Thermostatic Radiator Valves (TRVs): Types, Installation, and Smart TRV Comparison for London Properties

What Is a Thermostatic Radiator Valve?

A thermostatic radiator valve (TRV) controls the flow of hot water into a radiator by responding to the air temperature in the room. When the room reaches the temperature set on the TRV dial, the valve closes, reducing or stopping flow to that radiator. When the room cools below the set temperature, the valve opens again. This allows different rooms to be maintained at different temperatures without manually adjusting the heating system for each room. TRVs do not control the boiler — they respond to the boiler running — and they work in combination with a room thermostat, not as a replacement for one.

Building Regulations Part L requires TRVs to be fitted on all radiators in new and replacement heating system installations, except in rooms with a room thermostat (typically the living room or hallway). This requirement has been in place since 2006, but many London properties — particularly older terraced houses and flats — still have radiators without TRVs or with seized TRVs that no longer function. Fitting TRVs to all radiators in these properties is one of the simplest and most cost-effective heating upgrades available, typically paying for itself in energy savings within one to two heating seasons.

Types of TRV Available

Standard TRVs use a wax-filled capsule or liquid-filled bellows that expand and contract with temperature changes to move a pin that opens and closes the valve. They have a numbered dial (typically 1 to 5 or with a frost setting) that sets the desired room temperature range — position 3 typically corresponds to approximately 20 degrees Celsius, depending on the manufacturer. These are the most common type and are available from all plumbing merchants for £5 to £20 per valve body. Angled, straight, and corner TRVs are available to suit different radiator positions and pipework entry angles.

Pre-settable TRVs include an adjustable maximum flow limiter that can be set by the installer, allowing the maximum flow rate to each radiator to be balanced at commissioning. This is particularly useful in London properties with long pipe runs where the radiators closest to the boiler tend to receive disproportionately more heat than those at the far end of the circuit.

Smart TRVs replace the standard wax capsule with a motorised actuator controlled by a smartphone app or a smart home hub. Popular options available in London include Tado, Netatmo, Hive, Drayton Wiser, and Honeywell Evohome. Smart TRVs offer individual room scheduling (setting different temperature profiles for each room at different times of day), remote control via smartphone, open window detection (cutting heat when the window is open), and integration with weather forecasts to adjust heating proactively. The trade-off is a higher upfront cost (typically £40 to £70 per smart TRV versus £10 to £20 for a standard TRV) and the need for a hub or gateway device in some systems.

Smart TRV System Comparison for London Homes

Tado is compatible with virtually all boilers via OpenTherm or simple on/off switching. It operates via a gateway connected to the router, with individual smart TRVs in each room. Auto-assist features (geofencing, weather adaptation) require a subscription. Good choice for properties with multiple zones and an existing compatible boiler. Drayton Wiser is a strong choice for properties with multiple heating zones where the user wants room-by-room control without a subscription fee for core features. Hive integrates well with British Gas boiler controls and offers a broad ecosystem of smart home devices. Honeywell Evohome is the most comprehensive multi-zone system and is well suited to larger London properties with many rooms, but it has a higher installation complexity and cost. For a small London flat, a single smart thermostat controlling the boiler combined with standard TRVs on each radiator is often more cost-effective than smart TRVs throughout.

TRV Installation in London Properties

Replacing a standard radiator valve with a TRV requires draining the affected radiator (or, on a sealed system, draining the section of circuit), removing the old valve, fitting the new TRV body and head, and refilling with inhibitor. A Gas Safe or qualified plumber can typically replace all TRVs in a two-bedroom flat in two to three hours. For a Victorian London terrace with eight or more radiators, allow four to six hours. The cost is typically £80 to £120 call-out plus £20 to £40 per valve in labour, plus the cost of the valves themselves. Upgrading all radiator valves at the same time as a boiler installation is the most cost-effective approach, as the system will be drained down and under access at that point.

TRVs and Boiler Efficiency

TRVs interact with boiler efficiency in a way that is worth understanding. On a traditional on/off boiler, TRVs that close across multiple rooms reduce the flow through the heating circuit, which can cause the system pressure to rise and the boiler to cycle on and off more frequently — a phenomenon called short cycling that reduces efficiency and accelerates wear. On a modern condensing boiler with weather compensation or load compensation, the boiler modulates its output in response to the heating demand, which works much more effectively with TRVs. If you are installing TRVs on an older system, ensure the boiler has a bypass valve or a bypass radiator (typically the towel rail or the bathroom radiator without a TRV) to allow minimum flow when all TRVs are closed.