Pocket HVAC
What is ACH (air changes per hour)?
Air changes per hour (ACH) is how many times an hour a room's volume of air is supplied or exhausted. It's the airflow divided by the room's volume: in CDC's example, a 12 × 10 ft room with a 10 ft ceiling holds 1,200 ft³, so 80 CFM of exhaust is 80 × 60 = 4,800 ft³ an hour, or 4 air changes.
One air change doesn't swap out all the air, because the new air mixes with the old as it comes in. That's why clearing a room takes several air changes: CDC's table puts it at 69 minutes to remove 99% of an airborne contaminant at 4 ACH, in an empty room with perfect mixing.
Many ventilation targets are given in air changes per hour. A target in air changes turns into a CFM you can set on the equipment and check at the grilles.
Typical air changes by room
For hospitals and outpatient facilities, CDC's 2003 health-care guidelines (Table B.2, reprinted from the AIA's 2001 design guidelines) list these minimum air changes per hour, in total and from outdoor air.
| Room | Total ACH | Outdoor air ACH |
|---|---|---|
| Operating room | 15 | 3 |
| Airborne infection isolation room | 12 | 2 |
| Toilet room | 10 | – |
| Food preparation center | 10 | – |
| General laundry | 10 | – |
| Patient room | 6 | 2 |
| Exam room | 6 | – |
| Pharmacy | 4 | – |
| Patient corridor | 2 | – |
A dash means the table sets no outdoor air minimum. A patient room can drop to 4 total with supplemental heating or cooling. These are the 2001 edition's numbers. The 2010 edition of the guidelines replaced this table with ASHRAE Standard 170, so for hospital work check the edition your code adopts.
In a home, ASHRAE 62.2-2013 asks for 5 ACH of the kitchen's volume from a kitchen fan that runs continuously, and sets bathroom fans and whole-house ventilation in CFM (the older ASHRAE 62-1989 asked for 0.35 ACH for the whole house). To cut down airborne viral particles, CDC's rough guide is 5 or more air changes per hour of clean air.
How it's calculated
Room volume = length × width × ceiling height (or floor area × ceiling height) CFM = volume in ft³ × ACH ÷ 60 min ACH = CFM × 60 min ÷ volume in ft³ Metric: m³/h = volume in m³ × ACH; L/s = m³/h ÷ 3.6 Airflow per floor area = airflow ÷ floor area Time to remove 99% = ln(100) ÷ ACH × 60 min (CDC Table B.1: empty room, perfect mixing)
Sources
- CDC/NIOSH, "How Much Ventilation Is Enough?": ACH from airflow, floor area and ceiling height, and the target of 5 or more air changes per hour of clean air
- CDC/NIOSH, Ventilation FAQs #7, #8, #9 and #14: worked ACH examples, mixing factor, MERV 13 filter efficiency, air cleaner sizing (the 2/3 rule) and clean airflow through filters
- CDC, Guidelines for Environmental Infection Control in Health-Care Facilities (2003), Appendix B: Table B.1 (time to remove airborne contaminants by ACH) and Table B.2 (minimum air changes for hospital and outpatient rooms, reprinted from the AIA's 2001 guidelines)
- Facility Guidelines Institute, "About FGI": the history of the Guidelines for Design and Construction, including the 2010 edition replacing its ventilation table with ANSI/ASHRAE/ASHE Standard 170
- Walker, I. (Lawrence Berkeley National Laboratory), "The Best Way to Meet ASHRAE 62.2 in Multifamily Buildings", Building America meeting 2013 (U.S. DOE): ASHRAE 62.2-2013 ventilation and exhaust rates, with CFM and ACH examples
If a number doesn't match your chart, email me what you entered and what you expected, and I'll check it against the source. You can also read how the numbers are checked.
Checked by Paul Williams · Updated
Questions
How do you calculate CFM for a room?
Multiply the room's volume in cubic feet by the air changes per hour you want, then divide by 60 minutes. A 2,000 ft² home with 8 ft ceilings is 16,000 ft³, so 0.35 ACH (the old ASHRAE 62-1989 rate for homes) takes 16,000 × 0.35 ÷ 60, about 93 CFM. In metric, the volume in m³ times ACH is the airflow in m³/h; divide that by 3.6 for L/s.
How do you calculate air changes per hour?
Multiply the airflow in CFM by 60 and divide by the room's volume in cubic feet. A 1,500 ft² home with 8 ft ceilings is 12,000 ft³, so 75 CFM of ventilation gives it 75 × 60 ÷ 12,000, about 0.38 ACH. In metric, divide the airflow in m³/h by the volume in m³.
How many air changes per hour does a room need?
It depends on what the air is for. CDC suggests aiming for 5 or more air changes per hour of clean air to cut down airborne viral particles, counting outdoor air, filtered air and air cleaners together. It calls that a rough guide rather than a hard-and-fast rule: a large space with very few people, like a warehouse, may not need 5, and a space with a lot of people or higher-risk people may need more. Hospital rooms and kitchen fans have their own rates, in the table above. A house is different: ASHRAE 62.2 sets its ventilation in CFM.
How much ventilation does a home need?
In its 2013 edition, ASHRAE 62.2 sets a total ventilation rate of 3 CFM per 100 ft² of floor plus 7.5 CFM × (bedrooms + 1). For a 2,000 ft² three-bedroom condo that's 90 CFM, about 0.34 ACH with 8 ft ceilings. In an apartment or condo the fan has to supply all of it, because 62.2-2013 gives multifamily units no credit for air leaking in. The older ASHRAE 62-1989 asked for 0.35 ACH, about 93 CFM for the same condo. Check which edition your local code uses.
What size bathroom or kitchen exhaust fan do I need?
ASHRAE 62.2 (2013 edition) asks for 50 CFM on demand or 20 CFM running continuously in a bathroom. A kitchen needs 100 CFM on demand, or 5 air changes per hour of the kitchen's volume if the fan runs continuously. Enter the kitchen's size and 5 ACH here to get that CFM.
Which airflow should I use, supply or exhaust?
The larger of the two. Don't add them, because it's the same air coming in and going out: CDC's example room has 65 CFM of supply and 80 CFM of exhaust, so it uses 80 CFM. A portable air cleaner's clean air delivery rate (CADR) is in CFM too, and it does add: a HEPA air cleaner with a 120 CFM smoke CADR takes that room to 200 CFM, or 10 ACH. Filtered air counts by what the filter catches: as a rough best case, CDC multiplies the airflow by the filter's efficiency, so 300 CFM through MERV 13 filters (at least 85% efficient for 1 to 3 micron particles) gives about 255 CFM of clean air.
How long does it take to clear the air in a room?
In CDC's table for an empty room with perfect mixing, it takes 69 minutes to remove 99% of an airborne contaminant at 4 ACH, 46 minutes at 6 ACH and 23 minutes at 12 ACH. Real rooms don't mix perfectly, so CDC multiplies those times by a mixing factor between 1 and 10. Its example room at 4 ACH with average mixing (a factor of 5) takes 5 hours 45 minutes.
How does CFM per square foot relate to air changes?
CFM per square foot of floor is the ACH times the ceiling height in feet, divided by 60. CDC's 2/3 rule for sizing an air cleaner asks for 2/3 CFM per square foot in a room up to 8 ft tall, so a 300 ft² room needs 200 CFM, and at least 275 CFM with an 11 ft ceiling. That works out to 5 air changes per hour. With an 8 ft ceiling, 1 CFM per square foot is 7.5 air changes per hour. The calculator shows both.
Is this the same as the duct velocity calculator?
No. This one works out the airflow a room needs, or the air changes an airflow gives a room. The duct velocity calculator finds the CFM moving through a duct from a velocity reading, and the ductulator sizes the duct to carry it.
