How to Replace a Condensing Fan Motor

Service calls about condensing fan motor failure are quite common. Even though fan motor replacement is a standard procedure for residential split systems, it requires careful attention to safety and detail. 

This article will give you a step-by-step guide to replacing condensing fan motors. Along the way, we will also explain a few best practices for safety and enhanced understanding of the process.

 

1. Diagnose the REAL problem

One of the reasons why motor failure is so common is because there are many ways that it can fail.

Before you replace a motor, you should know what caused it to fail (and that you’re dealing with a motor failure in the first place). Some causes may include failing shorted, windings failing open, and bearing failure. The image above shows highlighted bearings, which often fail due to improper lubrication or environmental contamination.

Remember: just because the motor isn’t running, that doesn’t mean it failed. There are a few other conditions that can cause a motor not to run. For example, wiring problems and failed capacitors are other issues that may cause the motor to stop running. Replacing the fan motor is not going to help anything in those situations.

Heat pump systems also stop the fan motor on defrost, which is not a motor problem at all. It’s important to know what kind of unit you’re working with so you can diagnose and troubleshoot effectively.

 

2. Find a suitable motor replacement

This best replacement motor is an original equipment manufacturer (OEM) motor. OEM motors are specifically designed to fit the unit’s specs and are adapted to its model’s blade, static pressure, and size constraints. 

However, there will be times when you won’t have an OEM motor and will have to use an aftermarket motor on your truck. During this step, you must consider a few different criteria for finding a replacement motor.

Size and frame

Clearly, you’ll want to use a motor that fits the system. Frame and size are going to be the two most obvious factors to consider when selecting a motor. 

Size is pretty intuitive. You can’t install a motor that doesn’t fit. However, depth is what you really want to pay attention to. Even if the new motor positions the fan blades just a little deeper or shallower than they initially were, the blade placement may adversely affect the airflow. You’ll want the fan blades to remain as close to their original position as possible to prevent airflow and high head pressure complications from occurring.

While you technically can modify the application to fit a motor’s frame, we don’t recommend it. If the new motor fails due to an incorrect installation, the original motor type may no longer fit because of the modifications you made for the aftermarket motor. 

Specifications to check

When replacing motors, you’re going to want four other values to be nearly the same as the old motor: RPM, voltage range, horsepower, and amperage. 

RPMs, or revolutions per minute, should be an exact match, if not extremely close. RPM is usually related to the number of poles, which is important for the motor’s form and function. It’s worth noting that some manufacturers may list 1075 RPM as 1100 RPM or vice versa. Those work fine interchangeably, as they are both six-pole motors. However, it is not okay to use a motor with 1075 RPM to replace a motor with 875 RPM; the latter is an eight-pole motor, so the two are incompatible.

You’ll also want the voltage range to be an exact match. For most residential and light commercial applications, the voltage range on single-phase motors will be 208-230.

Horsepower and amperage measure roughly the same thing: power at a given voltage. It’s best to find a replacement with the same horsepower and amperage, but some technicians replace motors with slightly higher horsepower and amperage, which is fine within reason. However, we NEVER recommend going below the original horsepower.

If you want to learn more about replacement fans and motors, check out our articles on aftermarket motors and fan blade depth.

 

3. Pull the condenser fan

BEFORE you pull the condenser fan, pull the disconnect and use a voltmeter to make sure there’s no potential present. Check leg to ground on each side and leg to leg.

Take the top off and set it on the ground, fan blades up. This is a good time to inspect the blades. Check for damage, corrosion, or other conditions that will affect its performance. If any blades need to be changed out, go ahead and do that. Three things should match the original fan blade: the pitch, the number of blades, and the diameter. 

If you’re going to reuse the original blade, pull the set screw out before removing it. As you can see in the picture above, we like to clean the blade shaft with a lubricant. We then scrub it away with sandpaper or a wire brush and remove the blade. The blade should slide off more smoothly when the shaft has as little grime and mechanical wear as possible.

Another trick to help remove the fan is to hold a crescent wrench on the backside between the motor and the fan blade. Then, use your hand to rotate the blade a little bit in each direction. As you hold the end shaft in place with the wrench, the opposing rotational forces may loosen the blade up and make it easier to remove.

 

4. Remove the nuts and the failed motor

To access the motor, you’ll have to remove the nuts that bolt it to the top of the unit. You will need to replace these later, so take note of the nut type; they may be acorn nuts or open-back nuts. Regardless of what they are, remove them.

Once you remove the nuts, you may start replacing the motor. Flip the top over to expose the motor. At this point, you can go ahead and remove the failed motor.

 

5. Replace the motor

During this step, you’ll want to pay attention to the parts you have and their orientation.

Pay attention to the wiring on the new motor. You don’t want the wires to be pinched or awkwardly placed within the unit. In the picture above, you’ll notice that the wires have been correctly aligned with the conduit (highlighted in white).

When you put the motor in place, turn the top back over. You may notice that the studs are quite long and won’t allow the cover to fit back on. You’re going to cut them, but be sure to screw the nuts back in first. If you try to put the nuts back on after you’ve guesstimated the stud length, you might not be able to align the threads properly.

In many cases, you’ll need to cut the studs on the bottom side by the shaft as well.

 

6. Consult manufacturer literature for drainage port information

The use of drainage ports (or weep holes) varies by manufacturer and model. However, some units can fail if you don’t unplug these ports and allow condensate to drain out. Read the unit’s manual or look up the manufacturer’s recommendations to see how many drainage ports you should unplug for optimum performance.

 

7. Tighten down the fan

Regardless of how many set screws your fan blade has, you’ll only want to tighten them down on flats. If you only have one flat, you only tighten one screw. Be careful not to overtighten the set screw. The set screw should be snug, but overtightening could cause it to break or create a bur on the shaft that will make the blade hard to get off later.

Check that the fan’s rotation is correct. This can be tricky, but we recommend placing the top back on the unit with the rotation wires sticking out of the top of the fan. Ensure that the blades spin in the correct direction for the unit and that they don’t hit anything.

While the rotation wires are exposed, we recommend encasing them together with some heat shrink tubing. Then, use tie wires to fasten the insulated rotation wires to the inner side of the unit’s top. We secure the wires to that part of the unit to keep the rotation wires out of reach without interfering with the fan blades. When the rotation wires are exposed, they may attract children or animals and shock them if they touch the wiring.

 

8. Wire in the motor

This is one of the more critical parts of the process.

First, you’ll want to route your wires. Ideally, your wires have already been aligned with the conduit or channel since step #5. Now’s the time to run them through the conduit or channel and into the electrical area.

NOTE: We are referring to wire colors here on universal motors that are by far the most common in this application. Colors can vary, so always read the info on the box or data tag on the motor.

You’ll make your connections in the electrical area. There are four wires that we use in those connections: a white or yellow wire, a brown wire, a black wire, and a brown-and-white wire. You can make a connection using one of two configurations: 3 wire and 4 wire.

It’s worth noting that the brown-and-white wire is the same as the white/yellow wire, as they connect inside the motor. That’s why you can let the brown-and-white wire go unused in a 3 wire configuration.

In a 3 wire connection, the white/yellow wire connects the condenser fan motor to one side of power on the contactor (the terminal side doesn’t matter). It’s also jumped to one side of the fan capacitor. The connection occurs at T1. The black wire connects the condenser fan motor to the other side of power on the contactor (T2). The brown wire connects to the other side of the capacitor from the jumped side, and you can cap off the brown-and-white wire, as it will remain unused.

In a 4 way connection, you use all four wires. The white/yellow wire connects the condenser fan motor to one side of power on the contactor (this is T1; again, the terminal side doesn’t matter). You DO NOT jump the wire to one side of the fan capacitor. Instead, the brown-and-white wire connects to one side of the capacitor. Just like before, the black wire runs from the condenser fan motor to the other side of power on the contactor (T2), and the brown wire connects to the other side of the capacitor.

As always, make sure to tidy up your wiring when you’re finished. We know you don’t like it when other techs leave you a clump of spaghetti wires, so please don’t do that to other techs (or yourself).

For a more comprehensive review of the wires and connection configurations, check out this article.

 

9. Perform a final inspection

Even though you’ve been working through each step carefully up to this point, you still want to make one final check before you test the voltage of the new motor. We’ve put together a checklist for you:

  • The blade doesn’t hit any other parts
  • Everything is wired properly
  • The blade spins freely
  • Appropriate weep/drainage ports have been removed
  • The blade should be at an appropriate height in the shroud
  • The top is securely mounted and fastened
  • The rotation wires have been insulated and fastened securely

 

10. Run test the unit

When you test the unit, you’ll check the voltage and amperage to make sure that your new fan motor is running properly.

Check the voltage at the load side of your contactor (as seen above). Make sure that the applied voltage is in the proper range and doesn’t drop more than a few percentage points (which would be very rare).

Next, you’ll check your amperage. You measure the current on the black (common) wire as it feeds power to the condenser fan motor. It should operate in range, but it might be difficult to determine the amperage if your multimeter’s resolution is poor. You must know your meter and take the amperage measurement in a location far enough away from other wires to get a precise reading. Otherwise, the other wires may interfere. (Your brand new motor might not be overamping; it’s likely that the compressor wire’s amperage interfered with your reading.)

Of course, if you continue to pick up high readings, double-check your motor to make sure that the RPM and voltage match the original.

Once you’ve taken the readings and confirm that the new motor is running properly, all you have to do is make sure the unit isn’t making any strange noises. If you don’t hear anything out of the ordinary, then the motor replacement is complete.

Go ahead and take the time to check the full system operation while you are there to make sure everything is working properly.

HVAC/R Refrigerant Cycle Basics


This is a basic overview of the refrigeration circuit and how it works. It isn't a COMPLETE description by any means, but it is designed to assist a new technician or HVAC/R apprentice in understanding the fundamentals.

First, let's address some areas of possible confusion 

  1. The Word “Condenser” Can Mean two Different Things Many in the industry will refer to the outside unit on a split air conditioner, heat pump or refrigeration unit as a “condenser” even though it will often contain the condenser, compressor, and other parts. It's better to call the outside component the “condensing unit” or simply the “outside unit” to reduce confusion.
  2. Cold and Hot are Relative terms Cold and Hot are both an experience, a description, a comparison or an emotion. Cold is a way to describe the absence of heat in the same way that dark describes the absence of light. We will often use the words cold and hot to compare two things “Today is colder than yesterday” or to communicate comfort “It feels hot in here”. These are useful communication tools, but they are comparisons not measurements.
  3. Heat and Absolute Zero Can be Measured We can measure heat in BTUs and light in lumens, we cannot measure cold or dark. Absolute cold is the absence of all heat.  -460°F(-273.3°C) (cold) is known as absolute zero, -460°F(-273.3°C) is the temperature at which all molecular movement stops. Any temperature above that has a measurable level of heat. While this is a known point at which all molecular movement stops, it has not (and likely cannot) be achieved.
  4. Boiling Isn't Always Hot When we say it's “boiling outside” we mean it's hot outside. This is because when we think of boiling we immediately think of water boiling in a pot at 212°F (100°C) at atmospheric pressure, which is 14.7 PSI (Pounds Per Square Inch)(1.01 bar) at sea level. Boiling is actually just a change of state from liquid to vapor, and the temperature that occurs varies greatly based on the substance being boiled and the pressure around the substance. In an air conditioner or a refrigeration system, refrigerant is designed to boil at a low temperature that corresponds to the design of the system. On an average air conditioning system running under normal conditions with a 75°(23.88°C) indoor temperature, the evaporator coil will contain refrigerant boiling at around 40°F(4.44°C). In air conditioning and refrigeration when we refer to “boiling”, “flashing” or “evaporating of refrigerant” we are talking about the process of absorbing heat, otherwise known as cooling.
  5. Cooling and Heating Cannot be “Created” We are not in the business of making heat or creating cool; it cannot be done. We simply move heat from one place to another or change it from one form to another. When we “cool” a room with an air conditioner, we are simply absorbing heat from the air into an evaporator and then moving that heat outside to the condenser where it is “rejected” or moved to the outdoors.
  6. Heat and Temperature Aren't the Same  Imagine a shot glass of water boiling away at 212°F(100°C). Now imagine an entire lake sitting at 50°F(10°C). Which has a higher (hotter) temperature? That answer is obvious-I just told you the shot glass had 212°F(100°C water in it so it is CLEARLY hotter. But, which contains more heat?  The answer is the lake. You see, heat is simply energy and energy at its basic form is movement. When we measure heat we are measuring molecular movement; the movement of molecules–atoms stuck together to make water or oxygen or nitrogen. When molecules move FASTER they have a HIGHER temperature and when they move SLOWER they have a LOWER temperature. Temperature is the average speed (velocity) of molecules in a substance, while heat is the total amount of molecular movement in a substance. The lake has more heat because the lake has more water (molecules).
  7. Compressing Something Makes it Get Hotter (Rise in Temperature) When you take something and put pressure on it, it will begin to get hotter. As you pack those molecules that make up whatever you are compressing, they get closer together and they start moving faster. If you drop the pressure the molecules will have more space and will move slower causing the temperature to go down.
  8. Changing the State of Matter Moves Heat Without Changing Temperature  When you boil pure water at atmospheric pressure it will always boil at  212°F(100°C). You can add more heat by turning up the burner, but as long as it is changing state (boiling), it will stay at 212°F(100°C). The energy is changing the water from liquid (water) to vapor (steam) and the temperature remains the same. This pressure and temperature at which a substance changes state instead of changing temperature is called its “boiling point”, “condensing temperature” or more generally “saturation” point.
  9. Superheat, Subcool, Boiling, and Saturation Aren't Complicated  If water is boiling at sea level it will be 212°F(100°C). If water is 211°F(99.44°C) at sea level we know it is fully liquid and it is 1°F(-17.22°C) subcooled. If water is 213°F(100.55°C) at sea level we know it is vapor and superheated. If something is fully liquid it will be subcooled, if it is fully vapor it will superheated, and if it is in the process of change (boiling or condensing) it is at saturation.

Where to start 

Take a look at the diagram at the top of this piece and start at the bottom left. Are you looking at the part at the bottom left? OK, now read this next line OUT LOUD:

Compressor > Discharge line > Condenser > Liquid Line > Metering Device > Expansion Line > Evaporator > Suction line and then back to the Compressor

When I first started in HVAC/R trade school this was the first thing my instructor forced me to LITERALLY memorize forward and backwards before he would allow me to proceed.

While I am not always a huge fan of rote memorization as a learning technique, in this case, I agree with committing this to memory in the proper order.

These four refrigerant components and four lines listed above make up the basic circuit that every compression refrigeration system follows. Many more parts and controls may be added, but these basics are the cornerstone on which everything else you will learn is based. Once you have these memorized we can move on to describing each.

Compressor

The compressor is the heart of the refrigerant circuit. It is the only mechanical component in a basic refrigeration system. The compressor is like the heart that pumps the blood in the body or like the sun that provides the earth its energy. Without the compressor to move the refrigerant through compression, no work would be done and no heat would be moved.

The compressor creates a pressure differential, resulting in high pressure on the high side (discharge line, condenser & liquid line) and low pressure on the low side (suction line, evaporator and expansion line).

There are many different types of compressors, but you will most likely see Scroll and Reciprocating type compressors most often. A reciprocating type compressor uses pistons, valves, and a crankshaft. Reciprocating compressors operate much like car engines; pulling in suction vapor on the down-stroke and compressing that vapor on the up-stroke. A scroll compressor does not have any up-down motion like a reciprocating compressor. A scroll compressor uses an oscillating motion to compress the low-pressure vapor into high-pressure vapor.

The compressor pressurizes low-pressure vapor into high-pressure vapor, but it also causes the temperature of the gas to increase. As stated in the gas laws, an increase in pressure causes an increase in temperature and a decrease in volume. In the case of refrigerant cooled compressors, heat is also added to the refrigerant off of the kinetic (bearings, valves, pistons) and electrical (motor windings) mechanisms of the compressor. Compressors require lubrication; this is accomplished through oil that is in the compressor crankcase, as well as oil that is carried with the refrigerant. Liquid entering the compressor through the suction line is a very serious problem. It can cause liquid slugging, which is liquid refrigerant entering the compression portion of the compressor. Liquid slugging will most likely cause damage to the compressor instantly. Another problem is bearing washout or “flooding”. This occurs when liquid refrigerant dilutes the oil in the compressor crankcase and creates foaming, and it will greatly reduce the life of the compressor because it will not receive proper lubrication and too much oil will be carried out of the compressor and into other parts of the system. The compressor also (generally) relies on the cool suction gas from the evaporator to cool the compressor properly, so it's a delicate balance to keep a compressor from being flooded and also keep it cool.

Condenser

Condensers come in all different types, shapes, and sizes. Regardless, they all perform the same function: rejecting heat from the refrigerant. The refrigerant entering the condenser was just compressed by the compressor, and this process increased the temperature by packing the molecules together which added heat to the vapor refrigerant due to the motor and mechanical workings of the compressor. This process in the compressor also greatly increased the pressure from a low-pressure in the suction line entering the compressor, to a high-pressure vapor leaving the compressor.

The condenser has three jobs:

  1. Desuperheat the refrigerant (Drop the temperature down to the condensing temperature)
  2. Condense (saturate) the refrigerant (Reject heat until all the refrigerant turns to liquid)
  3. Subcool the refrigerant (Drop the temperature of the refrigerant below the condensing / saturation temperature)

The condenser’s job is to reject heat (drop the temperature) of the refrigerant to its condensing (saturation) temperature, then to further reject heat until the refrigerant fully turns to liquid. The reason it must fully turn to liquid is that, in order for the refrigerant to boil in the evaporator, it must first have liquid to boil.

The way in which the condenser removes the heat from the refrigerant varies. Most modern condensers flow air over the tubing where the refrigerant is flowing. The heat transfers out of the refrigerant and into the air. The cooling medium can also be water. In the case of a water source system, water is circulated across the refrigerant in a heat exchanger.

In either case, the condenser relies on the removal of heat to another substance (air, water, glycol etc..). For instance, if you turned off the condenser fan so that no air was flowing over the condenser coil, the condenser would get hotter and hotter. This would cause the pressures to get higher and higher. If it kept going that way it would trip the internal overload on the compressor or cause other damage.

The hot vapor from the compressor enters the condenser and the superheat  (temperature above condensing temperature) is then removed. The refrigerant then begins to change state from vapor to liquid (Condense). The refrigerant maintains a constant temperature until every molecule of vapor is condensed. The temperature of the liquid again starts to fall. This is known as subcooling. When we measure subcooling we are measuring degrees of temperature rejected once the refrigerant has turned completely to liquid.

Temperature above the saturation temperature is called superheat. Temperature below the saturation temperature is called subcool or subcooling. So when something is fully vapor (like the air around us) it will be superheated, and when it is fully liquid (like the water in a lake) it is subcooled. 

Metering Device

The metering device is a pressure differential device that creates a pressure drop to facilitate refrigerant boiling in the evaporator coil.

The metering device is located between the liquid line and the evaporator. The liquid line is full of high-pressure liquid refrigerant. When the high-pressure liquid hits the small restrictor in the metering device, the pressure is immediately reduced. This drops the pressure of the refrigerant to such a degree, that the saturation temperature is lower than the temperature of the air surrounding the tubing that the refrigerant is in. This causes the refrigerant to start changing from liquid to vapor. This is called “boiling” or “flashing”. This “flashing” brings the refrigerant down from the liquid line temperature to the boiling (saturation) temperature in the evaporator, and in this process a percentage of the refrigerant is immediately changed from liquid to vapor. The percentage of the refrigerant that changes during flashing depends on how great the difference is. A larger difference between the liquid line temperature and the evaporator boiling temperature results in more liquid lost to flashing and reduces the efficiency of operation.

There are a few different types of metering devices. The most common ones being the Thermostatic Expansion Valve (TXV/ TEV) and the Fixed Orifice (often called a piston)– as well as electronic expansion valves, capillary tubes, and others.

Evaporator

The evaporator is also known as the cooling coil, because the purpose of the evaporator is to absorb heat. It accomplishes this through the refrigerant changing from liquid to vapor (boiling). This boiling process begins as soon as the refrigerant leaves the metering device, and it continues until the refrigerant has absorbed enough heat to completely finish the change from liquid to vapor. As long as the refrigerant is boiling it will remain at a constant temperature; this temperature is referred to as saturation temperature or evaporator temperature. As soon as the refrigerant is done boiling, the temperature starts to rise. This temperature increase is known as superheat.

When the indoor air temperature or the air flow going over the coil is higher, the evaporator pressure and temperature will also be higher because more heat is being absorbed into the coil. When the air temperature or airflow over the coil is lower, it will have lower pressure and temperature in the coil due to less heat being absorbed in the coil.

The refrigerant leaves the evaporator, travels down the suction line and heads back to the compressor where the cycle starts all over again.

Refrigerant Lines 

Suction Line = Line Between the Evaporator and the Compressor

The suction line should contain low-pressure superheated suction vapor. Cool to the touch on an air conditioning system, and cold to the touch in refrigeration.

Discharge Line = Line Between the Compressor and the Condenser 

The discharge line should contain high temperature, high pressure superheated vapor

Liquid Line = Line between the Condenser and the Metering Device

The liquid line should be high pressure, slightly above outdoor temperature subcooled liquid

Expansion Line (When applicable) = Line Between the Metering Device and the Evaporator

On most systems, the metering device will be mounted directly to the evaporator making the expansion line a non-factor. Some ductless mini-split units will mount the metering device in the outside unit making the second, smaller line and expansion line. The expansion line is full of mixed vapor/liquid flash gas.

Yes, this was long, but more than anything else just keep repeating over and over: compressor>discharge line>condenser>liquid line>metering device>expansion line> evaporator>suction line and on and on and on…

Jim Bergmann did a great whiteboard video on the MeasureQuick YouTube page that explains the basic refrigerant circuit

-Bryan

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