When you visit your local physician’s office, they normally perform a few routine tests. It doesn’t matter whether your visit is for a yearly exam or sudden illness; they take these tests each time. Without exception, they measure your blood pressure, weight, and temperature.

These tests, along with a few more, help the physician discover invisible health issues that could cause you problems later. Their goal is to be proactive, not reactive. The doctor wants to find and address any issues you have before they turn into something worse. The key is prevention. Could you follow the same line of thinking for an HVAC system?

Poorly installed HVAC systems work twice as hard as they should and suffer from a wide variety of invisible health problems. Hidden defects cause them to run excessively and fail prematurely. There are three HVAC system vital signs you can use to help your customers see there is more to their HVAC system problems than meets the eye. Let’s look at how you can identify hidden problems before they turn into something worse.


Blood Pressure Is Static Pressure

Blood pressure is one of the first tests your physician uses to gauge your overall health. If your blood pressure is high, the physician will dig deeper and ask questions to figure out what is going on. Sometimes high blood pressure connects dots for you about why you have headaches, chest pain, difficulty breathing, or pounding in your ears.

Total external static pressure (TESP) is the blood pressure of an HVAC system. It can help you determine overall system health. It reveals hidden airflow issues that lead to premature equipment failure and comfort issues. To diagnose TESP, compare measured TESP to the maximum rated TESP found on the equipment nameplate. If measured TESP exceeds rated TESP, test further to uncover system restrictions.

A simple blood pressure comparison will help customers understand what you’re measuring. High static pressure, just like high blood pressure, means major issues exist. To keep your explanations simple, you can use the NCI Static Pressure to Blood Pressure Tables. If you would like a digital copy, send me an email request.

Common causes of high static pressure are air filters, indoor coils, and duct systems. You can locate airflow restrictions by comparing pressure measurements against manufacturer specifications or NCI pressure budgets. Your customers can see how much pressure they should have across a system component and then compare it to the results. Once they see the restriction, the next question will probably be: how will you correct it?


Weight Is Airflow

Weight is also a vital sign your physician uses to gauge your health. Each person has a recommended weight range based on their height and age. If you are underweight or overweight based on those parameters, your physician will try to discover why.

Fan airflow is like weight. You can calculate it in pounds per hour, but for simplicity, we use cubic feet per minute (cfm). It’s the second vital sign measurement that helps customers understand the health of their system.

Proper airflow is key to a well-performing and long-lasting HVAC system. Without it, you won’t achieve rated energy efficiency, and the system will suffer long-lasting negative effects. Many customers have endured low airflow for a long time, but never had it identified through measurement.

After you measure TESP, add in the fan speed setting and a fan table to plot fan airflow. This helps the customer see approximately how much air the fan is moving compared to what it requires for proper operation.

Use the fan chart as a customer aid to visualize plotted fan airflow and required fan airflow. The customer can see how much their system needs to improve to get the airflow it needs. Let’s say required fan airflow is 1600 cfm for a four-ton system at .50” w.c. Measured static pressure readings reveal the system operating at .80” w.c. and only moving 1,120 cfm in its current operating condition. To improve airflow, you’ll need to lower pressure.


Temperature Is Dry Bulb Temperature

Temperature is another vital sign measurement your physician uses to gauge your health. If you have a fever, your physician knows you’re fighting off an infection.

A few months ago, we looked at how to measure system temperatures to diagnose uncomfortable rooms. You can use the same principles to give your customer a glimpse into how much influence their ducts have on their system. You’ll need to measure the following temperatures with the system running in cooling mode:

  • Temperature entering the equipment
  • Temperature leaving the equipment
  • Temperature at the farthest supply register
  • Temperature at the farthest return grille.

These four temperature readings and some simple math will help you estimate how much system capacity is lost through the duct system. A properly operating duct system shouldn’t have a fever.

First, subtract the equipment leaving temperature from equipment entering temperature. This is the equipment temperature change or equipment delta T (∆T). Next, subtract the farthest supply register temperature from the farthest return grille temperature. This is your system temperature change or system ∆T.

Finally, compare equipment ∆T to system ∆T. The difference is the percentage of duct system temperature loss. Ideally, equipment ∆T and system ∆T should be the same. Here’s a cooling example:

  • Temperature entering the equipment is 75°
  • Temperature leaving the equipment is 55°
  • Temperature at the farthest supply register is 60°
  • Temperature at the farthest return grille is 70°.

The formula completed with the temperature readings above looks like this:

75° - 55° = 20° equipment ∆T

70° - 60° = 10° system ∆T

10° system ∆T ÷ 20° equipment ∆T = 50% temperature loss through the duct system.

Consider this: If you had a four-ton system with these numbers, it is effectively operating as a two-ton system because of duct system temperature loss. If you only looked at equipment ∆T, this problem would remain hidden. Have you ever had a customer who thinks they need larger equipment? Use this test to show them why they don’t.


Establish a Baseline

Blood pressure, weight, and temperature all have acceptable values a physician can compare your measurements against. When the measurements are too far off, they know you need further tests or immediate action. It depends on the severity. A properly operating HVAC system also has acceptable operating values.

TESP should be close to the nameplate rating and fan airflow should be within ± 10% of manufacturer specifications. Compare temperatures in the heating mode of a furnace to the furnace nameplate temperature rise range. The temperature change of air source equipment such as a heat pump will depend on outdoor air temperature and airflow across the indoor coil.

Whatever equipment you’re using to measure the vital signs, you need a baseline to compare those measurements against. Otherwise, you don’t have a target to aim for.


Translate the Technical

When you discuss readings with your customers, keep the explanations simple, like your physician would. Avoid technical talk and use basic analogies a customer can tie to their problems.

Base your questions on your readings. For instance: If static pressure is high, ask questions tied to airflow issues such as comfort, hot and cold spots, or excessive utility cost.

As you continue assessing your customer’s problems, they’ll think you have a crystal ball. Seeing is believing, and your test instruments and readings make their problems visible. Once you’ve discovered the source of their pain, you can focus on finding a solution for it.


Nothing but the Facts

Your physician won’t offer you any opinions about your vital signs. They won’t use any high-pressure sales or scare tactics to get you to lower your blood pressure, lose weight, or take your medicine.

Instead, they will state the facts. They will give you the findings and their recommendations to correct your problem. Their attention is on how to fix the cause of your problem, not treat the symptom. They also discuss the consequences if you don’t follow their advice. It’s all about the consequences and you decide.

Your customer must also decide about their HVAC system. If they choose not to follow your recommendations, their HVAC equipment will ultimately suffer the consequences of their decisions. It’s your job to present the facts, and it’s your customer’s responsibility to decide.