13 SEER... (Oh, Really?) Measure System Btus

by Rob Falke

Many contractors assume that if they install a piece of air conditioning equipment rated at 13 SEER, that their customers will automatically get a 13-SEER system. Take a look at your last change-out proposal -- isn't that what it says? Most proposals make that claim.

Test for yourself and see if your systems make the grade or not.

You'll learn that the effect the duct system has on the equipment can reduce efficiency by 10% to 50% under extreme weather conditions.

The Challenge

If a five-ton air conditioning unit rated at 14 SEER is supposed to remove 60,000 total Btus under laboratory conditions, would it become a 9.3 SEER device if it were only removing 40,000 total Btus?

Let's find out. Let's field measure system Btus using the following procedure.

Total Btus are the total of the sensible and latent Btus removed by a cooling system. In simple terms, cooling system Btus normally exist as 70% sensible and 30% latent. For example a typical 4-ton system would remove 48,000 total Btus of which 33,600 would be sensible Btus (these lower the temperature in the building) and 14,400 Btus would be latent Btus (that remove water vapor, or humidity from the building).

In heating mode all the Btus are sensible Btus. (We'll discuss how to measure just the sensible Btus later.)

The Formula

Calculating total Btus involves measuring airflow and wet bulb air temperatures in and into a conditioned space. Total Btus are calculated using the formula:

Total Btu =

4.5 X CFM X Enthalpy Change

The 4.5 factor represents the weight of 1 cubic foot/minute (CFM) of standard air per hour.

CFM is the total supply airflow entering the building from the system.

Enthalpy change is measured using a hygrometer and converting the readings to Enthalpy.

Total Btu Procedure

Start the system in cooling mode by setting the thermostat to 55F with the fan in the "on" position. This will cause the system to continue running throughout the test. Allow the system to run for 10 minutes or so until the coil is wet and the system has stabilized.

Measure airflow using a reliable commercial air balancing hood, or an anemometer. In other words, you are taking airflow traverse readings of the air entering the building. Measure each of the supply registers, then add the CFM readings together to determine the total airflow entering the building from the system.

Next determine enthalpy change. First, use a good quality hygrometer to measure the wet bulb temperature of the average air temperature entering the building. Then measure the average building air temperature. The hygrometer should read within 110th of a degree.

Convert the wet bulb readings to enthalpy using an enthalpy chart (see Figure 1). The enthalpy values are
subtracted from each other to find the Enthalpy Change caused by the system.Calculate the system total Btus using the test data you've collected by multiplying the CFM by the enthalpy change by 4.5. Compare the total Btu delivery that you measured from your system to the manufacturer's rated Btu. How did you do?

While you have the airflow figures, go ahead and calculate the sensible Btus and latent Btus your system is
producing. This should take just another five minutes.

Sensible Btu Procedure

To calculate sensible Btu change through the system, use the formula 1.08 x CFM x st (temperature change).

The 1.08 factor represents sensible Btu that comes from the airflow density multiplied by 60 minutes/hour.

The CFM is the same supply airflow that you used for the total Btu formula.

For the st or temperature change portion of the formula, measure the average dry bulb air temperature entering the structure. Then measure the average dry bulb temperature in the building. Subtract to find the dry bulb temperature change.

Then multiply 1.08 x CFM x st to find the system sensible Btu delivery.

Latent Btu Procedure

Finding the system's latent Btus delivery takes less than a minute once you've measured the systems total and latent Btus. Simply subtract the difference between the system total Btu and the system sensible Btu. Or,

Latent BTU =

Total Btu - Sensible Btu.

Helpful Hints

To prevent confusion when you begin testing total, sensible and latent Btus in the field, here are a few helpful hints:

  • During extreme weather conditions, the effect of a duct system on equipment will be significant. A return duct leak can easily reduce system efficiency by 50%, if it's located in a very hot or cold attic.
  • You can compare the field-tested system efficiency to actual operating equipment efficiency by using the same tests. Just determine equipment airflow by carefully measuring fan static pressure, then plotting the fan CFM based on the manufacturer's fan data, and finally measuring the wet and dry bulb temperature changes over the equipment. This test isolates the effect of the ducting from the operation of the equipment.
  • An enthalpy change of about 6 is what you'll find most often.
  • Airflow has a drastic effect on system performance. Make no mistake about this. Remember, airflow is a multiplier in each of the Btu formulas. It must be measured accurately with top quality instruments and procedures.
  • Don't be surprised at humidity levels of 90% plus from the supply side of the system. The air is very cold and can hold little moisture. When the air mixes with the building airflow, the result is building dehumidification.
  • Don't be discouraged if your systems don't operate perfectly. It takes a great duct system to get within 10% of the equipment's rated Btus.
  • Use this testing to teach customers about their duct systems before recommending an equipment change-out. The testing will often add a duct system renovation to a job. Margins on duct renovation work can be significant and can more than triple the net profit on a project.
  • If a cooling system has a hefty return duct leak from a hot attic, don't be surprised to find a 55/45 sensible/latent BTU ratio. The ratio never shows up in the cooling performance charts because these operating conditions are unimaginable in the laboratory, but they happen on every hot afternoon in attics in the field. n

Rob Falke is an industry author and trainer and has served his customers as an HVAC contractor and air balancer for over 20 years. He is the president of National Comfort Institute, and can be reached at [email protected].

For a free procedure and report form showing how you can field-measure HVAC system Total, Sensible and Latent Btus call Rob at 800-633-7058.

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