Tag: box fan

Post #1859: Why all bathroom fans suck. A corollary to Post #1843

 

Answer:  Because they’re small.  That’s it.  It’s just basic physics.  And there’s nothing that can be done about it.

Background

In Post #1843, I figured out and explained why ceiling fans are vastly more efficient that box fans.  Where efficiency is measured by cubic feet of air moved per minute, per watt of power used (CFM/watt).

The answer turned out to be remarkably simple:  To move the same volume of air, a smaller fan blade has to move that air much faster.  That’s just arithmetic.  (If the area swept by a 20″ box fan blade is one-seventh the area swept by a big ceiling fan, the box fan has to move the air seven times faster, to keep up with the volume moved by the ceiling fan.)

Moving air faster takes much more energy than moving it slowly.  Not due to the energy-wasting turbulence that might create ( though that can be a factor), but merely because it takes more pressure to move air faster, and overcoming that pressure takes more energy.

Roughly speaking, CFM/watt should scale inversely with the size of the fan.  Given identical designs and motors, a box fan that is one-seventh the size of a ceiling fan should take seven times the wattage to move the same amount of air.  Roughly.

That’s all laid out in Post #1843.

And now on to bath fans

Today the penny dropped, and I realized that this same phenomenon explains the poor performance of bathroom vent fans.  Seems like bath fans take forever to clear a bathroom.  And I include all bath fans, almost regardless of make or quality.  Where a box fan stuck in a window could clear the air in a bathroom in a couple of minutes, an in-ceiling bath fan might take half an hour.

At best, a bathroom vent fan might have 6″ blades, feeding a 6″ diameter duct.  (Although 4″ duct for bath fans is far more common).  Since the area of a circle goes as the square of the radius, the area swept by the blades of a 6″ bath fan would be about ( 3-squared / 10-squared = ) 9% of the area swept by the blades of a 20″ box fan.  And so, to move the same volume of air as a box fan, a hypothetical 6″ bath fan would require (1 / .09 =) 11 times the wattage.

Let me now put that to the test, via virtual shopping at Home Depot.

And, sure enough, the median bath fan from Home Depot moves about one-tenth as much air per watt, compared to a box fan.

Bottom line:  A bath fan that could clear a bathroom as fast as a box fan would draw ten times the wattage of the box fan.  If you could squeeze that much air, that fast, through the ducts, you’d need to have a 500-watt bath fan*, in order to clear a bathroom as fast as a box fan sitting in the window.  That, before we even consider whether or not you could move that much air through a small duct without undue losses due to turbulence.  That, before we consider how much noise that would make.

* That’s 2/3rds of a horsepower, more or less.  A big electric motor, in this application.

And so, the apparent poor performance of bathroom fans is not a figment of my imagination.  Bath fans move air quite slowly, compared to (e.g.) common box fans.  It’s not a design flaw, or an intentional choice.  It’s just physics.  The smaller the fan, the more power it takes to move a given amount of air.  And bath fans — typically restricted to 4″ ducts — can only move a tenth of the amount of air that box fans can move, per watt of power.

Post #1843: Why are ceiling fans vastly more efficient than box fans?

 

In a nutshell?  To provide the same flow (CFM or cubic feet per minute), a small fan (like a box fan) has to move air a lot faster than a larger fan (like a ceiling fan).  And to move air fast, it takes disproportionately more pressure — and hence energy — than it takes to move it slowly.

The rest is just arithmetic.

I’m not talking slightly more efficient.  It’s well-established that ceiling fans are the most efficient type of home fan you can buy (reference).

I’m talking on-order-of five times as efficient as a box fan.  That, comparing the elderly ceiling fans in my house, against the most efficient modern box fan currently sold at Home Depot.

My main point is that the efficiency advantage of ceiling fans is rooted in basic physics.   It’s purely a consequence of their larger size.  It has nothing to do with (e.g.) the grilles on the box fan or the efficiency of various styles of electric motors.  It is simply that to achieve some given rate of air movement (cubic feet/minute), it takes far less energy to move a large volume, slowly, than to move a small volume, quickly.

Not only are ceiling fans more efficient than box fans, they always have been, and always will be.  It’s not the motor, or the housing, or the grille, or any of that.  It’s just physics.

Edit:  This also explains why bathroom fans are so slow at clearing the air.  If you wanted a bathroom fan that could move as much air as a box fan, it would require a 500 watt motor (Post #1859).

 

Continue reading Post #1843: Why are ceiling fans vastly more efficient than box fans?

Post 1798: Forest fire smoke and easy air cleaning.

 

With smoke from the Canadian forest fires continuing to generate air pollution alerts in the U.S., my wife suggested that I re-up my articles on using a box fan as an air cleaner.

This is a re-telling of Post #1792 and Post #1794.  Refer to those posts if you want more background information.

Three simple points

Point 1:  A standard 20″ box fan and a high-end 3M Filtrete HVAC filter together make a simple and effective air cleaner.  Get a 3M 1900 filter (rated MERV 13), place it on the back of the fan, and turn the fan on.

The key here is that the 3M electrostatic filters produce little “back pressure” or resistance to air flow.  That’s why you can have the low-powered fan draw air through that filter and still have significant air flow.

You can do the same thing with standard high-resistance MERV 13 filters, but you would need to construct a “Corsi Box” to provide enough surface area.  That is, tape four together into a hollow box, to provide enough surface area to allow for adequate air flow.

The 3M filters are expensive, but in my experience they last for months.  Arguably, this being almost July, you’d only need one for the entire summer.

 

Point 2:  This is more effective than a typical room-sized HEPA filter.  The reason is that with heavily-polluted outdoor air, filtering a lot of air reasonably well (fan + filter) beats filtering a small amount of air extremely well (HEPA unit).

Above is the labeling on that Filtrete (r) 1900 filter. In a single pass through the filter, it removes

  • 62% of the tiniest particles (0.3 to 1.0 mircons)
  • 87% of the mid-sized particles (1.0 to 3.0 microns)
  • 95% of the larger particles (3.0 to 10 microns).

That’s nowhere near as good as a HEPA filter, which removes on-order-of 99.97% of all such particles in a single pass.

So why does the fan + filter win?

First, outdoor air infiltrates into indoor spaces at a fairly rapid rate.  Typical tight older construction has one air exchange per hour.  That is, every hour, enough outdoor air enters the building to replace the entire volume of indoor air.

In the current situation, that means smoky outdoor air is more-or-less pouring into your living space, continuously.  Even with the windows and doors shut.

Second, a box fan moves a lot more air per minute than a typical room-sized HEPA unit.  A box fan on high can move about 2000 cubic feet of air per minute.  Depending on the fan, a box fan on low can move on order of 1000 cubic feet per minute.  A typical room-sized HEPA unit might move just over 100 cubic feet per minute.

The end result is that the slower HEPA filter can’t keep up with the steady inflow of dirty air.  Or, more properly, can’t keep up as well as the fan-and-filter combination.

On the left, you see the results of a numerical simulation of the two types of filtration.  Left is the box-and-filter, right is a typical HEPA unit.  Horizontal axis is time, vertical axis is the density of particulates in the air.  (See prior post for full details of simulation).

The equilibrium level of particulates in the room is vastly lower with the high-volume, lower-efficiency filter (left graph above).  Why?  Because the slow pace of the HEPA filter (right graph) can’t keep up with the level of outside-air infiltration that is typical in older construction.

Point 3: Availability.  As we learned during the pandemic, if there’s a sudden surge in demand (e.g., for N95 respirators), the shelves are soon stripped bare.  So if everybody goes out looking for an air cleaning device, those will soon become unobtainable.

As of today, my local Home Depot has well over 100 20″ box fans in stock, on the floor, ready to be purchased.  By contrast, they have just five room-sized HEPA units in stock. 

Which makes sense.  Those fans are commodity items costing about $25 each.  The Honeywell HEPA unit, by contrast, goes for just about $300.    Home Depot couldn’t afford to keep 100 of those in stock, on the off chance that there might be a run on air cleaners.

Summary

Sometimes, simple and cheap is what you want.  In this case, a box fan and a 3M 1900 air filter together cost much less than a room-sized HEPA filter.  And in this situation — where you are trying to filter pollution arriving from outdoor air — the much higher air flow of the fan-and-filter combination actually works better than a typical HEPA air cleaner.

Nothing prevents you from dealing with this problem by wearing an N95 respirator inside.  But note from the simulation above, the fan-and-filter combination provides air that is almost as clean as you would get, breathing through an N95 respirator.  So you get almost the full benefit of that, without the hassle of wearing a mask 24/7.

As a bonus, while the mask protects your lungs, the fan-and-filter combination protects both your lungs and your eyes.  If eye irritation is an issue for you, filtering the indoor air is the only way to go.