"What type of A/C filter should I use?"
This is the single most frequent question I am asked. Should you go with the dirt-cheap ($1.94 each) Flanders EZ-Flow because it lets your system breathe better? Or should you use the MERV 13 Filtrete Healthy Living Elite (a whopping $21 each) because it filters pollutants better?
The short answer: You want the best filtration you can get without hurting your air-conditioner's airflow. But how to do this?
• Increase the size of the existing return duct and filter housing;
• Add more returns in the home, ideally while increasing the length of the return plenum at the air handler. This can help solve room-pressure issues if the returns are properly installed in bedrooms.
The return side of an air-conditioner sucks air from the house, through a filter, and into the system. As it does so, it creates a negative pressure across the filter. Generally speaking, we'd like that pressure to be 0.25 inches of water column ("w.c.) or less.
If the pressure increases on the return side, the airflow decreases. The main things that cause increases in return static pressure:
• a “good” but restrictive filter;
• a dirty filter;
• an improperly set return damper;
• a return duct that is kinked, compressed (laterally or longitudinally), crushed or too small for the system.
Imagine stepping on your garden hose a little at a time. You’ll create a pressure increase upstream of your foot, and reduced flow downstream of your foot.
Here’s another analogy: Imagine running a marathon while breathing through a snorkel. Then add a filter to the snorkel. Then kink or crush the snorkel. Bear in mind that I *routinely* find more than one of the factors listed above in the homes I audit. (A full 90 percent of the homes I audit have high return static pressure.)
But what if you added another snorkel? Now you’d be breathing better, even if you installed a “good” filter that restricted airflow.
To get more detailed: Let's say that a 4-ton air-conditioner moves roughly 1,600 cubic feet per minute (CFM) with a measured return static pressure of 0.2" w.c. without a filter in place. Then we put a new, cheap EZ-Flow filter in place. The static pressure increase is a stated 0.08” w.c. But the higher the filtration quality, the greater the increase in static pressure and the more airflow is hampered. A system that moves 1,600 CFM at 0.2” w.c. might only move 1,325 CFM at 0.9” w.c. If your house needs that 1,600 CFM on a 115-degree day, then your A/C won’t keep up.
The moral of the story: You need more return ducting if you want better filters.
