by Rob Falke
The next time you walk into a restaurant, stop for a moment when you’re opening the front door. Crack it open about an inch and feel the air being pulled into the restaurant from outside. Check for an overabundance of heat lamps at the staging area where the servers pick up the plates of cooked food. Then watch the steam travel off your meal towards the kitchen. These are all classic symptoms of kitchen exhaust and makeup air systems that are completely out of balance, and that means it’s a prime business opportunity for the savvy HVAC contractor.
It’s not uncommon to have 12,000 cfm of air moving through a 1,000 square foot restaurant kitchen. Typical makeup air units are short of airflow by 30%, causing the exhaust fan to steal 3,600 cfm from the dining room. You can imagine the effects of this scenario on comfort and energy costs.
Balancing kitchen exhaust systems is a complex procedure that requires more training and experience than you can get from one article. Even the best kitchen exhaust balancers in the country confess that it takes experience and commitment to master the art. But here’s an overview to get you started, or to add to what you’re already doing.
Here’s a short list of what we usually find in restaurants because of out-of-balance kitchen exhaust systems:
- Extreme discomfort. Yet no one knows who to call or what to do. Airflows are invisible, and loads and temperatures are constantly changing throughout the day
- Super high utility bills. However, it’s difficult to trace the cost back to the HVAC systems, unless the systems are tested and diagnosed.
- Poor food quality. Uneven cooking surface temperatures, excessive grease accumulation, high velocity over food staging areas, irritable cooks due to discomfort, and shortened food life from poor open storage conditions are the culprits here.
- High employee turnover. They can’t stand the heat, so they find a more comfortable kitchen.
- Loss of business. Poor food quality and an uncomfortable dining room mean customers just don’t come back.
Kitchen Exhaust Components
The kitchen hood captures smoke, grease, and other particles, mixed with air, above the kitchen appliance. An exhaust fan pulls the air from the hood through an exhaust duct. The amount of air exhausted must be replaced by air brought into the building from a make-up air unit, or through the air conditioning or heating system.
There are also diffusers, filters, and controls that complete the system. The purpose of these components is to move proper and equal amounts of exhaust and makeup air in and out of the building, to carry out unwanted heat, grease, smoke, and steam.
When all these systems and components operate properly, the “typical symptoms” listed above are replaced with comfort, reduced operating cost, good food, happy employees, and prosperity.
Exhaust And Hood Design Requirements
The requirements for manufacturing, installation, and balancing of kitchen exhaust systems may vary depending on the location of the project and local jurisdiction. Some states, such as California, still use the Uniform Mechanical Code, while most other states and counties have adopted the International Mechanical Code. The requirements differ, so check with local building officials. Always read airflow and velocity requirements from a set of approved plans.
Remember, there are two types of kitchen hoods. Type One handles grease and smoke. Type One hoods require more airflow than Type Two hoods that handle only heat and steam.
Air velocity in grease ducts is one issue currently under fire. It’s expected that most jurisdictions will soon follow the NFPA 96 Section 220.127.116.11 version, which only requires velocity in grease ducts to exceed 500 ft./min. The Uniform Mechanical Code, on the other hand, requires 1,500 to 2,500 ft./min.
Some areas of the country require hood airflow to be 100 cfm/sq.ft. of hood opening. However, most hood manufacturers now have their hoods UL tested and rated for specific airflow per linear foot. This often lowers the required airflow to less than 50 cfm/sq.ft.
You should be safe if you simply read the hood’s required airflow from the manufacturer’s engineering data, or from the nameplate data, and balance the system to match what is specified.
Makeup Air Requirements
Makeup air delivery is critical to the performance of the hood. Before determining the amount of makeup air necessary, you must first calculate the required fresh air needed in the restaurant. Take the maximum seating capacity of the restaurant, multiply that by 70%, then multiply that number by 20 cfm per person. This is the amount of fresh air that should be brought into the restaurant through the HVAC equipment.
Here’s a rule-of-thumb to calculate the makeup air cfm: Take the hood exhaust cfm, subtract the required fresh air cfm, then add bathroom exhaust cfm and about 250 cfm building pressurization airflow for a typical sized restaurant.
Have the system cleaned. If the hood and grease ducts have been in use, require the restaurant owner to have the system cleaned before you begin testing. A greasy system is very unpleasant to work on, and your odds of a successful balance are reduced. The system should be balanced with the appliances below the hood turned on, so the temperature of the air is near what it will be under actual operating conditions.
Gather and record nameplate data. Record design information from the plans, and then collect equipment and motor information from the field to be sure the proper equipment was installed. Be sure to measure and record the hood size, grease filter size, and the exhaust duct dimensions. List all required airflows. Make a sketch of the floor plan, hoods, grease filters, registers, grilles, and equipment locations.
Visual inspection and start-up. Check that all ductwork is connected, filters and controls are installed, and that all grilles and registers are in place with dampers open. The system should have been previously started up and checked out. Drill static pressure and temperature test holes. Start-up the exhaust and makeup air systems.
Initial testing. Before measuring airflow through the equipment, measure building pressures. Be sure all systems — kitchen exhaust, makeup air, heating, cooling, and bathroom exhaust fans — are operating when this test is performed. Attach a 25-foot length of tubing to a low-pressure manometer, and place the tube in a protected area outside of the restaurant. Be sure not to pinch off the tubing. The open port will read the pressure in the restaurant. Read the difference between the indoor pressure and outdoor pressure on the manometer.
Building pressure reveals the sum of all the airflows. A positive pressure means the total makeup air exceeds the total exhaust air. The goal is for the restaurant to have a positive pressure of .02-in. to .04-in. w.c.
Measure make-up airflow. If the air is delivered through registers and the airflow is within the capacity of your air balancing hood, measure each of the registers and add them together to find the total airflow for the makeup air. Often, the air is delivered through the kitchen hood or through a high capacity registers. These must be measured using an airflow traverse at the register, or, better yet, in the duct delivering airflow to the register.
Part of measuring makeup air is documenting the amount of fresh or outside air entering the air conditioning and heating systems. This can be done by traversing fresh air inlets or by using temperature calculations.
Measure the exhaust hood airflow. Exhaust hood airflow measurement requires practice and a touch of art. You need to compare the measurements you’ll take here to other system values that you’ll take during final testing to verify that your airflow reading is correct.
Average Velocity X Filter Area X Correction Factor = cfm.
Add the cfm through each of the grilles together to determine exhaust airflow through the hood.
Another option is to measure the pressure drops through each grease filter and interpret its airflow. Compare your pressure drop reading on each grease filter to the pressure drop/cfm data provided by the grease filter manufacturer to determine the filter’s cfm.
New methods of measuring airflow offered by select manufacturers include UL-listed, calibrated volume dampers and airflow measuring pressure ports that make reading airflow a breeze (pun intended). These are valuable new inventions that significantly increase the reliability and accuracy of reading kitchen hood airflows.
An airflow traverse in the grease duct may seem to be the ideal way to measure exhaust air from a kitchen hood, but be careful. Type One hoods require a minimum 16 gauge welded grease duct. Many states require that these be wrapped in a fire retardant jacket, or enclosed in a sheetrock chase. This is a fire-rated shaft that must not be penetrated. If the duct is so enclosed, a traverse isn’t possible.
To calculate the velocity in the grease duct, divide the hood cfm by the area of the duct in square feet (Velocity = cfm/Area of the Duct).
Contact local code officials to determine what velocity is required in your system.
Final equipment testing. Once you’ve achieved required airflow through the exhaust and makeup air systems, you must measure pressure, electrical, and rpm values at each piece of equipment. This is done to verify the equipment is operating properly and within specifications.
Pressure is taken by drilling test holes in ductwork and fan housings. Remember, the grease duct can’t be penetrated for a pressure test. Try a 18-in. piece of stainless steel or copper tubing slid between the exhaust fan and the curb attached to your manometer. This method works especially well if the fan is hinged.
Measure electrical values on each motor including voltage and amperage. Compare the amp reading to the FLA listed on the motor nameplate to verify the motor isn’t running over full load amps.
Measure motor and fan speeds. Use this information with horsepower and the static pressure readings to plot airflow on the manufacturer’s fan performance chart to verify your airflow readings. Record pulley and belt sizes and model numbers.
Complete the balancing. There are a few final tests you can perform that will make a big difference in system performance.
Smoke testing using titanium tetrachloride, incense, or a match makes air visible and allows you to see the air patterns entering and exiting the kitchen hood.
Watch air patterns from nearby make-up air registers and adjust flow patterns as needed so they don’t disturb the exhaust air patterns. There are times additional airflow is necessary to ensure adequate performance. Remember, grease filters will load up and reduce airflow, so if the system isn’t running well now, it never will.
Check the control settings and sequence. All to often, we find the make-up air unit heating because it’s cold outside, while the cooling system is running in the kitchen because it’s too hot indoors. Check settings and take the time to educate the system owners and operators on how to use the kitchen ventilation system.
Check for a clear path for the fresh air to pass from the HVAC systems in the dining room into the exhaust hood. Building pressures can change drastically if airways become obstructed. Also check carbon monoxide levels in the kitchen under all possible conditions to verify the negative pressure from the exhaust fan won’t pull combustion gasses from the appliances or flues.
Educate your customers about similar hazards they may encounter if the system is used improperly, such as shutting down the make-up air unit on a cold day.
Complete a report for your customer, along with any comments or recommendations that may be appropriate. Include traverse forms, instrument calibration records, and copies of your certifications or information on your company.
The Importance of Assurance
Air balancing and diagnosing kitchen exhaust and other HVAC systems assures you and your customer that the system is performing as it should. Both of you can know the job has been done right. Air is invisible. Measurement is the only way to verify that a system is working.
Testing is the next level in the advancement of the HVAC industry. It can provide what our customers want, and get us the profit we need.
Rob Falke is president of National Comfort Institute (NCI) a national training company providing more than 100 public and private technical and sales seminars annually for the HVAC industry. Rob can be reached by phone at 800/633-7058 and by e-mail at [email protected]. Visit the NCI website at www.ncinstitute.com.
Call National Comfort Institute for a no cost Kitchen Exhaust Balancing Report and companion balancing procedure in Microsoft Excel® format.