HVAC Battles COVID-19, Part II


Air balancing is required for any air system modifications.

With proper ventilation in place and required modifications made to existing systems, building operators can now provide safer indoor spaces for occupants

By Roy Collver

We are still learning about COVID-19. In the month since the first part of this article, the Government of Canada revised quite a good chunk of their online COVID-19 resources, especially important with the newer strains now found in Canada.

For the purposes of this article, when discussing exhaust air and intake air, please consider them as tandem processes— meaning when one increases, the other also increases. Additionally, the term ventilation is used to describe this duality.

To help your clients provide safer indoor environments, consider which strategies listed might work for your situation. Proper assessment requires examination of the entire building structure, existing HVAC systems, and any usage details. This process will highlight limitations and suggest ways to modify existing equipment operation or configuration when possible. It will also point to situations where adding equipment might be the best option.

For example, when looking at existing HVAC systems to see how ventilation can be increased, you will see restrictions in either the amount of air that can be moved by existing equipment, or in the amount of air make-up heating or cooling capacity that is available.

Significant mechanical modifications may be necessary to meet goals. If you lack the knowledge to measure airflow, find someone that can. Most of the following strategies can cause serious harm if air balancing is not done in lockstep with system modification.

Isolation and separation

Plan A begins with isolation. This requires finding ways to keep pathogens out of a space and putting the necessary security measures and barriers in place. If Plan A fails, move on to Plan B—seal off isolation areas and provide ventilation that exhausts viral particles. If only some occupants within a room have been infected, the uninfected should quarantine elsewhere. If that’s not possible (think a single-family dwelling), then create isolation zones within the building. Isolation zones may be impractical in buildings using a central air handler. These mechanical systems will need to be modified for isolation to work. There can be no return air contamination from infected zones. Blocking off return air grilles and providing enhanced filtration will reduce airflows, however, it can seriously damage heating and cooling equipment if taken too far.

Be on the lookout for situations where air may short circuit, making exhaust air less effective.

Hydronic air handlers have an advantage over combustion furnaces and compressive refrigeration equipment in this regard. Their airflows can usually be reduced without harm to the equipment.

Measuring an appliance heat rise/fall or external static pressure should tell the tale and remember to follow the manufacturer’s instructions. In many cases, supply and return ducts (including door grilles and undercuts) will need to be completely blocked within isolation areas and a dedicated alternative HVAC method provided—think electric space heaters. For a really cool economical high-level filtration hack, search “comparetto cube” in your browser.

If a building has a zoned HVAC system using multiple appliances, then Plan B might be easy to pull off. HVAC supply and return ducts, grilles, and diffusers, all need to be self-contained within the isolation zone—no exceptions. The clean zones of the building should be maintained at a slight positive pressure compared to the isolation areas. Having a knowledgeable HVAC professional quarterback this process will be essential to ensure cross contamination is avoided.

Zoned hydronic heating systems, like radiant floors, can often be easily divided up into isolation zones. Any ventilation modifications needing to be studied should be done so often on a room-by-room basis.

Keep filters clean and in working order. Filter maintenance might be needed to ensure that it’s able to capture pathogens.

Dilution and elimination

Ventilation will also reduce viral spread in indoor spaces. Bringing in fresh air dilutes pathogens, which reduces the viral load occupants are exposed to. The displaced air and its entrained contaminants are eliminated from the space via exhaust strategies, decreasing the dwell time of aerosol particles. If a building has a balanced ventilation system with a capacity for variable volumes, operators should increase ventilation rates toward the higher end during occupied periods and ramp up to full capacity for building pre- and post-purge during unoccupied periods. Dedicated outdoor air systems that can’t increase volume should be set to run during unoccupied periods. It can be quite simple for operators of commercial and institutional buildings to reprogram their building automation systems to maximize ventilation—others will need to physically adjust controls and dampers.

Designers need to re-think many ventilation strategies going forward in light of the pandemic. Enhanced ability to vary the volume of ventilation combined with increased make-up air heating, cooling and humidification all need to be studied, as does consideration of displacement exhaust designs for faster flushing of spaces.


Don’t forget the control of airflow velocities, direction, and patterns. Large open spaces found in big box stores are ideal for diluting aerosols and can take advantage of this strategy. By reducing the number of occupants, social distancing, wearing face masks, and diverting the airflow up and away, operators can make these spaces safer. Modifying HVAC equipment to make crowded gathering spaces, like courtrooms or restaurants, safer is very difficult. Facility usage is perhaps more important than any other design factor when confronted with viral spread.


If enhanced exhaust of contaminated air is too difficult, the capture of pathogens with fine air filters might be the next best tool available. Filtering viral particles requires a MERV 13 or better filtration to be effective.

Retrofitting existing equipment can be successful provided appliance manufacturer’s minimum airflow is maintained. The common one-inch plain filters can be changed to pleated, or it may be possible to replace filter racks and upgrade to two-inch pleated or even four-inch pleated lower resistance filters. This will give you more static pressure wiggle room while reducing the frequency of filter changes. Adding fan driven air scrubbing units with HEPA filtration can be effective when integrated into ducted systems or provided as a stand-alone solution. Hydronic radiant floor and radiator systems may be good candidates for this approach if ventilation options are limited. Filters need to be inspected and replaced often—finer ones, more often.


Disinfection strategies that inactivate COVID-19 include chemical disinfectants, UV light, time, heat, and humidity. Chemical disinfectants are not a practical solution in most HVAC equipment due to their corrosive and sometimes flammable nature. UV light can be effective with limitations. Time also kills viruses. Currently, the medical community hasn’t agreed on the exact amount of time it takes for the virus to become inactive. The consensus, so far, is within a few hours to days—good to know, but not entirely useful.

When it comes to heat, there are a few predictable parameters. It is well established that heat speeds up the inactivation of SARS-CoV-2 dramatically. Hydronic coils, convectors and radiators could be given a high temperature boost occasionally to kill active viruses on their surfaces. This would be similar to the way some domestic hot water system boosts are used to kill Legionella bacteria.

Some studies indicate that COVID-19 can be killed by soaking the virus at three minutes at temperatures above 75C, five minutes for temperatures above 65C, and 20 minutes for temperatures above 60C. Experimental research still needs to be done for HVAC components to qualify and quantify the parameters needed to ensure success. Boosting a baseboard convector, panel radiator, or fan heating coil up to 75C for three minutes will not elevate room temperature significantly and should be easy to do with the controls we have available. Those of us who champion low temperature hydronics need to be creative. Radiant floor won’t work for obvious reasons. For low temperature air handlers, we can perhaps hold off the fan while we boost the coil if the equipment is capable of that type of control. Hopefully, people are already looking into this. Why not give it a try?

Humidity control

It is recommended that 40 to 60 per cent relative humidity is used to decrease the risk of infection and speed up the inactivation of COVID-19, while slowing the evaporation of droplets into aerosol particles. Beware cold climate effects—increasing ventilation will make indoor relative humidity plummet as cold outdoor air is heated. Adding moisture to the air will be necessary—steam humidifiers highly recommended. Keep humidity tightly under control and watch the dewpoint like a cat watches a mouse. When high humidity meets cold structural assemblies, the situation can get really sad, really fast.

I will be continuing to monitor the COVID-19 pandemic as it develops and plan to weigh-in with important updates as things evolve. The plumbing and HVAC/R industries cannot solve this mess, but we play an important role in helping to keep people safe and slowing the spread of this beast.

The industry needs to look at a complete rethink of our technology and design parameters for the long-term. We can use this current crisis to accelerate the urgent need to de-carbonize and increase the energy efficiency of our built environment. We find ourselves in perhaps the most dynamic and exciting time ever in HVAC history—embrace it.


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