This post is an expanded guide from a previous Reddit post that had some purifier-related calculations.
What is CADR
CADR (clear air delivery rate) is a value reflecting the effective filtration rate of a purifier where larger values means the purifier can filter larger rooms. Manufacturers will usually disclose their claimed CADR value, which is often given as three values representing different air contaminant types.
In addition to effective filtration rate, CADR is also used in determining sizing purifiers to rooms as it is a figure of merit that quantifies the effective volumetric flow rate of clean air. WARNING: While this is a figure of merit, it also represents a theoretical volumetric flow rate of 100% clean air which means that it can be in units of cubic feet per minute (CFM) or cubic meters per hour (m^3/h).
CADR values should be independently verified by a third-pary. The AHAM database contains the CADR ratings of many different purifier manufacturers. The CADR values for these purifiers can be reliably trusted as they follow the ANSI/AHAM AC-1 technical guide for CADR measurement. While illegitimate purifiers are never have claimed CADR values certified, there still may be legitimate purifiers that are not certfied by AHAM.
Though CADR is the best metric for comparing purifiers, there are still some differences that have some effect on performance despite having equivalent CADR ratings.
What is ACH
ACH (air changes per hour) is a rating of how many times the total volume of air in a living spaces passes through an air filter every hour. This number is chosen by you, the person reading this guide, to represent how strictly clean the air must be. For most people the deciding factor affecting this value is if they have breathing disabilities or not. There are other niche cases that would require higher values, such as applications where there is high particulate generation occuring in the room.
Most manufacturers rate the room size of their purifier using an ACH of 4.8, which is typically agreed upon to be too low. Most recommendations are a value of 6-8 where values towards 6 being casual purifier use and 8 for people with breathing disabilities. Other niche cases may be aggressive dust control, where you may want ACH values greater than 8.
| ACH | Use Case |
|---|---|
| 4.8 | Value used by manufacturers for room size rating |
| 6 | Minimum ACH value recommended for general applications |
| 8 | Minimum ACH value recommended for breathing disabilities and other health issues |
Sizing an Air Purifier
Variables
ACH: Air changes per hour for a room.
CADR: CADR value used to represent air volumetric flow rate, in cubic-feet-per-minute.
CeilingHeight: The height of the ceiling, in feet.
RoomSize: Floor space of the room, in square feet.
Calculations
All of the values in the calculations below use either room dimensions or purifier ratings. The exception is ACH, which is chosen by you, the person reading this guide, depending on use cases in the previous section “What is ACH”.
Max Room Size
Calculates the room size supported by a particular purifier.
RoomSize = (60 * CADR) / (ACH * CeilingHeight)
Airflow For a Specific Room
Calculates the required CADR for a particular room.
CADR = (RoomSize * ACH * CeilingHeight) / 60