Many of our homes have been built or retrofitted with energy efficiency in mind. That means tightly sealed walls and doors that don’t let in much outdoor air, which keeps air conditioning or heated air inside. This approach saves power and money, but can cause other problems when outdoor air can’t get in and indoor air can’t get out.
There are many sources of indoor pollution, cooking, cleaning, dust, artificial scents and more all contribute to the fact that the concentration of some pollutants is two to five times higher indoors than outdoors. Any occupied room has at least one source of air pollution; the occupant, who exhales carbon dioxide that, at elevated levels, can cause drowsiness and impair decision making. Household activities compound the issue, cooking in particular can be a serious air quality hazard, potentially sending the air quality deep into the purple or “unhealthy” territory for a few hours.
Some of the best advice is just to ventilate with more outdoor air, but that can waste energy spent on keeping the air cool, warm, or at the right humidity. Fortunately there is a device known as an air exchanger to fix this exact problem. These devices can bring in more fresh air and control humidity while conserving energy spent on climate control. Let’s take a look at how to use one and if an air exchanger might be right for your home.
Trade air, not conditioning
Air exchangers work on simple principles, but their design is not simple. The basic idea is that indoor air gets swirled through a heat exchanger along with outdoor air. The temperature of the indoor air is transferred to the outdoor air and vice versa. Both streams of air are filtered as they enter the device, and it also serves as a dehumidifier by removing moisture from the air.
This diagram shows an air exchanger as it would operate if the indoor air had been heated up by a furnace. The temperature flow would be the opposite if indoor air had been cooled by an air conditioner.
In cold winter months, warm indoor air holds a lot more water vapor than cold outside air. Whenever that indoor air hits a window or other cool spot, the vapor condenses into liquid water, which can damage wood or metal and also provide a habitat for mold to grow. So keeping the right relative humidity (about 30% to 50%) is important during this time.
There are two general types of air exchangers, heat recovery ventilators (HRVs) and energy recovery ventilators (ERVs). HRVs are an older technology and a little more reliable, but they can’t cool off incoming air to save on air conditioning, they can only warm it up to save furnace power. ERVs can do both, but are a little more complicated and require more maintenance.
How to use an air exchanger for the best air quality
People with allergies or asthma are going to get the most immediate benefit from an air exchanger because it will control humidity to prevent mold growth and also replace indoor air laden with dust and built up chemicals with cleaner outdoor air.
Air exchangers have a dial or slider to control the humidity called a dehumidistat, and is set for relative humidity exactly like a thermostat would be for temperature. Whenever the humidity exceeds the set level, the air exchanger will activate. It should be set around 40% to prevent mold growth.
If your home is feeling dry, up to 55% should still be dry enough to prevent mold, but there will be more moisture in the air so out-of-the-way corners and windowsills might still get wet. On the other end, if you’re concerned about mold or are allergic to dust mites, reducing the dehumidistat to 30% can help to curb the growth of both. Do keep in mind that pretty much all homes have at least some mold and some dust mites.
While they usually are set to respond to high temperature, dehumidistats are also available for air conditioners so they respond to high humidity. Air coming out of a traditional air conditioner is generally fairly dry due to the way most air conditioners operate, though swamp coolers or evaporative coolers add humidity to the air. A dehumidistat on an air conditioner is likely to require special installation, but are otherwise inexpensive at around $30 to $50.
Air exchangers can also be a great way to reduce allergy and asthma triggers. The incoming air is filtered, and many air exchangers can use a high-efficiency filter like a HEPA or MERV 16 that removes pollen and mold spores from outdoor air. This would be in addition to removing air from indoors that could be laden with dust and other allergy and asthma triggers.
What kind of air exchanger to get
There are a few different ways to move energy from one air stream to another, with varying degrees of efficiency. It depends on how your home is configured, but many air exchangers can reach an efficiency of 70%, so don’t settle for anything less than that. The Home Ventilating Institute has great information on how to think about adding an air exchanger to your home, and also tests and certifies ventilation products like air exchangers. So do a little research or get a professional opinion from an HVAC technician before buying.
It will cost somewhere around $3000 to purchase and install an air exchanger, so if you’re looking to reduce your power consumption by conserving heat or cold, some simple math with your utility bill can tell you if it’s worth it over the long run. If you’re looking to keep moisture in check to prevent mold growth, an air exchanger is almost certainly worth it. Replacing moldy drywall can easily run thousands of dollars, fixing a moldy HVAC system is almost certainly going to cost more than the air exchanger, and there is no price to put on healthy breathing.
Header photo by Peter Klauss