What is mixed mode ventilation?
In much in the same way as a hybrid car operates on both petrol and electricity, a hybrid ventilation system is one that uses both natural and powered ventilation to produce the best cooling for the lowest cost.
How does mixed mode ventilation work?
Mixed mode ventilation can either be manually operated, or automatically controlled. A manually operated mixed ventilation scheme might be as simple in its operation as turning on an exhaust fan at the same time as you open windows for cross ventilation.
Automated systems are usually a fair bit more complex. These types of systems use sensors that recognise when natural ventilation is not doing a good enough job, and accordingly switch over to powered ventilation. If the outdoor wind speed and temperature were to change again, the powered ventilation would switch off and the system would revert to natural ventilation.
Improved ventilation, control and efficiency
While there are negatives to this approach (added cost, requires careful programming etc.), an automatic mixed-mode ventilation system actually gives you greater control over your ventilation requirements. Proper ventilation for improved air quality requires a given amount of 'air changes' in your house within the space of an hour. The number of air changes you need depends largely on the volume, position and surroundings of your home. The amount you can achieve depends on the flow rate of your ventilator.
In a typical brick-veneer home that features eaves and has trees near the windows, for example, around two to three air changes per hour should be sufficient. In an older style home with no eaves and no trees, it may require up to five changes an hour. A hybrid system will allow you to consistently achieve this rate in the most energy efficient way possible. It also allows you to change your requirements based on the time of day, or time of year.
Providing that your house meets various other passive design requirements (including control of thermal mass, infiltration and shading), mixed mode ventilation may allow you to cool your home for less than the cost of running an air conditioner.