Radiator valves control the flow of water through each radiator on a wet heating system.  Each radiator has 2 valves – one to control the water coming in and one to control the outgoing water.

Typical Manual Valve

The simplest valves on the incoming side are “wheelhead” or “manual” valves.  These basically have two positions – on or off.  In the off position, no hot water will flow through the radiator; in this case it will not heat the room however the thermostat and controls have been set.  You can turn the valve to be partially on but there is no means of judging how much water it is letting through.

A Typical TRV valve suitable for a smart valve.
Typical TRV Valve

TRV (Thermostatic radiator valves) have sensors, containing a wax or liquid, that expands as the room temperature goes up and contracts as the temperature goes down. When the sensor contracts, the valve opens automatically to allow more hot water to flow to the radiator, and when it expands, the hot water flow is restricted. 

A TRV has multiple positions marked on it, usually in the form of a dial.  Most are manually operated – you turn them to alter the flow of hot water, and hence the heating to the room.  However, the settings are not very precise.  The * position is a good indicator of the setting required to avoid freezing, but the difference between, say, 4 and 5, is not clear.  It could vary by several degrees.

Honeywell Evohome Smart Valve

Smart TRVs, such as the Honeywell Evohome ones installed by Homesmart, will only work with TRV valves.

These smart valves respond to wi-fi signals.   They measure the room temperature accurately (to within half a degree) and then adjust the radiator valve automatically via their in-built motor.  In this way, they are busily adjusting the temperature on your radiators to ensure exactly the temperature you want room by room. 

As they are wi-fi they can be controlled from anywhere via an app, wireless hub or another controller. At the touch of a screen, you can adapt schedules, temperature and settings for your heating system

Note that sometimes “wheelhead” or “manual” valves are also referred to as “lockshield”.  It is used as a verbal shorthand to differentiate other valves from those that are TRVs