Duct Dynasty: Verifying Proper Design through Performance Measurements
Tips to Ensure the Accuracy of Your HVAC Installations
|It’s your job to test systems and ensure customers are getting the best system available. Make sure, as a professional, you’re providing what you really intend to provide.|
A couple of months ago, I addressed the topic of how proper HVAC system design could not guarantee the installed performance of a system. Since that article, I have received a number of emails requesting I go into further detail about the measurements I mentioned and how to verify the installed performance of a system.
These measurements enable an individual to verify whether or not the installed system actually performs as the design said it should. This is accomplished by measuring static pressure, airflow, temperature changes, and delivered Btu. Once these basic measurements are obtained, you can begin to determine if the system is operating as designed.
Moving from Design to Reality
Many in our industry tend to place emphasis on proper system design, yet never verify the end result. Beyond the important starting point of ensuring the design software was used correctly, the next step involves looking beyond the software’s produced values and quantifying those values were accurately delivered inside the customer’s home.
This may take an installer to an uncomfortable place, as he or she now has to question the installation practices. If there are deficiencies in the installed system, testing will identify them.
Verification is a huge turning point in the lives of many HVAC professionals. They either see it as an opportunity or ignore it and continue forward with business as usual. Let’s take a brief look at the measurements that can help you determine if your designs really deliver what you promised.
Static pressure is one of the foundations of airflow and should be one of the first test readings taken on any HVAC system. Think about that last visit to the doctor’s office. Regardless of whether you had a cut finger or a broken leg, one of the first readings taken was blood pressure. Static pressure serves a very similar function when it comes to the performance of an HVAC system. With total external static pressure readings, you can determine if the fan in the equipment is being subjected to excessive pressures that will cause it to function improperly and fail prematurely. The majority of equipment manufacturers’ engineering data list airflow in furnaces and air handlers based on total external static pressure.
The typical furnace is designed to operate at a maximum of 0.5 inch of water column (wc). If the measured total external static pressure is more than the allotted 0.5 inch, the fan is typically unable to move the industry standard airflow of 400 cfm per ton, in cooling mode.
By taking additional pressure readings, you can also compare the actual restriction to airflow of the evaporator coil and the air filter compared to design. This ensures these components are within tolerances specified by the manufacturer. If these pressures exceed listed tolerances, then further investigation is needed to determine the reason for restriction.
To verify system performance, you will need to determine airflow at the fan and at the grilles. At the fan, airflow needs to be verified to ensure the proper volume of air is available to be delivered to individual rooms. Total external static pressure, the fan speed being used, and the manufacturer’s fan performance data can be used to approximate the amount of air that is moving through the HVAC equipment.
Verifying delivered airflow from each supply register and return grille ensures the designed amount of airflow is being delivered to the proper areas of a home. This is a complete change from assuming the delivered airflow based on a particular duct size and a duct calculator. The use of a balancing hood allows a technician to verify conditioned air is making it to its intended point of delivery. The delivered airflow readings can then be compared to the design airflow numbers from a load calculation. This will allow you to verify your airflow design was actually achieved. How cool is that?
The next step in verification is to ensure the proper temperatures are being delivered the equipment.
By measuring the supply temperature and return temperature at the equipment, you can obtain a temperature change (delta t) across the equipment. These measurements help in determining the equipment’s performance and isolate the equipment performance from the system performance.
Remember, system temperatures are entirely dependent on the system airflow. Low airflow equals higher temperatures and too much airflow equals lower temperatures. This is why you measure airflow first, so you can correctly interpret your delta T’s.
A crucial set of temperature readings used in determining system performance are average supply register and return grille temperatures. With these temperatures, you can see how much temperature is being lost through the duct system and how much is truly making it into the conditioned space.
Subtract the difference between the average supply register temperature from the average return grille temperature to find the system delta t. Then, compare the equipment delta t to the system delta t. The difference is the amount of heating or cooling you have lost through the duct system. This new test will blow your socks off.
It’s easy to assume ducts inside a conditioned space won’t have much of an impact on comfort. This test helps you see the impact of those duct locations. These temperature readings play a big role in determining delivered Btu.
The real truth verifying your design intent has been met or not lies in measuring delivered Btu into the conditioned space. You can calculate system-delivered Btu by multiplying airflow and temperature readings to verify the design was actually achieved. There are two basic formulas typically used to accomplish this feat. There is a formula for cooling, and a formula for heating. (In next month’s article, we’ll examine these formulas in depth.)
Compare the actual system delivered Btu to the required system designed Btu to verify if design intent was met or not. If the calculations are way off, you may need to take a closer look at the installation process. It takes real attention to detail when installing an HVAC system to get close to the manufacturer’s rated output.
It’s your job to test systems and ensure customers are getting the best system available. Make sure, as a professional, you’re providing what you really intend to provide.
Publication date: 2/16/2015