Ventilating a home: It’s about the numbers, not the windows
By Mark and Alexandra Parliament
While writing this article, we have the windows in our home open to let in the crisp autumn air, giving us plenty of natural ventilation. Each year, as we fall into winter’s grip, however, our windows close and our indoor air quality has a tendency to falter.
Thirty years ago, this would not have posed much of a problem as older homes are not sealed like the houses today. These older structures allowed for natural air changes to occur, which would flush out adverse air quality in our homes.
In our attempts to make buildings as energy efficient as possible, newer building techniques and product technologies have created increasingly air-tight homes. The downside to this is that we tend to trap more Volatile Organic Compounds (VOCs), Respirable Suspended Particulates (RSPs) and water vapour, all of which can contribute to poor Indoor Air Quality (IAQ).
Acceptable strategies for improving the IAQ for your clients depends on where in the country they reside or work. An exhaust- only approach is still being used in some provinces, whereas other provinces mandate the use of an HRV or ERV by code.
Ventilation: The process of “changing” or replacing air in any space to provide high indoor air quality.
A radioactive gas that comes from the breakdown of uranium in soil and rock, radon is invisible, odourless and tasteless.
When released from the ground into the outdoor air, it is diluted and is not a concern. In enclosed spaces, like homes, however, it can accumulate to high levels and become a health risk.
While present in all areas of Canada, concentration levels will vary from region to region and house to house. The only way to be sure of the radon level in a home is to conduct a radon test.
An Exhausted Approach
An exhaust-only system is simply a bathroom fan installed in the highest bathroom in the house.
While the fan is running, it exhausts air from the house, putting it into a negative pressure situation. This air imbalance will then be equalled out by infiltration air that is drawn into the home through cracks or crevasses in the building.
While relatively inexpensive to install, this system can cause significant inconvenience for the homeowner, especially if the house is sufficiently air tight as it can cause natural draft combustion appliances to spill. It can also cause soil gas issues, like radon, to be drawn into the house.
One of the more recommended strategies for mechanical ventilation is a balanced system with heat recovery. A Heat Recovery Ventilator (HRV) or an Energy/Enthalpy Recovery Ventilator (ERV) can offset the disadvantages of supply-only or exhaust-only systems.
An HRV is a factory-assembled unit that incorporates a means to circulate air for balanced ventilation. It also provides a way to transfer sensible heat between the two different air streams.
It accomplishes this by drawing stale air from within the house (usually from kitchens and bathrooms) and forcing it through a heat exchanger before exhausting the stale air from the building.
On the opposite end, it draws fresh air from outside through this same heat exchange to take advantage of the heat from the exhaust air, transferring it onto the incoming air, warming it up.
An ERV looks and operates much like an HRV, except that the ERV core will exchange both sensible and latent heat. This has led a significant number of HVAC contractors to believe that the HRV is to be used solely in colder climates and the ERV in warmer climates.
While it is true that the HRV will remove more moisture from a home during the winter months than an ERV, it will cause the air conditioner to work harder in the summer because, unlike the ERV that deals with latent load, the HRV does not.
It is important to remember that most manufacturers and codes require that the airflows be balanced within plus or minus 10 per cent of each other. Therefore, no matter which equipment is used, the most important aspect of any system is that it is balanced.
When choosing which method to use to gain the required ventilation for a home, contractors need to take into consideration not only the specific requirements for the amount of air to be supplied to the house but also how much is removed.
Pushing, rather than pulling
A supply-only system is another ventilation strategy that can be used, preferably in mild climates.
Here, a fan is used to force air into a building envelope in order to positively pressurize the building. This will cause the stale air to be pushed out of the structure.
A downside to this approach is that moist air can be driven into the building, which can cause mould propagation, but the positive pressure does help to disperse soil gas issues.
Sizing the Equipment
The capacity of a ventilation system in a home is based on the Total Ventilation Capacity (TVC). This is the minimum amount of indoor air that the ventilation system must be capable of providing.
In Canada, the TVC is determined using a room count method, where every room is assigned a specific airflow in CFM. Once the room count is complete, a ventilation approach can be selected to meet the code requirements. One of the main codes used is CSA-F326.
This code specifies that the system used to meet the TVC must be capable of continuous operation at the TVC, and has the ability to operate at 40 to 60 per cent of the TVC.
Planning a ventilation system in a modern home is not as simple as installing a fan in a bathroom or an HRV in the basement. Look at the building as a whole, since what gets installed in one area of the building can have a negative impact on another section of the house.
By approaching ventilation projects with the “House as a System” strategy, you can ensure that the ventilation system will not adversely affect other components in the home, like furnaces and water heaters.