Energy

Mechanical ventilation systems may be necessary where passive ventilation is not sufficient to meet Building Code requirements.

On this page:

• Components of an air supply ventilation system
• Fresh air heating
• Ventilation system types

Some air supply ventilation systems provide heating as well as ventilation. With air supply ventilation systems, only a small amount of energy is required for air movement.

Components of an air supply ventilation system

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

The air intake should be located where it will not draw in polluted air.

To minimise energy use, fans should be sized and controlled to move only the amount of ventilation air required. Most fresh air ventilation supply systems are designed to run continuously, with fresh air flow rates at around 3–20 litres per second.

Fresh air heating

Many fresh air supply ventilation systems provide some heat to the supply air flow before it is drawn indoors. Heating air brought from outside can consume substantial amounts of energy, particularly in winter when the outdoor air temperature is low. This can be reduced by using a heat recovery ventilation system, or 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, e.g. on the sunny side of the building, near warm surfaces or 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 not heated more than to make the occupants comfortable.

Ventilation system types

Heat recovery ventilation systems

Heat recovery ventilation and energy recovery ventilation systems are ducted ventilation systems consisting of two fans – one to draw air in from outside and one to remove stale internal air. An air-to-air heat exchanger, generally installed in a roof space, recovers heat from the internal air before it is discharged to the outside, and uses that heat to warm the incoming air. These are not heating systems but do reduce heat losses from the building.

See heat recovery ventilation systems for more detail.

Positive pressure/forced air systems

Positive pressure/forced air systems bring air inside to create a positive, internal pressure that forces air movement outwards. The air will leak out through gaps and open doors and windows.

For these systems to be effective, the supply air flow rate must be higher than the leakage air flow rate. The more airtight the home, the more effective the system.

Solar energy heating/ventilation systems

Solar energy heating/ventilation systems use solar energy that has been absorbed by solar panels 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 quite expensive to install in houses.

Ducted warm air transfer systems

Ducted warm air transfer systems take warm air from the roof space or some other interior space and transfer it to the space to be heated. In BRANZ’s view, these systems do not meet the minimum ventilation requirements of the NZBC clause G4 Ventilation as they do not introduce outdoor air to indoors.

These systems require adequate sunshine to heat the heat source area, ambient temperatures that are not too low, sufficient roof space to provide a heat source, limited moist air penetration from the spaces below, and air filters to ensure particulates from the ceiling space aren’t brought into the home. Filters commonly require replacing every 12 or 24 months.

A ducted warm air transfer system that utilises roof s/e warm air during the daytime when the sun has been shining. On overcast days and at night, the heat source (roof space) air temperature may be colder than the room air temperature. They must therefore be able to be slowed or stopped to prevent heat loss at these times and supplementary space heating will be required.

EECA and the University of Otago funded research into ventilation systems that use roof space air. Among the findings: “The majority of the time, it was calculated that pumping air from the roof space into the house would provide no heating or cooling benefit. In fact, this would often actually act to push the internal temperature further away from the desired level rather than closer to it.

Updated: 25 February 2021