Testing ventilation systems

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Forced air ventilation systems are increasingly being promoted as the solution to house performance problems such as dampness and cold.  However, very little research has been done to show what sort of houses, climates and situations they work in. Given the large investment required to install one of these systems, and the need to address cold and damp in New Zealand homes, Beacon commissioned a study to understand if these systems are providing a solution and, if so, under what conditions.

Ten homes with forced air ventilation systems in Christchurch and Wellington were selected as case studies.    Eight of these houses were tested for airtightness using blower door testing, and all ten had their temperature and humidity levels monitored to establish how effectively the systems make use of “free heat” and the impact they have on humidity levels within the house.


Did the homes need ventilation?

The airtightness testing revealed that, rather than needing a mechanical system for ventilation, nearly all the homes were draughty.  Draughty is defined as approximately 0.9 air changes per hour.



Air changes

/ hour



































As the case study homes were also part of the HomeSmart Renovation project, their condition was assessed as part of the In-Home Assessment.  This showed that the cold and dampness in the homes was not because of lack of ventilation but rather caused by:

  • Lack of insulation
  • Not enough heating
  • Uncontrolled moisture sources (leaks, ground water, unvented wet areas, unflued gas heaters)


Measuring humidity

Rather than just using relative humidity which takes the percentage of moisture in the air, the researchers also used the humidity ratio - the ratio of the mass of water vapour in moist air to the mass of dry air.  This makes a difference because warmer air will have a lower relative humidity than colder air; however, heating air does not add or remove moisture to the room so that the humidity ratio will be the same.


Did the systems improve dampness and temperature?

Using humidity ratio measurements, monitoring of the roof space air and the air delivered into the house shows that in winter during the day there is more moisture in the roof space than in the house. However, the system is turned on by a thermostat based on roof space temperature which will increase with solar heating over the day.  This means that the increased operation of the ventilation system during the day actually increases the moisture in the house during the day.

Equally, operating the ventilation system at night takes drier air from the roof space, thereby reducing the level of moisture in the house. However the roof space temperatures are colder at night and will cool down the house as well.

The control systems in forced air ventilation systems are controlled by temperature and not by the humidity ratio or moisture content of the air. These sensors do not provide the necessary information to the system to reliably reduce the moisture levels in the house.  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. There is clearly an opportunity to develop forced air ventilation system controllers that consider all of these aspects of building performance. It may be that the controller is set to a particular function, such as managing relative humidity (and therefore condensation risk), moisture removal, harvesting heat or increasing ventilation for a short time to remove odours (‘burnt toast’ setting).

  • 29-Jun-2010 (Report IEQ7570/3)

    Forced Air Ventilation Systems (PDF 2MB)

    Andrew Pollard, S. McNeil

    This report discusses the methodology and results from monitoring  ten forced air ventilation systems in Christchurch and Wellington. Results showed most homes were already draughty and needed no further ventilation.  Additionally, operation during the day would increase the moisture levels within the house while night time operation would tend to reduce moisture levels, but also reduce house temperatures. Forced air ventilation system controllers could be more effective if they are better able to distinguish when favourable times to operate are.

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