Temperature controller

This is the mechanical room thermostat for fan coil units and small air-conditioning systems: a dial, a vapour-filled sensing element and a set of contacts that switch the fan and the valve or the compressor.

Working range is 10 C to 30 C, with a control accuracy of plus or minus 1 C at 20 C, which is the accuracy that matters in a room where the setpoint is somewhere between 20 and 26 C. Five models (J001 to J004) cover the combinations of cooling only, cooling and heating, and the versions with a power switch or a working indicator.

The thermostat contact is rated 2 A at 220 VAC and the three-speed fan switch 3 A at 220 VAC, both at plus or minus 10 per cent and 50/60 Hz - enough for the fan motor and a small solenoid valve directly, so no contactor is needed on a fan coil.

The sensing element is a metal bulb rather than a bimetal strip, which gives a slower but more stable response and avoids the short cycling that bimetal room thermostats produce on small units.

Practical points: mount the thermostat about 1.5 m above the floor on an internal wall, away from direct sun, draughts, and the discharge of the unit it controls - most "thermostat faults" on fan coils are siting faults. Set the fan to a fixed speed where the room is small, since automatic fan switching with a mechanical thermostat tends to hunt. And check the mode switch is not left between cooling and heating: on a mechanical unit that position can leave both circuits open.