There are three main types of ventilation systems.

Forced-air ventilation systems

These blow dry air into your house from the roof space above the ceiling. They work best where there is a decent amount of space in the roof. The roof space must also be dry and reasonably warm on sunny winter days.

Note: Strictly speaking, roof space air does not comply with the building code requirements. Good filtration is essential.

Most forced-air systems make a big thing of the ability to recover heat from the roof space. This is often possible - but not all the time. There won't be much warm air up there on frosty nights. To cope with this, most systems automatically slow or even stop the airflow when the roof space temperature falls below certain levels. That means your ventilation slows down or even stops, unless there is a heater fitted.

Heat-exchanger ventilation systems

These extract warm damp air from living spaces, remove the heat using a heat exchanger, transfer this heat to cold dry air from outside, then pump it back into the house. These can work even where there is no roof space, for example in an apartment.

These heat-exchanger systems have two advantages. They run all the time, regardless of air temperature in the roof. And they fully meet the building code's requirements for ventilation with outside air. But a properly designed heat-exchanger system for your house can be quite expensive.

Solar-powered ventilation systems

These use the sun to heat up panels, which are mounted on the roof or an outside wall. These panels then warm the incoming air, with solar cells being used to power the fan. So there are no running costs with solar-powered ventilation, and you get fresh outside air that's nicely warmed up. There's also an optional cooling kit for summer ventilation.

But solar power depends on getting adequate sun, and won't suit some locations. You'll also still need additional heating in the house at night - and on very cold wet days.

We've found only one solar-power model suitable for a typical-sized New Zealand house, and it's quite expensive.

"Heat recovery" units

There is quite a lot of confusion over so called "heat recovery" systems. The most effective type of "heat recovery" unit is the heat-exchanger system, which extracts warm damp air from living spaces and passes this through a heat exchanger to heat up dry air which the system brings in from outside. Inline heaters are not required.

This type of system conforms to the heat recovery definitions of the American Society of Heating, Refrigeration and Air-Conditioning engineers (ASHRAE). Local ventilation standards are based on ASHRAE standards.

Another type of "heat recovery" unit, sold by HRV, claims to recover heat from the roof space. In our opinion, this is a forced-air system. The HRV unit can undoubtedly extract warm air from the roof space on sunny days (like a forced-air system does). But HRV also offers the option of inline heaters "for reduction of relative humidity", which means that the unit doesn't operate on true heat-exchanger principles.

Which system is best?

The best system for you depends on the design of your house, its floor area, the location, how much sun the house gets, the type of roof ... even the local climate.

So before you install any system, do some homework. We've outlined how the different designs work, and some of their pro's and cons which should let you decide if your house is suitable for any of the different types of system available. Consider what you want to achieve against the types of system on offer - and also consider any extra features you might need, to meet your particular requirements.

For best results, a system should be designed specifically for your house and your needs. But this can be quite costly, especially if you choose a fully automated system with multiple outlets or other options.

We recommend getting quotes from several installers who are familiar with your local climate.