Room pressure measurement inquiries have been increasing at NCI tech support. Many of you are receiving customer requests to help prevent COVID-19 airborne infection migration from patient’s rooms into the rest of their homes. Let’s take a look at what’s required to create and measure negative room pressures.
A Ventilation Solution
The simplest way to isolate infection in a home is to change a patient’s room ventilation patterns by installing an exhaust fan. Airflow moving through an exhaust fan to the outdoors must exceed the maximum supply air delivered to the room by a forced-air heating or cooling system.
For example, if a patient’s room receives 100 cfm from the supply register, an exhaust fan with perhaps 150 cfm may be installed. Since 50 cfm more air is being exhausted than the amount entering it, the room will operate at a negative or suction pressure. Negative room pressure helps prevent airborne infectious contaminants from drifting out of the patient’s room into the rest of the home.
All ventilation design requires field verification by airflow and pressure measurement once system modifications have been completed.
All ventilation design requires field verification by airflow and pressure measurement once system modifications have been completed. The final test is to measure and verify the patients’ room negative pressure remains near .03-in. w.c. concerning neighboring rooms in the home. Testing should be done with all home ventilation systems operating.
Additional testing and system modification may be needed to assure compliance with local residential code requirements (if any). To learn about room pressurization in health care facilities see THIS LINK. (Not sure the reference to health care facilities fits here)
Test Instruments
To measure room pressure differences, a quality digital manometer (pressure gauge) with two ports is required. Its test range should be from 0 to 5 in. w.c. or less, with a readout accurate to .01-in. w.c.
You’ll also need 20 feet or more of 3/16 in. ID 5/16 in.” OD pressure tubing or tubing sized to match your manometer.
Units of Measurement
Traditionally, inches of water column is the U.S. pressure measurement Pascals is the preferred European pressure unit but is often used in the U.S. for low-pressure testing. Most good quality manometers display both units at the push of a button. There are about 250 Pascals in one inch of pressure.
Room Pressure Test Procedure
First, verify the amount of negative pressure required by local code and assure you slightly exceed the required room negative pressure. If no code requirement is available, use .03 in. of negative pressure as your target.
Before testing, assure doors are open or closed to reflect typical operating conditions and that all associated fans and equipment are in full operation before taking pressure readings.
Before testing, assure doors are open or closed to reflect typical operating conditions and that all associated fans and equipment are in full operation before taking pressure readings. You might 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 inside the patient’s room, turn on the manometer without hoses attached and assure the manometer reads zero pressure. Most manometers have a button that zeroes it out.
Stand in the patient’s room. Connect the pressure tube to the positive (usually the top or left) port on the manometer. Place the other end of the tube in a neighboring room or hallway.
If there is a door between the rooms, carefully close the door without pinching the tube to avoid a false reading. Then allow the room pressures to stabilize for about 30 seconds.
Read and record the pressure differential between the two rooms. Since a manometer is a differential pressure gauge the difference between the pressures in the two rooms will appear on the readout screen.
Compare the measured negative room pressure difference to .03-in. w.c. If the patient’s room negative pressure is less than .03-in. or as code requires, either increase the exhaust airflow or reduce the room’s supply register airflow. This can be done by adjusting the air volume dampers.
If the patient’s room negative pressure exceeds .04 in. or .01 in. more than code requires, you can change room pressure as you reduce exhaust or supply register airflow.
Once required room negative pressure is achieved, lock and mark dampers to assure the room’s negative pressure is maintained. Assure the exhaust fan operates continuously.
When the home’s air conditioning and heating system’s fan cycles off, the room’s negative pressure will increase, which is normal and appropriate.
Many Applications for Room Pressure Testing
The principles of pressure testing are used throughout the HVAC industry. In addition to room pressure testing and diagnostics, building pressure compared to outside pressure is used constantly in residential and commercial applications.
Avoid making diagnostics and verification decisions about room and building pressure differences from readings below .01 in. unless the readings are verified several times.
Medical facilities, commercial kitchens, clean rooms, office buildings, industrial and commercial applications are in demand. Multi-unit residential building standards are also evolving to improve indoor air quality, safety, and occupant comfort.
Many building factors may make slight pressure change from moment to moment and may not be detected at your test immediate test location. Avoid making diagnostics and verification decisions about room and building pressure differences from readings below .01 in. unless the readings are verified several times.
As your experience increases by regularly testing room and building pressures, your judgment and ability to interpret readings and adjust pressures will grow rapidly.
Codes and standards are rapidly changing to keep up with the need for increased safety, building cleanliness, and sanitation. These ongoing changes will drive the demand for more pressure and airflow testing and verification opportunities.
Rob “Doc” Falke serves the industry as president of National Comfort Institute, Inc., an HVAC-based training company and membership organization. If you're an HVAC system designer, interested in a free copy of a residential and/or commercial air balancing specification to include in your designs, contact Doc at ncilink.com/ContactMe or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, articles, and downloads.