Jump to content
Click here to check out our structured, online appliance repair training courses for rookies and experienced techs.

FAQs | Repair Videos | Academy | Newsletter | Contact

Stay connected with us...

Subscribe to our YouTube channel for lots of appliance repair tips and help! Subscribe to our MST Radio podcast to learn secrets of the trade. Sign up for our free newsletter and keep up with all things Appliantology.

Son of Samurai's Blog

  • entries
    50
  • comments
    193
  • views
    5,421

How to Correctly Measure Dryer Vent Airflow

Son of Samurai

1,566 views

The general rule for dryer vent airflow is that, if the airflow feels like a breath (even a strong one), then you have a problem. This rule of thumb will serve you well a lot of the time, but sometimes a "calibrated palm" just isn't enough. Sometimes, you need to get an actual measurement of the airflow so that you can compare it to the specifications.

What you really want to know is the volumetric flow rate of the dryer exhaust. That is, how much air it's pushing out over a particular period of time. That's a difficult thing to measure directly, so what we do instead is measure velocity and use that to calculate volumetric flow.

Let's break down how we do that.

The easiest way to measure a vent's velocity is by using an anemometer. You can get these relatively inexpensively at places such as Amazon -- for instance, look at this one: https://amzn.to/2uLJjBP

We use miles per hour (MPH) when we're talking about velocity. And that's exactly what your anemometer will tell you: how fast is the air moving. But you can't calculate volumetric flow rate if you only know velocity. You need to also know how wide your vent tubing is. Fortunately, dryer venting is pretty standard. The tubing is almost always 4 inches in diameter.

Once you know both the diameter of the tubing and the air velocity, you're ready to calculate your volumetric flow rate, which is measured in cubic feet per minute (CFM). You can calculate it easily by using an online calculator, like this one: https://www.engineering.com/calculators/airflow.htm

NOTE: When that calculator asks for the width of a circular duct, it asks for the radius, not the diameter. The radius is always half the diameter, so if you have a 4 inch wide duct, that's a radius of 2 inches.

So now you know how to calculate how many CFM that vent is pushing out! But what spec do you compare that to? Check out the circled bits below:

Screen Shot 2020-03-12 at 12.27.01 AM.png

This is from a Whirlpool dryer manual, but these specs apply to basically all dryers. The technology just doesn't vary that much.

In yellow, you can see that 4 inch diameter vent spec. In green, the maximum CFM that the venting system needs to be able to handle. And then in red, you can see the minimum CFM that your dryer vent should be able to push out.

So, for a healthy, 4 inch diameter vent, you should get at least 105 CFM. And that's a general spec you can apply to any model of dryer. You now know your spec!

Let's run through a quick example.

My anemometer reads 10 MPH coming out of the dryer vent. I've confirmed that my dryer vent is 4 inches in diameter. Is that measurement within spec or not? Well, a quick calculation using the tool I linked above (making sure that I put in 2 inches for the radius), and I get... 76.8 CFM. That's way below spec!

So there you have it: a simple and objective way to determine dryer vent health using one inexpensive tool and a tiny bit of math. Want to learn more about troubleshooting laundry appliances with precision and certainty? Click here to check out the Washer and Dryer Repair Course over at Master Samurai Tech.

  • Like 5
  • Thanks 2


13 Comments


Recommended Comments

Got my new airnerd meter today!

  • Like 1

Share this comment


Link to comment
EthanRanft

Posted

5 hours ago, Coley said:

Got my new airnerd meter today!

Got mine last week!

  • Like 1

Share this comment


Link to comment
EthanRanft

Posted

Hoping to get some ideas or opinions on how to properly use that tool to give good information.

For one thing, how would you accurately calculate the CFM if checking at this style vent?

F3909ADB-02B3-488F-B836-415EAC355AA0.thumb.jpeg.f9d379930aba877b2fa2eb09c2f39071.jpeg

I can’t imagine it’s the same size hole as a 4” circle.

I bring this up because the other day I was wanting to try out the anemometer. So I stuck it down at the end of this vent. By the way, it makes a big difference how exactly you’re holding the meter. I just went with the highest speed I could get, which was 16mph. Then I went directly on the other side of this wall and disconnected the flex. At the end of the flex I measured 22mph. Now I can calculate that pretty accurately as 169cfm.

Obviously this style vent is restricting the airflow by quite a bit, but how much actually? Do I measure the dimensions of the bottom opening? But then that’s not the same as the area that’s open when that flapper is pushed open.

Am I missing the point and getting too technical? Just use it as a general reference?

I have to admit, when I bought it I thought it would give me good definitive numbers to give the customer to prove their dryer is operating within spec. But maybe I’m asking too much out of a $30 meter

Share this comment


Link to comment
Son of Samurai

Posted

18 hours ago, EthanRanft said:

For one thing, how would you accurately calculate the CFM if checking at this style vent? I can’t imagine it’s the same size hole as a 4” circle.

Great observation! Yep, you're correct that the size of the vent hood opening affects your reading. What you can do is either a) remove the vent hood so that you can get a clear measurement on the air coming out of the 4 inch diameter vent tube, or b) measure the width and height of the rectangular vent hood opening, then use that to calculate your CFM from MPH.

19 hours ago, EthanRanft said:

By the way, it makes a big difference how exactly you’re holding the meter. I just went with the highest speed I could get, which was 16mph. Then I went directly on the other side of this wall and disconnected the flex. At the end of the flex I measured 22mph. Now I can calculate that pretty accurately as 169cfm.

To get the most accurate reading possible, you want to hold the propeller blades of the anemometer as perpendicular as possible to the direction of airflow. The best thing to do is like what you did: slowly move the impeller from side to side and then go off the highest reading you get. The highest one will be the most accurate.

  • Like 1

Share this comment


Link to comment

I have the Whirlpool vent tester I bought years ago. It has a tube that goes into the dryer and measures vacuum maybe?

l‘ve used it with good results so far.

What are your thoughts on this tool?

701C8BFE-7131-4926-87EE-552717C5A4DF.jpeg

Share this comment


Link to comment
AQAppliances

Posted

Amazing. I bought one of those whirlpool vent testers years ago and was not happy with it. I’ll order one of these. 

Share this comment


Link to comment
BearsFan4Eva

Posted

I bought that Whirlpool tester as well and wasn’t happy with it as it says it is only good for WP dryers. 

Share this comment


Link to comment
AQAppliances

Posted

34 minutes ago, BearsFan4Eva said:

I bought that Whirlpool tester as well and wasn’t happy with it as it says it is only good for WP dryers. 

I believe it needs to be calibrated yearly as well. 

Share this comment


Link to comment

I didn’t know about the calibration!

I’ve used it on many brands though with decent results. At first I would back it up by watching the temperature cycle on and off with my thermocouple and temp meter. The rate of temperature drop was a good indication of a clear vent. They matched results often enough I stopped using the temp drop meter.

What I have failed to understand is how does the venting for a gas dryer differ from an electric one? I’ve never understood why they had two different settings. 
I always looked for the “good all dryers”.
 

 

  • Like 1

Share this comment


Link to comment
allen63

Posted

I use the whirlpool airflow tester on all dryers works great for me.

Share this comment


Link to comment

I just got the whirlpool tester.  I also have the extent Mini Thermo-anemometer.  It gives The FPM /cfm etc.. and, the temp. Mainly used  in the past for a/c but, I am going to try it out on the dryer vents

IMG_7354.JPG

Share this comment


Link to comment
Wx4usa

Posted (edited)

I always also pre-test with the anemometer and post-test with the anemometer and compare the variance as a percentage increase or inverse - percentage of blockage. These numbers combined with the poodle I just pulled from the vents is a great selling tool.
 

Do you realize what lint is? Cremated missing socks. 

Edited by Wx4usa
  • Like 1

Share this comment


Link to comment
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.