In a recent public forum, system performance testing was criticized by a manufacturer as being unnecessary because the equipment would always work just fine as it was shipped. I was delighted when before I could respond, another manufacturer declared that system performance testing was essential because it allowed a contractor to adjust the system so it would actually meet and perform to manufacturer’s specifications. Without field testing and adjustment a system cannot perform as intended.
Do your systems perform to manufacturer’s specifications? With the national average system performance hovering below 60% of capacity odds are your system’s performance, efficiency and comfort have considerable room for improvement.
Let’s take a look at which specifications can be checked in the field to verify that an installation is delivering the performance that we indicate in our sales proposals. Testing and adjusting the system will insure that it comes as close as it can to meeting the performance indicated in the manufacturer’s claims published in their sales literature. Without testing and verification, don’t count on it performing as intended.
Fan rated total external static pressure specifies the maximum amount of pressure a fan can operate under and still push and pull the specified airflow through the system. If pressure exceeds the specified maximum, the system performance will often decline proportionally.
Total external static pressure is published on the nameplate and more accurately in the equipment engineering data. It can and should be measured upon start-up and each time the equipment is serviced as an indicator of adequate system airflow.
Excessive static pressure increases can be attributed to air filters that have too much restriction to airflow as they load up with dust, pollutants or insulation. Some filters are too restrictive due to their design, fresh out of the box.
Coils load up and become too restrictive as well. Both filter and coils can be cleaned or replaced.
Undersized or poorly installed ducting is also reflected in the measured total external static pressure reading. Duct renovation to repair design or installation defects would increase system performance on more than 80% of residential systems in the US.
Assuring total external static pressure is within the limits of the manufacturer’s specifications is a quick and simple test that takes less than five minutes.
Measuring the pressure drop of system components and comparing the pressure drop to manufacturer’s specifications can also identify system defects that need attention.
A minimum and maximum temperature change in air temperature is specified on the nameplate of nearly all gas fired air moving equipment. With most cooling equipment 18F to 22F is the standard temperature drop. Temperature change will increase or decrease as airflow decreases or increases. This is why static pressure and temperature testing are inseparable.
As humidity increases, the temperature drop through a cooling system will also decrease as latent BTU removal replaces sensible BTU removal.
Temperature change through the equipment should be measured at start up and at each service. Care should be taken on the supply side to assure that the temperature probe cannot "see" the coil, heat exchanger or cooling coil, because radiant heat or cold can cause the probe to misread air temperature by several degrees.
Temperatures outside of specifications are caused by a poor refrigeration charge or a lack of combustion efficiency adjustment as well as improper system airflow.
Although few industry standards exist, temperature change through the duct system can also be measured as a powerful diagnostic tool. Temperature loss through the duct system is easily measurable and has an immediate impact on the system efficiency often decreasing system performance by over one third during extreme weather.
Service techs learn early on to interpret the effects of electrical values in HVAC equipment. Electrical has a direct and immediate impact on the performance of a compressor, heat strips or a blower motor.
Comparing measured electrical voltage and amp draw to manufacturer’s engineering data and nameplate data can quickly identify defects and enable a service tech or energy professional to identify and take or recommend corrective action.
One significant variable that’s becoming a recognized issue in the energy field is blower motor watt consumption. It’s not uncommon to find variable speed blower motors operating at two to three times the watt draw used in the laboratory when the equipment’s efficiency was rated compared to actual watt draw in the field. This can quickly turn an 18 SEER piece of equipment into a 12 SEER in the field due to a poorly designed and installed duct system or the installation of a filter that has nearly as much pressure drop as the fan is rated.
Airflow is the life blood of an HVAC system and can track system performance with ease. Learning to accurately use an air balancing hood or an anemometer to measure system airflow at the equipment and at the registers and grilles where airflow enters and exits a building enables to technician, salesperson or energy professional to immediately identify system losses and flaws that can solve comfort and efficiency issues that have existed undetected for many years.
Once airflow and temperature are known, the delivered BTU of the equipment and system and easily be interpreted and compared to the published manufacturer specification. This is the greatest system performance indicator of all time and can be measured directly and accurately in the field.
Combine Test Data to Know for Sure
As you see system performance can be interpreted by pressure, electrical, temperature airflow and BTU measurement directly in the field. Each of these test values can be then compared to the manufacturer’s specifications to know the performance of an HVAC system.
Are live field measurement’s perfect? They are only as perfect as the instruments will allow and that test conditions provide for. The accuracy and usefulness of the test data multiplies with the skill level of the tester. Effective testing and one’s ability to interpret and apply test results can quickly turn a laborer into a professional. Mastering field testing and effectively interpreting test results is the surest way to build job security and increase income in the HVAC profession.
Rob “Doc” Falke serves the industry as president of National Comfort Institute an HVAC based training company with membership organizations. If you're an HVAC contractor or technician interested in a free static pressure or other live field test procedure contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, articles and downloads