Airflow and coils

Poor airflow causes freezing, weak cooling and high head pressure. Check it before anything else.

The short version

• Pull the filters. Dirty filters are the number one cause of weak cooling.
• Look at the evaporator coil for dirt and ice. A frozen coil means low airflow or a refrigerant issue.
• Check the condenser on the roof is clean and the fan turns freely.
• Measure the temperature drop between return air and supply air. That tells you if the system is actually doing its job.

Numbers

• Temperature drop return to supply: ~16–22°F after 15+ min; check the manufacturer's spec

Try it on a service call

Put the lesson to work on a hands-on troubleshooting case.

• AC runs but barely cools

Keep going on air conditioning

Learn it

• Rooftop AC
• Capacitors
• Thermostats and AC control boards
• Heat pumps and heat strips

Prove it

• AC hums and trips
• AC runs but barely cools
• AC ices up and barely blows
• AC kicks off on hot afternoons
• AC does nothing, thermostat is blank

See it in 3D

• Rooftop AC 3D wiring model

On the job

• Air conditioner
• Thermostat
• Heat pump / heat strip