Airflows and HVAC

[cardcrimson]

Not sure this is the right place for this, but an answer to M_T's question in the car airflow.

There has been a ton written about HVAC and Covid, though at this point, there are no clear solutions.

Background: 

• Sick building syndrome has been around for decades, and costs $60 billion in lost productivity annually per the EPA.
  
• Causes: VOCs, viruses, mold, bacteria, pollen, outdoor pollution brought indoors, etc.; brought indoors through three mechanisms: bringing in outdoor air; people and what they bring in (food, pets, viruses, etc., poor hygiene); the building itself (outgassing of materials, leaks creating mold, rodent infestation, etc)
  
• Energy savings tend to tighten buildings and exacerbate the issue.
  
• HVAC systems are designed primarily for comfort, energy efficiency, and installed cost; minimize outside air, install low pressure drop filtration (MERV 8)
  

Enter Covid:
CDC and ASHRAE (look it up) recommend bringing in more outside air and tighter filtration (HEPA); unfortunately, that won't really work.

• Systems aren't designed to bring in increased levels of outside air, requiring significant more energy and equipment capacity to cool in the summer and heat in the winter
  
• Systems aren't designed for the added contaminants either (smoke from fires, pollutants from cities, pollen)
  
• Systems aren't designed for high grade filtration; inadequate airflow across the filter will ultimately damage the compressors; adding increased fan capacity is an expensive retrofit
  

The alternatives:
HEPA Filtration

• Will trap contaminants, including the virus
  
• Confusingly, filters are measured on their effectiveness in trapping .3micron particles, which for some reason are the most difficult to trap. Smaller particle, such as Covid (around .1 micron) are easier to trap due to Brownian motion. Disappointingly, many new entrants to the market are stretching the claims of how good their filters are, because of the Brownian motion. "Our filters remove 99.9999% of all particles". While that may be true, the standard metric for comparison is how they perform with the .3 micron particles and they are misleading the uninformed masses. Shame. . . .
  
• As mentioned, systems aren't necessarily designed for the pressure drop over a HEPA filter (MERV 8 has a drop of less the .4 inches of water, HEPA over 1 inch of water)
  
• Importantly, filters in the duct work don't help at all with person to person transmission within the space, and the same can be said for many of the centralized purification strategies. As mentioned in the other post, a major hospital, working in concert with HVAC contractors, found that there is zero evidence of transmission of Covid through an HVAC system.
  

UVC

• Dangerous to humans, okay in ductwork. 
  
• Dwell time is key (the time the airflow is exposed to the UVC). Multiple bulbs that are 5-6 feet long are required to actually kill the virus as it's moving through the system. Systems are on the market and probably will kill the virus.
  
• UVC LEDs may be fine to kill the virus, if it is stuck on the evaporator coils, but they are certainly not strong enough to kill in the air streams. Not even close yet.
  
• Far UVC LEDs. Safe for humans (allegedly), but not strong enough at this point, either.
  

Bi-polar Ionization

• Positive and Negative ions attach themselves to impurities in the air, and drop them to the floor or attach them to walls.
  
• Half life of an ion is 5 to 90 seconds. Probably not very effective in large spaces where the virus may exist. Imagine an in duct ionizer with long duct runs. The ions will capture stuff in the duct, but won't survive long enough to protect those in the space themselves.
  
• Localized bi-polar ionization may indeed provide some protection, especially in smaller spaces
  

Ionized hydrogen peroxide

• Cold plasma created hydroxyl radicals and H2O2 ions appear to effective on the virus and other pathogens
  
• Half life is a question mark, as ions don't tend to survive for long
  
• I'll know more in a couple of days, as a client is developing some new tech here.
  

Ozone

• Very effective at killing mold, bacteria, viruses
  
• Considered a pollutant, and dangerous to humans at higher levels, okay at lower levels
  
• Much longer half life than ions
  
• California has strict regulations regarding purifiers that use ozone
  
• Ozone purification is common in Asia and Europe
  

So what to do now?

• I’m a big believer in localized purification; though there is a lot of snake oil out there (see Molekule)
  
• I think a combination of the above can be helpful, especially when combined with a device that pushes ions/ozone that can go into the space locally and attack the contaminants
  
• Forgot the most important suggestion, focus on CFM and air changes. The more the better, and at least 4X per hour, preferably 8X for a conference room, and higher for small spaces.
  
• The airflows of the purifiers should positioned to pull in from the inhabitants, and blow air down, away from people. Pushing air up or out is not good.
  

Long term

• There is a $15trillion installed base of HVAC equipment in the US. Changing that will not be easy.
  
• Eventually, new designs will be similar to that in planes. Air will flow from the HVAC system through diffusers in the ceiling and exit the space through returns on the floors or lower walls. That way, air will not flow readily from person to person as it does today. (Currently both the diffusers and returns are typically in the ceiling, and many homes have the registers on the floor and the returns in the ceiling. No bueno)