Measuring HVAC Efficiency

• By Michael Fickes
• 01/01/06

Chuck Felland recently reviewed heating, ventilating and air-condition (HVAC) performance in a building that had, through the years, tripled in size. The building’s owner had failed to upgrade the facility’s air handling equipment.“That’s wrong,” says Felland, a product manager with TSI Incorporate of Shoreview, Minn., a company that makes instruments that measure indoor air quality and ventilation system performance.

When did you last measure the efficiency of your building HVAC systems? Have your buildings changed since then? Felland recommends incorporating seven efficiency measurements into scheduled building maintenance work to make sure your buildings continue to operate at peak efficiency. Here’s a quick look at each.

Flue Gas Analysis: Specifications for burners in boilers, furnaces and hot water heaters note the quantities of gasses that will be exhausted when the equipment is generating the most possible heat from the least amount of energy. Flue gas meters measure exhaust gasses and help determine whether or not equipment is operating efficiently.

Air Velocity: Various pieces of HVAC equipment carry specifications related to how fast air should move through a ventilation system. Air velocity and ventilation meters take these measurements. According to Felland, the rules of thumb set minimum velocity at 15 to 20 cu. ft. per minute (cfm).“Below that level, people will think the room is stuffy,” Felland says. “And a room feels drafty when air velocity exceeds 50 cfm.”

Air Volume: Devices called air capture hoods measure air volumes when positioned on an air diffuser. The measurement is taken in cfms. By multiplying the result by 60, it is possible to determine the number of cubic feet of air per hour flowing through a space. By dividing that number by the room’s cubic feet, a maintenance technician can determine the number of air changes per hour. Suppose an air capture meter measures air volume flowing into a room at 200 cfm. That would equal 12,000 cu. ft. per hour (60 minutes multiplied by 200 cfm). If the room were 10 ft. by 10 ft. by 10 ft., it would contain 1,000 cu. ft. By dividing the room’s volume, 1,000 cu. ft., into the number of cubic feet per hour of air flowing into the room, it turns out that this room undergoes 12 air changes per hour.

Measuring ASHRAE Standards: Standard 62, as recently revised by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), recommends minimum amounts of outside or fresh air per occupant in particular kinds of spaces. To compute this figure, you need three measurements. You must measure carbon dioxide outside, carbon dioxide at the diffuser (air coming into the space) and carbon dioxide at return (air moving out of the room).

According to Felland, these measurements can be taken as either carbon dioxide or temperature. “Whether you use temperature or carbon dioxide, you plug the results into a formula,” says Felland. “Return air measurement minus the supply air measurement (at the diffuser) divided by the return air measurement minus the outside air measurement. Finally, multiply by 100 to turn the result into a percentage.

Suppose the equation says that 25 percent of the air flowing into the space is outside air. Think back to the previous example of a room through which air flows at 12,000 cu. ft. per hour. Twenty-five percent, or 3,000 cu. ft. of that total, is outside air and the other 75 percent of the air in the space is re-circulated air.

“This is another way to save energy,” says Felland. “You can reuse the air already in the building. Since it has been heated, filtered and conditioned, you don’t have to heat or cool it as much as outside air. The more inside air re-used, the higher the carbon dioxide levels will go. As long as carbon dioxide levels do not exceed 700 parts per million or ppm over the outside level of carbon dioxide, the building will remain comfortable. Above that level, while it is not dangerous, people may feel tired.”

Humidity Measurements: Felland also recommends using a hygrometer to regularly check the humidity inside a building. For the comfort of the building’s occupants, he says humidity should be kept at 50 percent.

Differential Pressure Measurements: Use a micro manometer to measure the difference in pressure between two different areas of a building. “We want to keep rest rooms, cafeterias, chemical labs, gymnasiums and places that may be sources of odors under negative pressure with respect to the areas around them,” Felland says. “Odors, gasses and particles always move from higher pressure areas to lower pressure areas. Overall, a building must be positive. If it isn’t, outside odors will come in, even if all the windows are closed. A good rule of thumb is: good buildings blow, and bad buildings suck.”

Counting Particles: If a building has sources for particulates indoors, a facility manager might want to add a real-time particle counter to his or her toolbox. For example, a building that houses an underground parking garage may leak fumes into the general building. Likewise, a leaky gasket on a boiler may let fumes into occupied building spaces. “Particle counters give you a quick way to track down problems like this,” Felland says. “Particle counters also provide a good way to check the integrity of air filters — not how well they work, but if there is a hole in one or another.”

Felland recommends making HVAC measurements part of the regular maintenance routine and not a special auditing activity that tends to get put off. “Set a schedule that meets your needs based on your experience with your buildings,” Felland says. “If you check heating coils every six months and always have to spend a day and a half cleaning them in certain buildings, why not check the coils every three months? It’s also important to include measurements in the maintenance schedule so that you don’t let problems go for too long.”