Portable vs. In-Duct Air Cleaners

Question:
What is the difference between a portable air cleaner and an air cleaner added into my central heating and cooling system? I can’t tell whether central or portable air cleaning systems are better.

Response:
Portable air cleaners are small appliances that typically are placed on the floor of a room in your home and plugged into an electrical outlet. They are designed to clean one room in your home. A whole house or in-duct air cleaner is added to your central air handling system and is installed by an HVAC contractor. Whole house air cleaners clean air inside rooms that are served by the central air handling system.

To use a whole house air cleaner, you will need to have a central forced air handling system. If you do have such a system and you want to clean the air in your whole house, an in-duct air cleaner will likely do a better job than portable air cleaners. This is because the in-duct air cleaners tend to have a much higher Clean Air Delivery Rate or CADR, which tells you how much air the device can clean. For best performance, the CADR should be approximately equal to the square footage of the indoor space that you want to clean. [For more info, see my earlier entry about portable air cleaners.] Also, whatever air cleaner you choose should be labeled as at least 98.98% efficient or HEPA, which means the device performs as a high efficiency particulate air cleaner. A HEPA-grade whole house air cleaner will typically have CADR ratings of about 1200, while the CADR for portable air cleaners is much smaller -- about 250. Theoretically, five portable air cleaners would provide the same performance as one in-duct air cleaner (250 times 5 equals 1250). However, testing has shown that this is not true. In fact, five portable air cleaners together have a CADR of only about 600, since portable air cleaners are not as effective at cleaning air outside their immediate vicinity.

For more detailed information about whole house and portable air cleaners, you may want to check out a report of a study funded by the air cleaner manufacturer TRANE, which compared the performance of several air cleaners, including their own in-duct air cleaning system. Since I know the person who led the study very well (and since I advised in the design of the study), I believe that the data and results are fairly and accurately reported, even though I am usually suspect of studies funded by a group that has something to gain by the results. This report can be found on the internet at www.trane.com/Residential/Downloads/EHE_study.pdf.