If you want the best system on the market have a look at this site http://www.cleanaire.co.nz/

They market a true HRV system not one of the fake systems that suck warm air out of the roof space. Basically they have a vent at one end of house that sucks air from outside into ducting which is connected to a stainless Heat Exchanger. Every room in house has an input and output in ceiling which are ducted back to exchanger. The warm air in house is sent through the exchanger and the air is then exhausted out of the house at opposite end from fresh air inflow. The warm air in the exchanger heats the cold air from outside and which  is then injected into each room.

An amazing system that actually works. If you dont have heating in house you can have inline heaters in the fresh air line.

If you want to completely get rid of condensation/mold  and have a continuous flow of fresh air into every room this is the device.

t0ny:

 

Been doing some additional research. Some interesting facts:

 

The New Zealand Building Code requires homes to ventilate using outdoor air to maintain air purity. Ventilation systems that draw air from the roof space and not directly from outside do not comply with ventilation standard NZS4303:1990 "Ventilation for acceptable indoor air quality". They cannot be used to comply with the Building Code Acceptable Solution for ventilation.

 

and, a research report on ventilation systems states:

 

An ideal control system for forced air delivery from the roofspace clearly needs to take account of the temperature and humidity in both spaces. With this data, the system will be able to determine when operation will reduce the relative humidity in the building.

 

My PPV system doesnt appear to be getting the outdoor air at all :)