How to Measure Building Pressures

May 1, 2006
The heat is on. Summer is bearing down on us, and its bound to stick around for a while. As youre in and out of buildings day-to-day, could a five minute

The heat is on. Summer is bearing down on us, and it’s bound to stick around for a while. As you’re in and out of buildings day-to-day, could a five minute building pressure test provide the answer your customer has been longing for?

All buildings unintentionally operate under some level of positive or negative pressure. These pressures are usually caused by a defective duct system. Complaints from building pressure problems include indoor humidity issues when the fan runs, and systems that work in the cool of the morning but will not cool the building in the afternoon.

Ideally, most buildings should have a positive pressure of about 0.02-in. w.c. To ensure positive building pressures, normally we have to introduce fresh or outside air into the system. This pressurizes the building, because if a 1,200 cfm fan returns 200 cfm through an outside air duct, it can only pull 1,000 cfm from the building. Meanwhile, the supply side of the fan is delivering the full 1,200 cfm. This leaves 1,000 cfm return and 1,200 cfm supply, causing a 200 cfm positive pressure in the building. With more supply air than return air in the building, the building is under a positive pressure.

This pressure condition can be designed, but to be sure it actually happens requires air diagnostic testing. However, it’s hard to say how great the positive pressure reading in the building will be. It depends on how tight (or leaky) the envelope of the building is, and what other pressure generating forces exist, including the wind, appliances, exhaust fans, and the stack effect of the building.

Because pressures are invisible and variable, we must measure them to understand their effect on a building. In addition to the overall building pressure, pressure measurements can be taken between rooms in the building to further identify the effect of the HVAC system on the performance of a building.

Room To Room Pressures

Pressures between rooms are a different issue than the overall pressure of a building, although they’re often related. Measuring pressure between rooms will reveal pressure imbalances that may have a significant impact on the comfort and air quality in the rooms.

For example, a room with an extremely negative pressure will be drawing in outside air through leaky doors and windows. A room with too much positive pressure will be forcing conditioned air out of the building, leaving and uncomfortable room.

Rooms with a negative pressure containing combustion appliances may result in serious injury or even death due to carbon monoxide poisoning. It pays to measure room pressures.

All pressure balancing requires a differential pressure measurement instrument. The manometers we use all have two pressure ports; a manometer reads the difference between the pressures exerted on either of the pressure ports on the tool. We use hoses to channel the pressures from a duct or a room into each of the ports on a manometer.
Readings are often expressed using the term “With Reference To” (WRT), such as, "The mechanical room pressure was -0.05-in. w.c. WRT the outside of the building, because of the large kitchen exhaust fan located in the neighboring room."

When measuring room pressures, it would be common to have the bathroom 0.03-in. negative to the living room because of the suction of a strong exhaust fan in the bath area.

Let’s look some typical residential pressure scenarios. A living room could have equal supply and return airflow. This would leave a neutral or zero pressure in that room. Or, if the living room had no return air grille, the room would have a positive pressure. If the return was located in the hallway, the hall would be under a negative pressure. But since the pressure in the living room is zero and the hall is under a negative pressure, the living room would have a higher, or positive, pressure with reference to the hallway.

In hospitals, for example, some rooms require negative or positive pressures. Rooms containing cleaning fluids and chemicals must be kept under a constant negative pressure to avoid fumes polluting clean areas of the facility.

Of course, measuring the total of the supply registers and the total of the return registers in a room, then comparing them to each other, is another way to measure the effect of the HVAC system on the pressures of a single room. If the supply register cfm exceeds the return grille cfm, the pressure in the room from the system fan is positive, and vice versa.

Inside and Outside Pressures

To measure the pressure difference between indoors and outdoors, place the end of a long hose outdoors, away from the effects of the wind, and attach it to your manometer. Leave the other port on the manometer open to measure the indoor building pressure. The pressure difference between inside and outside will appear on the manometer.

A good exercise is to then repeat the test on all four sides of the home to see the differences in building pressures. Turn fans on and off and see how the pressures change, and evaluate the effects on the building. Then, isolate fan airflows by measuring them with a balancing hood and use puffs of smoke to follow the air patterns and see where the air is going to and coming from.

Air pressure differences, when undesirable, can be changed by adjusting and balancing airflows throughout the building to achieve the results that you desire. Be cautious when interpreting room pressures however. Making a decision from a 1/100th of an inch pressure can lead you down the wrong road . . . someone may have sneezed in the next room.

Good pressure diagnostic testing includes a careful consideration of the various fans operating around the house such as a dryer or kitchen exhaust fan, and the forces of the wind on the on the upwind or downwind side of the house.

When the occasion arises and you suspect severe building pressure issues in a home, apply building pressure testing to solve the mystery. Then prescribe duct renovation, air balancing, and fresh air ducts to fix the building deficiencies you have found.

Remember, just because you have found the problem and offered the solution, that doesn’t ensure your customers are willing to get out their checkbooks. Educate them first by using theatrical fog to make the defects visible, or show them the testing that lead to your discovery. Once a consumer understands what you do, they are most likely to pay you to provide the solutions they need.

Rob “Doc” Falke serves the industry as President of National Comfort Institute a training company specializing in measuring, rating, improving and verifying HVAC system performance. If you're an HVAC contractor or technician interested receiving a free building pressure testing procedure, contact Doc at [email protected] or call him at 800/633-7058. Go to NCI’s website at for free information, technical articles and downloads.