New Zealand’s houses have traditionally been ventilated (often to great excess) by air flows through gaps around windows and doors and by flows through penetrations in the building envelope, especially up the chimneys of open fireplaces. They were also a reason for houses being difficult to impossible to keep warm.

Reducomg the air flow in and out of homes can improve living conditions and cut energy consumption.  But it is also likely to exacerbate moisture build-up, with the risk of mould growth and damage to the building structure, furnishings and fittings. Mould is also thought to be closely associated with the incidence and severity of asthma (rates of which are high in New Zealand by international standards).   Greater awareness of moisture as a problem has lead to increased sales of dehumidifiers and mechanical ventilation systems. 

This study evaluated mechanical ventilation systems in terms of:

1. How the various broad types of system work, including how they controlled and how frequently they run;
   

2. Impact on the condensation risk in cavities of lightweight construction;
   

3. Impact on natural infiltration rates of houses (i.e. does it change due to the pressure difference created by some of these systems?);
   

4. The  benefits or problems of using the various types of systems in both uninsulated, poorly-heated homes and insulated, well-heated homes;
   

5. Potential risks of using these systems including their behaviour in the event of fire either in the house or in the roof space, and any appropriate safety systems that the different types of ventilation system use; and
   

6. Calculation of the typical payback economics of the key types of system if increased heating efficiency or other benefits can be identified.
   

The simplest of these systems take air from the ceiling space, pass it through a filter and blow the air into a single ceiling mounted diffuser, usually located near the centre of the house.  More sophisticated systems blow the air into several rooms from the one device, with more than one filtration and fan system for the largest systems.    A feature of these systems is that they only supply air into the house.  No specific provision is made for the corresponding flow of outgoing air.  Air leaves the house through whatever gaps and penetrations exist through the building’s external envelope. These are typically sub-optimal from the ventilation viewpoint and airflow through the house is likely to be quite uneven.  

In terms of energy consumption, the promotional material distributed by the suppliers often make bold statements about their heating benefits, such as: “Dramatically reduce your winter heating and summer cooling bills …”, and “Your roof is a giant solar panel heating the air in your roof space even during winter days”.

These statements can be highly misleading: 

1. New Zealand winters typically have many cloudy days when little or no heat will be obtained from the roof;
   

2. Useful amounts of heating will be gained only on those days when the sun is shining, while the sunny days that do occur are the ones when least heating is needed.  Even then heating will be obtained at most, only from mid-morning to mid-afternoon; 
   

3. Roof spaces can become very cold at night, due to outward radiation from its external surface. There are likely to be times when a unit could supply air at a temperature below that of the outside air.  Most systems have controls to switch the unit either completely off or at least turned down to a low flow rate;
   

4. The actual heating effect depends on the sunshine hours at the specific location of the house, and the time of year. For example, in Wellington sunlight is recorded for 14% of the hours in June and 16% in July. Only houses not affected by any shading by hills or trees will experience this degree of sunlight.  Also, the sunlight at the beginning and end of the day is at too low an angle to provide useful heating;
   

5. Many houses are frequently unoccupied during the day, so the occupants will not fully benefit from the heat which is gathered during the day, while still experiencing the unheated air provided at night.  Some heat may be carried over due to thermal storage in the building fabric, and some energy savings may be obtained if heating is left operating even when the house is unoccupied, but only if the heater is thermostatically controlled;
   

6. A limiting factor on the heating benefits is that when warm air is being introduced from the roof cavity it is released into the room at ceiling level.  If this air is warmer than the air in the room it will tend to hug the ceiling.  This may result in stratification and not provide the degree of comfort that could be provided by a floor level heater. 
   

Some of the advertising claims made for these systems are of concern. Assertions are made, for instance, that “customers can save money on buying expensive heaters”.  Since the heat supply from a roof space source ventilation system is very intermittent in winter, and non-existent during winter evenings, the need for a primary heating system is not changed appreciably.  Claims that these systems “can act as an air conditioning unit, providing you with the perfect inside temperature all year round” and that “when you come home from work on a freezing cold evening, you will not have to worry about warming up the house, as it will already be cosy and dry” far exceed what is possible on a reliable basis from an intermittent energy source.  Overall, there may be be no reduction in energy required for room heating at all, and any heating gains that do occur may be offset by the not inconsiderable amount of electricity used in running the fan(s). 

Other aspects of the evaluation also highlighted issues with these systems including risks of condensation in walls; the drawing into the occupied rooms of air from a space that can be quite dirty (including residues from birds, rodents, spiders and insects), and the potential to exacerbate the spread of gases within the home in the event of a fire when, depending on the duct material, such systems may provide a route for a fire into the roof space earlier than  might otherwise occur.