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Furnace Fan Motor


Fan Motor Not Working

Why is my fan motor not working?

Why is my furnace or air conditioner fan motor not working? The fact is that most fan motor’s don’t just die, they’re murdered. What do I mean by that? I mean that most fan motors stop working because of issues with air flow. In fact, most furnace repair problems and air conditioning repair problems that we come across are the result of improper air flow.

Furnace Fan Motor

The fan motor in a furnace is the heart of your system. It is responsible for pulling the return air back to the furnace as well as pushing the supply through the system. The air flow in your home is a lot like blood pressure in your body. When we measure the static pressure we are essentially measuring the blood pressure of your system. High pressure in a HVAC system will kill the fan motor.

Air Conditioner Fan Motor

On an air conditioning unit the fan motor is essential to proper refrigeration cycle operation. The air conditioner fan motor is responsible for cooling the hot gas leaving the air conditioner compressor. As the hot refrigerant gas leaves the compressor is condenses (hence the name condenser unit) into liquid refrigerant. The amount of liquid refrigerant in the condenser is known as the subcooling. Most system’s require 8 to 10 degrees of subcooling.

Most Common Fan Motor Failure

A dirty blower motor can result in a system that can only move 60-70% of it’s rated airflow CFM. When the fan blower wheel becomes dirty on an ECM, Variable Speed or constant torque motor it makes the fan work much harder. This is the reason why so many variable speed or electrically commutated motors are replaced after only a few years of operation. The motor didn’t just die, it was murdered. It was murder by high static pressure. Air conditioner fan motors are frequently murdered by weak capacitors and dirty condenser coils.

Frequently Asked Questions

These are some of the most common questions about static pressures and fan motors not working in furnace, air conditioners, and heat pumps.

Are my ducts properly sized?

The proper size air ducts must be calculated by using an ACCA manual J.

Is there a minimum static pressure?

If the inlet and the outlet of a motor is left open ended without any static pressure it will end up overheating and killing the motor. Some level of static pressure is necessary. Low static pressure can also result in a lot of duct leakage.

How do you measure static pressure?

Static pressure can be measured with a manometer using static tips. There are three systems. Total pressure is a measure of

Does Static pressure change with flex duct?

Static pressure can change dramatically with flex duct. When we measure static pressure we are measuring resistance to air flow. Flexible duct can be damaged under the insulation which can result in increased pressure even though the duct looks fine on the outside of the insulation.

Does tube length matter when measuring static pressure?

For the distances that we are going the length of the tube doesn’t matter. We are measuring systems that most of the time have a maximum static pressure rating of .5″ water column.

How much airflow is lost by a dirty blower wheel?

Our in home research indicates that 1/8″ dust can cause a 30% loss in airflow cfm.

How do you recommend checking a secondary heat exchanger?

The best way is to look at the blower wheel. If the blower wheel looks really dirty, then the second . High temperature rises and high flue temperatures are common problems we see with a dirty secondary heat exchanger.

A dirty secondary

Variable speed furnace and air conditioning systems can show a

How do I fix an undersized duct system in a slab?

The best way to solve this problem is to relocate the equipment. This can be a difficult and expensive process. Our team of Comfort Heroes will always try to do the best that we can. We will always provide you with solutions.

What happens when ducts are dirty?

Dirty ducts will act as restrictive ducts. It depends on how dirty the ductwork is. The more dirt in the ductwork the higher the static pressure will go.

Does high static pressure result in less humidity removal?

Yes, higher static pressures can result in what is known as evaporator coil “blow off.” This is where the condensate blows off of the coil. Another problem common on downflow HVAC systems is that the evaporator coil can hold water on the return side. This water will not drain until the system turns off.

How much does the actual design of the plenum affect static pressure?

The airflow has to get out of the system to begin with. We have experimented with plenum designs and found it to be a very important problem. If your plenum is less than four feet tall it will have a negative impact on airflow.

In super insulated homes how do you combat humidification issues?

There are two different issues here. We like to use a separate dehumidification system as well as a separate ERV. High humidity levels in a super insulated house can destroy the home from the inside out.

Is round duct or square duct better?

We like both round duct and square the duct. The most important factor is which duct delivers the proper air flow. We like to size system for about 350 to 400 CFM per ton of cooling and 150 CFM per 10,000 BTU on a 90% furnace. On an 80% furnace we try to hit 100 CFM per 10,000 BTU.

Can you have too much return air?

The answer is no, you cannot have too much return air. In our experience return air, or cold air return, is normally too small.

Does an energy recover ventilator reduce duct pressure?

Yes, an energy recovery system will lower the overall static pressure.

Where do I measure static pressure?

The proper place to measure static pressure is after the filter and before the evaporator coil.

What is internal static pressure?

Internal static pressure is the pressure already account for in the system from the factory. The longer an HVAC system runs, the dirtier the internal components will become.

Can you remove the limit switch to measure static pressure?

Yes, make sure that you turn the power off to the furnace before removing the high limit switch.

Do undersized filter grilles affect static pressure?

Yes, undersized filter grills will affect static pressure. You always want to measure with the filter in the grille. The best way to measure is to drill a 3/8″ hole through the 1″ filter and insert the tip into the air stream through the filter.

If you add return air, how much static pressure shifts to the supply?

The static pressure will shift as the air flow moves across the fan motor.

Is duct sealing like AeroSeal worth it?

Products like Aeroseal are great for sealing leaking ductwork, however, if your duct system is already undersized then sealing the ductwork will just make the static pressure higher.

What are the most common fan motor failure reasons?

Check out this article to learn more about common fan motor failures.

“I Need to Find Furnace Fan Motor Repair Near Me!”

You work hard to keep your home comfortable and operating smoothly. Don’t let an HVAC emergency undo all your work and cost you hundreds, if not thousands, in repairs! Call (605) 206-3303, or schedule a visit from our Sioux Falls HVAC heroes online!


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