A true heat recovery ventilation system is a ducted ventilation system that consists of two fans: one to draw air in from outside and one to remove stale internal air. An air-to-air heat exchanger, often installed in a roof space, recovers heat from the internal air before it is discharged to the outside, and warms the incoming air, as the schematic drawings further down the page show:

Heat recovery systems typically recover 67–95% of the heat in exhaust air.

Advantages of heat recovery systems include:

• as these systems reduce heat loss rather than create heat, they are relatively cost effective
• they can readily be combined with heat pumps
• they allow effective ventilation where open windows are a security risk and in windowless rooms (eg interior bathrooms and toilets)
• they can operate as ventilation system in summer, by bypassing the heat transfer system and simply replacing indoor air with outdoor air
• they can reduce indoor moisture in winter, as cooler air outside will have lower relative humidity.

Remember, these are not heating systems, so some means of warming the house such as heat pumps may still be required.

Some domestic ventilation supply systems which draw air into a house from the roof space are advertised or promoted as heat recovery systems. When specifying, check that the proposed system actually incorporates a heat recovery device.

Similarly, some heat recovery systems allow moisture to pass to incoming air; if removal of moisture is required, specify this carefully.

Heat recovery systems meet the requirement of fresh outdoor air ventilation in Building Code Clause G4 Ventilation.

Heat recovery system size

The Building Code requirement for fresh outdoor air ventilation requires mechanical ventilation for occupied spaces in accordance with NZS 4303. It sets the rate at 0.35 air changes per hour, which is equivalent to approximately one-third of all the air in the house being changed every hour.

To determine the size of the ventilation system required, calculate the internal volume of the house or part of the house that is required to be ventilated and multiply the volume by 0.35 to get the minimum volume of air changes per hour, as in these examples:

• for a house with a floor area of 80 m² and an internal volume of 192 m³,
  multiply 192 x 0.35 = 67.2 m³/h
• for a house with a floor area of 250 m² and an internal volume of 600 m³,
  multiply 600 x 0.35 = 210 m³/h.

You need to allow for airflow resistance through ducting when calculating the required capacity.

Ducting

Ducting should always be the largest size possible, as the larger the ducting diameter, the better the airflow performance and the quieter the airflow noise. A typical duct size is 200 mm diameter – this should be used as much as possible, then reduced to 150 or 100 mm diameter if required to the ceiling vents or grilles, as in these examples:

• a 100 mm ceiling vent can supply fresh air to a room with an internal volume of 40 m³
• for a larger room, the ceiling vents or grilles, both exhaust and supply, should be at least 150 mm in diameter, or two or more ceiling vents of 100 mm diameter each could be used.

When designing the ducting system, ensure:

• the internal surface is as smooth as possible to minimise air flow resistance
• there are no tight bends as these can cause significant air flow resistance
• when bends are unavoidable, they are as large diameter as possible, and the number of bends is kept to a minimum
• the ducting is insulated to reduce heat loss and duct noise
• there is a condensate drain for the exhaust ducting, to allow the removal of moisture formed when the heat is removed from the air.

Air supply and exhaust vents

Locate air supply and exhaust vents (or grilles) to maximise the system’s performance:

• locate supply vents in living areas such as living room, dining room, study and bedrooms
• locate exhaust vents in kitchens and bathrooms, so that kitchen odours and moist air are not drawn through the living areas before being vented
• alternatively, supply vents can be located on opposite sides of the house with an exhaust vent in the hallway or a central location in the house – the fresh, warmed air is delivered to the perimeter of the house (living rooms and bedrooms) and flows through to a central exhaust vent
• indoor supply and exhaust vents should be located some distance apart inside rooms, to maximise the fresh, warm air circulation through the space
• outdoor air supply and the exhaust air discharge vents must be far enough apart separated to ensure that exhaust air is not collected by the fresh air intake – if possible, they should be located on opposite sides of the house.

For your clients

To help your clients make decisions about space heating options, refer them to www.smarterhomes.org.nz/energy/heating.