Over the last three decades the test methods we use to measure the performance of HVAC systems has undergone sweeping changes. As a teacher and an author, I’m often present when individuals and organizations are introduced to a higher level of HVAC testing and struggle to defend obsolete test methods they’ve used for most of their careers. If you find yourself questioning various HVAC test methods, this article is for you.
Balancing Hood or Blower Door
Let’s start at the beginning. A balancing hood measures airflow at the grilles, a blower door measures airflow through a calibrated fan. When testing with a balancing hood, the system is operating under live conditions. When testing the HVAC system with a blower door, the HVAC system is turned off and the grilles are sealed from the inside of the home.
To gain perspective, let’s go back to the 1970’s. Ernie Shortridge invented the air balancing hood as a new method of measuring the performance of an HVAC system. Believe it or not, in the early years of NCI training, many classes had technicians who would argue that unless the water moved in the incline manometer, airflow was not being measured accurately. Some claimed that “new fangled balancing hood was a sissy’s way of measuring airflow.”
Decades later, while still a valid test, liquid filled u-tube manometers and a Pitot tube have been replaced by modern digital instruments and airflow testing using balancing hoods is here to stay.
While blower doors are the primary sustainable method used to measure the performance of a building envelope, and are valid and effective, they were never intended to be an HVAC test instrument. Certain testing protocol has been developed in an attempt to measure HVAC performance as an element of the building, but the accuracy of the test is questionable at best as published by the manufacturer.
The two test methods are conflicting because the test results measuring an HVAC system never agree. However, without the use of a blower door it’s impossible to correctly size the HVAC equipment. Are the two test methods co-dependent? Sure they are. Each test must be used for the purpose it was intended. When used together as intended, the two test methods will deliver the best diagnostic results and efficiency possible.
Static Pressure or Pressure Pan
Each piece of air moving equipment has rated total external static pressure listed on its nameplate. Each manufacturer publishes fan performance data intending the contractor or rater to measure total external static pressure and plot fan airflow. Without adequate airflow, equipment cannot operate at the efficiency it’s rated to and doesn’t meet specifications. Airflow is essential to efficiency, the performance of an HVAC system cannot be effectively verified without interpreting it correctly.
Pressure pans are a plastic cake-pan device with a handle. They are hooked to a manometer where the pressure difference from inside the pan to the outside of the pan is measured as the pan is pressed tightly over a can light or an electrical outlet. The pressure difference identifies if the object that’s covered has any connection to outside the building envelope.
Pressure pans have also been used to interpret a duct leak near a register or grille. The problem occurs when it’s believed that flow can be measured with a pressure pan. It cannot. It’s a pressure fan, not a flow pan. Remember, air takes the path of least resistance. If in doubt, read the manufacturer’s user manual.
Duct Temperature Loss or Duct Leakage
Measure the air temperature at the return grill. Say it’s 75F. Measure the air temperature entering the fan, and it measures 85F. Subtract to find the air temperature increased 10 degrees. If the temperature drop over the cooling coil is 20 degrees, 50% of the system’s BTUs are being lost. Measuring duct temperature loss (or gain, if you want to wax technical) is an effective HVAC performance test. The result of this test indicates two losses in efficiency; one loss due to duct leakage, the other loss due to ineffective duct insulation.
Duct leakage can be measured using a fan/pressure test method where the grilles and registers are taped shut and the duct system is pressurized. Then the airflow is measured through a calibrated fan and interpreted as duct leakage. While this is a widely used test, the problem is the results in no way are represented to be close or equal to actual duct leakage under live conditions. So, we’re stuck with “deemed” savings.
Don’t get me wrong, ducts need to be tight and they must be sealed. But tight ducts, as shown by HVAC testing above, are only part of the efficiency picture.
Operating Efficiency or Rated Efficiency
The operating efficiency of an HVAC system can be measured. If it can be measured in the laboratory, it can be measured in the field. The same tests, standards and requirements for accuracy in the field are the same as the requirements of accuracy in the laboratory. The reality of field measurement is that customers aren’t interested in paying for laboratory accuracy. As imperfect as field measured operating efficiency is, it is repeatable and reliable enough.
By measuring airflow, pressures, temperatures, fan speed and other essential elements of system performance, the operating efficiency of an HVAC system can be measured live in the field. Your customers want it and your technicians will love the ability it gives them to diagnose, repair and verify the improvement in efficiency that was made.
The bad news is that when operating efficiency is measured in the field, it’s often disappointing. As an industry we have been so conditioned to focus on the laboratory rated equipment efficiencies that our ears become closed to the idea that operating efficiency is often more than 40% lower and we reject the notion that the systems we work on may still remain substandard. Unfortunately the integrity it requires to accept and deal with the realities of hidden inefficiencies escapes many. Welcome to the new frontier ladies and gentlemen.
One of the primary strengths that each testing group has is that we are passionate about our work. I am confident that as the two groups of practitioners blend together, one technician at a time, that each can learn the best application of each test instrument and use the two approaches to deliver better HVAC systems and better performing buildings. I am grateful to experience the melding of two great industries daily and I delight in participating in the advancement of energy professionals.
Rob “Doc” Falke serves the industry as president of National Comfort Institute an HVAC based training company and membership organization. If you're an HVAC contractor or technician interested in a free report on measuring duct temperature loss, contact Doc at email@example.com or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, articles and downloads.