How it Works - Thermostatic Radiator Valves | Castrads
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Thermostatic Radiator Valves

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How they work, when and where to use them and a little on smart heating controls.

How TRVs work

A thermostatic radiator valve (TRV) is made up of two parts. The thermostatic head contains an actuator that expands and contracts as the temperature rises and falls. The valve body has a sprung plunger inside it that closes and opens the supply of water (or steam, in two-pipe installations) into the radiator.

Above - thermostatic head (left) and valve body (right). 

When the two are put together the supply of heat into the radiator is automatically controlled with respect to the temperature of the room. When the room reaches the desired temperature, the valve closes and the radiator stops heating. The room then cools, the valve opens and the radiator begins to heat again.

Above: Technical drawings of thermostatic head (actuator) and thermostatic valve body. 

Turning the handle of the thermostatic head adjusts the distance that the actuator must expand by to close the valve. Many TRVs have a 1 to 5 scale - with 1 representing the smallest distance and 5 the greatest. The larger the distance, the warmer the room must be before the valve is closed.

Thermostatic valves are designed to let a small amount of water pass through when the room is cold to protect against frost. If you need to shut the valve off completely, when removing the radiators from the system in order to decorate, for instance, use the decorator’s cap supplied with the valves.

Function with Form

Not all TRVs have a 1 to 5 scale. Our Windsor TRVs - our single most popular valve - works exactly the same as traditional-looking thermostatic valves but with a more discrete scale. Rings on the neck denote the 1 - 5 settings. Once set, you usually don't need to touch the TRV much at all so the lack of visibility isn't a problem.

Windsor thermostatic radiator valve in Natural Brass
Windsor TRV Natural Brass

A stable internal temperature, whatever the weather

The beauty of a TRV is that they don't need adjusting when the outside temperature changes. The thermo-sensitive element automatically adapts to the changing temperature and keeps the radiator heating longer when the outside temperature falls.

Do TRVs control the boiler?

We're asked this a lot and the answer is no, not at all. Regular TRVs (read about Smart TRVs below) are a simple mechanical valve that have no electronic parts and cannot communicate with the boiler. All heating systems need some sort of electronic thermostat that tells the heat source when to supply hot water or steam to the radiators. We recommend a learning thermostat such as Nest.

Thermostatic radiator valves work completely independently to the boiler and don't have any direct control on its operation. 

There is an indirect link - TRVs control the temperature of each room, and therefore have an effect on the temperature that's sensed by the wall thermostat. This is why it's often recommended that thermostatic valves aren't used in the room where the wall thermostat is located.

Florence 3 Column in Little Greene Cordoba with Niva Antique Brass
Florence 3 Column in Little Greene Cordoba with Niva Antique Brass and Genius Smart Valve

A Connected Home With Smart TRVs

The biggest update to TRVs since their invention in the 1970s is smart tech - the internet of things. It's now possible to use an electronic actuator to control radiators, giving granular control from a smartphone or laptop anywhere in the world.

The basic concept is the same as traditional TRVs but the level of control is far more detailed. We work with Genius Hub who offer an intelligent control system that works seamlessly with most boilers and heat pumps, and of course with our radiators too.

TRVs on One-Pipe Steam

Unlike two-pipe steam and hot water radiators, one-pipe steam radiators are controlled not at the inlet side but instead on the outlet. Adjusting the size of the opening into a one-pipe steam radiator causes problems with condensate drainage so instead, we control the air that leaves it.

Before a heating cycle starts one-pipe radiators are full of air. If that air can't get out, the steam can't get in. Adding a thermostatic valve between the radiator and the air vent adds control to the amount of air that can escape and, as a result, the amount of steam that enters the radiator.

If no air can get out, no steam can get in.

Thermostatically-controlled steam radiators require a vacuum breaker. Our discrete VAB05 vacuum breaker is designed specifically for this purpose.