How to Replace an A/C Condensing Unit - Scotts Global Ltd

The contractor should begin by determining why the unit needs to be replaced. The answer may
impact the design of the new system. For example, if the compressor is defective, it is important
to know whether the problem is the result of a mechanical or an electrical failure. A mechanical
failure suggests a number of possible causes, not the least of which is the original application
may not have been correct. Other possible influences would be poor maintenance, which may
have created an airflow problem leading to floodback, electrical failures that may have been tied
to voltage issues or even a lightning strike. Understanding the reason for failure and then taking
the steps to correct it can prevent premature failure of the replacement unit.
After learning why the condensing unit is being replaced, it’s important to determine the cooling
load for the home by performing a load calculation. Consider using the Air Conditioning
Contractors of America’s Manual J, the load calculation required by an increasing number of
municipalities prior to issuing a building or occupancy permit. A soon to be released Abridged
Version, along with well-designed and easily available software, has simplified this once
complex task.

The load calculation considers the entire home as well as the habits of the homeowner who
impacts the amount of cooling required to maintain a comfortable environment. The design and
construction of the home, insulation value, color of the shingles, angle of the sun, size and
orientation of the windows, number and placement of overhangs, location of trees and shrubbery,
electronic equipment inside the house, number of houseplants and number of family members all
affect the temperature inside the home.
An important element to remember about load calculations is that a contractor shouldn’t add in
“fudge” factors. Even when accurate input data is used, a Manual J load calculation will more
than likely be 15 percent higher as a result of assumptions built into the program.
Chances are, if the homeowner complained about being too warm with their old unit when
outside temperatures reached 95° F, the load requirement for the house may have changed and a
larger unit is probably required. On the other hand, if the customer has been sweaty or clammy in
the past, the old unit may be too large with respect to the load calculation, causing short run
cycles. This leads to improper latent cooling, resulting in a cool, yet uncomfortable environment
for the homeowner.

Both cases demonstrate the need to perform a new load calculation prior to selecting and
installing the replacement condensing unit. Improper sizing of the new unit can impact both unit
efficiency and occupant comfort.
The existing evaporator coil also can impact the efficiency of the new condensing unit,
especially if it is not matched to the new unit. Almost without exception, coils that are not part of
a matched system that has been ARI rated will not deliver the rated performance of the
condenser. This means that a 13 SEER unit matched with an older, less efficient evaporator coil
will not achieve the efficiency that a 13 SEER unit is designed to deliver.
This holds true even if you could replace the metering device in the existing coil with one you
“think” would be sized correctly. As a result, replacing the old evaporator coil with a new coil
matched to the new condensing unit is usually the most prudent step to take.

Refrigerant’s role
The refrigerant in the new condensing unit also will influence the fate of the existing evaporator
coil. If the existing unit uses R-22, instead of the R-410A typically found in 13 SEER units, the
evaporator coil will not meet the elevated pressures of R-410A.
Some older coils were tested for 250 to 350 pounds per square inch gauge (psig), while new coils
are tested for 500 psig, making them suitable for R-410A applications. As a result, most hvac
manufacturers advise against matching a new 13 SEER unit that uses R-410A refrigerant with an
old evaporator coil that was part of an R-22 system.
Replacing the old evaporator coil with a new coil also eliminates concerns associated with signs
of corrosion or contaminates left behind as a result of the original unit failure. Additionally,
replacing the coil removes the question of existing leaks, which may not have been detected
during the operation of the old system.
In addition to the evaporator coil and the lineset, the installer should inspect the air handler to
check its physical condition as well as its size in relation to the new condenser. This inspection
also should include an examination of the electrical service and connections on the unit, blower,
motor bearings and blower wheel.
It’s important for the contractor or technician to look closely at the existing indoor coil for size
and location. Newer coils designed for use in 13 SEER applications typically have more coil
surface area and are physically larger. This means, at the very least, ductwork transitions will
need to be made.

The new coil also may have a higher pressure drop, which can impact airflow. The existing
blower may be operational and may be part of a furnace that will not be replaced. But if the
blower motor does not have the capacity to deliver the needed airflow due to the additional load,
it may have to be replaced.
Another part of the hvac system worthy of inspection is the ductwork. Just as other system
components need to be properly sized, so does the ductwork. What is appropriate for an older
condenser may not be appropriate for a new, more efficient 13 SEER unit. Improperly sized or
poorly maintained filters are another source of problems with a system, as are grilles and
registers that are not open or clean and free of furniture, carpet or window treatments.
An inadequate electrical system presents the potential for additional problems. Therefore, it
makes good sense to examine the electrical service to the home, checking the amperage to make
certain it meets the needs of the new condenser and verifying the existence of a properly sized
grounding rod and wire to the electrical fuse or breaker box.

In some cases, it may be necessary to enlist the advice of a licensed electrician. By taking this
step, the contractor is in a position to make recommendations regarding the size of the power
lines from the fuse or power box to the condenser unit, as well as the size of the indoor control
transformer and the circuit for the indoor air handler.
Although it may not be required, it is strongly recommended that the lineset be replaced when a
condenser is being replaced. Proper sizing is a concern as is the presence of contaminants
trapped in low spots of the lineset. R-410A associated with new 13 SEER condensers uses
polyester oil (POE) rather than the mineral oil used with R-22. Therefore, if a replacement unit
uses R-410A, any mineral oil left in the lineset from the R-22 refrigerant may slug the new
compressor.

Kinks or visible corrosion are also valid reasons for replacing the lineset. Regardless of how
diligent the technician is in sizing the unit, improper lineset sizing will lead to a loss of capacity
and negatively impact the efficiency of the new condenser. In fact, the rebate offered by some
utilities for 13 SEER units can be denied if proper line sizes are not used.

The match game
In the event a lineset and/or an evaporator coil cannot be replaced or a customer refuses to do so,
it is a good idea to explain to the homeowner the relationship between the condensing unit and
the other components that comprise the hvac system. You need to emphasize that they work
together to achieve the rated efficiency of the condensing unit. This relationship makes
compatibility and proper sizing of the units critical to the success of the system.
If the decision is not to replace the existing components, the contractor is faced with several
daunting tasks that must be performed if a system is to approach its rated level of efficiency:
First, perform an acid test and recover the refrigerant charge from the old system. If acid is
present, flush out the lineset and coil with nitrogen and a U.S. EPA-approved flushing agent.
Remove the existing metering device from the evaporator coil and replace it with a thermostatic
expansion valve. Since you will be working with an unmatched system, your best chance to
avoid over or under feeding the coil is using a TXV that can adjust the refrigerant flow as
conditions change.

If acid was present, install an alumina-activated suction line drier in the suction line near the
condensing unit. Remove it if the pressure drop across it reaches 3 psi or after 50 hours of run
time, whichever comes first. Most new units ship with liquid-line driers installed at the factory.
However, if the unit does not have one, add it to the system. Remember, when brazing in the new
unit and driers(s), always run nitrogen through the lines to prevent copper scaling.
Pressure test the system using nitrogen and a bubble solution. If the system is sealed, evacuate to
below 500 microns. If it is not sealed and a leak exists, the vacuum level will rise to atmospheric
pressure. In this case it becomes necessary to find and repair the leak; then repeat the vacuum
process. A vacuum level that rises, but stabilizes below atmospheric pressure points to the
presence of non-condensables in the system. It becomes necessary to break the vacuum with
nitrogen and repeat the process until the vacuum holds below 500 microns.
With a matched system, weighing in the additional refrigerant charge is easy. However, the
disadvantage is an unmatched system means the amount required will only be an estimate.
Start up the system, verify the achievement of airflow requirements, and allow the system to run
for a minimum of 15 minutes. Determine the subcooling and adjust the charge as needed, using
either a subcooling calculator or the actual manufacturer’s performance charts. Again, a
mismatched system means the charts will be estimates, at best. As a result the charge may not be
adjusted properly to achieve the system’s rated efficiency.
Asking homeowners appropriate questions and educating them on the operation of the new
system, as well as the benefits of 13 SEER, will enhance that success.