What is the Typical Amp Draw of a Gas Furnace Blower Motor?
The typical amp draw of a gas furnace blower motor can vary depending on the motor’s horsepower and the specific operating conditions. For a standard residential system, a 1/2 horsepower blower motor typically draws between 4 to 5 amps at maximum load. On high speed, most blower motors will draw 70-100% of the Full Load Amps (FLA) if they are moving the required airflow, such as 400 CFM per ton.
How Does Amp Draw Differ in Different Modes?
Idle Mode
When the blower is not running, the amp draw is zero.
Operating Mode
When the blower is running, the amp draw can be measured in different speeds. For example, on high speed, the motor might draw close to its FLA rating, while on low speed, it will draw significantly less. For instance, an ECM (Electronically Commutated Motor) blower motor might draw less than 100 watts (approximately 0.8 amps at 120 volts) on low speed.
What Factors Can Cause High Amp Draw?
Undersized Ducts or High Static Pressure
- Static Pressure: High static pressure in the duct system can cause the blower motor to work harder, leading to higher amp draw. This is because the motor must overcome greater resistance to move the air through the ducts. Proper duct sizing and ensuring that all zone dampers are open can help maintain normal static pressure and reduce amp draw.
- Duct Restrictions: Restrictions in the ductwork, such as kinked or undersized ducts, can increase the load on the blower motor, causing it to draw more amps. Ensuring that the ductwork is properly sized and free from obstructions is crucial for maintaining normal amp draw.
Other Factors
- Motor Condition: A faulty or worn-out motor can draw more amps than usual. For example, if a motor is drawing significantly more amps than its FLA rating, it may indicate a problem with the motor itself.
- Capacitor Issues: A malfunctioning capacitor can also cause the motor to draw higher amps. Ensuring that the capacitor is the correct type and in good condition is important.
- Shaft Alignments and Bearings: Misaligned shafts or worn-out bearings can increase the drag on the motor, causing it to draw more amps.
How to Calculate Energy Consumption and Measure Amp Draw?
Calculating Energy Consumption
To calculate the energy consumption of a blower motor, you need to know the amp draw and the voltage. For example, a 5-amp motor running at 120 volts would consume 600 watts (5 amps * 120 volts). Running this motor 24/7 would consume approximately 12 kWh per day.
Amp Draw Measurement
To measure the amp draw accurately, it is crucial to follow proper procedures, such as disconnecting power, checking the rotation of the fan, and ensuring the blower compartment is closed during the measurement to avoid false readings.
References
By understanding the typical amp draw of a gas furnace blower motor, the factors that can cause high amp draw, and how to properly measure and calculate energy consumption, you can ensure your gas furnace is operating efficiently and identify any potential issues that may be causing excessive power consumption.
