When airflow increases 10%, motor amp draw will increase 33%. It’s because of this little known fact that thousands of blower motors are cooked each year as technicians adjust fan speed to increase fan airflow. The good news is that anyone can predict amp draw in less than a minute if they are equipped with Fan Law Three. Let’s review the math and function of this fan law and learn to avoid ongoing blower motor damage.

In the last two issues of CB Hotmail, we’ve learned to use Fan Law One to calculate the size of a pulley and Fan Law Two to predict the changes in system pressure drops. This review of Fan Law Three will complete this series about Fan Laws and arm you will the ability to predict the behavior of any forced air HVAC system before you ever pick up a wrench. These skills are what differentiate a laborer from an HVAC professional and enable a technician to apply brainpower over muscle power in solving problems that you face daily in the field.

When to Use Fan Law Three

Suppose you are testing and diagnosing the performance of an HVAC system and find the fan specified to move 2000 CFM, but measurements reveal it’s only moving 1660 CFM. You inspect the blower motor nameplate to learn the Full Load Amps (FLA) of the motor is rated at 9.0 amps. You measure the motor’s amp draw to find it is currently pulling 7.3 amps at 1660 CFM.

Now for the million dollar question: what will the amp draw increase to if you adjust the pulley or fan speed to increase the fan airflow to 2000 CFM?

A Full Load Amp rating of 9.0 amps indicates the blower motor should not to be operated at more than 9 amps or the excessive heat it will generate may damage the motor. Before adjusting the pulley, apply Fan Law Three to your test numbers and accurately predict what the blower motor amps will increase to before you make the pulley adjustment.

Fan Law Three

Let’s take a closer look at the math involved in working Fan Law Three. We’ll discuss each component of the formula and then move through it step by step. Hopefully by the time you reach the end of this article you’ll be able to work Fan Law Three in the field next time you adjust fan airflow.

Fan Law Three is shown below. If you are not familiar with it, the formula may look a little threatening, but the math of this formula is quite simple to calculate and is very suited for fieldwork. The math involved in this formula is quite simple: divide once, and multiply twice. There’s nothing too fancy going on here.

To understand this formula better, let’s take a look at the abbreviations contained in it. First is AMP, this represents the amperage of the system before and after the airflow is increased. The CFM in the formula represents the fan airflow in Cubin Feet per Minute.

Notice each abbreviation has a subset number following it. (That’s the short number following each abbreviation) of 1 or 2. If the abbreviation has a 1 behind it, this means you’ve already measured this amperage or airflow in the formula. In this case, you measured the blower motor amperage and verified it was pulling 7.3 amps and you measured the fan airflow of 1660 CFM.

When the abbreviation has a 2 behind it, that represents the new or future values the system will operate under. The 2000 CFM will be the airflow after the fan speed is increased and the AMP2 is the answer to the formula.

In algebraic language the parenthesis in the formula are saying, “Do this part of the formula first.” The last feature of this formula is the 3 at the end, and is what you do next. In this case the 3 means to cube the answer in the formula. To cube a number is to multiply it by itself three times.

Work the Formula

Take the formula and replace the abbreviations with the real numbers from the job you are working on. In this case replace the AMP1 with the blower motor amperage that you measured of 7.3. Then replace the CFM1 with the fan airflow that you measured of 1660. Replace the CFM2 with the airflow that the fan will be delivering of 2000. Now your formula should look like this:

The first step is to do the math inside the parenthesis. Divide 2000 by 1660 to discover the rate of increase on fan airflow of 1.20, or airflow will increase 120%. This is the foundation of the formula:

The next step is to cube the 1.20. Or 1.20 x 1.20 x 1.20. This equals 1.73. Let’s stop and take a look at what we just learned from this formula. In the last step we saw that airflow increase 120%. In this step we see that amperage will increase 1.73 or 173%. Amperage increases at the cube of airflow! That’s why when you’re working fan laws you’ll quickly begin to understand that amp draw changes the most of any of the values in the system and so it’s the one you really have to watch out for. This is why you can overheat or cook a motor in a very short period of time when you are increasing system airflow.

The next step is to multiply the amp draw of the blower motor by the 1.73:

The problem is solved by revealing that after the fan airflow is adjusted from 1660 CFM to 2000 CFM, that the blower motor amp draw will increase from 7.3 amps to 12.6 amps.

Remember that we checked the blower motor nameplate and learned that this motor was only rated for a maximum FLA of 9.0 amps? What would have happened to this blower motor an hour after you increased fan airflow to 2000 CFM and left the job?

The blower motor would have overheated and been damaged because it would be pulling 12.6 amps or 3.6 amps more than the motor is rated. And who’s to blame?

Knowing how to use Fan Law Three to predict the change in amp draw can serve you well. It can enable you to look into the future and solve problems before they even happen. Knowing the amp draw will increase this much will allow you to look for other changes you can make in the system to prevent the problem before it happens.

By knowing in advance what problems adjusting fan airflow can create, you can solve the problem before the damage occurs. You can now adjust the fan speed less to protect the blower motor. Or you can offer your customer the solution of a new, more efficient variable speed blower motor that can handle the increase amperage requirements. This will avoid cooking their old blower motor and damaging your relationship with this customer. In essence, you stop acting like a laborer and begin performing like a true professional.

Learning to use fan laws takes study and practice, but it yields a fluid, yet firm knowledge and understanding that will multiply your ability to solve everyday problems and prescribe solutions that will leave your customers delighted with you and the solutions that you provide to them.

Remember Fan Law Three’s bothers: Fan Laws One and Two, and become familiar with these indispensible tools of our trade.

Rob “Doc” Falke serves the industry as president of National Comfort Institute an HVAC based training company and membership organization. If you're an HVAC contractor or technician interested in a free Fan Law Calculation Report Form that contains all three fan laws, contact Doc at robf@ncihvac.com or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, articles and downloads.