These are designed to clip onto the pipes that feed the fan coil. The pipe thermostat acts as a switch and prevents the fan motor from starting until the pipes become hot (around 50 deg C) thus preventing cold air from being blown into the room. They are usually used in conjunction with the capillary action thermostats below.
Capillary Action Thermostat
The capillary action thermostat can be used in a number of ways but is usually used as a stand alone control to switch the fan deck off when a user set temperature is reached. They can also be wired in series to automatically control the speed of a fan unit. When a room is cold the thermostat will select a high speed to blow warm air quickly into the room, once the room has reached a user set temperature the thermostat will switch the fan motor automatically to a slower speed giving a more gentle background heat. The second thermostat switches the fan unit off when a user set temperature is reached.