Cooling Calculator

This is a simple calculator to help determine whether an air conditioner is performing as expected.

Use this at your own risk. 

This uses rules of thumb for average residential HVAC equipment.

Assumptions:

• Return air (into evaporator) is between 68 and 88 degrees
• Proper airflow through the system (fan speed correct, no airflow restrictions)
• Supply and return air temperatures measured at evaporator/air handler. Temperature differentials will be lower when measured at indoor registers because of heat gain in ductwork (especially when in attics).

1. Start system in cooling mode.
2. Place thermometers at return and supply air locations. Ideally, this would be done at the air handler by sticking “meat probe” type thermometers into the ductwork. If this is not feasible, place thermometers on or into supply and return air registers as close to the evaporator as possible.
3. Measure the outdoor temperature (or use a local weather app).
4. Measure the relative humidity at the return register.
5. Let the system run a while and stabilize. Ductwork takes time to cool down to the indoor temperature.
6. Record the supply and return air temperatures.
7. Input data into the calculator.
8. Seal any holes you made in ductwork with a piece of aluminum duct tape.
9. Remember to set the thermostat back the way you found it.

• Residential systems are generally sized to cool up to 20 degrees below the outdoor temperature over the design range. The maximum indoor temperature differential is affected by this limitation but is not taken into account in the calculation.
• Thermometers with magnets can be placed onto metal registers using an extending “grabber” tool. No ladders needed!
• An infrared camera or gun thermometer can be used to measure register temperatures as a last resort. This won’t be as accurate because of emissivity, temperature of the metal register vs. air, and other factors.
• Thermometers with MAX/MIN features are really handy, especially if the thermostat reaches the setpoint before you can take readings.
• Use the same setup for measuring heat mode, but outdoor and humidity readings aren’t necessary. 
• When measuring heat performance with a heat pump, ensure that auxiliary heat isn’t running. This is tricky to determine. Setting the thermostat more than 1 to 3 degrees above ambient (thermostat dependent) will cause auxiliary heat to start. The best approach is to leave the thermostat set to room temperature and wait for the heat to come on at some point. Thermometers set to MAX hold will grab the highest recorded temps.
• Recommend that your clients have the system checked and serviced by an HVAC pro. True system performance can only be determined by using pressure gauges or by calculating parameters such as superheat