How Much Air Filter Does a Home Really Need?
More is better, but consumers need to understand limits, trade-offs
How much filtration does a home need? After the pandemic, many homeowners would say, “As much as we can get!” And some businesses are talking about very high-performance filtration. But is there such a thing as too much filtration for a home environment?
Home builder PulteGroup Inc. recently announced a plan to build communities across the country with numerous health features. This includes MERV 16 filters for all the HVAC systems. Pulte’s press release calls this “hospital-grade air filtration.” The release said the offerings came about from a survey that found 60% of respondents say how a home can support health and wellness was the most important attribute.
“As a direct result of the pandemic, consumers are seeking homes that will help them stay healthy, and Pulte Homes is leading the way,” said John Chadwick, PulteGroup’s CEO, in the release.
Tech mogul Elon Musk talks about going even further. When he floated the idea of a Tesla HVAC system last fall, he said it would include HEPA filters. Tesla has been offering HEPA filters as an option on its vehicles for several years, part of what the company calls its “bioweapon defense mode.”
But higher filtration comes with its own challenges. High-performance filters cost more and may need to be changed more often. If installed improperly, they offer no more protection than lower performance filters. And they can tax an HVAC system.
Trade-Off Between Filtration and Airflow
For people worried about threats to their health from a virus, allergens, or pollutants, a filter that removes these particles is needed. Traditionally, filters did little to remove these, said Kathleen Owen, a member of ASHRAE’s epidemic taskforce. Those filters were designed to keep the coils clean and the system functioning.
Over the last thirty years, most home filters have been improved to MERV 6-8 levels; these will remove some of the particles related to health issues, Owens said. With COVID, though, the recommendation is to increase filtration levels significantly.
In some cases in the past, technicians downgrade filtration to improve airflow, said Tom Piscitelli, vice president of distributed markets for SecureAire.
“A service technician cares about equipment and doesn’t want a callback,” Piscitelli said.
He said HVAC contractors need to discuss the trade-offs with customers and determine what their higher priority is — filtration or performance. A fuller filter will capture more particles, but it will affect airflow. Of course, a dirty system will also decrease airflow.
An HVAC contractor could set up a maintenance plan based on static pressure readings. A technician can take a reading on the initial visit and then take another during the maintenance contract visit six months later. If the static pressure is within a certain range, the technician can sell the homeowners a filter and tell them to replace it in three months. Some newer HVAC systems can even measure static pressure and send a message when the filter needs replacement.
“Air quality has always suffered because of air pressure concerns,” Piscitelli said.
Consumers should worry about the effect of increasing pressure drop across a filter as may happen with higher MERV filters because that means it takes more to push air through the system. The system will need to run more to cool or heat a house, may use more energy, and could hurt the fan. At the same time, Owen said, increasing pressure drop often means the air will pass through the filter less often and the filter will capture fewer contaminants.
“If you’re not running it, you’re not filtering,” she said.
Higher performance does work, Owen said. A MERV 13 filter, when properly installed, will capture 85% of viruses in a single pass. The more air changes, the more it captures. This clean air also helps dilute the virus in the air we breathe. However, the virus comes from people
“If we are near to the infected person, the exposure is going to occur long before the air has time to get to the filter,” Owen said. “Your filter may be good, but it may not do much for what you’re breathing, because you’re breathing so close to the source. Hence the need for distancing and airflow into the occupied space for dilution.”
Pollen is similar. Pollen is usually 30 to 40 microns, which seems small, but is actually so large that it’s rarely airborne indoors where the air velocity is less than outside on a breezy day. It enters a home on shoes and clothing. Keeping pollen out is likely to work better than an HVAC filter. In-room air cleaners near the people will also help cleaning the air near the people.
Wildfires Are Biggest Threat to Indoor Air Quality
The real benefit from high performance air filtration has to do with wildfires. Even a MERV 15 filter only captures up to 75% of the contaminants from a wildfire on the first pass.
“Because it’s a combustion aerosol, a lot of it is much smaller, so you really want a better filter for that,” Owen said.
Wildfires seem like more common occurrences. There were more than 50,000 wildfires in the United States last year, an all-time record according to the National Interagency Fire Center. That’s a lot, but these fires were spread over a fairly large geographic area, so the risk to any individual remains relatively low.
Still, consumers want to reduce any risk. High-filtration is, at least for now, a marketing technique being used by some companies. HVAC contractors need to make sure a system can handle a higher performance filter and explain to consumers what the trade-offs may be in terms of a system’s overall performance. They will also need to make clear what a filter can and cannot do on its own, and they need to set up a maintenance plan that takes into account the effect of static air pressure from increased filtration.