A popular type of air conditioning system uses an outdoor condensing unit coupled with an indoor evaporator coil. The coil is mounted downstream of a furnace located in a closet or attic. Split-system heat pumps are similar, but they incorporate a blower to move the air instead of a furnace.

In a properly operating residential system, the air from the home is forced across the evaporator and cooled to below its dewpoint. Moisture is literally squeezed from the air like water from a sponge. The combination of cooling and dehumidification produces (hopefully) very comfortable results.

Too often, however, when a system like this is upgraded, only the outdoor unit is replaced. The evaporator coil is usually not replaced in order to keep the total job price competitive.

When this happens, the new unit cannot function as well as it should.

The cost to operate the system is higher than it would have been had a matched evaporator been installed. Not only will the system cost more to operate, premature failure of the compressor can result as well.

Let’s look at what can happen to a system if just the condensing unit is replaced.

What if

Let’s say that, at your customer’s house, you find a 4-ton split system that’s about 12 years old and not functioning.

Return air to the system passes through a disposable filter that has been changed when the unit required service or when the homeowner remembered; in other words, not often enough. The system problems are severe enough, and the old unit is inefficient enough, that the customer decides to have you install a new system.

The refrigerant piping is properly sized and a new, 10-SEER condensing unit is installed.

The suction pressure is a little low, so some refrigerant is added to bring the suction pressure to 69 psi. The return air temperature is 80°F, the supply is 57°, and everyone is happy — except the compressor.

The suction line temperature is 44°. The head pressure is 294 psi, and it’s 100° outside. The liquid line temperature is 106°.

Let’s look at what’s happening.


The evaporator fin surface is covered with a fine layer of particles that have bypassed the air filter over the past 12 years.

The refrigerant circuit and metering orifice in the old evaporator is a little small for the new condensing unit. This was demonstrated by the low suction pressure on start-up.

The superheat at the compressor is less than 4°. The subcooling is 23.5°. If the manufacturer supplied a chart that lists amp draw at various ambient conditions, you will find the unit is drawing higher amps than listed.

Note: Don’t use compressor rated load amps for this. You won’t draw RLA until the ambient is over 115° and the indoor wetbulb is off the scale.

The system subcooling is high because the metering device is too small. This restriction, combined with the excess refrigerant that was added, is backing up liquid refrigerant into the condenser.

The system superheat is low because it is overcharged and the evaporator is dirty. Sure, you have good TD across the indoor coil, but at what cost? The system is going to crash; it’s just a matter of when.

Convincing the customer

Homeowners will consider operating efficiency when purchasing a system to keep their home comfortable. It’s no different than evaluating MPG when they consider a new automobile.

How do you sell against the contractor who tells the homeowner you are trying to “rip them off” because there is nothing wrong with the coil?

Simple. You guarantee, in writing, on your proposal that your system will achieve the efficiency and Btu capacity in the manufacturer’s spec sheet.

Ask the homeowner to get the same assurance, in writing, from the competition.