🧵 Answers to what the different strategies will result in term of air change rate
Example used throughout the thread:
👉Classroom: 960 ft2: 30' x 32’
👉Number of occupants: 30 (this is the design number, taking into account average classroom density).
Ventilation by code, this classroom should have 415 CFM airflow of outside air or 14 CFM/person.
👉This airflow is being forced through the HVAC system (unit in classroom or mechanical room).
When we check ventilation, the answer is not “yes” or “no”, the answer should be how much?
Many classrooms are at full capacity, meaning that social distancing of 6 feet is not adopted - further highlighting the importance of masks/ventilation/filtration MERV HEPA/UVGI.
Exploring what different combinations of outside air and filtration and UVGI will result in terms of air change rate:
What is the target of air change rate? General agreement 5 or above ACH.
Check different combination ventilation/filtration/HEPA mitigation strategies and the resulting ACH:
Here is some good news:
Mechanical Filtration cost (if there is a place/capability to install filters)
The Yearly cost including two replacements of MERV 13 filters will be $28.
Let's assume it takes $5 of labor cost to install a filter.
Total cost/Year = $48 to get ~5 ACH.
HEPA air purifiers are also a cost-effective measure to get to 5 ACH:
For a 960 ft2,
👉5 ACH: purchasing 2 x air cleaner with CADR = 320 CFM costs ~$17/student for first year.
OR
👉5 ACH: MERV 13 costs $1.6/student each year.
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I looked at many classrooms. Similar findings. Most do not have outside air and problems with exhaust fans of bathrooms (unless I am missing something - no reports it was fixed?).
Most schools have windows that can be open but windows are not a reliable method to get you a specific air change rate. For more information about ventilation:
COVID Safety Plan (1/2)
- People density was at 50% (by design)
- Other measures below.
Everything was followed properly, except social distancing.
Masks were enforced, even outside. One might ask how can you enforce masks if people are drunk? I saw 2 people holding a person to his room while a third person holding by two fingers his mask in place.. commitment!
There are three types of air cleaners:
- Subtractive: like filters, will remove the contaminant when it comes in contact with the media
- Additive: like ionizers, will emit ozone/radicals/ROS/etc.. to react with the air (will emit something to remove something)
- Hybrid
Filter media (MERV, HEPA), UVGI are proven because Standards/guidelines/>decade of testing exist to test efficiency and by-products.
NO Standard exist today to test additive/electronic air cleaners (none, nada, zero, nul, nula), therefore they are Unproven.
🧵 about ventilation
- Mechanical versus Natural ventilation
- ASHRAE Standard 62.1 historical rates
- Why schools are under-ventilated?
- What is the cost of outside air?
- What we should do about it?
Note 1: According to ASHRAE Standard 62.1 (most prominent ventilation standard), ventilation is not only about outside air. Rather, it is the sum of outside air and return air that is cleaned.
In this thread, I will focus only on outside air.
Note 2: schools are either designed to have mechanical ventilation (intentional OA from HVAC unit) or natural ventilation (intentional OA through windows and doors).
Most schools in North America are mechanically ventilated (for many reasons mainly for comfort and safety).
1) Misleading marketing:
The filter used is called "HEPASilent". It is not a HEPA filter.
It is a common pleated filter + ionizer.
2) By-products:
The company claim to produce low ozone.
It is certified by CARB to produce < 50 ppb.
They have a carbon filter which will capture some ozone - although % will decrease with use (we don't know how much % versus age).
Indoor ozone levels should be < 5 ppb
3) As we discussed previously, with every ionizer, there is a potential production of formaldehyde (and other VOCs) and ultrafine particles. Example of one ionizer tested by Lawrence Berkeley lab: