By: George Zawacki

Section 12 of the NFPA Handbook displays Figure 12.9.7.  It is located under the heading “Manifold Exhaust Duct System”. This diagram shows five hoods identified as System A through System E. All five hoods are shown connecting to a manifold through branch ducts. The manifold then connects to one common duct that eventually terminates at the exhaust fan. The diagram is an example, not a recommendation.

The 2003 ASHRAE Handbook devotes more space under the heading “Multiple-Hood Systems”. No diagram is included. The first sentences indicate that several design challenges are present for multiple hoods over a single hood system. Air balancing being the primary challenge and the point we wish to make. We do understand the constant challenges the architect has with cost. In some cases it may be exterior roof appearance but cost is always a factor when adding another shaft.

Engineers when designing a new commercial kitchen ventilation system are usually under considerable pressure from their architects to limit the number of stacks and chases to the roof. One is ideal, but too many hoods terminating in one shaft to one exhaust fan can be difficult to balance.

As the number of hoods increase, many factors come into play that make it very difficult to balance out a system and provide the required amount of exhaust air at each and every hood. Hoods further from the main exhaust stack may lack the volume of air necessary to properly operate. Hoods closer to the main stack or common duct may be prone to having too high an exhaust rate with resultant drafts being created that can be annoying to all. Another annoying problem can be the sound or noise level in the hood area within earshot.

The problem can be exaggerated when one large main stack is at or near the end of a number of hoods and draws too heavily from the closest, starving hoods further away. In addition, normal duct run situations such as a duct having to be redirected in the field due to other trades installing their equipment before the black iron is ready to go in adds static pressure that had not been taken into consideration during the mechanical engineer’s calculations.

You may be aware that balancing dampers are not permitted within the exhaust duct. However, many listed hood manufacturers have developed a listed adjustable panel built-in to the exhaust collar of their hoods. These are available as an extra, but too few designers and engineers seem to take advantage of the opportunity to specify them. In some difficult cases, they may be able to be retrofitted, but it is far more expensive than including this option at the very beginning for a very modest cost.

A great variety of grease filters exist with varying degrees of rated static pressure. Some are adjustable. However, we find that it is difficult to impossible to train employees to put back certain special filters in their correct location. When special filters are needed and they are reinstalled in the wrong location after washing, the entire balancing process is defeated.

Another method that may be employed is replacing a filter with a blank panel. That increases the static pressure in that particular hood causing the air stream to seek a lesser path of resistance. However, adding a blank panel in place of a filter changes the dynamics of that hood and may cause it to operate at less than optimum.

The point to remember is that as the number of hoods increases that feed to one stack, so do the balancing problems. We commonly see two, three and sometimes four hoods on one stack that operate and have balanced out very well. If everything has gone well with the engineering, the installation and the T&B, no particular challenges are incurred and each of the hoods operates properly.

The challenges grow exponentially as the designer works with several hoods and only one shaft to the roof. Just because four hoods work OK, putting seven, eight or more hoods into one duct or stack is definitely a challenge.

But we also see situations were the mechanical engineer decided only one duct is needed for multiple hoods. Perhaps the architect insisted he only had floor space above the kitchen in his multiple floor building for one shaft. Then when problems develop the usual scenario is all parties pointing at the hood for not working. No one is ever ready to take the blame when a particular hood fails to capture the smoke coming off a broiler full of fatty meat, ribs or chicken slathered with barbeque.

When multiple hoods with one stack and exhaust fan are the problem, solutions can be time consuming and expensive. That expense radiates out to all who are involved while the root cause is looked for. Everyone loses. Why? Because all parties get pulled into the resultant field visits, inspections, meetings, modifications and failed attempts to resolve the smoke issues.

Balancing multiple hoods properly is a challenge. We suggest adding a second or third stack or at least strongly considering the multiple hood issue before issuing final construction documents.

We strongly recommend that the foodservice design consultant or dealer designer discuss this issue with the architect and the mechanical engineer early in the project. There will probably be resistance but when the downside is understood, the final results with a trouble free installation will be more than worth it. Be proactive.