How to replace a radiator with a new one

Measure before you buy

Most failed radiator swaps go wrong in the shop, not on the wall. Three measurements decide whether a new radiator will simply hang in place of the old one.

Pipe centres are the distance between the middle of one pipe coming up from the floor and the middle of the other. Measure that, not the width of the radiator, because the valves and tails add length at each end. Then measure the height from the floor to the centre of the valve connections, and note the depth of the old radiator and how far it sits off the wall.

Choosing a radiator that fits

Radiators are sold by height, width and type. Type 11 has one panel with fins behind it, type 21 has two panels with one set of fins, and type 22 has two panels with two sets of fins. A type 22 gives much more heat than a type 11 of the same size, which is useful when a room has always been cold but means it sits deeper off the wall.

Some manufacturers sell replacement ranges designed to match the pipe centres of older imperial radiators, which can save moving pipes in an older London house. If you cannot get an exact match, a small difference can sometimes be taken up with extended tails or offset valves. Larger differences need the pipework altered, which is a job for a plumber.

Check the heat output, listed in watts or BTU, against the room. If the old radiator struggled, go up a type or a size rather than copying it. Our guide to choosing radiator size and output explains how to estimate what a room needs.

Changing from a horizontal to a vertical radiator is more than a swap. Our guide to vertical vs horizontal radiators explains what else is involved.

What you need

A new radiator does not usually come ready to hang. It comes with blank ports that you fit yourself, and the brackets rarely match the old ones.

• The new radiator with its brackets, bleed valve and blanking plug
• Two new radiator tails, or the tails supplied with new valves
• PTFE tape or a thread sealant suitable for heating systems
• Two adjustable spanners, a radiator key and a hex key or tail tool
• Drill, spirit level, tape measure and fixings suitable for your wall
• A shallow tray, towels and a dust sheet
• Central heating inhibitor, if you drain more than the one radiator

Step 1: drain and remove the old radiator

Turn the heating off and let it cool. Close the thermostatic valve (fitting a manual cap is more reliable than turning it to zero) and close the lockshield, counting the turns so you can set it back later. Loosen the union nut on one valve while holding the valve body with the other spanner, open the bleed valve a little, and let the radiator empty into the tray. Our guide on how to drain a radiator goes through this part in detail.

Once it is empty, undo the second union, lift the radiator straight up off the brackets and tip the last of the dirty water into the tray. Plug the ports before you carry it through the house, because what is left inside will stain carpets.

If water keeps running from a closed valve, retighten the union and stop. That valve is not holding, and the job now needs the system drained or the valve changed first.

Step 2: prepare and hang the new radiator

Take the old brackets off the wall. Wrap PTFE tape round the threads of the new tails, the bleed valve and the blanking plug, or use sealant, and screw them into the new radiator: the tails in the two bottom ports, and the bleed valve and blanking plug in the top two.

Hold the radiator in place with the tails lined up with the valves and mark the bracket positions, or measure them from the radiator. Fix the brackets level and firm, with fixings suited to the wall. Plasterboard needs heavy-duty cavity fixings or a timber behind it, since a full radiator is much heavier than an empty one. Hang the radiator and check it sits level, or very slightly higher at the bleed end so air collects there.

Step 3: connect, fill and bleed

Line up the valves with the tails and tighten each union nut by hand first, then with a spanner while you hold the valve body. Open the lockshield by the number of turns you counted and open the control valve. The radiator fills from the system. Bleed the air out at the top until water appears, and close the bleed valve.

Check both unions and the plug for drips while the radiator fills, and again once the heating is on and hot. On a sealed system, the pressure gauge will drop as the new radiator fills, so top it up through the filling loop to about 1 to 1.5 bar cold, or the figure in your boiler manual. An open-vented system with a loft tank refills itself.

When a swap turns into a bigger job

A like-for-like swap is a reasonable job for a competent DIYer. These are the situations where it usually is not.

• The pipe centres do not match and the pipes need moving or extending
• The valves will not close, or leak when closed, so the system has to be drained
• You want the radiator in a different position on the wall or in another room
• The old radiator is cast iron, which is very heavy and often has imperial fittings
• The water coming out is black, which points to sludge in the whole system
• You are changing several radiators at once, when draining and refilling the system properly makes sense

Having a radiator replaced for you

Our engineers replace single radiators and whole sets, alter pipework when the sizes do not match, change valves at the same time and leave the system refilled, bled and treated. If the new radiator is part of a bigger plan, such as a new boiler or moving a radiator, it is usually cheaper to do it in one visit.

Loyal Plumbers has 10 vans working 24 hours a day across London and everywhere inside the M25. Our call-out charge is £60. When you get in touch, send the measurements from the table above, a photo of the old radiator and its valves, and whether you already have the new radiator.