Btu Explained

    Practical Standards to Measure HVAC System Performance

    April 1, 2006
    You can depend on them to evaluate how well your HVAC system is performing.

    EDITOR'S NOTE: This article was updated with a new video link.

    Practical industry standards are rock-solid principles that you can depend on to evaluate how well your HVAC system is performing. You can measure any of these in the systems that you work on and use the test results to see how your systems measure up.

    In a day when most HVAC standards are written by committees in scientific language that many of us can barely pronounce, it’s refreshing to know that real, practical standards exist that each of us can use to evaluate our work every day.


    If we were artists, Btus would be our paint. It’s our job to move Btus around. We take them out of a building or put them into a building. Each room needs just the right amount in and out of them, or it’s not comfortable.

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

    I’ve heard it’s about the amount of heat released by one wooden match. Although I’ve never measured it, I like this description of a Btu and so do my customers.


    One ton of nominal cooling equals 12,000 Btu. We use the term nominal because no one makes equipment that delivers 12,000 Btu per ton anymore. Under ideal conditions, most cooling equipment delivers around 11,700 Btu per ton these days. Years ago, you could find equipment that would deliver the goods, but the price point and competition must be pretty tough out there.

    Total, Sensible, and Latent Heat

    The kind of Btu that makes up 12,000 Btu in a ton of cooling is called total 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 we hear so much about. Latent heat can be simply defined as the removal of humidity from the air that passes through the coil. Or, the cold water that runs down the condensate drain.


    Airflow is the fluid we use in most of our systems to move the Btus around. There are nasty rumors out there claiming the amount of airflow doesn’t matter all that much, but to deliver the Btus the way most equipment is built today requires 400 cu. ft. of air through the system in a minute in cooling mode. The fudge factor is 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.

    If you hear the term “airflow over the indoor coil” mentioned, this is an indication that someone is only focused on the equipment performance only, not the system performance. The airflow through the system is all that really counts. It should match the airflow over the coil, minus a few cfm for duct airflow loss, which is an unfortunate reality to some degree in nearly every system.

    In heating mode, airflow is a little tougher to figure. Here’s the best method we’ve found:

    1. Divide the heating equipment rated Btu input by 10,000.

    2. Then multiply by a factor based on the type of equipment. The factor for natural draft furnaces is 100 cfm per 10,000 Btu. Induced draft furnaces require 130 cfm, and condensing furnaces require 150 cfm per 10,000 Btu. Give this a try and always compare calculated airflow delivery to the required airflow specified by the manufacturer.

    Keep an eye on your temperature rise through the heat exchanger as well. You’ll find if the airflow is too low or too high, the temperature of the flue gasses will skyrocket. Where else can the Btus go, if they don’t make it into the system airstream?

    Static Pressure

    Nearly every installation instruction out there requires the measurement of total external static pressure at startup. This reading 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 than 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 are pretty sorry and shouldn’t be included in your equipment selection options.

    About 50% of the available equipment out there is rated at 0.50-in. maximum total external static pressure. Most variable speed fans are rated from 0.90-in. to 1.2-in. available fan pressure. (Buy them, you’ll need the extra fan capacity) The sad news is that ARI tolerates some equipment with fans rated as low as 0.20-in. Unless you’re not using a duct system with these air handlers, don’t buy this type of equipment.

    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 the duct system. Make sense? Of course, the live total external static pressure after a system is built is the real test of system performance from a pressure perspective.

    BTU Delivery

    Total heat removed by a cooling system can be measured. To do so requires a very high-quality hygrometer that measures wet bulb temperatures with extreme accuracy. It’s the wet-bulb temperature (the measures of both heat and humidity) that enables us to measure total Btu.

    The formula that guides this test is timeless. It is simply CFM x Delta-T x 4.5. In other words, total BTU equals delivered airflow (cfm) times the change in temperature and moisture content of the air through the system (Delta-T), times the constant of 4.5.

    The trick here is to measure the Delta-T through the system. This is easily and accurately measured by taking accurate 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 the ageless formula CFM x Delta-T x 1.08. To measure heating Btu, multiply the measure 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.

    For those of us who like all our questions answered, let’s look at the constants in these Btu formulas. Understand that all through we call them constants, actually field conditions require them to be altered constantly.

    The 1.08 is a constant in the sensible Btu formula is based on the air density under standard conditions. These are air at 70°F, 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.

    System Performance

    I spoke with a government official recently who was surprised to learn that NCI’s acceptable standard for delivered Btu in a live system was only 90% of rated equipment Btu capacity. I could tell that she had never considered a system delivered less than 100% of the rated equipment Btu into the building. “Why this would lower a 13 SEER system performance to only 11.7!” she exclaimed. Imagine her surprise when she learned that 13 SEER equipment connected to a typical duct system in the U.S. measures a delivered effective rating of less than 8 SEER on a hot afternoon.

    Ninety percent is a tough standard for heating and cooling system performance. Currently, few contractors in the U.S. can achieve this level of performance in the systems they service and sell. In one European country, if heating equipment cannot achieve 90% efficiency, it is removed by a government official and must be replaced by law. God bless America!

    Our hope is that by understanding these practical standards our industry can voluntarily improve the performance of our systems and the product each of us delivers to our customers. For now, the question is: What percent of rated equipment Btu do your systems deliver?

    Rob “Doc” Falke serves the industry as president of National Comfort Institute a training company specializing in measuring, rating, improving and verifying HVAC system performance. If you're an HVAC contractor or technician interested in a no-cost copy of a quick reference sheet containing many of the HVAC industry formulas, contact Doc [email protected] or call him at 800/633-7058. Go to NCI’s website at www.nationalcomfortinstitute.com for free information, technical articles and downloads.

    Tonnage, as explained by HVACdirect.com