System 1 – Background ventilators and intermittent extract fans
Extract fans are used to serve moist spaces such as kitchens, toilets, and utility rooms for fast intermittent air extraction in this form of decentralized ventilation scheme.
The substitute air is supplied by a background ventilators. The phrase background ventilators relates to tiny air vents which enable air into a building, for example trickle vents in the windows or air vents in internal walls. Manual or automatic control can enable this system.
System 2 – Passive stack ventilation (PSV)
The use of a passive stack ventilation (PSV) system is one alternative to provide domestic ventilation. This utilizes a mixture of the air flowing across the ceiling and the natural buoyancy of hot humid air to raise the humid, stale air from the kitchen bathroom, cloakroom, etc. up to the point of the roof ridge where it flows into the atmosphere.
The trickle vents in the windows and doors etc. draw fresh air into the building. PSV systems are energy efficient without the need for any electrical fans or control. The quantity of ventilation obtained mainly relies on the amount of external air movement and the external temperature. PSV systems offer very little control, although humidity-controlled dampers (which do not need electrical energy) can be installed in the ducting to avoid over ventilation.
A Passive Stack Ventilation system requires thorough design and installation as the air movement is governed by the temperature/moisture in the air. The ducting must be comparatively big (compared to systems that use fans to move the air) and as close as possible to the vertical without sharp bends.
Installing Passive Stack Ventilation in a new build or during major renovation is really only practical. In addition to the ducting, a roof ridge level ventilation for external air movements is included in the system to help extract stale air.
In double glazed units, trickle vents are needed to allow fresh air to enter the building to replace the stack’s stale air.
System 3 – Continuous mechanical extract (MEV)
Air from moist spaces like kitchen, toilets and utility rooms is extracted from this form of ventilation system.
The replacement air is supplied through background ventilators. Alternatively, when the building has a design permeability greater than 5 m3/(h.m2) @ 50 Pa, background ventilators (trickle vents) are not required. The system can either be a centralized system with a single fan connected to various rooms or a decentralised system (dMEV) with air extracted from each room by individual fans. There are two airflow rates, trickle and boost. Air heat pumps for exhaust air would be regarded as a central MEV form.
System 4 – Continuous mechanical supply and extract with heat recovery (MVHR)
MVHR is a form of centralized ventilation scheme that combines supply and extract in one system.
Typically, hot, humid air is obtained by a duct system from wet areas, such as kitchens, toilets and utilities, and is exchanged via a heat exchanger. Fresh air enters through the heat exchanger before being provided to spaces such as the living room or the bedrooms. The air is preheated. There are two airflow rates, trickle and boost.
Other types of ventilation systems are permitted to be used in the UK, such as demand controlled ventilation (DCV) and positive input ventilation (PIV). In order to satisfy the building control body, these schemes need to show that certain performance criteria, such as moisture removal or indoor pollutants, are met.
