Great restaurant indoor environmental quality
With an energy penalty
For decades, I have been looking up from my restaurant seat to check out the HVAC system. It is a bad habit made worse by my tendency to give a free analysis of the observed HVAC System to my unfortunate dining companions. I think this is because my very first job after graduating from college was in the air distribution side of HVAC, that part of the air conditioning system - ducts and air outlets- seen inside every building. Indoor spaces need ventilation air and finding the best ways of delivering ventilation air was my first professional role in the industry.
I am currently seated at a favorite cafe in South Minneapolis, alone, so why not share my free analysis with readers? I took a picture of the cafe interior and included it below for reference. The first observation is of an unusual drip tray attached to the bottom of the supply air duct. The drip tray is a response to a problem, rather than an engineered design feature. That’s because ducts aren’t normally designed with drip trays, in fact, I have never seen this before. Ducts are expected to stay above dewpoint, so that humid air never condenses on the duct resulting in dripping. I’ve been in a few other restaurants where water was dripping from the ducts, but this is the first to address the problem with a drip tray attached to the bottom of the ducts.
The cafe is in a converted automobile shop. The garage doors have been upgraded, and they are opened when the weather is right. I am not sure what the criteria for opening the garage doors is, probably manager’s choice. But one thing I do know, I’ve been in this cafe when the doors are open and the HVAC is running. And I have enjoyed the vibe of sitting indoors listening to great music, eating a scone and occasionally feel a waft of fresh outdoor air.
The vibe is nice, but my pesky engineering intuition tells me that if the HVAC system is running, and the garage doors are open, energy is being consumed to nearly no benefit - a big opening like a garage door is an easy escape route for newly conditioned air to blow outside. Open doors are also an easy path for outdoor humidity to get inside. If the temperature of the duct is below the dewpoint of the air, moisture will form on the cold surface of the ducts just like it forms on a glass of cold water. When enough moisture forms on the duct, it will drip. This cafe owner has chosen to use drip trays to catch the drips before they hit the guests below. I don’t see any drainpipes on the trays, so it is possible that the drip trays can fill up and overflow. It is also possible for microbes to grow in any stagnant water in the drip tray.
An easier solution is to prevent condensation in the first place, by simply turning off the HVAC system whenever the garage doors are open, with the additional benefit of saving the cost of running the HVAC system. A sensor could be installed to signal the HVAC system to automatically turn off when the doors are open, but for some reason, the cafe has ended up without this sort of automation. It’s up to busy staff to remember to turn off the HVAC.
Of course, it’s more complicated than that. That scone I just enjoyed was baked in the kitchen. A wall separates the seating area and counter from the kitchen. A nice large window offers a view from guest tables into the kitchen, so I can see that the kitchen shares the HVAC system with the rest of the cafe. With stovetops, ovens and other equipment generating heat, it is safe to say that the kitchen always needs cooling, and the free cooling that is available to the guests via the garage door has no way to get into the kitchen. If I owned the cafe, and I wanted to lower my utility bills, I would pay to to design and install a control system so that the kitchen could be kept cool by the HVAC system, while the guests could enjoy the nice fresh air. That would solve the dripping duct problem too.
This cafe is owned by a successful restaurant owner who obviously knows how to draw a crowd of happy guests. The owner is probably happy to spend extra money on energy knowing guests enjoy the arrangement, and an easy solution has been found.
Clients invited me to dinner while I was in Phoenix not too long ago, and the outdoor air temperature exceeded 110F. Yes, it was a dry heat. This restaurant, like so many others in the desert southwest, was blowing jets of mist into the hot dry air. Thanks to the abundant dry heat, the mist evaporated quickly, absorbing enough heat to cool the air to more comfortable temperatures for those restaurant goers that prefer sitting outside (I was not one of those guests seeking outdoor seating.) Evaporative cooling is widely applied indoors and outdoors in this climate. Never mind that in Phoenix, water is scarce, and electricity is costly. Guests want a comfortable outdoor dining experience even when the temperature exceeds 110F. So, restaurateurs and others invest in conditioning the outdoors.
Evidence exists that restaurant owners aren’t too concerned by energy consumption. According to the U.S. Energy Information Administration, in 2018, the last year these statistics were compiled, “food service buildings were among the most energy-intensive commercial building types, using 263 MBtu per square foot. On average, commercial buildings used 70 MBtu per square foot. Also, food service buildings had the highest natural gas intensity (147.6 cubic feet per square foot), which was over four times higher than the natural gas intensity for all commercial buildings (32.7 cubic feet per square foot).1
This above-average energy consumption in restaurants has more to do with cooking and keeping kitchen air free of smoke and grease than it does with enhancing the vibe with open doors and misters. But conditioning the indoors with outside air while leaving the HVAC on or conditioning the outdoors with misters is an extreme measure of providing comfort that has an avoidable energy expense.
Although I don’t suggest readers develop the habit of evaluating the HVAC system of every food service establishment visited, I am sure that if you look around, you will find your own examples of restaurants using excess energy. If you do find examples, please feel free to share them here.
source: U.S. Energy Information Administration - EIA - Independent Statistics and Analysis (https://www.eia.gov/consumption/commercial/pba/food-service.php)