This article was written by Christopher Molnar, a licensed Florida mechanical contractor. While I'm not personally a practitioner of the “check static every time” doctrine, I certainly appreciate Chris and his passion for this topic. Thanks, Chris!

Why Check Static Pressures on EVERY call

You wouldn't go to the doctor’s office and walk away without having your pulse and blood pressure checked. To do so would be malpractice. Your pulse and blood pressures indicate how your blood is flowing through your body and how well your heart is pushing that blood around your body. Measurements outside of the standards are cause for further investigation.

Why is it acceptable to walk away from any HVAC service call without checking the air pressures in the ductwork? I would suggest that not taking these pressures on each and every service call is malpractice, and you are cheating your employer and customer by not taking these readings.

What is static pressure? The definition is the outward pressure of the air (without movement) in the ductwork. The number is either positive or negative based on which side of the blower/fan you take the pressure on. The air on the supply side of the system is always positive pressure; in other words, the pressure is greater than that of the air outside the duct. Duct pressures are generally measured in inches of water column (inch w.c./in w.c./” w.c.) in the US.

The air on the return side of the system is always negative, or a pressure that is below the air pressure outside of the ductwork. Think about it; it makes sense. To blow air into a space, the air in the supply duct must be above the pressure in the room we are moving it into. To draw air into the return duct, the air pressure in the return duct must be under that of the room we are pulling from. Air will always move from an area of high pressure to an area of low pressure.

What causes this static pressure? Ductwork is just a pipe, correct? So, why does pressure build up in this pipe? Shouldn’t it just “flow?”

The blower is designed to work against a certain amount of pressure in order to provide comfort to the customer and achieve peak heating/cooling efficiency.

The less air that moves through the ducts and over the coil, the greater the temperature of the air will drop and the more humidity it will give up. However, we also must maintain a proper level of airflow so that we do not overheat a furnace or freeze a coil. For air conditioning, this is about 400 CFM (cubic feet per minute) per ton in most regions.

On the data plate or in the specs of every air handling device or furnace, you will find the “total external static pressure” that the device can handle. For example, on the nameplate above, you may see that the total maximum static pressure is .5 inches of water column. So, what does this mean?

A technician will take two pressure readings using a static pressure tip. One will be in the supply duct above the coil in the case of a fan coil/air handler or between the blower and coil on a furnace. The other will be in the return duct right before the blower compartment, or it may sometimes be in the return plenum.

The number in the supply duct will be positive. The number in the return duct will be negative. To find the total external pressures, you will drop the sign (+ or -) and use the absolute value. So, a +.25 will become .25, and a -.25 will become .25. Add the two numbers together as .25 + .25, which equals .50 inches water column (.50″ w.c.).

To get these readings properly, you will need a minimum of one static pressure tip and a manometer, such as the Testo Smartprobe 510i that interfaces with a smartphone.

You want to see a measurement as close to but not over the “maximum external static pressure” on the data plate. If the number is too low, it most likely means that the blower is not operating at the correct speed or that there is a leak in the ductwork that is preventing static pressure from building up.

Low supply static pressure: Look for duct leaks. Look for missing register covers. Look for proper blower speed. Check for proper cooling (if the system has a cooling coil, that coil should be wet). Finally, double-check the supply duct sizing.

Low return static pressure: Look for duct leaks. Look for missing filters. Look for proper blower speeds.

High return duct static pressure: Dirty filter? Blocked return grills? Look for insulation or something else stuck in the return duct. Look for a collapsed return duct (if fiberboard or rusted steel ductwork).

High supply duct pressure: This indicates undersized supply ducts, closed registers and grilles, closed doors in buildings with central returns, collapsed ductwork, or that the blower has been set at too high of a speed.

Static pressure that is too low will affect latent capacity (moisture removal) on PSC motors in some cases.

Static pressure that is too high will cause noise, excess wear on the blower motor, high amperages, cooling and heating inefficiency, and, in many cases, can pull the condensation off of the air conditioning coil, causing leaks and damage.

We all learn to measure voltage, amperage, and the resistance of components in trade school. Most technicians understand what causes low superheat, high subcooling, and pressures. We understand what high amperages can do to a system. We know that motors will burn out if the amperages are too high. We know that a clogged condensate line can cause a flood. Taking the additional 5 minutes to take two static pressure measurements when the system is operational will allow you to do a quick air flow check and ensure that there is not something else happening that will impact system health, performance, and customer comfort. Not taking static pressure on each and every heating or air conditioning service call should be malpractice.

—Chris