It's that great time of year again, when outside air temperature falls to sub-zero temperatures. And, of course, this can leave ventilation plant vulnerable. So here we will describe the best method (as set out in CIBSE Guide H for Building control systems) for protecting building and plant from freezing conditions.

The frost thermostat is the primary means of preventing cold air from entering a running Air Handling Unit (AHU). The supply fans should be hardwired to stop upon activation of the stat. This should override all other control priorities. The stat should typically be set to 3°C and should be positioned downstream of the pre-heat coil. During normal running operation, the frost thermostat is prevented from activating by the pre-heat coil controlling to a setpoint above the setting of the stat (typically 10°C).

When not running, the pre-heat coil is protected by means of sensing of the outside air temperature and the LTHW return temperature. If the outside air temperature is detected to fall below a given setpoint (typically 3°C) then the circulation pumps should be enabled and the AHU valves opened. Should the LTWH return temperature fall to below a given setpoint (typically 20°C) then heat source (boiler, etc) should be enabled.

It is also recommended that the fabric of the building be protected by against the effects of low internal temperatures, outside of normal running times. This is achieved by monitoring the internal space temperature. On detection of any available internal space temperature sensors reading below a given setpoint (typically 10°C) then the ventilation, circulation and heat source plant should be enabled to maintain the buildings internal temperature above this setpoint.

Unfortunately, though these principles are very simple, they are often implemented incorrectly, or neglected completely. So before the weather turns really cold, it’s worth checking that the correct ventilation frost protection is in place. Just a simple check of your setpoints could help protect from major plant or fabric damage this winter.