"Technicians should approach static pressure testing from a scientific perspective, utilizing only empirical data to form a conclusion that will help determine a corrective action if needed.”
These words, from Martin Easley, lead HVAC instructor, Valley College, Martinsburg, West Virginia, may seem daunting for some. However, Easley is highlighting both the importance and general lack of understanding surrounding static pressure readings in the HVAC industry.
WHAT IS STATIC PRESSURE?
To fully understand the importance of static pressure readings, one must first know what exactly static pressure is.
According to Dennis Silvestri, lead instructor, MRS Educational Training, New Haven, Connecticut, static pressure can be broken down into three things:
1. The bursting pressure or outward force in a duct system;
2. The pressure of air pushing against the walls of a duct system; and
3. The resistance air encounters as it is traveling through a duct system.
High static pressure is often caused by blockages in ducts, improper duct sizing, closed dampers, and kinked flex ducts while low static pressure is caused by leaking ductwork, separated duct connections, missing filters, or low fan speeds, he said.
“Looking at residential and light commercial HVAC equipment, a more common use of static pressure is to set the proper blower speed on a new system installation,” said Silvestri. “It is a good idea to check total static pressure when replacing an existing HVAC unit as well as when a system is being checked out by an HVAC contractor who did not do the original installation, especially if a service contract is being asked for by the customer. There are times, too, when a homeowner is adding a room or some other addition to the home and may look to simply tap into the existing duct system. This could create issues with airflow. In this situation, when the homeowner finally calls in the HVAC professional, checking the total external static pressure can be useful.”
Static pressure testing is most appropriate when a system is initially commissioned, said Easley. “The overall external static pressure of the air system should be known to ensure that the duct system’s external static pressure is not greater than the equipment’s rated static pressure. This will serve as an overall mechanical system check that will expose the potential good or bad performance of the system. Competent technicians will gain the knowledge of any air-side restrictions, provided the technician performs individual static pressure readings with the understanding that the side of the fan with the greatest pressure will point to an airside restriction.”
PERFORMING A READING
The importance of actually conducting static pressure readings cannot be overstated, as the reading can accurately evaluate a system’s health and performance, expose poor installation practices, and determine what future steps need to be taken.
“Without the measurement of static pressure and its comparison to pressure budgets or manufacturer’s specifications, there is no way of knowing if a component is too restrictive to airflow or not,” said David Richardson, curriculum developer and a trainer for the National Comfort Institute (NCI). “Besides the comparison to known pressure values, it’s an invaluable aid in helping customers understand the air side of the HVAC system. Comparisons to blood pressure often work best for communicating high static pressure readings. When the results of static pressure are communicated in a simple way, it can be one of the best lead-generation tools you’ll ever use.”
Silvestri suggested that manometers and pressure gauges be used when measuring static air pressures. “Measure the entering and exiting static pressures and add them together — this is what is called total external static pressure (TESP). You can compare this number to the equipment manufacturer’s total external static pressure found on the unit’s data plate and/or in the unit’s installation instructions’ airflow section. You will then have a picture of the system’s performance.”
Silvestri also said it is important to note that static pressure measured on the inlet side of the equipment will be a negative number while the pressure leaving the unit will be a positive number.
“The proper test location for conducting static pressure readings is over a device that can potentially restrict the airflow,” said Easley. “For example, before and after the filter [pressure drop over filter], before and after the coil [pressure drop over coil], and, for the external static pressure, the technician should probe after the filter and before the coil [on gas furnaces] and before the coil and at the supply plenum on air-handling units the technician is testing. The locations will change based upon the component layout of the equipment.”
“In measuring static pressure, you will most often need to drill a couple of ¼- to 3/8-inch holes, so pay attention to where you are drilling, and remember to seal the test holes when you are done testing,” said Silvestri. “In a typical residential upflow heating unit with air conditioning, drill one hole after the air filter, where the airflow enters the unit. Make sure the air filter is clean. Drill the other hole before the coil as the air leaves the unit. With just the fan on, record the two pressures and add them together if you are using a device that only allows for one reading at a time. If your testing device allows for both readings to be taken together, then just look at the reading on the device for total external static pressure. Some manufacturers may recommend placing the probes before the air filter and after the coil. Always check what the unit manufacturer recommends. Remember, they designed and tested the units, and we install and work on them.”
Bryan Lee, HVAC instructor and program manager at Fresno City College in Fresno, California, added that one method to be avoided is to let the pressure gauge (manometer or Magnehelic) “do our math for us,” which is what happens when measuring TESP via a single measurement. “This method might tell us whether the TESP is greater than the blower TESP rating, but it does not reveal whether the return or supply ducting is the most restrictive to airflow.”
WHY IS IT OVERLOOKED?
The fact remains that many technicians are continuously overlooking static pressure readings, and Easley believes it is due to a multitude of reasons.
“First, adequate training is significant to the technician on the job,” he said. “Oftentimes, the subject of static pressure is brought up to seasoned technicians to no avail (you hear crickets). Not being able to answer the question on static pressure due to a lack of information speaks volumes in regards to our current training and education. Secondly, technicians who know how to evaluate static pressure and fail to do so may speak to the hustle and bustle of commerce. It fosters the attitude of ‘get the job done and get on to the next one.’ Perhaps the owner only cares about the margin. I have also witnessed a plethora of technicians who simply maintain the work ethic of ‘it looks good from my house.’ The thinking goes that as long as the customer feels the sensible effects of heating and cooling, then everything is good.”
Richardson said a lack of education and training on the subject is a big reason these readings are commonly overlooked. “If technicians, salespeople, managers, and owners aren’t provided the necessary technical skills, they won’t know any different.
“We’ve grown too confident in our duct designs, equipment choices, and installation practices,” continued Richardson. “When our industry shifted its focus from comfort to equipment efficiency ratings, we took our eyes off the system and placed them on the equipment. It became easier to assume static pressure is correct instead of measuring it to assure it is where it needs to be.”
Lee believes this is all part of a systemic problem, as those within the industry lost their sense of priorities years ago as basic engineering principles gave way to the drive for more efficiency and features.
“Looking back 50 years, HVAC was simply about the conditioning of homes and businesses. Labor and material costs were affordable, and a principled, engineered design was enough to satisfy the customer,” said Lee. “In the beginning, what we delivered as an industry was homes and businesses that were comfortable, without any other consideration … Over time, basic engineering principles, such as static pressure, were diminished in favor of more marketable features and terminology. Fortunately, static pressure has come full circle with manufacturers, contractors, and government beginning to understand that technology cannot minimize the importance of good duct design.”
SIDEBAR: INTRODUCING THE PANEL
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Publication date: 6/20/2016