More designers are becoming aware of the effects of using room pressures to control air flows in buildings, control odors and contain contaminants. When balancing these buildings, testing and reporting must include room pressurization measurements. Let’s take a look at what required to effectively measure room pressures.
Let’s start by taking a look at the required test instruments. A quality digital manometer with a 0 to 5-in. w.c. test range will do the job. The readout must be down to .01-in. for this testing. Avoid meters with higher ranges, or you may sacrifice accuracy at the lower pressure levels that you will use for this type of testing.
You’ll also need 50 feet or more of 3/16-in. ID, 5/16-in. OD pressure tubing or the size tubing that matches your manometer. Avoid the cheap tubing, or you will pay for it as it may kink or twist too easily and have a negative impact on your pressure readings.
If testing indoors to outdoors, you may also need a pressure pot or pressure filter to reduce the pressure effects of wind on your testing.
Units of Measurement
Traditionally, inches of water column is the U.S. pressure measurement, but you may see the specification express pressure in Pascals. Pascals is the preferred European pressure unit, but is being used more and more in building science testing. Most quality manometers will display both units at the push of a button these days. There are about 250 Pascals in one inch of pressure.
In addition to pressure unit measurement terms you will also need to become familiar with the term “with reference to,” often abbreviated as WRT. This refers to the pressure of the Subject Room with reference to the area where the opposite end of the pressure hose is located. For example, the pressure of the operating room, with reference to the hallway of -.02-in. w.c.
Also some rooms require a positive pressure and some rooms require a negative pressure. Generally speaking, a positive pressure room has more air entering than exiting the room, and a negative room has more air exiting the room than entering the room.
The following steps will apply to most room pressure testing, although it may need to be adapted to fit some circumstances. If testing in a clean room for example, additional steps may be required to meet the sanitation standards.
First, identify from the plans, specification or applicable regulation, what the required pressure difference is between the Subject Room and the neighboring room or area.
Before testing, verify that specified operating conditions exist. Assure the doors are open or closed as required and that all associated fans and equipment is in full operation before taking final pressure readings. You may take some sample tests to evaluate the changes that may occur as doors are opened and closed and appliances, equipment or auxiliary fans are turned off and on.
Before testing, turn on the manometer without any hoses attached just to assure the manometer reads zero pressure. If not, zero the instrument before any testing is completed.
Connect the pressure tube to the positive (usually the top or left) port on the manometer and then place the open end of the tube in the Subject Room and leave the negative port on the manometer open to the room.
Place the other end of the tube in the neighboring room or hallway. In the field, we call it the With Reference To room and we all know what we’re talking about.
If there is a door between the rooms, carefully close the door without pinching the tube to avoid false reading. Then allow the room pressures to stabilize.
Read and record the pressure differential between the two rooms. The manometer is a differential pressure gauge. The difference between the pressures in the two rooms will appear on the manometer.
Compare the room pressure difference to the required room pressure differential.
If the pressure does not meet the specification, airflow through an exhaust fan is increased or decreased until the pressure matches the specification. Supply air can also be increased or decreased accordingly to obtain design room pressures.
Room to Outdoors Pressure Test
When the room pressure is stated with reference to outdoors, a few steps may be added to the testing.
Follow the steps listed above, run the tubing from the Subject Room to the outside of the building in a location sheltered from the wind, or place the end of the tubing inside a pressure pot to limit wind effects if necessary. It’s much more challenging to run a tube under an outside door without pinching it.
A pressure pot can be made from a set of nesting plastic kitchen bowels with fitted lids. Drill a tight hole through the lids and slip the tubing through the holes. Then drill a 1/64th hole in each of the plastic bowels on alternating sides of each bowl.
Read and record the pressure difference of the room pressure with reference to outdoor pressure.
A smoke puffer can be used to make air visible as it moves from one room to another between the two pressure zones. This can help verify room pressure differences. If releasing smoke near a door, check at the top of the door, then again at the bottom of the door. These pressures are often slight and may change as air stratifies in a room and small air patters become visible with the smoke.
In certain clean room type environments, verify that releasing caustic chemicals are permitted before using a smoke puffer.
As your experience increases with room pressure testing, your judgment and ability to interpret readings will grow rapidly. Pressures are constantly changing throughout a building and between inside and outside. Many of the factors that make slight pressure changes may not be seen or heard from where you are testing. Avoid making decisions about room pressure differences from readings near .01-in., unless the readings are verified several times.
Rob “Doc” Falke serves the industry as president of National Comfort Institute an HVAC based training company with technical and business level membership organizations. If you're an HVAC contractor or technician interested in a free Room Pressurization Procedure and test report contact Doc at firstname.lastname@example.org or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, articles and downloads.