Thermostatic Radiator Valves: Types, Sizing, and How to Replace Them

Thermostatic radiator valves — TRVs — are one of the most cost-effective heating upgrades in a UK home. Fitting TRVs across a central heating system allows each room to be controlled independently, reducing energy waste in unoccupied rooms and bringing the system into compliance with Part L of the Building Regulations, which requires individual room temperature control on new and replacement heating installations. This guide covers how TRVs work, how to choose the right type, and how to replace an old valve — with guidance on the common complications a plumber encounters on the job.

How a TRV Works

A thermostatic radiator valve sits on the flow pipe connection to the radiator (or the return, depending on configuration). Inside the valve head is a wax or liquid element that expands as air temperature rises. When the room reaches the set temperature, the element expands, pushing a pin into the valve body and throttling the hot water flow to the radiator. As the room cools, the element contracts, the pin retracts, and flow resumes.

The head is calibrated to a temperature scale — typically 1 to 5, where 1 is approximately 12°C and 5 is approximately 21°C. Position * (frost protection) allows a trickle of flow to prevent freezing.

TRVs respond to air temperature in the room, not the water temperature in the system — this is an important distinction. They don't reduce boiler firing; they reduce heat output from individual radiators. The boiler continues to run until its own thermostat (room thermostat or programmer) is satisfied. For maximum efficiency, TRVs should be used alongside a room thermostat on the main living area, not as a replacement for it.

Taurus TRV — compression fitting, available at APM Electricals, Acton

TRV vs Manual Valve: When Each Is Appropriate

Manual (lockshield-paired straight valve) radiator valves have a fixed open/closed position — they either allow full flow or are shut off for balancing. They're cheaper and simpler but offer no temperature modulation.

Under Part L of the Building Regulations, TRVs are required on all radiators except in rooms with a room thermostat (typically the living room or hallway). In practice, most heating engineers fit TRVs throughout and a room thermostat in the main living area — this satisfies the regulation and gives the best temperature control.

Rooms where you might retain or fit a manual valve rather than a TRV:

• The room containing the room thermostat (TRV and room thermostat fighting each other can cause the boiler to short-cycle)
• Bathrooms with a towel rail — a manual valve allows you to run the towel rail continuously without it affecting room temperature calculations
• Plant rooms and utility areas where temperature control isn't needed

Types of TRV: Angled, Straight, and Corner

The physical configuration of the TRV depends on how the pipework approaches the radiator:

Angled TRV

The most common type in UK domestic heating. The valve body turns 90° — the pipe enters from below (floor or skirting) and the connection to the radiator is horizontal. Used with underfloor pipework and where pipes run along the skirting board level. The Embrass Peerless EDEN angled TRV is a standard 15mm angled valve suitable for the vast majority of domestic radiator connections.

Straight TRV

Pipe in and pipe out are in a straight line. Used where the pipe runs horizontally at the same height as the radiator tails — older properties with surface-mounted horizontal pipes often need straight valves.

Corner (Horizontal Entry) TRV

The pipe enters horizontally from the wall side, connecting to the radiator on the same horizontal plane. Used when the pipe comes out of the wall behind the radiator rather than up from the floor. The Taurus 15mm corner TRV with lockshield is available in ergonomic white and in chrome finish for exposed radiator positions where aesthetics matter.

The Lockshield Valve

Every radiator has two connections: the flow (where hot water enters) and the return (where cooled water leaves). The TRV goes on the flow side. The lockshield valve goes on the return — it's a manual valve with a plastic cap that requires a key or flat-bladed screwdriver to adjust. Its purpose is balancing: restricting flow to radiators near the boiler so that water circulates equally throughout the system, reaching distant radiators before giving up its heat.

On a new install or after adding a radiator, all lockshields should be balanced. The rough rule: radiators close to the boiler — close the lockshield down to one or two turns open; radiators at the end of the circuit — open fully. Fine balancing uses a differential thermometer across the flow and return on each radiator to achieve equal temperature drops throughout the system.

Note: Some TRVs are sold as paired sets (TRV + lockshield). Others are sold separately. The Taurus valves in our range include the lockshield — check the product listing before ordering.

Valve Sizing: 15mm vs 22mm and Connection Threads

Most domestic radiators use 15mm pipe connections. Standard TRVs are 15mm x ½" BSP — the 15mm refers to the pipe diameter and ½" to the radiator tail thread (which is ½" BSP on the vast majority of UK panel radiators).

Larger radiators (double panel, high BTU outputs) may have ¾" tail connections. Always check the radiator tail before ordering valves. A valve with the wrong tail thread can be adapted with a tail reducer, but it's cleaner to order the right thread from the start.

22mm radiator connections are rare in domestic heating — they appear on some older cast iron radiators and on some commercial units. If in doubt, measure the pipe and check the tail with a thread gauge before ordering.

How to Replace a TRV: Step by Step

Replacing a seized or failed TRV head is the easiest intervention — if the head mechanism fails but the valve body is sound, a replacement head (same brand or universal fit) can be swapped without draining the system. Many TRV heads are interchangeable via the M30 x 1.5mm thread standard.

Replacing the full valve body (body and head) requires draining down:

1. Isolate the radiator. Close the TRV fully (turn to 0 or close position) and close the lockshield (turn clockwise until seated). If you can't isolate the radiator with the existing valves, close the system at the central heating zone valve or drain down from the drain cock.
2. Drain the radiator. Place a bowl under the valve. Open the bleed valve at the top of the radiator to allow air in and water to drain. Collect the water in the bowl — expect 2–10 litres per radiator depending on size.
3. Remove the old valve. Use two spanners — one on the valve body (to prevent twisting the pipe), one on the union nut. Undo the union nut and disconnect the valve from the radiator tail. Repeat on the pipe connection. The valve body should come free.
4. Fit the new valve. Apply PTFE tape to the radiator tail thread (2–3 wraps, clockwise). Fit the new valve body: hand-tighten the tail union, then spanner-tighten 1¼ turns. Connect the compression fitting on the pipe side — olive, nut, and 1¼ turns past hand-tight.
5. Refill and bleed. Open the lockshield 1½ turns (starting point for balancing). Slowly open the boiler filling loop or the system isolator valve to re-pressurise. Bleed the air from the radiator until water runs from the bleed valve, then close it. Check all connections for leaks under pressure.
6. Fit the TRV head. Screw onto the valve body body (M30 x 1.5 thread or proprietary fit depending on manufacturer). Set to the desired temperature setting.

The Seized Olive Problem

The most common complication in TRV replacement is a seized compression olive on the pipe connection — particularly on older systems where the olive has been in place for decades and has deformed around the pipe. If the olive won't slide back when the nut is loosened:

• Try compressing the olive gently with a pipe vice or grips while turning the nut
• If the olive won't shift, cut the pipe 50mm behind the valve and use a straight compression coupling to join the new valve into a short section of new pipe
• On very old systems with lead or thick-walled steel pipe, the pipe may need cutting and a new section inserting regardless

TRVs and Smart Heating Controls

Smart TRV heads (Honeywell Evohome, Drayton Wiser, tado°) replace standard TRV heads on compatible valve bodies and allow per-radiator scheduling and remote control via smartphone. They're fitted to the same M30 x 1.5mm thread as standard heads. For installations where smart controls are being retrofitted, check whether the existing valve bodies have the M30 thread — if they use a proprietary thread, an adaptor ring is required.

For whole-system programmable control without smart TRVs, see our range of wireless programmable thermostats and 7-day wireless thermostats.

Buy TRVs and Radiator Valves at APM Electricals, Acton

We stock a range of TRVs and lockshield valves for 15mm pipework — angled, straight, and corner configurations in white and chrome finishes. Same-day collection from our trade counter in Acton. No minimum order. Trade accounts welcome.

Browse our heating and controls range, or visit us at 24 Western Avenue, Acton, London W3 7TZ. Call ahead on 020 8702 8080 to check stock on specific valve sizes and configurations.

Quick Reference: TRV Selection

• Pipe comes up from floor → angled TRV
• Pipe runs horizontally along skirting → straight TRV
• Pipe comes out of wall behind radiator → corner TRV
• Most UK domestic radiators → 15mm x ½" BSP
• One TRV per radiator, one lockshield per radiator
• No TRV in the room with the room thermostat
• Replacement head only (valve body OK) → no draindown needed
• Smart TRV heads → M30 x 1.5mm thread (most UK valves)