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The COVID-19 pandemic has made us all think more about remaining healthy. It also has forced many of us to stay put for more prolonged periods inside buildings. In nonpandemic times, mechanical engineers often heard that humans spend nearly 90 percent of our time indoors. With physical distancing requirements, it’s definitely a believable number, and it looks like more of that time has been at home for many of us.
It’s not surprising that HVAC professionals and our customers are thinking more about ensuring superior indoor air quality (IAQ) than before the outbreak. It sometimes feels as if customers are talking as much as manufacturers do about HEPA and PCO filters, and ultraviolet lights that kill germs.
According to manufacturers, UV lights can kill the COVID-19 virus on surfaces where it may have otherwise lived for up to nine days.
Although it would be very rare, ASHRAE fellow and former president William Bahnfleth said in March that it’s not impossible for infectious agents to become suspended in room air, primarily via a human sneeze or cough. Scientists say they could remain in room air for up to six minutes. In some conditions, they may even settle and proliferate on coils, air filters, ducts and drain pans.
UV and other germicides were effective in killing other varieties of coronavirus, such as SARS and MERS, so the hope is that they will work for the COVID-19 virus (technically called SARS-CoV-2). ASHRAE suggests that UV products, room air cleaners, better filters and relative humidity between 40 percent and 60 percent may all be helpful.
As we know, IAQ has been a hot topic for a few decades and has been working its way slowly into building codes and ASHRAE guidelines. Still, ASHRAE Standard 62.1-2010 Ventilation for Acceptable Indoor Air Quality and other versions contain metrics more applicable for comfort, so in hazardous situations such as the current pandemic, they would be seen as minimum design guidelines.
Along with ASHRAE, the International Building Code and International Mechanical Codehave specific criteria on ventilation rates for building or use type.
Ventilation professionals know that to remove contaminants, mechanical equipment must displace 100 percent of stale indoor air on average about three times per hour. This becomes a little more challenging as numerous cities and states double down on electrifying buildings. In the quest to save energy, we are reducing air changes per hour and re-using indoor air more.
Different combinations of heat pumps, ventilation equipment and heat pump water heaters are being employed with tight envelopes and lower loads. An obvious question is, does this approach provide good IAQ?
Insofar as we breathe 40 pounds of air into our lungs each day, about 2,000 pounds or 28,000 cubic feet of good quality air per person must be provided daily. Ventilation equipment and systems have a big job to do. Have they done it well in the past? Do the newer approaches minimize airborne hazards?
Managing indoor air properly means minimizing volatile organic compounds (VOC) and substances such as methane, propane, butane, acetone, formaldehyde, allergens and, of course, viruses.
Contaminants also can include dust, dust mites, fiberglass, asbestos, gases from building materials, vapors from cleansers, solvents, pesticides, disinfectants, glues, off-gas emissions from furniture/carpets/paints, microbial contaminants, fungi, molds, bacteria, ozone from electric motors, electrostatic air cleaners and photocopiers, carbon dioxide, perfume, body odors and tobacco smoke.
Do any of the modern devices protect against these perils? Do we even know which contaminants need flushing out?
Conditioning Energy Recovery Ventilator
A company called Build Equinox in Vermont thinks its product provides a “yes” to both of these questions. The conditioning energy recovery ventilator (CERV) fresh-air ventilation system incorporates carbon dioxide and VOC sensors that detect pollutants such as C02, methane, propane, butane and 19 high- and medium-high VOCs, including acetone, sodium hypochlorite, Windex and even 80-proof bourbon.
The sensors allow the unit to determine when ventilation is necessary and when the air is fresh enough to stop ventilating. It can adjust and optimize efficiency with a variable-speed compressor and fan control. It also can shut down motorized dampers connected to the outdoor grilles, circulating indoor air instead.
Used in super-insulated buildings, the CERV has proven effective for finding the right ventilation levels to dramatically improve the quality of indoor air. It has also contributed to a more efficient space heating and cooling strategy in small, tight homes when in recirculation mode.
Although it has the words “energy recovery ventilator” in its name, it does not include an ERV or heat recovery ventilator core. Instead, it uses its own 1/3-ton air-source heat pump to transfer heat from one air stream to another, meaning that both the condenser coil and evaporator coil are located indoors.
During the winter, the exhaust air from the house passes over the evaporator coil. This allows the heat pump to scavenge some heat from the exhaust air. The condenser coil is located in the fresh-air duct, so the CERV delivers heat to incoming ventilation air.
During the summer, the CERV’s heat pump removes heat and moisture from the incoming outdoor airstream. It delivers heat to the exhaust air stream, thereby lowering the temperature of the incoming ventilation air.
It comes with online monitoring and sophisticated controls for optimization and analytics, with three input channels and six output channels so it can trigger external devices. This allows the CERV to do zone control, as well as operate a bathroom heater or other external heaters.
Stupid, Sick and Tired
“At one time, asthma affected about 2 percent to 3 percent of North Americans. Now it affects up to 8 percent or 10 percent,” says Ty Newell, University of Illinois professor and inventor of the CERV. He has been studying indoor air quality for decades. He founded Build Equinox in Urbana, Ill., and spent about 10 years testing and developing the CERV and the CERV2.
“Poorly ventilated homes and buildings are making us stupid, sick and tired,” he explains, citing Harvard and ASHRAE studies that show improved cognition in buildings with better air quality.
In a Vermont community of 80 prefab passive house homes called High Performance Modular Homes near the White Mountain National Forest, 13 are equipped with CERV units. The builder has cooperated for many years in the collection and analysis of performance data.
The houses measure about 1,250 square feet and are heated and cooled using Mitsubishi mini-splits and ventilated with Build Equinox CERVs. They also use heat pump water heaters. Newell has data dating back to early 2015.
In winter, the mini-splits provide heat to the living room, dining area and kitchen. Hybrid water heaters remove heat from the surrounding air in the mechanical closet, while cool exhaust air is picked up by the CERV’s supply fan. The water heater-removed heat is replaced by a combination of the mini-split and the CERV.
Efficiency is higher than if an electric water heater had been used instead of the hybrid water heater — the mini-split and CERV lift outdoor cold temperatures to indoor room temperatures. The hybrid water heater lifts room temperatures to hot water temperature levels.
Newell says summer thermal performance efficiency is higher than in winter because the hybrid water heater has a dual benefit. It heats water with a useful byproduct of cool, dehumidified air that the CERV distributes to the rest of the home. If the CERV didn’t circulate air from the water heater, the mechanical closet would get colder, reducing water heater efficiency.
The primary energy usage in the 13 homes is 9 kWh/square foot/year. Homeowners save around $1,000 to $2,000 annually on utilities. Not bad for small houses in one of the most brutal U.S. climate zones.
In homes heated by a ductless mini-split, it can be tricky to ensure comfort. The CERV avoids over-ventilation and helps with comfort in remote spaces like that cold bedroom when you’re using point-source heating. If doors are closed or solar gains aren’t right, the CERV helps mix the air in the house.
The CERV is still pricey and complicated technology, so there may or may not be difficulties with maintenance, although the manufacturer has been rumored to be very responsive with support. Whether it’s exactly where technology is going remains to be seen. Still, it’s an excellent example of how modern energy-efficient, electrified building systems can address indoor air quality.
If you’re stuck at home with a CERV, you can probably breathe easy. And, of course, that’s saying something during a respiratory disease pandemic.