A thermostatic radiator valve, or TRV, is the small numbered knob fitted to one end of most modern radiators. Unlike a plain on/off valve, it responds to the air temperature around it and throttles the flow of hot water automatically. Once a room reaches the temperature you set, the valve closes down; when the room cools, it opens again. That simple feedback loop is doing more work than most people realise.
What TRVs Do
Inside the head of a TRV is a capsule filled with wax or liquid that expands as it warms. As the room heats up, the capsule swells and pushes a pin down into the valve body, restricting the hot water passing through the radiator. When the room cools, the capsule contracts and the pin lifts, letting more water flow again. No electricity, no wiring — just physics responding to the air in that particular room.
The point of this is room-by-room control. A boiler and its main thermostat decide whether the heating is on at all, but TRVs decide how much heat each individual room receives. A south-facing lounge that catches afternoon sun can be set lower than a north-facing bedroom, and neither will cook while the other stays cold. In older London terraces where rooms vary wildly in how well they hold heat, that distinction matters. Heating engineers such as WPJ Heating Battersea often point out that a properly balanced system with working TRVs can shave money off bills simply by not over-heating rooms nobody is using. The valve never sends more hot water than the room actually needs.
Reading the Numbers
The numbers on a TRV — usually running from a frost setting up to five or six — are not degrees, and that trips people up constantly. They are relative settings that correspond roughly to a target room temperature. A setting of around three tends to hold a room near 20°C, which suits most living spaces. The frost or snowflake symbol keeps the radiator just warm enough to stop pipes freezing without heating the room for comfort.
Because the numbers are approximate, the sensible approach is to set a valve, live with it for a day or two, then nudge it up or down until the room feels right. Turning a TRV to maximum does not make a room heat faster; it only raises the point at which the valve decides to close. All you achieve by cranking it to six is a room that overshoots and wastes heat. Think of the number as the temperature you want to settle at, not a speed dial.
Common Placement Mistakes
A TRV can only react to the air it can feel, so where that air comes from makes all the difference. The most frequent error is hiding the valve behind long curtains or a sofa. Trapped behind a curtain, the valve sits in a pocket of warm air, decides the room is hot, and shuts the radiator down while the rest of the room stays chilly.
Similar problems arise when a radiator sits inside a cabinet or beneath a deep shelf, both common in period properties around London. The enclosed space warms quickly and fools the valve. Draughts cause the opposite fault — a TRV next to a leaky door or window feels cold air constantly, keeps the radiator running, and overheats the rest of the room.
If your radiator is in an awkward spot, a remote sensor version lets the sensing head sit away from the radiator on a short capillary tube, measuring the real room temperature instead. It is also worth knowing that most systems leave one radiator, often in the room with the main thermostat, without a TRV. This gives the pump somewhere to circulate water when every other valve has closed, protecting the boiler.
The quickest win is to walk through your home and check each TRV is clear of curtains, furniture and cabinets. Free any that are blocked, and you will give the valves a fair chance to do the job they were fitted for.