Ventilation systems come is several different types.

Postive Pressure - The most common system type sold in New Zealand. They work by bringing in filtered air, usually from the roof cavity. They are an effective form of ventilation and many add-ons are available. Most common systems do not operate when the air temp is cold and switch off as they bring too much cold air into the house. Look for systems which continue to ventilate by using minimal airflows so as not to affect thermal comfort.

Balanced - These systems are design to supply and extract air from within your house. They supply fresh filtered air to the habital zones of your house, and extract stale, used air from those zones where it exists. These systems can be fitted with extra sensors to monitor Carbon dioxide, VOC gases, temperature and humidity. Most are fitted with a heat/energy recovery core to return this back from the extracted air to the supply air. Heat recovery units must have a core bypass for summer time, and use EC fans otherwise you will consume more energy running the unit than you recover.

Extraction - One of the most over-looked part of ventilation. Extracting the air at the source of the pollutants entering your home is of the utmost importance. It takes 10 times more air to dilute this air as it does if extracted correctly. Most bathroom fans are just not up to the task. Extraction from these zones is complex as too much extraction can also cause big issues. Look for products which monitor and control the extraction required to control these zones. A run-on timer helps but this is a fixed time. Extraction timez vary due to climate conditions and the reason why these spaces are used. Look for on-demand controlled products which automatically switch on and deal with condensation correctly. We believe these types of fans should operate between 10l/sec and up to 50 l/sec.

A effective filter is important not just for ensuring the air is clean, but to protect the system components as well. Most systems claim they have the highest, and most effective filter on the market. A good F7 filter is more then enough for most systems. In January 2017 internal filter ratings changed to the PM filter compliance. This new method of testing included filter effeicencies over the useful life of the filter. PM = particulate mater.

Only choose systems which have filters rated to the new regulations for filters; e.g. minimum should be PM2.5. Also multistage filters give you the best performance. Any system which costs you more than $120 to have a filter replaced is either poorly designed or you are being ripped off. Look for systems where filter replacements can be done by yourself.

AC fan systems are a lot more costly to run and should be avoided. It's not just a few more cents as claimed by those selling these systems.
AC fans cost up to $250-$300 a year to run. And an important fact is that even when using a variable speed AC fan, the power consumed remains the same. They just run less efficiently. AC fans are old technolgy and should be avoided
EC/DC fans - can provide you with the same airflows for about 60% less energy than AC fans.
Smarter EC fans are able to operate with much greater performance in both low and high speeds than AC fans. Typical AC fans can only run down to 60% of the full speed, whereas EC fans run down to 10% of full speed. This allows for much greater control of the airflows required. AC fan systems bring in too much cold air at low temperatures, affecting your thermal comfort, and are therefore designed to be turned off, usually when you require ventilation the most.

The ducting used in the system is important. There are 3 types:
Non insulated - Most common systems use non insulated duct as the air source and the space the ducting runs in is the same temperature and no losses in heat occur. Cheaper types of this ducting are used, and can have a large impact on airflows. Most systems only show you the fan performance and never the system performance. Cheap ducting can reduce the airflow performance by up to 60%. Ducting is required to be stiff enough to hold its form to ensure the inside is smooth. Ducting which cannot support itself will suck itself together under high airflows, consequently reducing the flow rate. This cheaper type of ducting also will not retain its shape once crushed
Insulated - If you plan to use add-ons like heat transfer or outdoor air then consider this type of ducting in your roof space. There is little benefit in installing this ducting if you are moving air inside the building's internal envelopes. It's just harder to install and takes up more space in wardrobes, adding no benefit. Acoustic ducting, if used, has a much lower insulated rating than non-acoustic insulated ducting. Expect heat losses up to 1 degree per 3 meters.
Acoustic ducting - All systems should include a minimum of 1.5 meters of this ducting after the fan. Any system without this ducting will transfer fan noise to the rooms. Acoustic ducting requires more care to install and is one of the major problems with systems. This ducting requires both inner and outer layers to be sealed. The outer sleeve is also prone to being torn during installation which causes leaks. This type of ducting also has a high pressure drop which reduces airflows, up to 40% compared to non-insulated ducting.

All fans make noise. While claims of the quietest fan on the market are made, you can't move air without making sound. AC fans with variable speeds due to electronic controls make a high buzzing sound at low speeds and should be avoided, as it also stresses the fan, shortening the life of the fan. Acoustic ducting of 1m should be included after the fan to remove fan turbulence noise. A system must include this ducting. Any more than 1.5 metres has very little effect on further noise reduction and just adds cost to the system. Fans which can run at under 30% fan speed generate noise levels under 20dB. This is a common speed for these fans at night when you are sleeping. Correct positioning and installation of the fan is also a very important factor.

Systems come with various warranties ranging from 3 to 7 years. Fan manufacturers rate fan motors in hours used. The highest rated bearing used in fans are rated at 50000 hours for AC fans and 70000 hours for EC fans. Ventilation systems are also required to be checked annually. A more important warranty, in our opinion, is proof the system is working correctly and doing its job. Airflows should be measured and checked against the original installation or, better yet, a monitoring system installed to test the system is operating effectively. Conditions in houses change, people change and the use of the house changes.