How to Replace a Radiator: What the Job Involves, Step by Step

To replace a radiator, you isolate it at both valves, drain it, undo the two union nuts, lift it off its brackets, fit the valve tails and bleed screw to the new radiator, hang it, reconnect, then open the valves, bleed out the air and check the system pressure. On a like-for-like swap, where the new radiator has the same pipe centres as the old one, that is a morning's work with no pipework changes. Most of the difficulty sits either side of that sequence: choosing a radiator that fits the pipes and the room, and dealing with what the old radiator tells you about the water in the rest of the system.

Before You Start: Is Replacement the Right Answer?

Replace a radiator when it is leaking from the panel, badly corroded, or the wrong size for the room. If it is only cold in places, a new one may not fix anything.

A radiator that is cold along the bottom is usually full of sludge, and a new radiator on the same dirty water will go the same way. A radiator cold at the top usually needs bleeding. A radiator that stays cold while the others heat is often a stuck valve or an unbalanced system. In those cases cleaning, bleeding or balancing is the cheaper fix, and we will say so. Replacement makes sense when the panel has pinholed, the seams are weeping, it is visibly rusting through, or the room has changed and the radiator no longer keeps it warm.

Choosing a Radiator That Fits the Pipes and the Room

The new radiator needs to match two things: the distance between the pipes, and the heat the room needs. Get both right and the swap is simple; get either wrong and it becomes a bigger job or a cold room.

• Pipe centres. Measure the distance between the centres of the two pipes where they come out of the floor or wall, not the width of the radiator. A new radiator with valve connections at the same spacing drops straight onto the existing pipes. Radiators fitted before metric sizes became standard often have spacings that are no longer made, which is common in older London homes.
• Output. Check the heat output in watts or BTU against what the room needs, not just against the old radiator. Rooms change: new windows, insulation or an extension alter how much heat is required. Our guide to radiator sizing and BTU works through the calculation.
• Type. A double-panel radiator gives out much more heat than a single panel of the same size, so you can often gain output without a bigger footprint.

If the spacing does not match, there are three routes: extendable or offset valve tails that bridge small differences, altering the pipework under the floor, or choosing a different size that does match. Small differences are routine; large ones mean pipe alteration, which is a job for an engineer.

Isolating and Draining the Old Radiator

You only need to drain the radiator being replaced, not the whole system, as long as both valves close properly. Turn the heating off and let it cool first.

Close the control valve fully, whether it is a thermostatic radiator valve set to off or a manual valve turned clockwise. Then close the lockshield valve at the other end, counting the turns so it can be reopened to the same setting later; that setting is what keeps the system balanced. With both closed, the radiator is drained into a tray through a loosened union nut or the drain-off point, with towels down, because system water is black and stains. Our step-by-step guide on how to drain a radiator covers this part in detail, including what to do when a valve will not shut off.

Once it is empty, undo both union nuts, tip the radiator slightly to catch the last water, and lift it off the brackets. Keep it upright while carrying it out; there is always more sludge left inside than you expect.

Fitting the New Radiator

New radiators do not come with valve tails fitted, so the first job is to transfer or replace them. Fitting new tails is better than reusing old ones, which are often corroded.

1. Tails and bleed screw. Screw a valve tail into each bottom connection, sealed with PTFE tape or a thread sealant suitable for heating systems. Fit the bleed screw and blanking plug in the top corners.
2. Brackets. If the new radiator uses the same bracket positions, the old brackets may be reusable. More often, new brackets are fixed so the tails line up with the valves at the right height. Check the wall fixings suit the wall; a full radiator is heavy, and plasterboard needs proper cavity fixings.
3. Hang and connect. Lift the radiator onto the brackets, line up the tails with the valves and tighten the union nuts, holding each valve body with a second spanner so the pipe underneath does not twist.

This is also the moment to decide on the valves. If the old ones are stiff, weeping or decades old, replacing them now, while the radiator is off and drained, avoids a second drain-down later.

Refilling, Bleeding and Checking the System

Open the lockshield valve back to its counted setting, then open the control valve, and the new radiator fills from the system. Air trapped inside it has to come out through the bleed screw.

Bleed the new radiator until water comes out steadily, then check every joint with dry tissue for weeps. On a sealed system with a combi or system boiler, filling the new radiator will lower the pressure, so check the boiler gauge and top up through the filling loop to the level your boiler manual gives. Open-vented systems with a tank in the loft refill themselves. Run the heating, check the joints again once they are hot, and bleed once more after the system has cooled, because more air usually collects.

If the old radiator was full of sludge, add inhibitor. Draining one radiator loses only a little, but a sludged radiator is a sign the protection in the water has run out.

Can You Replace a Radiator Without Draining It?

You cannot avoid draining the radiator itself, but you can usually avoid draining the system. Closing both valves isolates the radiator so only its own contents come out.

Where a valve will not close fully, the options are to drain down the whole system, or to freeze the pipe either side of the valve with pipe-freezing equipment so the valve can be worked on without losing system water. Freezing is quick on a single job but not suitable for every pipe or situation. On an old open-vented system the alternative is draining down from the lowest point and refilling, which then means bleeding every radiator in the house.

Doing It Yourself or Booking an Engineer

A like-for-like swap on matching pipe centres, with valves that close, is within reach of a confident DIYer. It is not gas work, so it does not need a Gas Safe registered engineer, although anything touching the boiler itself does.

It stops being a simple job when the pipe centres do not match, a valve will not close, the pipework is corroded or microbore, the radiator needs moving, or several radiators are being replaced at once, where one planned drain-down, a system clean and fresh inhibitor make more sense. That is where an engineer earns the fee. Through our radiator installation and replacement service we size each radiator for its room, deal with awkward pipe spacings, rebalance the system afterwards and treat the water, and our guide to radiator replacement costs explains what moves the price.