Since the start of the COVID-19 pandemic, many eyes have turned to buildings’ HVAC systems to help mitigate the spread of coronavirus indoors. There are a range of options designed to prevent the virus from traveling through a system. But how effective are they? At the SMACNA Edge event this fall, Steve Taylor, principal of Taylor Engineering in Alameda, California, attempted to answer that question.

One of the biggest questions to be answered: Which is better at preventing the spread of viruses — filtration or outside air? Taylor said there is plenty of evidence that more frequent air changes help, but that air doesn’t have to come from the outside. A study of the flu offers some insight into which of the two is more effective. This study finds that filters are the best defense. And MERV 13 filters work as well as higher level filters.

Taylor said many contractors argue they cannot place a MERV 13 filter into the 2-inch space available. They say the pressure drop is too tight as a result. Taylor said most pressure drop calculations are conservative.

“If you had MERV 8 filters and you replace them with MERV 13 filters, it would simply work, fans would speed up all by themselves, and you'd have plenty of motor horsepower to handle it because we engineers are very conservative,” he said. “So we really think that MERV 13 is a very practical requirement, something that most systems will be able to do without any changes being made.”

UV Lights, Increased Humidity Cost More

Another tool promoted to fight the virus is UV lights. Taylor said they are effective, but not recommended for air handlers. He said better filters are just as easy to use, less expensive, and as effective while using less energy. The filter housing is already there.

Some object that filters only capture the virus rather than kill it. Taylor said that’s untrue. The virus dies without a living host.

What about humidity? Taylor said early studies showed that a higher relative humidity, between 40% to 60%, helped prevent spread. Further studies found little effectiveness from higher relative humidity.

“So all in all, there's not enough evidence supporting humidification is a good mitigation measure,” Taylor said.

Humidification can also create a lot of problems on its own, he said. It can cause condensation in the ductwork immediately downstream of the humidifiers, as well as humidification within the walls. If the vapor barrier is not in the right place, it can certainly cause it on windows if they're only single glazed, Taylor said.

“And wherever there's condensation, there can be microbial growth,” he said.

He concluded that the best solution for improving an HVAC system to prevent the spread of coronavirus is improving air filters to MERV 13 or better. Taylor said this move has benefits in many parts of the country, even if the pandemic ends. It improves IAQ in general and is especially useful when regions are dealing with the smoke from wildfires.

Of course, the best filter is the one closest to a building’s occupants — in other words, masks.

“There's no practical amount of ventilation that's going to protect you from viruses unless you have masks,” Taylor said. “They are mandatory.”