Adding a new radiator to an existing central heating system
Check the steps below first — if you're not confident, get it fixed safely today.
Post a job — we'll find you an engineer →⚠️ Adding a new radiator changes the hydraulic balance of the whole system. If done incorrectly, some existing radiators may receive insufficient flow. A competent plumber will rebalance all radiators after adding a new one.
Not sure if this matches your problem?
Use our interactive tool — answer a few questions and get a personalised diagnosis.
Most likely cause & what to check
Adding a new radiator is a straightforward plumbing job but requires some planning. You need to select the correct radiator size using a BTU calculation (use our BTU calculator to work out the heat output needed for the room), plan the pipework route from the nearest flow and return pipes, and choose the valve type (TRV on flow, lockshield on return).
Locate the nearest flow and return pipes — usually under the floor in a ground floor room, or through the ceiling in an upstairs room. On a standard two-pipe system, flow and return pipes run separately. On an older one-pipe system, different pipe connections are used — this matters for positioning the valves.
Mark and cut the flow and return pipes at the tee positions. Fit two tees (compression or push-fit) and run 15mm or 22mm pipe (depending on the existing system) to the new radiator position. Use the boiler pressure circuit to plan the route.
Fix the radiator brackets to the wall at the correct height and spacing (the bracket positions are shown on the radiator packaging). Hang the radiator, fit the TRV to the flow connection, and the lockshield valve to the return. Use PTFE tape or fibre washers — not both.
Open the system valves, fill the radiator, and bleed out any air. Check all joints for leaks, including the tee fittings you inserted.
Rebalance the system: close all lockshield valves fully, open them all by the same number of turns (start with 1 turn), run the heating for 20 minutes, then check all radiators are heating evenly. Adjust the lockshield valves — closing slightly on those that get hot fastest, opening on those that are slow — until all reach temperature together.
Not confident doing this yourself?
Post the job and we'll match you with vetted local engineers. Free, no obligation.
🛠 Tools & materials you may need
Was this guide helpful?