Re: RabbitAir SPA-421A vs Honeywell 50250: CADR and Room Size

A Reader writes:

Ed,

Thanks a lot for your website, as it has been very useful in our process of determining which purifier to buy.

We are college students on a budget and have run into the problem of a smoking neighbor seeping through the walls in our Baltimore row home rental.

We think it is mostly in the basement (as that is where it is very pronounced) with an occasional whiff of it in one of the bedrooms two floors up.

We're hoping that putting a purifier in the basement will keep the basement from saturating and the smell from moving up.

If that doesn't work, we will put the purifier in the bedroom.

For cost reasons, the two products we have been looking at are the Honeywell 50250 and the RabbitAir SPA-421A.

I am, however, a little confused by the combination of CADR and the rated room size.

Is the CADR independent of room size?

If so, then how does CADR factor in combination with the rated room size?

Is the CADR a measure of how much the purifier removes when a fixed volume of air is passed through it, and the rated room size is a function of how much air is pushed through in a certain amount of time?

That is, if the RabbitAir is placed in a smaller room, does it have a higher "effective" CADR?

I am just wondering why, other than the significant additional noise volume ofthe 50250, we should spend $320 for the RabbitAir when the Honeywell seems to do a better job of cleaning the air for $150.

Ed's Reply

Hey Reader;

CADR and Room Size

The Association of Home Appliance Manufacturers (AHAM), Clean Air Delivery Rate (CADR) is discussed in depth, and criticized heavily, in this air-purifier-power page;

About AHAM and CADRs

AHAM also rates every CADR certified air cleaner for room size.

But look for yourself at the AHAM certifications;

AHAM CADR listing and Room Size Ratings

Each certified air cleaner has four numbers to the right, the dust, smoke, and pollen CADRs, and then the room size rating.

You will notice that AHAM's recommended room size is generally just a numeric constant, roughly 1.52, multiplied by the average CADR.

My room size ratings are based on my estimate of a machine's actual utility. Things like quietness and automatic controls, like the RabbitAir SPA421 possesses, allow a larger practical room size than the noisy and manually switched Honeywell 50250.

CADRs are solely about particle capture, they ignore odor and chemical capture capability - despite the "smoke" rating.

My general rule is room size equal to CADR, with additions or subtractions for safe oxidizers (plasma-ion/UV/photocat), sound emissions, automatic operation, chemical/odor capacity, particle filter efficiency, and air circulation effectiveness.

CADR is, to oversimplify, the product of airflow throughput and efficiency of mid-sized particle capture. So a product with 90% one pass capture of 9 micron particles and an airflow rate of 250 cubic feet per minute would get a CADR around 250 x .9 = 225.

Placing an air cleaner in a smaller room allows it to run at lower speeds, as the vast majority of all purifiers will. CADRs and room sizes are based on continuous noisy top speed operation, an unrealistic scenario.

The RabbitAir has about 2 pounds of activated carbon which is lacking in Honeywell 50250, making the RabbitAir 431 significantly better at removing subtle smoke odors.

I own three good 'ol 14-inch round Honeywells, and still run my 50250 almost every day - often in response to smoke from neighbor's wood stoves - it will clean air well. Just be ready to change the carbon prefilter blanket frequently to get rid of smoke odor.

The RabbitAir is actually a price class above 50250, a nicer and more effective air cleaner - quiet, electronic and automated, and stylish.

But, as you note, the main detraction with the Honeywell is noise. Basement installation being the plan, that may not be a major consideration.

But don't expect quiet, even on low speed, if the bedroom option manifests itself.

As students, you will graduate within the life expectancy of this purchase, and, if economic disaster is forestalled, get an upscale machine for the bedroom then.

Best Wishes for a safe and healthy future,

Ed