How to “Hack” your HVAC system to Improve Air Quality

Residential HVAC manufacturers, just like any other product manufacturer, are driven by profitability.  Basic “plug and play” HVAC installers and builders look for the best value for cooling and heating capacity, and this type of demand does not drive manufacturers to design for better air quality.  Thus, it’s left up to the homeowner, often when they start to have a mold or allergen problem in their home, to discover that they need to make some changes to the HVAC system–quickly.  Here are some aspects of HVAC systems we’ve seen in clients’ homes that typically cause problems, and how you can resolve them.   

As in all aspects of homebuilding, adding these principles to your design before building or renovating is the most affordable way!  Making changes after you start building or installing takes time and of course, more money.

1. Air handlers installed inside the home’s envelope are going to have better air quality than those installed outside the envelope.  For example, an air handler sitting in the unconditioned attic or even worse, crawl space, will be subject to extreme heat/cold.  It will be a “weak spot” for enabling dust and microbes from these areas to be sucked into the system.  It will be more difficult to access for maintenance, which is important for the life of the system and your air quality.  In short, find a way to get your air handler inside your home’s envelope.  If you’re considering replacing your AC system, a small remodel to make a “closet” for your air handler to live in is WORTH IT.  I did this in my own home and by blocking off a space at the top of my tall stairwell, I was able to move the air handler from the attic to a new “closet” that I could access through my bedroom.

2. Air handlers that are made to install vertically or horizontally will have more mold when installed horizontally.  Why do we say this?  Because whenever the air going through the air handler is humid, condensate is going to accumulate outside of where it’s supposed to, and without “gravity” to help drain the condensate, it will tend to pool on horizontal surfaces.  Moisture that stays in one place is going to help feed mold.  When possible, install the air handler vertically. 

3. Internal insulation is a place for dust and dirt to attach, which combined with humid air, facilitates mold.  Most air handlers have about 1” of foil-faced or fibrous insulation inside them. First of all, 1” of insulation is not a lot when you consider the extreme temperatures that they face when installed outside the home’s envelope.  Secondly, this insulation can be located in a place where condensation collects (see #2) such as the bottom of a horizontal installation, and it will be constantly saturated with water.  Foil-faced insulation is better in that the surface can be cleaned, but the fiberglass under the foil will still hold standing water.  If we had our way, internal insulation would be removed and the entire unit “blanketed” instead.  This makes the unit a bit harder to access for maintenance, but cleaner on the inside. 

4. Likewise, internal insulation in the supply plenum and ducts is a breeding ground for mold.  It’s impossible to clean thoroughly.  For this reason, specify only an externally-insulated plenum and ducts.  Replace a moldy supply plenum with a new externally insulated plenum.  You will be thankful you did!

5. Optimize the return: this is the “suction” side of your air handler.  

1. Increase the HVAC filter area so you can install a larger, higher MERV filter (see our article for calculating the size you need), or install a filter cabinet for a thicker high-MERV filter (MERV 13 is the minimum for good filtration for smoke particles).  This is a high-value, not high-cost upgrade to consider when you're installing a new system.

2. Make sure the return duct is tightly sealed, especially if the air handler is in an unconditioned space.  Since the return duct and air handler all the way up to the fan are in negative pressure, any leaks in this part of the system will pull in humid air from surrounding areas.

6. Consider whether you need a whole-house dehumidifier to take the humidity out of your air handler. (Should You Install a Whole-House Dehumidifier?)

7. If you are replacing your system, consider an air handler that can provide reheat dehumidification (see our article here).  

8. Otherwise, consider installing or replacing with a high-SEER inverter system, which are generally better at dehumidifying than standard, fixed-speed systems.

Photo by Sam Jotham Sutharson on Unsplash