Fan Speed Measurement Basics

May 31, 2011
Your customer is suffering from low airflow. Your job is to identify the problem and prescribe a solution. A quick, but effective test to get you started with your diagnostics is to measure the amp draw of the blower motor.

Your customer is suffering from low airflow. Your job is to identify the problem and prescribe a solution. A quick, but effective test to get you started with your diagnostics is to measure the amp draw of the blower motor.

Amp draw is a measurement of the work that a blower motor is accomplishing. It is the amount of power that a fan is using to move the air. Testing amp draw or ‘amperage’ gives us a peek at what’s going on in the system.

Before measuring amp draw, you need to know the capacity of the blower motor that’s turning the fan. This is found in the nameplate of the motor and is called ‘Full Load Amps.’ It is usually listed next to the abbreviation ‘FLA.’

When a motor operates at 100% of its Full Load Amps it is operating at 100% of its designed capacity. The goal of this test is to compare the measured amps that the motor is using to the Full Load Amp capacity of the motor.

If the measured amp draw exceeds the capacity of the motor, the motor will fail prematurely. Excessive heat is what usually causes damage to a motor, and when a motor is working beyond its Full Load Amps, its life expectancy is decreased significantly.

If the measured amp draw is only half of the FLA, the chances are that you have a serious air flow problem that will require additional work.

Test Instrument

Most of us carry one of many electrical multimeters that are on the market that allows us to test electrical properties. On these meters, there’s a selection for AC amperage that allows for a wide range of testing sufficient for all sizes of residential blower motors. An electrical meter will range in cost from $80.00 to $400. As with all tools, you get what you pay for.

Some meters come with a built in AC current clip that looks like two fingers at one end of the meter that wrap around the wire being tested.

Other meters have an AC current clip accessory that’s like a clothespin with two insulated wires that attach it to the meter. This clip wraps around the power lead of the motor and measures the flow of the current through the wire.

Remember to have your test instruments calibrated annually so that you can have confidence in your readings and diagnostics.


Before we discuss taking measurements, let’s look at safety. Amperage measurement is taken with everything live. With some systems your fingers and test leads may be dangerously close to a moving fan. Keep in mind that you’re measuring electricity. If this is a new test for you or if you’ve successfully performed it a thousand times, please pay attention to your personal safety.

Use extreme safety when measuring a live fan. Also be aware when moving wires with your bare fingers to attach your test clip. Some wires may have had the back of the insulation stripped off during installation.

To help us take advantage of this quick and simple diagnostic procedure, let's take a look at measuring amp draw of a residential direct drive blower. Even if you often deal with large commercial systems, read on, the same principles apply.

Test Procedure

Step One: open up the blower compartment of the furnace or air handler. Find the Full Load Amp (FLA) rating of the blower motor. If the motor is direct-drive, these numbers can be difficult to read. They’re located on the side of the motor, which is located inside the squirrel cage. An inspection mirror may be required. (Be careful not to read the numbers backwards.

Most air movement equipment have the FLA of the blower motor listed right on the nameplate of the equipment.

Step Two: inspect the rotation of the fan. ‘Bump’ the fan by starting then stopping it quickly with the disconnect or the blower compartment safety switch. Check that the blower is rotating in the right direction, that it is tight on the shaft and that there’s no restriction or grinding as the motor turns.

Step Three: locate the power wire leading to the blower motor. This is normally the wire leading to the air conditioning or high-speed wire on the speed tap or circuit board. (Remember to test in cooling mode.) If you have a single phase multiple speed motor with speed taps, place the unit in cooling mode with the fan switch in the ‘on’ position and the temperature set to 55F.

One rule of reading amp draw is that only one wire at a time can be measured. Two or more wires wrapped in a housing such as a cord, will cancel each other out or the reading will be false.

Step Four: clamp the amp reading clip from your electrical meter around the power wire.

If the only access to the wire is in the blower compartment, you must be sure that the test is taken with the blower door on, or at least closed as much as possible.

If the door is left off during the test, the fan will move far more air than it would if it had to pull all its air through the return air ducting. Therefore, the fan will work harder and the amp draw will be higher than it would be under normal operating conditions.

Step Five: read the amp draw of the blower motor. Record the reading and compare it to the FLA listed on the motor.

Check the amp draw against the manufacturer's fan performance data to verify the airflow that the fan is producing. A rule of thumb is that on high speed most motors will run 70-100% of the FLA if it is moving the required 400 CFM per ton that matches the cooling rating of that air handler.

Rob “Doc” Falke serves the industry as president of National Comfort Institute an HVAC based training company with technical and business level membership organizations. If you're an HVAC contractor or technician interested in a free Amp Draw Test Procedure, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at for free information, articles and downloads.