Radiator Valves: TRVs, Lockshields & Smart

Radiator valves are a small but mechanically significant component of any wet central heating system. The valve controls how much hot water flows through a radiator; the lockshield balances flow across the whole system; and a thermostatic or smart valve can automatically adjust output to maintain a target room temperature. Done well, valve specification and balancing is one of the most cost-effective ways to reduce heating bills in an existing home.

The Three Types of Radiator Valve

Every radiator in a UK wet central heating system has two valve connections, one on each side at the bottom (or occasionally at the top on older column radiators). The two valves perform different roles:

Manual valve, a simple on/off or flow-adjustable valve. Turned by hand to set how much water flows through the radiator. Once set, it stays at that position until someone manually adjusts it. No automatic temperature control. Used on its own, it is the most basic form of radiator control.

Thermostatic Radiator Valve (TRV), replaces the manual valve on one side of the radiator. A wax or liquid capsule in the valve head expands as room temperature rises, gradually closing the valve and reducing flow to the radiator. When the room cools, the capsule contracts and the valve opens again. TRVs do not turn the boiler on or off, they modulate the heat output of individual radiators to maintain the set temperature in each room independently.

Lockshield valve, the valve on the opposite side of the radiator from the manual or TRV. The lockshield is used once during commissioning (balancing) and then left in that position; it has a protective plastic cap to prevent inadvertent adjustment. Its role is to restrict flow through individual radiators so that all radiators in the system receive proportionally correct flow, without balancing, radiators closest to the boiler get too much heat and distant ones too little.

How TRVs Work and BS EN 215

TRVs are standardised under BS EN 215 (current edition 2019), which sets requirements and test methods for thermostatic radiator valves and specifies their dimensions, materials and connection details.¹ A good TRV typically holds a room within a degree or so of its setting, although draughts, sunlight and furniture near the head all affect what it senses.

TRV heads are marked with a scale, typically 1–5 (some with a frost symbol at the low end and a maximum symbol at the high end). The numbers do not mean the same temperature on every model, but as a rough guide:²

• Frost / * symbol, minimum position; allows just enough flow to prevent freezing
• 1–2, approximately 12–16°C
• 3, approximately 20°C (common living room setting)
• 4–5, approximately 22–26°C

The head is replaceable independently of the valve body, useful when upgrading from a standard TRV head to a smart TRV head, as long as the body adaptor is compatible.

Pipe Connection Sizes and Valve Body Types

The vast majority of UK domestic radiator systems use 15 mm copper pipe with either compression fittings or push-fit connections. Valve bodies are manufactured with:

• 15 mm compression connections, the standard; compatible with copper and most plastic pipe (with a pipe insert); microbore systems on 8 mm or 10 mm pipe need valves or reducers to suit
• 1/2” BSP (British Standard Pipe), threaded connection used on the radiator inlet/outlet; standard UK radiator valves are sold as 15 mm × 1/2” BSP, with the 1/2” tail screwing into the radiator

Valve body orientation must match your pipework layout:

• Straight valve, pipe enters horizontally from the side; used where pipework runs in the skirting or along the wall to the radiator base
• Angled valve, pipe enters vertically from below; the most common configuration on modern installations where pipes drop through the floor
• Corner valve, pipe enters horizontally but at 90° to the valve body; used on corner installations and some period properties

When replacing valves, confirm the pipe entry direction before ordering. An angled valve on a horizontal pipe run requires an additional elbow fitting, adding cost and a potential leak point.

Building Regulations Part L

Part L of the Building Regulations (Conservation of Fuel and Power, England) was significantly revised with effect from June 2022.³ Paragraph 5.20 of Approved Document L says that when a boiler is replaced in an existing dwelling, each room should be provided with thermostatic room controls.³

The usual way to meet this is a thermostat in one room plus a TRV on every radiator outside that room, and Approved Document L adds that TRVs should not be used in the same room as the thermostat.³ Separate room thermostats, or a networked control on each radiator such as a smart TRV, are accepted alternatives.³

In plain terms: if you have a boiler replaced in England after June 2022, your installer should leave every room with its own thermostatic control, which in most homes means TRVs on all radiators except those in the room with the main room thermostat.

A boiler replacement is normally self-certified by the installer through a competent person scheme such as Gas Safe Register, so it falls to the installer to make sure these controls are in place.⁴

In practice, some homes with older systems and no TRVs received a new boiler without this upgrade. If that happened in your case, it is worth raising with the installer, and retrofitting TRVs is a sensible energy efficiency measure in any case.

Smart TRVs

Smart TRV heads replace the standard wax-element head on an existing TRV valve body, provided a compatible adaptor is used. The most common valve bodies, including Drayton, Honeywell, Pegler and Myson, have an M30 × 1.5 mm connection, while Danfoss RA valves need their own adaptor.⁵

Smart TRV heads use Zigbee, Z-Wave, or Wi-Fi radio to connect to a smart home hub or proprietary bridge. Features typically include:

• Scheduling, set different temperatures by time of day and day of week
• Individual room control, each radiator can have a completely different heating schedule
• Open window detection, some models detect a sudden temperature drop (window opened) and close the valve to avoid heating the outside
• App control, adjust temperature remotely
• Integration with room sensors, use a separate temperature sensor in the room for more accurate control, particularly where the TRV head is in a draught or affected by direct sunlight

Protocol compatibility: Zigbee smart TRVs require a Zigbee hub (or a hub supporting Zigbee such as Home Assistant with a Zigbee dongle). Z-Wave units require a Z-Wave hub. Some Wi-Fi TRVs work without a hub but are cloud-dependent. Matter-compatible TRVs are beginning to appear (2025–2026) and offer the best long-term interoperability.

Energy Savings

Independent tests at the University of Salford, commissioned by BEAMA (the trade association for heating controls manufacturers), put the average energy saving from TRVs in homes at 18% of heating costs.⁶ The Energy Saving Trust (EST) estimates that installing heating controls, including a programmer, room thermostat and TRVs, could save around £100 a year in Great Britain.²

Real-world savings depend heavily on:

• How many rooms are genuinely unoccupied during heating periods
• Current boiler and thermostat settings
• House insulation levels

The saving is greatest where rooms would otherwise overheat, so a home where several rooms can be turned down during the day will benefit more than one that is already heated evenly.

Balancing the System

Fitting TRVs changes the hydraulic balance of the system. With multiple TRVs closing simultaneously (all rooms up to temperature), the system pressure can rise and flow may be diverted to remaining open radiators. This is why:

Balancing after fitting TRVs is essential. Balancing means adjusting each lockshield valve so that all radiators reach their set temperature at approximately the same rate. Without balancing, some radiators overshoot their target (valve closes late) and others underperform.

Bypass valve: on systems with a high proportion of TRVs, when most valves close simultaneously, the circulating pump has nowhere to push water. A bypass valve, a spring-loaded valve typically fitted near the boiler, opens when system pressure rises, allowing water to circulate even when most radiators are closed. Most modern boilers include a bypass; older systems may need one fitting. Confirm with your heating engineer.

Combi boiler compatibility: TRVs are fully compatible with combi boilers. However, the bypass requirement is particularly important with combis, which are sensitive to very low flow rates.

Radiator Valve Comparison

Typical Installed Costs

Radiator valve replacement is DIY-accessible on a system that has been fully drained and depressurised, but many homeowners prefer to use a heating engineer to avoid the risk of leaks. A Gas Safe-registered plumber is not required for valve replacement (it is not a gas appliance), but a competent plumber is recommended.

Material costs (2026, per radiator, supply only):

• Standard TRV + lockshield pair: £10–£45
• Smart TRV head (fitted to an existing TRV body): £30–£75⁹ (2023 price, brought to 2026)

Typical labour to replace one radiator’s valve set: £60–£120

Whole-house retrofit (10 radiators, supply and installation):

• Standard TRVs throughout: £350–£700
• Smart TRVs throughout (new valve bodies plus smart heads): £650–£1,450

The Energy Saving Trust puts the cost of adding TRVs to all the radiators in a home that already has a programmer and thermostat at around £370.²

Balancing the system (separate service, often done when TRVs are fitted): £80–£150 for a heating engineer.

Practical Buying Advice

Check the thread size on your existing TRV body before buying a smart head, M30 × 1.5 mm is the most common UK standard and is compatible with the majority of smart heads. Most smart TRV manufacturers include a range of adaptor rings in the box.

If your boiler was installed before June 2022 and you have no TRVs, Part L does not require you to add them now, but doing so is likely to reduce your heating bills measurably. For a connected home with a smart hub, smart TRV heads add scheduling and zone control that standard wax-element TRVs cannot provide.

All costs include VAT at 20%. Regional variation applies, London and South East labour rates are typically higher than the national figures shown.