Airflow imbalances can affect restaurants, homes

A retired HVAC engineer walked into an Italian restaurant and immediately knew something was wrong.The restaurant owner welcomed his family, guided them to a table and apologized for the unusually warm dining area explaining their air conditioning systems were malfunctioning. It was 101 degrees outside, but the engineer knew from decades of experience, that the problem was likely not their AC, because it was hard to open the front door.After enjoying a fine pizza dinner, he mentioned to their waitress that he knew what was likely causing their AC problem after which the owner soon showed up at their table to learn more.She had spent thousands on various AC contractors with no improvement, so she was very interested to hear what this older gentleman had to say. The engineer demonstrated how difficult it was to open the front door and how forcefully air rushed in when the door was cracked open. He explained that this indicated their kitchen had an imbalance of exhaust and make-up air."Exhaust air" is air drawn from the restaurant, typically through a hood, to capture heat and fumes from pizza ovens and cooking surfaces. "Make-up air" is outdoor air piped into the kitchen to replace the exhaust air. Generally, fan-induced make-up air should be about 80% of exhaust air. Too little make-up air likely caused the restaurant's negative pressure. That vacuum pressed the front door closed and drew hot, humid outdoor air through every opening and crack in the exterior envelope – walls, ceiling, doors, windows. With 101-degree humid air being drawn in, it's little wonder the AC couldn't keep up.This restaurant's kitchen is a show piece, open to the restaurant, so they couldn't simply open the back door (as many restaurants do) to allow passive make-up air in.The engineer decided to speak up because this problem of air imbalance is expensive and potentially dangerous. Uncomfortable conditions hurt business. Needless carbon dioxide and other pollution damage the Earth. Could this drive her out of business from lost customers and excessive electricity bills? In his research, he's seen restaurants where air imbalance caused flames to shoot out the bottom of a gas water heater due to negative pressure drawing air down its exhaust vent. Also, hot moist air drawn in through relatively cool exterior wall cavities can cause moisture and mold build up.Our homes can also suffer airflow imbalance. If return-air vents are primarily in the central zone, then when the system is operating, closed rooms develop positive pressure which pushes air out of the house while the depressurized central zone draws outdoor air in, impacting indoor humidity and wasting energy.The fix? Install return ducts, transfer ducts or transfer grills to allow airflow between zones. The International Code Council recommends return vents be 1.5 times the size of the supply ducts serving a space.The retired HVAC engineer would be pleased to see you more comfortable while saving money and our planet.•••Rebecca Cummings is a retired engineer living in Unicoi who, along with her husband of 40 years, enjoys exploring the beauty of the mountains, streams and waterfalls. Her deep Scottish heritage feeds a strong drive to conserve money and resources and to protect the area's beautiful environment for future generations. She writes her "Conservation in Mind" column two times a month.