Specifying Short-Circuit Hoods

What You Get for Free When Specifying Short-Circuit Exhaust Hoods

KitchenVentilation.com is pleased to have guest Blogger, Steve Brown, Member of ASHRAE provide his take on the origins of short-circuit exhaust hoods.

The history of short-circuit exhaust hoods.

The price controls and energy shortages of the 1970’S “Dark Decade for Energy” began in 1954 with the United States Supreme Court’s decision in “Phillips Petroleum Co. v. Wisconsin, which resulted in regulation and price control of natural gas sold outside the state in which it was produced. The implementation of the regulations was so slow that, by 1970, most areas of the U.S. were limited to pricing natural gas at 1950s levels. According to the National Gas Suppliers Association, “Three eras of producer regulation ensued, each with its own difficulties until finally, wellhead price control culminated in the natural gas shortages of the 1970s.”

In her book “Panic at the Pump,” historian Meg Jacobs asserts that the OPEC oil embargo of 1973 created the shortages. But in his 2016 critique of her book, Institute for Energy Research founder Robert Bradley, Jr. correctly asserts that (among other internal issues) it was price controls that led to the shortages. History has shown the 1954 price controls and regulations caused the 1970’s natural gas shortages, while the oil embargo only exacerbated the shortages.

These shortages so limited natural gas supply that, in many areas, it was no longer available for traditional heating purposes, including commercial kitchen MUA units. At the very time industry was struggling to heat outdoor air NFPA Standard 96 introduced new mandatory exhaust rates, with no supporting research or test methodology, which resulted in extreme increases in the hood exhaust and resulting MUA airflows. Unfortunately, model mechanical codes, such as BOCA (Building Officials and Code Administrators), also without research or test data, adopted the new Standard 96 exhaust requirements. An example of the magnitude of the change would compare the Veterans Administration’s 250 FPM perimeter slot velocity formula, in use at the time requiring 3,000 CFM exhaust for a 10’ long x 10’ wide double-island hood to the new NFPA and Code exhaust airflow formula (Q = 150A or 150 CFM per square foot of face area) requiring the exhaust rate for that same 10’ x 10’ double-island hood to be increased five-fold to 15,000 CFM.

As a result, the 1970s designers of kitchen hood systems were facing up to five-fold increases in code-required hood exhaust airflow, while struggling with price-controlled natural gas that was, in many areas, not available leaving many areas with electricity as the only, excluding heat recovery, heating option. Everyone was desperate for a code-approved solution! Did it have to work? Well, at that point nothing was working very well, so any solution that was code compliant was worth a try. One of those solutions offered was the short-circuit exhaust hood.

CKV system 80/20 design concept

In the airflow example above, 15,000 CFM minus 3,000 CFM resulted in the goal for the new hood design to introduce 12,000 CFM of “free”, or unheated, outdoor air directly into the hood interior, thus short-circuiting the excessive exhaust requirements. 15k CFM at 100%, 12k CFM at 80% and the originally required 3k CFM at 20% resulted in no change to the HVAC systems that supplied the original 3,000 CFM. It also created a new stand-alone CKV system 80/20 design concept. A stand-alone hood system requiring no additional HVAC outdoor air.

A more complete description of the operation of the short-circuit exhaust hood is available in the Air Distribution section of ASHRAE Applications Handbook Chapter 34 “Kitchen Ventilation” – Internal Makeup Air (aka short-circuit).

The 1970s also introduced the “Classification” of CKV hood construction and performance by UL under the UL test procedure “Grease Extractors for Exhaust Ducts,” which resulted in hoods being “classified in accordance with NFPA Standard 96.” This “PROCEDURE,” (UL terminology) was used primarily as a method of test for alternate hood construction methods, including assembly by means other than fully welding and/or using lighter gauge materials. The UL “PROCEDURE” also included a new visual hood capture test.

This “PROCEDURE” would align with NFPA when, in response to negative public response, NFPA quickly (in code time) revised Standard 96 to delete mandatory airflow rates and required instead airflow rates “adequate to provide for capture and removal of grease-laden cooking vapors.” Model codes did not follow NFPA’s revision and would continue to require the excessive exhaust airflows for decades.

Most hoods tested for compliance with the NFPA capture test attempted to lower the exhaust rates as much as possible. Unfortunately, the visual capture test method used to develop “Classified” airflows was lax and easily manipulated by methods such as custom configuring one heating device to create the required cooking surface temperature while producing artificially low-intensity cooking plumes and optimizing test stand size and configuration. Also, “creative” airflow measuring methods were often used. Decades later ASHRAE research project 1376 would validate that many hand-held instruments used by UL and manufacturers during the 1970’s testing tended to under-represent exhaust airflow and overrepresent MUA airflow. 

The designers of many short-circuit exhaust hoods unknowingly used these measurement discrepancies along with additional known manipulations such as controlling MUA at an optimum delivery temperature to improve airflow ratings. Even though “Classified” exhaust rates for exhaust only hoods were often 50% lower than the exhaust rate for the same hood with internal short-circuit MUA the concept of the stand-alone 80/20 hood system that required no heated MUA unit was a powerful sales message.

Unfortunately, when these classified airflow rates were applied in the field to exhaust a full complement of actual cooking appliances and introduce outdoor at ambient conditions they failed. To overcome these failures field start-up techniques often included increasing exhaust above design and reducing MUA airflows below design until hood capture was achieved. These changes to the “free” MUA airflow required expensive increases to the outdoor air supplied through the building HVAC units. The changes often resulted in buildings that were extremely negative, cold in winter, hot and humid in summer and consuming considerably more HVAC energy than they were designed to.

The discrepancies between model codes and “Classified” hood airflows, construction methods, and materials created considerable AHJ pushback to the rollout of “Classified” hood products. AHJ’s saw many of the field modifications that were attempted but at this time, few AHJ’s required a certified hood system air balance and no industry-standard method of field-testing kitchen hood capture existed, which resulted in AHJs reluctantly approving many field-installed systems that would eventually fail to perform.

Eventually, some code jurisdictions either would no longer allow the installation of short-circuit exhaust hoods or some jurisdictions, such as the State of Michigan, required special testing of field-installed hood systems to verify performance.

Validating the performance

Decades of poor performance eventually led to laboratory research designed to validate the performance of hood exhaust and MUA systems including the short-circuit MUA design. “Makeup Air Effects on Commercial Kitchen Exhaust System Performance” research reported an average of only 11% to 14% internal makeup air could be introduced when exhausting under-fired broilers into hoods operating at “threshold” exhaust rates. Consistent with that finding, there are manufacturer designs that introduce room air internally to the hood to improve capture and containment but this airflow is limited to approximately 10% of hood exhaust airflow. In addition, PIER funded research, conducted by Architectural Energy Corp. and Fisher Nickell, Inc., also found, over some appliances, an increase of one CFM of internal MUA actually required an increase of “greater than” one CFM of exhaust air. The 1970s concept of using internal “short-circuit” hood MUA “free” of conditioning was proven to not only not be free but to actually be quite expensive. This research led to the clearly stated summary of PIER Design Guide 2 “Optimizing Makeup Air” – “Don’t use short-circuit hoods.”

At the peak of the U.S. natural gas shortages, Congress enacted the Natural Gas Policy Act of 1978, which began the deconstruction of regulations and price controls. In 1989, this deconstruction was completed by the Natural Gas Wellhead Decontrol Act. Wellhead prices were deregulated, and the decades of natural gas shortages ended. But unfortunately, the national concern about future energy shortages and misguided energy savings methods, such as the short-circuit exhaust hood, did not end with energy deregulation.

So, what do you get for free when specifying short-circuit hoods? Well, to quote the brilliant scientist Mr. Willy Wonka (Gene Wilder) in Willy Wonka & The Chocolate Factory, “It’s all there in black and white, clear as crystal … you get nothing. You Lose! Good Day Sir!”

Published by
Steve Brown, Member of ASHRAE

Have questions? Ask the expert or leave us your comments below.

See another article relating to Make-Up Air – How not to ruin a good thing, Make-up Air 101 

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