Utilizing Balancing Dampers to Reduce Night Air Infiltration from Exhaust Hoods

It is generally accepted that the cost of energy will continue to rise and some forecasters see no end in site. With this bleak forecast in mind, we have to take an extremely hard look at how we do business and our operating costs in order to make any profit. This holds true for any business, whether it is a school, a hospital, a country club or any B&I operation. One of the ways to reduce operating costs in new construction is with the LEED (Leadership in Energy and Environmental Design) program that builds more energy efficient buildings thus lowering energy usage (i.e. operating costs). Even though the buildings are more efficient, one of the weakest areas in this program has been the kitchen.

Kitchens use 5 times more energy than any other part of the building. Kitchens use more electric on demand, more gas on demand, more water, and more sewage. Kitchens affect waste management and overall HVAC costs of the building. More and more effort is being applied at evaluating these costs through such organizations as Foodservice Technology Center (http://fishnick.com/) and Energy Star. However, there is still more work that needs to be done. There has to be focus on the high-energy consumers, improve their efficiency and incorporate those improvements in the design specs. One of the biggest consumers of energy is still the exhaust hood. Even with the advancements of demand ventilation, total kitchen ventilation and displacement ventilation there is still a great big hole in the building roof that is being left unattended. It’s like leaving a window open in your home 24 hrs/day and 360days/yr. What is it? It’s the exhaust duct.

There are 960,000 foodservice kitchen operations in the United States (National Restaurant Association, 2011 ¹). It is estimated, that of those kitchens 17% (CBECS, 2003 ²) operated 24 hours a day and the remaining 83% turn off their exhaust hoods and shut down during the unoccupied hours at night. Why is this important? Because in 83% of the operations, outside air is infiltrating the buildings down through the exhaust duct and into the kitchens conditioned space. This is creating an additional load on the cooling and heating systems which requires them to run at night resulting in increased energy costs and an uncomfortable kitchen climate. During the winter and in northern climates where the temperatures drop well below 0° degrees at night the kitchens are freezing in the mornings and in the southern climates the kitchens are humid and musty. Until just recently nothing could be done about this dilemma, since fire codes for Type 1 grease hood applications prevent any type of fire dampers and back draft dampers from being installed in the exhaust duct work (NFPA 96).

In the middle of a difficult economy, the City of Akron, Ohio, home of the Inventor’s Hall of Fame, continues to reinvent itself. Nowhere is this more evident than in the string of new public schools that have been popping up all over town. These beautiful, state of the art, structures are the result of collaboration between the Akron Public School Board, Child Nutrition, and a group of talented Architectural Designers and engineers. While these new structures are striking on the outside, it is the cutting edge technology on the inside that makes these new schools a true testament to this innovative city.

On one of these projects Blasco Associates LLC, who is the foodservice consultant of the Akron Schools, was presented with an interesting design problem. The two Life Skills areas had six exhaust hoods each that needed to be ventilated and due to the building locations and restrictions each area had to operate on just one exhaust fan per area. The normal and accepted design criteria would be to throw a switch and all six hoods would come on regardless of how many hoods needed to be in use. The consultant, realizing that this was an inefficient design method, assembled the Halton design team headed by Matt Lewis of Thermal Products to assess the situation. The two worked together to enable the hoods to operate independently of one another. The hoods were fitted with UL710 listed automatic balancing dampers allowing each hood to function by itself. The dampers closed off the hoods not in use and a logic panel communicated to a variable frequency drive motor telling the exhaust fan how much air to exhaust. This design enabled the teachers to turn on any exhaust hood they desired for instruction, reduced the exhaust volume in the area, and eliminated the night infiltration of cold air in the rooms. Realizing that night infiltration is a serious problem in all kitchen projects, the consultant applied the UL710 automatic balancing dampers to the rest of the project.

The bottom line is that these motor driven dampers should be applied to the following exhaust hood applications, all Type 1, and hybrid Type 2 (Type 2 hoods with Type 1 ducting and fan). The energy savings are significant and quantifiable. Using the Akron, Ohio electric rates of 10¢/kWh with electric heat pumps, Akron Schools will be saving $9,619.00 dollars a year on energy costs with a return on investment of only 3.95 years and most importantly a comfortable working space.

According to Bill Blasco- “As far as I am concerned the School Board just hit the ventilation lotto! This is what we as consultants are supposed to do. Take new technology and apply it where it is needed. Doing things the same old way just doesn’t cut it anymore.”

Note: For supporting Data on the project please contact the writer.

The Kitchens of Today

We are constructing LEED buildings to last 70 to 100 years. We are building them, even though they cost more to build, and the reason is to lower the operating costs. However, we are not even looking at the operating costs of the kitchen, even though the kitchen uses 5 times more energy than any other part of the building. It uses more electric on demand, more water, and more HVAC/per square foot than any thing else.

Why are we sticking our heads in the sand when it comes to this area? Is it because we don’t know how to measure the energy use? Is it some enigma that mechanical doesn’t know how to deal with? No one has bothered to ask the question? Or are we operating under the idea that it has always been installed this way why change it? The answer is all of these. We are building a better building but then we take away all the advances we have made by not dealing with the kitchen.

Energy costs are skyrocketing and will continue to do so in the future. This is why LEED was instituted in the first place. However, what we are currently instituting is only the tip of the iceberg. Lets take a look at just one of the staggering usages of a kitchen. Do you realize that dishwashers use 125gal/ per hour of water to rinse dishes, that disposers use 8gal/ per minute of water down the drain lines, water cooled ice machines uses 100 to 150 gal/per 100 lbs of ice? That three-compartment sinks hold 120 gallons of water. It is mind-boggling.

In our heat load calculations are we taking into account the latent heat of the dishwasher, 110° degree water in the three compartment sinks, the added heat generated from self contained refrigeration systems both in the kitchen and out in the serving areas? How about the unchecked heat and moisture introduced into the kitchen by the roof top makeup air unit?

Yes, adding more Energy Star equipment will help conservation of energy as equipment is made available, however, what about the air quality of the kitchen, the safety of the kitchen, and the efficiency of the exhaust system? Lets look at some possible solutions for this area. Instead of turning the exhaust hood on at full capacity whether there is equipment operating or not, the better way would be to have a system that monitors the surface temperature and flue temperature of each piece of equipment, along with the exhaust duct temperature, the hood pressure differential and the room temperature. It would then increase the exhaust volume, from base line, as each piece of equipment is turned on and used and reduce the volume of air as the equipment is turned off and cools. This is called Demand Ventilation. This enhances the hood efficiency by as much as 50% with a return on investment of as little as 2 years.

What about the air quality in the kitchens? In the winter, when you first come into the kitchen are you freezing and in the summer is it hot and moist? Is the equipment cold to the touch or do you notice a musty smell? If so, it could be from your ventilation system. Since the fire codes prevent the installation of any back draft dampers on UL type 1 hood systems, in the winter, warm kitchen air is being drawn out through the exhaust duct to the outdoors, reducing kitchen temperatures significantly and the same scenario happens in the summer raising temperatures in the conditioned space. There has been no answer to this dilemma until recently. We are now able to utilize a UL listed hood accessory called an Automatic Balancing Damper. This piece is usually used to sync up multiple exhaust hoods to one exhaust fan but can also be used as a back draft damper, since it automatically closes when not in use. This takes a tremendous load off the A/C in the mornings to bring the kitchen temperatures up to normal. This single item is a significant advancement in energy conservation and will reduce A/C tonnage just by itself. This can be used on Type 2 applications with multiple hoods as well.

What about the Makeup air unit? The makeup air unit brings outside air into the kitchen to help balance the exhaust air taken out of the kitchen by the exhaust hood. The outside air is filtered but is usually left untreated, except for the winter where is it heated. Regardless of whether it is the summer or the winter the air is moist. Independent of the delivery method it has been proven, that the kWh usage is actually higher in a conventional exhaust system since the AC unit has to handle unintended loads generated by untreated make up air. When air is supplied outside the exhaust hood envelope, it will mix with the air in the kitchen thus affecting kitchen temperature and humidity. Moisture travels independently of the airflow and will migrate quickly throughout the space. The answer to these problems is called Total Kitchen Ventilation. This is a dedicated outdoor air unit that provides heating and cooling as well as dehumidification. This unit replaces the traditional makeup air unit in conjunction with the A/C unit reducing construction costs and eliminates simultaneous heating and cooling of the space. Automatic control of dehumidification functions remove excess outdoor air moisture before it enters the building protecting building components and again reducing A/C tonnage.

The final cog to this system is how the replacement air (makeup air) is delivered to the hood. It is universally accepted that an exhaust hood works more effectively as an exhaust only application, however, the additional tonnage required by the HVAC to makeup all the exhausted air makes this impractical. Thus the development of makeup air hoods and the gradual progression from front discharge, to short circuit, to the latest front air curtain. Again it is widely accepted that providing the makeup air away from the exhaust hood and allowing the air to mingle with room air and be drawn to the hood is the most efficient and effective method. However, until now there has not been a method to introduce this air without the HVAC providing the full load. This new method is called Displacement Ventilation. A series of low velocity perforated diffusers can be placed throughout the kitchen area, serving area and the seating areas to provide transfer air to the exhaust hood. The diffusers can be placed in kitchen corners, angled from the ceiling, along the walls or even at the floor area to provide the necessary transfer air. It especially works well in areas with high ceilings providing comfortable temperatures along the occupied zone. The diffusers are connected to the Total Kitchen HVAC unit with a series of flexible ducts providing heated/cooled and dehumidified air. This system has been proven to increase safety by accelerating the drying of wet floors and maximizing productivity by improving indoor environmental quality.

So there you have it, an exhaust system that is energy efficient in every way and solving the indoor air quality issues left unresolved over the years. The utilization of these exhaust system methods in your projects will provide significant operating cost savings that can be quantified. The savings will be ultimately large enough to petition for a variance from OSFC and require the school boards to vote this application in as an approved single line spec. In addition, the application of any or all of these methods qualifies for innovation credits for LEED projects. The credits and points fall under two of the six categories of the LEED rating system, namely: Energy and Atmosphere (EA) and Indoor Environmental Quality (EQ).