I forgot about this method until my friend Jason Nikkel, with JN Electrical Temperature Control, from Bowersville, Georgia, reminded me. Our conversation was the spark for this article. Let’s review Fan Law Two and what you’ll need to use it to set blower speeds.

Reviewing Fan Law Two

Fan Law Two is a mathematical formula commercial air balancers use to predict static pressure changes with airflow adjustments. There are a couple of formula variations, but the one we’ll use in this article is:

(CFM₂ ÷ CFM₁)² x SP₁ = SP₂

Once you know the Fan Law Two formula breakdown and flow, it’s easy to apply. The two abbreviations are the first pieces to understand. CFM stands for cubic feet per minute, or airflow, and SP stands for static pressure. In our application of the formula, we’ll use the total external static pressure (TESP).

Each abbreviation has a small subscript number at the bottom right. Abbreviations with a one (₁) are the measured value in the formula. Abbreviations with a two (₂) next to them are the design or target value you’re trying to determine.

Parentheses in the formula tell you to, “Do this part of Fan Law Two first.” There is also a superscript number two (²) outside the parentheses. This number means you must “square” the answer from inside the parentheses, or multiply the answer times itself. As an example, if you squared the number five, it equals 25 (5 x 5 = 25).

Gather the Essentials

Before you can use Fan Law Two to adjust blower speeds, you need to gather some essentials. First, you’ll need a quality static pressure test kit to measure TESP. A kit with all the basics should include:

• Manometer (digital or analog);
• Tubing;
• Static pressure tips;
• Step bit; and
• Test port plugs.

Once you have test equipment, you’ll need the manufacturer’s fan table for the air-handling equipment you’re testing. Depending on the brand, you may find this information in the installation instructions, on the back of the blower panel, or through an internet search. If the equipment is obsolete or you can’t find the original manufacturer’s fan table, you can use NCI’s generic fan tables.

Airflow data (CFM per Watts)

Click table to enlarge

FAN TABLE: You can see the changes in fan speeds as outlined in the article on this fan table. Red shows the “as-found” settings while blue shows the new speed to use according to Fan Law Two. (Courtesy of National Comfort Institute)

Finally, you need to know the heating and cooling airflow requirements. If you’re setting up a heat pump air handler, heating and cooling airflow are the same. However, if you have a gas furnace with cooling or dual fuel, you must set up blower airflow for each mode of operation.

We will use 400 cfm per ton as the cooling airflow example in this article. This airflow value will change based on equipment capacity and location. Make sure you know what works in your area. Gas heating airflow can be a little trickier. If you need help, here are four steps to estimate gas heating airflow that simplify the process.

Applying Fan Law Two to Blower Speed Adjustments

Let’s say you’re a service technician dispatched to a complaint about noise and poor cooling on a newer installation. When you arrive, you perform a visual inspection and find a split system with a two-ton outdoor unit. Inside, there is an 80% furnace with a constant torque (X-13) blower motor.

You look at the furnace nameplate and notice the maximum-rated TESP is 0.50-in. w.c. (inches of water column). As you record the furnace model number and look it up in the installation instructions, you notice the blower has five speeds and can handle a 3-ton outdoor unit. Further investigation shows that the installation crew set the cooling speed tap to the high-speed setting.

Next, you measure TESP at 1.10-in. w.c. and plot it along with the high-speed blower setting on the manufacturer fan table. This blower moves 1,275 cfm at 1.0-in. w.c. and consumes 504 watts. You know that 1,275 cfm is too much airflow for a 2-ton outdoor unit. It only needs around 800 cfm in your area.

The clues all lead to an improper blower setup. Now it’s time to estimate what TESP will be at 800 cfm, so you can predict the best blower speed choice. Your readings would fit into Fan Law Two as follows:

(CFM₂ ÷ CFM₁)² x SP₁ = SP₂

• CFM₂ = 800 cfm (the target airflow);
• CFM₁ = 1275 cfm (the plotted airflow from the manufacturer fan table);
• SP₁ = 1.10 in. w.c. (the measured total external static pressure); and
• SP₂ = ?? in. w.c. (the static pressure you can expect to see at 800 cfm).

First, place all the numbers from above into the Fan Law Two formula. It should look like this:

(800 ÷ 1275)² x 1.10 = SP₂<>

Next, divide CFM₂ (800) by CFM₁ (1275). This equals .627. We’ll round the answer to .63.

0.63² x 1.10 = SP₂

Then, square your answer by multiplying 0.63 times itself (0.63 x 0.63), which equals 0.396. We’ll round the answer to 0.40 to simplify the math. Ideally, you will jump straight to the next step on your calculator.

0.40 x 1.10 = SP₂

Finally, multiply your measured TESP, or SP₁ (1.10) times the squared airflow ratio, to determine SP₂, or the new static pressure you can expect to see at 800 cfm.

0.44 in. w.c. = SP₂

Now look at the manufacturer fan table to see which blower speed setting will move around 800 cfm at a 0.44 TESP. Since fan airflow is ± 10%, you can go as low as 720 cfm or as high as 880 cfm and still be within an acceptable range.

Fan table static pressure readings only show tenths of an inch of water column. You won’t find one that has a 0.44 column, so you’ll have to round and choose the closest option, 0.40. Your closest speed tap options in the 0.40 column are medium speed at 950 cfm or medium low at 775 cfm.

Since the medium-low speed option is within ± 10% of 800 cfm, you change the blower speed setting to medium-low. Now the blower should move approximately 775 cfm at 124 watts.

You button up the equipment and turn it on to recheck your airside measurements and calculations. As an extra precaution, you also check your superheat and subcooling to make sure the refrigerant charge doesn’t need any adjustments at the new airflow setting.

Put Fan Law Two to Work

The example above is just one use of Fan Law Two. Another potential application is for equipment setup and commissioning. If the installers used this procedure in the story above, there would have been no customer complaint or callback.

If you have a static pressure kit, you can use Fan Law Two now. If you don’t have a static pressure kit, that is your first step. Once you have one, test your own installations during startup. It’s also a great learning experience to test your office systems and work the math to make speed tap adjustments.

I hope you see the potential for this formula. It will make your life easier and help you predict changes to blower speed settings with more accuracy and reliability.