Measuring HVAC System Performance

You can measure capacity for any system that you work on and use the test results to see how your system measures up.   Remember equipment is rated at ARI conditions including 95 degrees F. outdoor for cooling.  That means capacity will be higher below 95F and lower when outside temperature in above 95F.

BTU Delivery

Total heat removed by a cooling system can be measured. Tools required are a high quality hygrometer that measures wet bulb temperatures.  It’s the wet bulb temperature that measures of both sensible heat and latent heat (humidity) that enables us to measure total Btu.

You can measure CFM by determining velocity in FPM (feet per minute) x duct or grill square feet or measuring static pressure  and referencing the manufacturers fan curve.  



The formula is CFM x Delta-T x 4.5. Total BTU airflow (cfm) times the change in temperature and moisture content of the air through the system (Delta-T), times the constant of 4.5.

Delta-T through the system is measured by taking wet bulb readings and converting the values to enthalpy. Subtract the two enthalpy readings to find the Delta-T.

Sensible heating Btu is measured by CFM x Delta-T x 1.08. To measure heating Btu, multiply the measured supply airflow (in cfm) by the temperature change from the average supply register temperature to the average return grille temperature, and multiply this total by the formula constant of 1.08.

The easiest way to find latent Btu is to subtract delivered sensible Btu from the delivered total Btu. A simple formula to find latent Btu is total Btu minus sensible Btu.

The 1.08 is a constant in the sensible Btu formula is based on the air density under standard conditions. These are air at 70F, at sea level , and at 50% relative humidity. That weight is .075 lbs/cu.ft. times the specific heat of standard air of 0.24 Btu/lb., times 60 minutes in an hour. We calculate Btu per hour because since we express Btus in per-hour units.

The total Btu constant of 4.5 is found by multiplying the weight of standard air of .075/lb. by 60 minutes in an hour.  


Btu.  Btu equals the amount of heat it takes to change the temperature of one pound of water one degree Fahrenheit.

Tons.  One ton of nominal cooling equals 12,000 Btu.

Total, Sensible, and Latent Heat.  Total heat is made up of two kinds of heat: sensible and latent. In heating mode, all of the Btus are sensible heat. An 80,000 Btu output furnace should deliver 80,000 Btu into the duct system.

Cooling removes two kinds of heat. The 12,000 Btu per ton is made up of about 8,400 Btu of sensible or dry heat (about 70% of total Btu), and about 3,600 Btu of latent heat (about 30% of total Btu). This makes up is the 70/30 Sensible/Latent heat ratio. Latent heat can be simply defined as the removal of humidity from the air that passes through the coil.

Airflow.  Most equipment requires 400 cu.ft. of air through the system in a minute in cooling mode. Plus or minus 10%, or 360 to 440 cfm per ton of cooling. Below 360 cfm/ton, the heat transfer through the coil falls off fast. That’s why any refrigerant charging method that ignores airflow through the system is bogus.

Static Pressure.  Measurement of total external static pressure  is used to plot airflow on the fan tables provided, with each system using a fan to verify that the proper amount of airflow is being delivered through the system.

In order for airflow to be delivered, static pressure should be less that the maximum total external static pressure listed on the nameplate data of the equipment containing the indoor fan. Check the manufacturer’s engineering data, because some equipment can handle static pressures up to 20% higher than the rated amount, while others don’t.

About 50% of the available equipment is rated at 0.50-in. maximum total eternal static pressure. Most variable speed fans are rated from 0.90-in. to 1.2-in. available fan pressure. ARI tolerates some equipment with fans rated as low as 0.20-in.

Design practical standards ideally limit pressure drop over the coil to 40% of the rated equipment static pressure. Pressure drop over the filter is recommended to remain below 20% of the equipment pressure. This leaves 40% of the available pressure for thetime. 

Energy Efficiency Ratio.   EER is the real time measure of energy efficiency.  SEER is seasonal EER and is measured over time.  EER is about .875 of SEER.  Use the actual capacity measured above and divide by the total energy input in watts.  12000 btu / 1000 watts is 12 EER.  To compare this EER with the published data the capacity must be measured at ARI standard conditions including 95F degrees outdoor temperature.