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.
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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.
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Frequently asked questions
Do I need to drain the whole system to add a new radiator?
Usually yes — you need to drain at least the section of pipework where the tees will be inserted. On a large system, you can sometimes drain only the affected circuit if there are isolation valves. Your plumber will advise on the best approach based on your specific system layout.
Will adding a radiator increase my heating bills?
Adding one well-sized radiator to a room that was previously underheated should not significantly increase your bills — it simply redirects heat from the boiler more efficiently to where it is needed. If the room was relying on borrowed heat from other rooms or electric heaters, a properly connected central heating radiator is typically cheaper to run.
What size radiator do I need for my room?
Use a BTU (British Thermal Unit) calculator to work out the heat output required — this accounts for room volume, window size, insulation level, and desired temperature. Our BTU calculator on this site gives instant results. Undersizing a radiator means it runs constantly without reaching temperature; oversizing wastes money and looks out of proportion.