Buying an Air Purifier? Why knowing coverage capacity is vital

Calculating Room Coverage Capacity
Mike Yurosek, a carrot farmer in California, wanted to stop throwing out his ugly carrots. In 1986 he bought a green bean cutting machine and used it to produce uniform 2-inch pieces of carrot, that we now know as baby carrots. It was win-win. Wastage was zero, and he could sell his crop. Baby carrots were a savior for a stagnant industry. Carrot consumption has skyrocketed in the years since. They are now close to 70 percent of all carrot sales.

Many consumers don’t know that ugly carrots from the soil are cut to be neat and tidy. And it probably doesn’t matter whether they connect baby carrots to their origin.

But unlike knowing the origin of the baby carrot, some things we do to protect our health demand deeper knowledge. Like buying the right air purifier to provide healthy air in our homes.

Unless we learn the facts about air purifier coverage capacity, we can easily end up buying the wrong appliance — one without enough oomph to rid the air of contaminants.

What do we mean by oompf?

The technical term is “coverage capacity” expressed as square feet. And the number of square feet quoted by a manufacturer indicates the maximum room dimensions for an air purifier model.
Is Your Measuring Tape Handy?
If the sales brochure says 200 sq.ft, it means the unit is right for a room of 10 by 10 ft or 20 by 10 ft. Or any length and width combination not exceeding 200 square feet. You get the square feet by multiplying length times breadth e.g. 20 times 10 equals 200. Do you have your measuring tape handy?

Note though that ceiling height can be an issue. We’ll talk about that later. In the meantime, let’s understand why we do the math.

Why does coverage capacity matter?

Coverage capacity is not the only factor determining how effective an air purifier is at cleaning the air.

But it is important:

  1. there is no point in buying a unit that has no capability to clean all the air in a room – you’ll waste your money.
  2. buying a unit rated for a larger room than yours means spending too much money – you’ll pay more upfront AND the running cost will be higher.
  3. because of how manufacturers estimate the number of square feet for a unit.

Let’s look deeper at manufacturers and their estimates.

How do manufacturers calculate the square feet coverage of an air purifier?

To explain how we have to go back to the objective of an air purifier.

They exist to clean air. No matter what method is used, the air purifier sucks air in one end, “cleans” it, then blows it out the other end.

But an air purifier can’t suck in all the room air at the same time.

And this is where the concept of “air exchange” comes in. The capacity to clean is determined by how many times it can exchange the dirty air in a room for cleaner air every hour.

The power of the motor is a crucial factor generating the number of air exchanges. The stronger the motor, the more air it can suck in, the faster it can pull the air through the filters and blow it out the other end.

Tests are used to rate the impact of these factors. The results identify how many air exchanges happen per hour for the amount of air in a room. Manufacturers quote the square footage for an air purifier based on these test findings.

An air purifier tested for a 250 square foot room with an exchange rate of only one or two has insufficient capacity for that room. However, if testing in a 150 square foot room generates an exchange rate of three plus for a 150 square foot room the manufacturer will quote 150 sq.ft as the “coverage capacity.”

But why do the calculation yourself when the sales brochure has the square footage?

It’s a question needing an answer. The trouble is the room coverage figure quoted by manufacturers use 8 feet as the ceiling height. If your ceilings are that height, it’s fine. But what if your ceiling height is higher? You’ll have to look for a higher capacity air purifier.

And know also that manufacturers err on the optimistic side when making their recommendations. In other words, the air purifier is not as effective as it should be for the square footage listed.

It pays, therefore, to calculate the figure yourself. If that is too much trouble, a good rule of thumb is to lower their square footage by 20%. So a manufacturer’s quote of a 250 sq ft room becomes 200 sq ft.

What if we know both the air exchange rate and the square footage coverage?

Even better.

As we saw, the higher the number of exchanges, the better a unit is at cleaning the air. The aim is a rate of at least four, possibly five.

If we know both figures, it is much easier to compare one air purifier with another. Imagine you’re considering two competing products quoting the same room size but with different rates. If one has a rate of 3 exchanges per hour and the other has 4, you’d buy the latter unit, wouldn’t you?

Knowing coverage capacity is crucial when you’re buying an air purifier.

It tells us whether a unit is right for the size of your room.

Knowing what the square footage quoted means and how it is calculated is invaluable as you compare products. The comparison is even easier when you know the square footage AND the air exchange rate.

You can differentiate manufacturers claims and buy the right air purifier.

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