Supplying Your Restaurant With A Sufficient Amount Of Return Air

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QUESTION FROM: Lisa in NJ

“In my new kitchen we replaced our ancient hood with a new system that has a short cycle make-up air unit.

I'm not familiar with them at all, but during my research when purchasing a hood I assumed it would take care of some of my HVAC issues.

The old hood system sucked all of the hot/cold air out if the building making it extremely cold in the winter and hot in the summer.

It was terribly drafty in the direct path of the hood suction area.

Anyway, now that we have had this new unit installed, I was expecting these issues to have greatly improved. However, after a few days of usage...it's no better than before.

The hood is creating so much pressure that it's preventing my swinging kitchen doors from closing.

The make-up air and vent are on separate switches. When I turn the make-up air on, I can reach up and barely feel any sort of air flow. I assumed it would be a much greater amount of air coming in- I have a 13' hood unit and it sucks a lot out.

Is it possible that maybe it's not installed properly? Or am I just not familiar with the units and that's all the make-up air that they produce?”


HH ANSWER:

A classic sign of an improperly balanced/negatively pressurized restaurant is when kitchen doors don’t close all the way and are “ leaning” Into the kitchen. If the doors aren’t closing all the way and leaning away from the kitchen, that’s a telltale sign that you have too much return air and/or a positive pressure situation on your hands. The key is to find a nice balance.

Although plugging up air leaks, properly insulating and generally “tightening up” buildings is a good way to make them more energy-efficient, builders and engineers need to remember that plugging air leaks makes it harder for air to enter a restaurant... and air entering your restaurant is a good thing.

Every time your kitchen exhaust removes air from your restaurant, an equal volume of air must enter or be “made-up”.

If your restaurant doesn’t have enough random air leaks around windows, doors, etc... make-up air can be pulled through water-heater flues or down wood-burning chimneys, or other vents to create a phenomenon called backdrafting.

Because the flue gases of combustion appliances can include carbon monoxide, backdrafting can be dangerous or even life-threatening.

So how do you get air into a building to replace the air a powerful hood system removes?

Overall, the restaurant should be positively pressurized in accordance with U.S. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (aka “ASHRAE”) standard 62.1.

Having too much supply or too much draw will result in positive or negative pressure and you really don’t want too much variation between the draw and supply.

Typical exhaust values for a commercial hood are around 100 CFM per square foot, but could be less or more depending on what equipment is below the hood.

Unless the building is as tight as a submarine, the supply fan should bring in about 80 to 90 percent of the air being exhausted AND, in order to prevent cold air being sucked out and replaced with warm air or vice versa, be conditioned .

More On That Here: https://www.contractingbusiness.com/archive/how-much-fresh-air-enough

If the kitchen area is being kept in a negative pressure status (to remove smoke and mitigate odors flooding the dining room/area) you’ll need the right amount of additional make-up air.
Too much supply (or make-ip air) will overload your AC units and too little will create negative pressure and drafts as described above.

The proper balance can be obtained by first bringing in a commercial HVAC team to evaluate the system in place (inspect the handler / blower compartment, AC coils, filters, assess the specific hood in use and check for return duct leaks) and/or what’s known as a “Test & Balance” firm. Most mechanical engineers or designers will be able to recommend one if you can’t find one.

They will measure the pressure in your space and with the expected occupancy, be able to determine the “occupancy ventilation rate” in order to supply the correct amount of outdoor air you need to flow inside and either adjust the exhaust fan’s speed/motor or determine alternate solutions that can include anything from appropriately sized wall penetrations / vents or adding additional return air inlets and ducts (in the kitchen) to increase airflow to the air handler to supplying more air via additional duct(s) that are activated by switch, automatically whenever the exhaust fan is activated or via “mixed mode”.

FOR MORE INFO, Check out these links:

https://www.trane.com/content/dam/Trane/Commercial/global/products-systems/education-training/engineers-newsletters/airside-design/admapn003en_0502.pdf


https://inspectapedia.com/aircond/Return-Air-Improvement.php


https://www.ac-heatingconnect.com/contractors/commercial-hvac-industry-glossary-of-terminology/