How to Install a New Radiator in an Existing Central Heating System

Adding an extra radiator to your home's central heating system is one of those jobs that sits on the boundary between DIY and trade work. It involves working with the plumbing system, understanding heating circuits, and making connections that need to be leak-free. But it is also genuinely within the capabilities of a confident and methodical DIYer — particularly if you are adding a radiator to an existing room rather than running entirely new pipework through multiple floors.

This guide takes you through the process step by step, from planning the installation through to balancing the system when the radiator is in place. Typical materials will cost £80–£250 depending on the radiator size and whether you need new pipework fittings.

Before You Start: Planning and Safety

Is This DIY or a Job for a Plumber?

Installing a radiator on an existing circuit — picking up from nearby pipes, fitting the radiator, and adding TRVs — is not notifiable work under Part P of the building regulations and does not legally require a Gas Safe engineer. However, if the installation requires significant alteration to the boiler, the pump, or the primary circuit, or if it is part of a larger system change, professional involvement is wise.

One important consideration: if your boiler is under warranty (from Worcester Bosch, Vaillant, Ideal, or another manufacturer), check whether DIY modifications to the system affect that warranty. Some manufacturers require system work to be carried out by a Gas Safe engineer.

Will Your Boiler Handle the Extra Load?

Every radiator added to a system places additional demand on the boiler and pump. A correctly sized modern boiler (which should have been sized to the existing heat loss calculation plus a margin) will typically accommodate one or two additional radiators without issue. However, if your system already struggles to heat all rooms adequately, adding another radiator may not solve the underlying problem and could make existing rooms colder.

Use an online radiator sizing calculator (Screwfix and B&Q both have good ones) to determine the correct BTU/Watt output for the room you are heating, based on room dimensions, insulation standard, and number of external walls.

Choosing Your Radiator

Standard double-panel convector radiators (Type 22, double-panel double-convector) are the most common and widely available. For a standard UK living room of around 15m², you would typically need 2,000–3,000 watts of output. Radiators range from around £35 for a basic white steel panel radiator from Screwfix or Wickes to several hundred pounds for designer or vertical models.

Tools and Materials

• New radiator (correct size for the room)
• Radiator brackets (often supplied with the radiator)
• Thermostatic radiator valve (TRV) — one for the flow connection
• Lockshield valve — one for the return connection
• Radiator bleed key
• 15mm copper pipe (if extending from existing pipework)
• 15mm compression fittings or push-fit fittings (SharkBite or JG Speedfit are reliable brands)
• Tee-pieces (compression) for picking up from existing pipes
• PTFE tape
• Pipe cutter (rotary type is easiest)
• Pipe deburring tool
• Spirit level
• Drill and appropriate wall fixings
• Bowl, towels, and a hose for draining
• Radiator flushing chemical (e.g., Fernox F3) if adding to an older system

Step 1: Drain the System (Partially or Fully)

You do not always need to drain the entire system — if you are using push-fit or compression tee-pieces with isolation valves, you only need to drain the system down to below the point where you are making connections. However, for most first-time installers, draining fully is simpler and reduces the risk of an unexpected flood.

To drain the system:

1. Turn off the boiler and allow the system to cool completely.
2. Locate the drain-off cock — usually a small brass fitting at the lowest point of the system, often near the boiler or on a ground-floor radiator.
3. Attach a garden hose to the drain cock and run it to a drain or outside.
4. Open the drain cock (turn anti-clockwise with a spanner or screwdriver).
5. Open the bleed valves on the upstairs radiators to allow air in as the water drains out. The system may take 30–60 minutes to fully drain.

Have plenty of towels to hand — there will be residual water in the pipes even after draining. Old heating system water is often black with iron oxide (magnetite) and will stain anything it touches.

Step 2: Mount the Radiator Brackets and Hang the Radiator

Use the radiator's bracket template (or measure carefully) to mark the bracket positions on the wall. Most radiators should be positioned 100–150mm from the floor and 50mm from the wall. Use a spirit level to ensure the brackets are level — a misaligned radiator looks poor and can affect valve function.

Drill, plug, and fix the brackets, then hang the radiator. Do not connect the valves yet.

Step 3: Fit the Valves

Install a TRV on one side of the radiator (the flow — conventionally the left-hand side when facing the radiator, but this depends on your system layout) and a lockshield valve on the other (the return). Both valves connect to the radiator using a 15mm compression olive and nut, or directly to BSP-threaded radiator tails with PTFE tape.

Modern TRVs from Honeywell, Drayton, or Danfoss are simple to fit and include fittings for various connection types. Always wrap PTFE tape clockwise around the thread before tightening.

Step 4: Connect to the Existing Pipework

This is the most critical stage. You need to pick up the 15mm flow and return pipes from the nearest accessible point — often beneath the floorboards in the room, or occasionally along the skirting.

1. Cut into the existing pipework at the pickup point using a pipe cutter. Make a clean, square cut.
2. Deburr the cut ends with a deburring tool or the back of the pipe cutter's blade.
3. Insert compression tee-pieces to create a branch for the new radiator feed.
4. Run 15mm pipe from the tee-pieces to beneath your new radiator. Keep runs neat and supported with pipe clips every 600mm on horizontal runs, 1.2m on vertical.
5. Connect the pipes to the TRV and lockshield valve connections.

Push-fit fittings (JG Speedfit, SharkBite) are quicker to install than compression fittings and reliable for concealed pipework — though some plumbers prefer compression for under-floor runs. Follow the manufacturer's instructions carefully.

Step 5: Refill, Bleed, and Check for Leaks

1. Close all bleed valves and the drain cock.
2. Refill the system slowly by opening the filling loop (the flexible connector between the cold mains and the system, usually near the boiler). The system pressure gauge should rise to 1–1.5 bar.
3. Once the system is full, go around each radiator and bleed any air using a radiator key — have a cloth ready. Start with the downstairs radiators and work upward.
4. Check the pressure again and top up if needed.
5. Inspect every connection for leaks. Pay particular attention to the new tee-pieces and the radiator valve connections. Tighten any weeping compression fittings by a quarter turn.
6. Fire up the boiler and allow the system to heat up fully. Check for leaks again when hot.

Step 6: Balance the System

Adding a new radiator changes the dynamics of your heating circuit. Without balancing, the new radiator (or those nearest the boiler) may heat up well while the furthest radiators run cold.

Balancing is done by adjusting the lockshield valves on each radiator to restrict flow proportionally. The radiators closest to the boiler need more restriction; those furthest need less. A simple approach:

1. Open all lockshield valves fully, then close all TRVs except the one on the new radiator.
2. Note the order in which radiators heat up (which comes up fastest).
3. With the system fully up to temperature, use a clip-on pipe thermometer (available from Screwfix for around £15) to measure the flow and return temperatures at each radiator. You are aiming for a temperature difference (delta T) of around 10–12°C across each radiator.
4. Close down (restrict) the lockshield valves on radiators that are running too hot relative to others.

This process takes some patience but makes a real difference to comfort and efficiency. Worcester Bosch and Vaillant both publish guides on system balancing for their respective boiler ranges.

Adding Inhibitor and Protecting Your System

Before final commissioning, add a central heating inhibitor such as Fernox F1 or Sentinel X100 to the system water. These chemicals prevent corrosion and scale build-up, protecting the boiler heat exchanger (the single most expensive component to replace). They are available from most plumbing merchants and Screwfix for around £15–£20 per bottle. Follow the dosing instructions based on system volume.

If the system water was very black when you drained it, consider a full powerflush before refilling — this is a job for a specialist, costing £300–£600, but can significantly extend system life and improve efficiency.