In New Zealand, sufficient cooling can usually be achieved with passive design options such as shading and allowing natural summer airflows and ventilation, given that indoor air temperature comfort levels range from about 18–24ºC, and our current outdoor air temperatures do not often exceed 25ºC.

Some situations may still require active cooling, to provide backup or boost a passive system when temperatures and/or humidity are well above average.

Active cooling is more likely to be required in rooms:

• with high solar heat gain or poor natural ventilation
• that are used by a lot of people, or used for long periods
• where people are very active (such as a rumpus room)
• with lots of electrical equipment (such as lights and computers in a study)
• in locations where weather conditions do not allow passive design to work effectively, eg strong hot or dry winds, dust, pollen
• in calm, hot and humid climates.

Mechanical cooling options are fans, heat pumps, evaporative coolers and air conditioners.

Fans

Fans move air and increase air velocity within a room. They don’t actually change the air temperature, but the increase in air movement makes people feel cooler. Fans can be fixed or portable. They are generally cheap to buy, relatively easy to install, and comparatively cheap to run.

Ceiling-mounted fans are often effective at moving the air in a room to provide cooling. They can also be useful in winter as they can be used to move the warmer air at ceiling level back down into the occupied space where it is more useful.

Heat pumps

In New Zealand, reverse cycle heat pumps have tended to be used to provide cooling only about 15% of the time, as they have primarily been installed to provide heating. However, as more people with heat pumps in place have begun to use them for summer cooling as well as winter heating, their use is increasing – along with the electricity costs of households that have traditionally not used any active cooling.

A heat pump is a very efficient method of cooling, typically providing 2.5–4.0 times the amount of cooling energy than the electricity it takes to run (this is the Energy Efficiency Ratio (EER)). The EER for a heat pump will be listed by the supplier along with its heating Coefficient of Performance (COP).

It is important to note (and to advise your clients) that heat pump energy efficiency decreases as the difference between the indoor and outdoor temperature increases, ie as it gets hotter outside it takes more time and electricity to cool the air inside the home.

Evaporative coolers

An evaporative cooler uses a fan to move hot, dry outside air into the house through a wet filter. It adds moisture to the air, and the air temperature drops as warmth is used to evaporate the moisture. Evaporative coolers are only efficient in air that is consistently dry. They aren’t commonly used in New Zealand, and should not be used in rooms that already have moisture problems. Evaporative coolers:

• are comparatively cheap to run as the only energy used is to run the fan – approximately 80% less than a refrigerated cooling system
• are simple and inexpensive to buy – installation costs are lower than refrigerated systems by about 50%
• use a significant amount of cold water
• do not use a refrigerant
• require regular filter maintenance
• may require windows or vents to be left open to allow airflow.

For your clients

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