Mechanical systems can be specified for ensuring sufficient air flow in very airtight buildings or in specific rooms. These systems generally use only a small amount of energy for air movement, but energy use will be higher if incoming air needs to be heated.

Components of an air supply ventilation system

Mechanical air supply ventilation is generally driven by a fan, which may be stand-alone or part of an air conditioning system or heat recovery system. To move the fresh air from outside into the rooms to be heated also requires an air intake, air filter, ductwork and room diffusers.

Fresh air quality

The fresh air being introduced needs to be better quality than the air it is replacing. This means:

• specifying an air filter that removes particulates
• locating the air intake to ensure it is not drawing in polluted air.
• Flow rates
• Most fresh air ventilation supply systems are designed to run continuously with low fresh air flow rates (about 3 to 20 litres per second), during the time the room is occupied. See passive ventilation for required air flow rates.

Energy use

In general, as more air is moved by the fan, more energy is consumed. To minimise energy use, fans should be sized and controlled to only move the amount of ventilation air required and for the time required.

Fresh air heating

Most fresh air supply ventilation systems provide heating on the supply air flow to take the chill off, particularly in winter. Heating cold air from outside can consume substantial amounts of energy, particularly in winter. This can be reduced by designing the system so that the fresh air:

• volume flow rate is kept to appropriate levels
• intake is located to capture warmer outside air – for example on the sunny side of the building, near warm surfaces, from spaces with solar gain storage
• is only heated when the outside air temperature is cold (below about 13ºC) or is 6ºC below the required room temperature – this depends on the heating system and level of heat gains from other sources in the room
• is heated no higher than the occupants comfortable room temperature.

Types of system

Positive pressure/forced air system

This type of system is designed to create a positive pressure in the rooms, so that all air movement is directed outwards. The supply air flow rate needs to be higher than the leakage air flow rate. The more airtight the home is the more effective the system.

In winter, the air will need to be heated before it enters the room. Energy use will increase as the outside air temperature drops and indoor temperature requirements increase. These systems may be specified with a heat recovery device to remove heat from the outgoing air. Any shortfall is made up by using a booster heater inside the system or a separate space heating system in the home.

Ventilation heat recovery system

This type of system uses a heat exchanger to absorb heat from warm room air that is being discharged outside. This heat is transferred by the heat exchanger to heat the incoming fresh air. This system relies on the room air being warmer than outside air. If the room air is cold, some other form of heating is required. See space heating: heat recovery systems for more.

Passive heat source systems

This type of system captures warm air from the roof space or some other interior space and transfers it into the space to be ventilated.

To be effective, these systems require:

• adequate sunshine to heat the heat source area
• ambient temperatures that are not too low
• sufficient ceiling space to give heat storage, and meet the air change rate needed in occupied spaces
• limited air movement through the ceiling space
• limited moist air penetration from the spaces below
• supplementary space heating when the heat source area is colder than the room temperature – these systems need to be able to be slowed or stopped when the fresh air supply's temperature falls (for example, a cold night, when the sun isn’t shining, or wind is moving more air through the roof space)
• air filters to ensure particulates from the ceiling space aren’t brought into the home
• a source fresh air from outside for when summer cooling is needed – this requires the specification of extra ductwork and a changeover feature.

Solar heated system

Solar energy ventilation systems use solar energy that has been absorbed by solar panels (see space heating: energy sources) to heat fresh supply air that runs through the panels. Solar cells can also be used to power the supply air fan. These systems are very energy efficient but still quite expensive to install in houses.