Replacing a Radiator: Draining Down, Valve Types, Sizing Up & System Refill

Summary

Radiator replacement is one of the most common plumbing tasks in UK domestic properties — whether it is upgrading an undersized radiator, replacing a corroded unit, converting from a single-panel (K1) to a double-panel (K2) for more heat output, or simply fitting a towel rail in a bathroom. The job looks simple but has several pitfalls: incorrect valve sizing, failure to add inhibitor after refilling, and — the classic — not closing the lockshield valve tightly enough and flooding the room.

The UK retrofit heating market has shifted significantly toward decarbonisation, and heat pump retrofits are creating new demand for correctly sized radiators (heat pumps run at 35–45°C flow temperature, requiring larger radiators than gas boilers at 70–80°C). Many existing radiator replacement jobs in properties undergoing heat pump installation will require upgrading to K3 (triple panel) or column radiators to achieve the required heat output.

Key Facts

• Radiator types — K1 (single panel, single convector), K2 (double panel, double convector), K3 (double panel, triple convector), P+ (single panel, double convector), column (architectural)
• Radiator sizes — Output measured in watts at ΔT50 (mean water temperature 75°C, room 25°C). For heat pumps, derate by factor: ΔT30 output = approximately 50% of ΔT50
• Valve types — TRV (thermostatic radiator valve) — flow side; lockshield — return side; manual valve — flow side (no thermostatic control)
• TRV installation — TRV must be installed horizontally (or use an angled head). Vertical installation causes thermostatic head to be affected by rising heat from the radiator
• Valve thread — Standard UK: ½ BSP (British Standard Pipe) on most domestic radiators. Older systems may have ¾ BSP. Always check before ordering valves
• PTFE tape — Apply 3–5 turns clockwise (looking at the male thread) on all threaded connections. Do not overtighten — hand-tight plus ¼ turn is sufficient for most connections
• Inhibitor — Always add corrosion inhibitor (Fernox F1, Sentinel X100, or similar) after any work involving system drain-down. Check existing inhibitor concentration first with test strip
• Magnetic filter — Opportunity to install or clean existing magnetic filter when draining down. See magnetic filters
• Bleeding — New radiator must be bled after refilling. Air at the top of a new radiator prevents heating

Quick Reference Table

Detailed Guidance

Isolating a Single Radiator

Most radiators fitted since the 1980s have:

• A TRV (thermostatic radiator valve) on the flow pipe (typically the bottom-left or bottom-right connection, depending on pipework routing)
• A lockshield valve on the return pipe

To isolate:

1. Turn TRV to position 0 (fully closed). If the TRV head has no 0 position, remove the head and close the valve pin with pliers
2. Count the number of turns to fully open the lockshield valve using a lockshield key or adjustable spanner. Note this number — you'll use it when you refit to restore the balance setting. Then close the lockshield fully
3. Allow the radiator to cool completely
4. Position a large bucket (15–20 litre) below each connection
5. Open the bleed valve at the top of the radiator to break the vacuum — the water level drops slightly
6. Using a spanner, crack open (do not remove) the union nut on the TRV side — water will drain into the bucket. Capture all water
7. Repeat on the lockshield side
8. Remove union nuts fully and lift the radiator off its wall brackets

Removing Wall Brackets

Radiators hang on wall brackets — either fixed single brackets that the radiator hangs on, or split-type brackets where one part is wall-fixed and the other clips to the radiator. The bracket fixings into plasterboard partitions must be screwed into studs or use appropriate plasterboard fixings for heavy objects (steel stud anchors, Gripit fixings). In masonry walls, use screws into plugs.

The bracket centres must match the radiator — measure the hanging bracket centres on the new radiator before committing to wall fixings.

Valve Selection

TRV: Standard domestic TRVs accept preset temperature settings 1–5 (approximately 10–28°C). For bathrooms, a manual lockshield or a frost-protection TRV (minimum setting) on both connections is appropriate — TRVs in bathrooms are turned off in summer and rooms can get very cold, triggering nuisance frost stat calls. An alternative is to leave one connection without a TRV and use the lockshield as a fixed-output control.

Smart TRV: Wireless thermostatic radiator valve heads (Tado, Honeywell Evohome, Drayton Wiser) replace the standard TRV head and communicate with a hub for zone control. The valve body is standard — only the head changes. Useful for individual room control without rewiring.

Angle vs straight valves: Angle valves connect to pipes coming out of the floor (most common in UK houses). Straight valves connect to pipes entering from the wall or skirting. Corner valves connect at a true 90°. Always verify pipe routing before ordering.

Refitting

1. Check the new radiator's wall bracket centres and drill/fix brackets at the correct height and position. Use a spirit level
2. Hang the radiator on brackets
3. Check valve bodies are tight in the radiator tails (hand-tight plus ¼ turn with a wrench)
4. Reconnect union nuts — apply PTFE tape to the male thread (not the nut), fit new olive if using compression connections
5. Open the bleed valve slightly, then slowly open the lockshield valve (to the same number of turns as noted during removal)
6. Open the TRV to approximately position 3
7. Allow the system to fill — you should hear water rising in the radiator
8. Close the bleed valve as soon as water appears (not air)
9. Check all connections for drips under pressure. Tighten any leaking joints gently

Sizing Up

If you are fitting a larger radiator to improve a cold room:

1. Calculate the required heat output: radiator btu or the simplified room heat loss calculation
2. Check the room's existing pipework can supply the flow rate — larger radiator needs more flow. Typically not an issue for a single room upgrade on a properly balanced system
3. Fit a larger radiator in the same bracket positions if the wall space allows. If wall space is limited, a K3 or column radiator gives more output in the same width
4. Re-balance the system after the replacement — the changed resistance affects all radiators on the circuit

Adding Inhibitor

After any drain-down, the system corrosion inhibitor is partially depleted. Test the concentration with an inhibitor test strip (Fernox or Sentinel brand). If below the manufacturer's recommended level, add appropriate quantity of inhibitor through the system filling loop or, if not possible, through the boiler feed/expansion vessel.

Do not skip this step — uninhibited heating systems develop sludge (magnetite) within 2–3 heating seasons, blocking radiators and damaging boiler heat exchangers.

Frequently Asked Questions

Can I fit a larger radiator without changing the pipework?

In most domestic installations yes — the pipes (typically 15mm or 22mm) have enough capacity to supply a modest increase in radiator size. A K2 replacing a K1 in the same position is fine. Fitting a very large radiator (over 3000W) on 15mm pipework will restrict flow — 22mm would be preferable.

Why won't my new radiator get hot?

Check in order: (1) TRV is open (not at 0), (2) lockshield is open, (3) air has been bled, (4) system pressure is correct (1.0–1.5 bar cold), (5) the radiator is being reached by the flow — confirm by feeling the flow and return pipes individually to check if one is flowing and the other is cold (indicates reversed flow). Reversed flow connections will cause poor performance.

My radiator leaks at the union nut after refitting. What do I do?

First try tightening gently — ¼ turn at a time. If it still leaks, drain the radiator, inspect the olive (compression fitting) or PTFE thread. Replace the olive if compressed (never reuse a compressed olive). Rewrap PTFE on the thread. If the valve body thread is damaged, the valve must be replaced.

Regulations & Standards

• BS 6880 — Specification for low temperature hot water heating systems (radiator and valve selection guidance)

• BS 7593 — Treatment of water in heating systems (inhibitor requirement)

• Approved Document L — Energy efficiency: replacement radiators should not reduce the thermal efficiency of the heating system

• Boiler Plus (2018) — New and replacement systems must include TRVs on all radiators

• Fernox Water Treatment Technical Guide — Inhibitor dosing and testing for heating systems

• Drayton Technical Support — Valve selection and balancing guidance

• Radiator Company UK — Radiator output tables and sizing tools

• radiator sizing — Calculating required radiator output by room

• radiator balancing — Balancing after replacement

• magnetic filters — Filter maintenance when radiator is removed

• radiator btu — BTU/W output reference tables