Many of us have changed over to snow tires, raked the leaves (after they were covered in snow first) and closed up our houses for the long Calgary winter, from which we had a short reprieve of but a few months. Sometimes I wonder why we bother to take off the snow tires for the summer only to put them back on a few months later, but we diligently perform this activity twice annually.
When our furnaces start to come on more frequently (many of us don’t bother to turn off the furnace in Calgary during the summer months) we close up the house tight in preparation for the long cold winter months. This is the right thing to do of course, because we don’t want the air that we paid dearly to heat to escape so quickly. Of course all of it ends up escaping at one point or the other, but we want that to be as slow a process as possible, thereby increasing our home’s energy efficiency.
Now that we have trapped that air inside to keep us warm and keep the elements out, there is another consideration. We know that air in the home can be as much as 3 to 5 times more polluted than outside air (because the EPS told us so), so what can we do about this? Of course we could buy HEPA filters to take out particles including dust and mold spores, very fine particles at that. We can buy house plants that actually remove a certain amount of contaminants from the air.
The fact remains, however, that humans require fresh air to be healthy. You may be surprised to know that building codes for commercial buildings require a constant supply of fresh air in the order of 15 cubic feet per minute per person, while homes do not have the same requirement. Folks that follow my writing and website are aware that our homes inevitably contain contaminants such as mould spores, formaldehyde, VOCs and the like. We can endeavour to remove all of the sources of these contaminants, but in the real world that would be very difficult to do in short order, unless one is planning to raze their house to the ground, and start again with all new and healthy materials and furniture.
Most homes built today have a fresh air intake installed on the furnace, which is rated to bring in, depending on the house, 65 cubic feet of cold air from outside into your cold air return stream to the furnace. The furnace heats up this cold air stream and distributes the heated air to your home via the furnace ducting. You might be tempted to think: I have 4 people in my family, 4 times 15 is 60, so that is enough fresh air for me and my family. That is tempting, but the reality is somewhat different. First of all, the ventilation is only occurring while the furnace fan is running, which in the case of most homes, is while the furnace is on and heating the air. Very few people run their furnace fan continuously, even when heating is not occurring. So one method to continuously bring in fresh air is to leave the furnace fan running all of the time. But wait, there is a fresh air intake, but if my home is sealed up tight like many new homes are today, how does the stale air get out? Very good question, and a difficult one to answer. Some air may be getting out through exfiltration (the process of air loss through the building envelope), but in reality less than 65 cubic feet per minute is escaping. This means that while the furnace is rated to bring in 65 cfm of fresh air, in reality it is doing no such thing, unless at least one exhaust fan is running in your home. Normally the 65 cfm rating is designed to work in conjunction with one of the bathroom exhaust fans in a bathroom on the uppermost floor of your home. So, to get the required ventilation on a continuous basis, you would need to run your furnace fan continuously while at the same time running the top floor bathroom exhaust fan continuously. In fact, in many homes, there is a switch right next to the thermostat called a “Principle Ventilation” switch. This switch does exactly that – it turns on the furnace fan and the bathroom fan at the same time to ensure proper ventilation. To sum up, if you want to get the full ventilation capacity of your furnace air intake, then run your furnace fan continuously and the bathroom exhaust fan continuously, or simply leave your principle ventilation switch in the “On” position at all times.
But what if you don’t have a fresh air intake on your furnace? For the untrained eye, it may take some time to determine if you do or not. Go into your furnace room, and look for a pipe (normally insulated) that goes from an exterior wall to the cold air return of your furnace. In some cases, on your furnace you will see a tag that indicates the ventilation rate in cfm. If you see neither of these, then there is a chance that you do not have a fresh air intake. Do not confuse this with the “canister” intake that may be attached to the side of your furnace. This intake is designed only to bring in enough fresh air for combustion of the natural gas in the furnace. It is NOT intended to supply fresh air for the occupants of the home.
If you do not have a fresh air intake on your furnace, then getting fresh air into your home is a challenging task. You can run your bathroom and kitchen exhaust fans all you like, but there is no source for “makeup air”. In other words, the fans may only be serving to depressurize your house, with the only air coming in being through “infiltration”, the process of air leakage into your home via cracks and holes in the envelope. However, if you open a window while you run the exhaust fans, then sufficient makeup air should be allowed to come into your home. Therefore the second ventilation method is to continuously run a bathroom exhaust fan while leaving a window open, for those home that have no installed ventilation. You might be inclined to think that this method is much worse than the cold air intake to your furnace, but it really accomplishes the same thing, but in a less controlled manner. In both cases, you are letting in cold air from the outside, and exhausting heated air through the bathroom fan. The furnace intake will probably let in more fresh air due to the pressure of the furnace fan, but you could overcome that without the furnace intake simply by running two bath fans continuously with a window open. Another caution is to make sure that the window you leave open is not directly next to contaminant sources, such as your neighbours furnace combustion exhaust. And you want to make sure that there is a clear path from the open window to the exhaust fan. If there is a door in between that closes very tightly, then the air flow is not guaranteed. Also, if you open the window in the bathroom where the exhaust fan is located then you are likely only ventilating the bathroom, and not the entire house.
But wait, the rabbit hole goes deeper. According to Natural Resources Canada, for proper ventilation and a healthy home, our home should have 0.3 complete air changes per hour (approximately one full air change every 3 hours). To accomplish this, the average home would need in the neighbourhood of 100 to 150 cubic feet per minute of ventilation (depending on size and natural air leakage). This could be accomplished in most cases with the above-mentioned methods, but would result in a significant amount of heated air being exhausted with a corresponding amount of cold coming in.
There is a solution that provides the best of both worlds. One that will allow you to bring in fresh air, exhaust stale air, without losing the heat of the exhausted air. This requires the installation of a ventilation unit called a Heat Recovery Ventilator (or HRV for short). This device exhausts stale air from your home and brings in fresh air (with fans), and the heat of the air that is being exhausted is transferred to the cold air coming in from outside. In the situation where you are building a new house, you would run separate ductwork for the HRV to rooms such as the kitchen and bathroom. In an existing house that method can be complicated an overly expensive, so the installation makes use of the furnace ductwork to withdraw the exhaust air from the home, while supplying fresh air. While this is the most ideal situation for your home, and is guaranteed to improve the health of the occupants, it does come at a price. The HRV itself costs in the neighbourhood of $1000, plus the cost of any ductwork. Installation is another matter, and could run anywhere from $750 to $1500, depending on the contractor and the complexity of the installation. There are some cost savings of having an HRV though. Consider the savings in any health care, days missed from work, and no requirement to have a HEPA filter installed in your home. Would you put a price on your health?For many of us, the cost of an HRV is just out of reach, but we all need to have fresh air to remain healthy.
Therefore, in the absence of an HRV, please use one of the other methods described above to ensure a continuous supply of fresh air to your home. While there will be a small cost in the form of extra energy usage, keep in mind that most homes today are much more energy efficient and airtight than they were 20 years ago.