How to Clean AC Evaporator Coils – evaporator coil inspection and cleaning with no-rinse spray foam coil cleaner. This project is continued from How to Clean AC Evaporator Coils – Part 1.

Table of Contents

  1. How to Clean AC Evaporator Coils
    Seasonal exterior coil surface cleaning with a spray-on foam.
  2. How to Clean AC Evaporator Coils – Part 2 (this project)
  3. Heavy Duty AC Evaporator Coil Cleaning
    Deep cleaning with professional coil cleaner, pump sprayer and brush.
  4. How to Clean Inside of AC Evaporator Coils
    Interior coil cleaning by removing the coil end plate, best for very dirty coils.
  5. AC Evaporator Coil Cleaning with Pump Sprayer and Brush
    Clean the coils inside and out.
  6. How to Install a Honeywell Ultraviolet Light Treatment System
    Prevent mold and algae with a disinfectant UV germicidal light.
  7. How to Install a Honeywell Ultraviolet Light Treatment System – Part 2
  8. How to Clean and Straighten AC Condenser Coils
    Outdoor compressor unit maintenance.

AC Evaporator Coil Inspection

The following is a photo of my central air conditioner evaporator coils before cleaning. The AC coils look very clean already because I service my system regularly. Of some concern is the rust on the ends of the frame. The unit is about 9 years old and rust is common in these areas. The greenish/whitish discoloration on the copper refrigerant lines is from dissolved copper deposits that have dried. The plastic drain pan looks in good condition with no cracking or leaks. Overall, not too bad for a 9 year old evaporator coil. I should expect to replace the coil and/or air handler in a season or two as it approaches the end of its expected life.

Air Conditioner Evaporator Coils

Air Conditioner Evaporator Coils

Evaporator Coil Icing & Freezing

If your coils look clean but are freezing over, it may be caused by:

  • Clogged air filter.
    A clogged filter can restrict the air flow through the coils causing condensate water to freeze on the coils. Coils need a large volume of warm air flow to stay above freezing. This is a vicious cycle because the ice grows blocking more of the surface.
  • Clogged interior coils.
    The exterior coils may appear to be clean, but the interior surface can be dirty. The interior surface is on the upwind side (closest to the blower motor) where dust, dirt, pet hair, loose insulation and such will stick. See Heavy Duty AC Evaporator Coil Cleaning for how to inspect the interior side of the coils.
  • Refrigerant Leak
    AC systems can leak refrigerant through failing brazed joints, pin hole leaks cause by corrosion, bad gaskets, etc. resulting in a low refrigerant charge. A low charge will cause the temperature of the liquid refrigerant flashed to a gas through the expansion valve to be much colder than normal causing them to freeze. The gas is much colder because there’s not enough of it to fill the coils; the gas over-expands inside the coils, resulting in lower pressure & lower temperature, hence freezing. The Joule Thomson Effect (highly technical) describes the process.These N style evaporator coils are freezing over, starting at the bottom closest to the expansion valve. An HVAC tech found the coils to be about 2 lbs low on R-22 refrigerant:

    Freezing AC Evaporator Coils - Low Refrigerant Charge

    Freezing AC Evaporator Coils – Low Refrigerant Charge

    My AC coils in this project were slowly leaking, needing a recharge of about 1 lb of R-22 per season. They didn’t freeze but the system grew less efficient and the air blowing from the vents wasn’t as cold as it should be.

Air Conditioner Evaporator Coil Cleaner

I purchased a can of Frost King Air Conditioner foaming no-rinse coil cleaner from Home Depot:

Frost King - Foaming Air Conditioning Coil Cleaner

Frost King – Foaming Air Conditioning Coil Cleaner

Update:

The Frost King coil cleaner has been replaced by the Thermwell Products AC-Safe Air Conditioner Coil Foaming Cleaner. (Thermwell owns the Frost King brand.) The AC-Safe coil cleaner is non-flammable and won’t damage plastic condensate drain pans.

Nu-Calgon Evap Foam No-Rise Coil Cleaner

The Nu-Calgon Evap Foam no-rinse coil cleaner is specifically formulated for cleaning air handler evaporator coils and approved for use in and around food processing areas. It’s available from Amazon.com. I like the Nu-Calgon Evap Foam because the manufacturer’s page (previous link) includes directions and literature. I used a similar Nu-Calgon coil cleaner in the Heavy Duty AC Evaporator Coil Cleaning project.

Clean the AC Evaporator Coils

The foaming coil cleaner is sprayed directly on the AC coils, coating the surfaces evenly and thoroughly. It’s best to do this on a warm day when the AC will be running to help rinse the coils with condensate water.

AC Evaporator Coil Cleaning with No Rinse Spray Foam

AC Evaporator Coil Cleaning with No Rinse Spray Foam

The spray foams nicely on the coils.

AC Evaporator Coil Cleaning: No Rinse Spray Foam

AC Evaporator Coil Cleaning: No Rinse Spray Foam

The no-rinse foam cleaner breaks down and liquifies quickly for a good rinsing action.

Evaporator Coil Cleaning: No Rinse Foam Breaks Down Quickly

Evaporator Coil Cleaning: No Rinse Foam Breaks Down Quickly

In minutes the foam is rinsing itself off and draining away. Always check the PVC drain line is free of algae and unclogged. I poured a quart of 50/50 solution of household bleach and water in the drain pan to keep the line clear. You can also buy time-release algae tablets to drop in the drain pan.

AC Evaporator Coil Cleaning: Plastic Drip Pan and Drain Line

AC Evaporator Coil Cleaning: Plastic Drip Pan and Drain Line

The evaporator coil access panel is reattached with the sheet metal screws. The top and bottom seams are then sealed with HVAC metal foil tape:

Reattach the Evaporator Coil Access Panel and Seal with Metal Tape

Reattach the Evaporator Coil Access Panel and Seal with Metal Tape

Take care not to tape over the manufacturer’s label on the access panel because the service technician may need this data:

Air Conditioner Evaporator Coil Access Panel Sealed with HVAC Tape

Air Conditioner Evaporator Coil Access Panel Sealed with HVAC Tape

The attic AC air handler is now ready for another hot summer of cooling. The last task is to turn on the thermostat.

Attic Air Handler - Ready for Summer

Attic Air Handler – Ready for Summer

HVAC Air Handler Basics

Now you’re familiar with the basic components of the attic air handler and the importance of changing the air filter and performing routine system maintenance. If your AC system needs professional servicing, you’ll know what the HVAC technician is talking about. Annual Service Contracts are available from HVAC companies and usually cost less than the sum of ad-hoc maintenance calls.

Heavy Duty AC Evaporator Coil Cleaning

The Heavy Duty AC Evaporator Coil Cleaning tutorial explains how to inspect and perform a heavy duty cleaning which is best for cleaning really dirty coils:

AC Evaporator Coils - Interior Clogged with Dirt and Mold

AC Evaporator Coils – Interior Clogged with Dirt and Mold

AC Condenser Coil Cleaning

It’s equally important for the outdoor AC condenser coils (also called “compressor coils”) to be clean for proper air flow and heat exchange. The fins on my condenser coils were also badly bent. That project is explained in How to Clean and Straighten AC Condenser Coils.

Stay cool!
Bob Jackson

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