Smart Inspector Science - Air Conditioning – What You Can’t See

By Tom Feiza, Mr. Fix-It, Inc., (

Home inspectors visually inspect and report on residential air conditioning systems. We operate the systems and check for water leaks, and may also measure the temperature drop across the evaporator coil. But we don’t get a chance to see several components.

Visible components of the AC system are shown in Illustration A032: compressor, condenser coil, expansion device and an expansion “A” coil.

A032 - Refrigeration Cycle - Residential AC

We can see the exterior of the furnace or fan coil unit (Photo 1).

AC Coil Connections at Furnace

Outside the house (Photo 2), we see the exterior of the condenser coil and maybe the fan.

But which parts of the system can’t you see?

AC Exterior Units

You can’t see much of the compressor

Compressors are buried inside the condensing coil within a metal housing (Illustration A039). Older compressors use a piston and crank shaft system, just like a small lawnmower engine. The piston moves back and forth in a cylinder to compress the refrigerant gas. This type of system is more sensitive to liquid refrigerant.

A039 - AC Compressor Types

Many modern systems use a scroll compressor. This compresses the refrigerant gas with a rotation scroll that spins inside a similar-shaped scroll. In general, this is more efficient and durable than a reciprocating piston.

You can’t see the “A” coil – evaporator components

Photo 3 shows a typical evaporator or “A” coil that’s buried in the ductwork near the blower fan. Normally there is no access panel to this coil.

Air Conditioning A Coil

Liquid refrigerant enters the coil through the small copper line. Refrigerant passes through a metering device that maintains proper flow by using a temperature-sensing bulb.

The liquid refrigerant is “boiled” in the coil with heat transfer from the air blown through the fins. Gas (vapor) refrigerant returns from the top of the A coil through the larger insulated copper tube to the compressor outside.

Because the humid home air is cooled below its dew point, water condenses on the coil and drips into the black plastic pan below. The water (condensate) is drained through a tube or pipe; the drain line and the plugged secondary drain line are visible here. All pans have a higher drain connection that is normally plugged.

The takeaway

You should understand the components and operation of an air conditioning system. While you don’t need to report on components you can’t see, you must report signs of failure. Signs of water leaks at the evaporator coil mean that you must recommend further evaluation. You should check for an air temperature drop of 14 to 22 degrees Fahrenheit across the evaporator. Outside, the coil should be clean, with a clear path for air flow. The exterior unit should be level.


Tom Feiza has been a professional home inspector since 1992 and has a degree in engineering. Through (, he provides high-quality books and marketing materials that help professional home inspectors educate their customers. Copyright © 2022 by Tom Feiza, Mr. Fix-It, Inc. Reproduced with permission.

To learn more, attend Tom’s technical presentations at educational sessions for ASHI chapters and local groups. Tom can also provide his knowledge for your educational event; contact him at

Photos, Illustrations, Captions
Photo 1. AC Coil Connections at Furnace
Photo 2. AC Exterior Units
Photo 3. Air Conditioning A Coil
Illustration A032 Refrigeration Cycle – Residential AC
Illustration A039 AC Compressor Types

1 comment

  • Great layout, easy to see flow.

    Ray Baird

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