is empty, undo the union nut on the other valve. Lift the radiator straight up off its brackets, keeping it level at first. There will still be some dirty water in the bottom, so tip it carefully into the tray on one side, then plug or cover both outlets before carrying it out. Keep the valves closed while the radiator is off; the tails and valve bodies will drip a little, so leave a towel under each one.

Refitting and refilling

Lift the radiator back onto its brackets and line the tails up with the valves. Tighten each union nut hand-tight first, then a little more with the spanners while you hold the valve body. If a union weeps once the water is back in, it may need a fresh seal on the tail thread rather than more force.

Close the bleed valve, then open the lockshield by the number of turns you wrote down and open the control valve fully. The radiator will fill from the system. Bleed the air out at the top as it fills, check both unions for drips, and refit the thermostatic head.

On a sealed system with a combi or system boiler, filling the radiator takes water from the rest of the circuit, so the pressure gauge will drop. Top it back up through the filling loop to the figure in your boiler manual. On an open-vented system, the loft tank tops the system up automatically.

Problems to watch for

Older installations do not always behave as the steps suggest. These are the situations where it is better to stop and get help rather than push on.

• A valve that will not turn, or keeps letting water through when closed
• A union nut that will not move without twisting the pipe
• Microbore pipes that are thin, bent or corroded where they come out of the floor
• Heavy black sludge, which suggests the whole system needs cleaning rather than one radiator
• Pressure that keeps falling after refitting, which points to a leak

When the job is bigger than one radiator

If you are replacing a radiator with a different size or moving it to another wall, the valves and pipework usually need changing too, and the room's heat requirement should be checked. If the radiator was cold along the bottom and full of sludge, a system clean and a magnetic filter may do more good than a new radiator.

Loyal Plumbers replaces radiators and valves, cleans heating systems and drains down and refills whole systems across London and everywhere inside the M25, with 10 vans working 24 hours a day. If a valve will not close, a pipe looks fragile or you would rather not risk the carpet, call us or send an enquiry with the number of radiators involved and what you are planning to do.