Uncontrolled ventilation, also called natural ventilation is based on air movement through trickle vents and through gaps in the building envelope (infiltration, otherwise called drafts). The ventilation rates change drastically, as they are based on wind speeds and temperature differences between inside and outside. It is not controllable, other than by opening and closing of trickle vents and activating of intermittent extract fans in wet rooms and kitchen.
Controlled or mechanical ventilation cuts out the extreme spikes of uncontrolled ventilation and thereby reduces the respective heat losses. There are different types used in the UK:
Positive Input Ventilation (PIV) and Multiple Input Ventilation (MIV)
Mechanical Extract Ventilation (MEV)
Mechanical Ventilation with Heat Recovery (MVHR) – single room or whole house systems.