Summary: A subduct is a duct inside a vertical exhaust duct routed between floors of a building. The subduct replaces the role of a fire/smoke damper at much lower cost. This article presents a discussion of issure surrounding the design and installation of a subduct exhaust shaft.
I could write a small book about subducts, but here is the short version. You either got to this web page because you don’t know what a subduct is, or your got here because you do and you are looking for more information about them. For the rookie, a subduct is simply a trick of duct construction to avoid a fire/smoke damper in an exhaust duct routed between floors of a building. As we all know, any shaft up through a building must maintain the fire rating of the floor assembly, So if you have a single duct routed in a shaft and there is an opening at each floor, the fire rating must be maintained. Since fire smoke dampers are expensive, the code offers an alternative as follows: provide an inner duct boot with a minimum 22 inch upward extension inside the main duct and maintain a constant negative pressure on the duct. The idea is that fire and smoke will not climb down the boot if exhaust air is being drawn up the main duct. It seems reasonable and I’m sure it has been tested. So why is this worth a blog entry?
Here is a photo of a real subduct before installation.
The reason I am writing this entry is because the code has so much room for interpretation, and the methods of constructing a subduct can vary significantly, with vastly different performance and cost implications. Let’s start with the subduct construction. The best construction is a sheet metal main duct with sheet metal subducts. The cross sectional area of the main duct is sufficiently large so that the subduct does not excessively restrict the airflow from below. Since the airflow is greatest that the top of a building, the air speed at the top subduct is a critical design point. The speed of the airflow around the top subduct should not exceed 400 feet per second. One trick is to reduce the size of the subducts at the upper floors to allow more free area in the main duct at the top where the airflow is greatest. This also has the benefit of accomplishing some degree of self balancing since the static pressure is greatest at the upper floors. Regardless, balancing dampers are recommended at all floors. The bottom floor balancing damper may be omitted since the balancing process should start with that damper fully open anyway.
A common VE recommendation is to eliminate the sheet metal duct and utilize the fire rated shaft as the main duct. The subducts remain as sheet metal elements. There are two major issues with this approach. First, there is always concern that the shaft material will develop mold, especially with the moist air of the showers in a hotel. The counter argument to this issue is that mold-resistant paint can be applied to the inside of the shaft walls. Also, since the shaft is always in exhaust (note, subducts are not allowed for supply systems), any mold will be exhausted. Also, since the exhaust air is moving 24/7, there might be a slight moisture build-up for a few minutes, but the moisture will rapidly be removed by the constant airflow. This may be true for dryer climates, but I would have some concern in the humid areas of the country.
The second issue is the integrity of the shaft which is generally gypsum board. The construction challenge is to get the shaft sealed so it does not leak. One can argue that the leaks will simply come from the same place as the bath air, but that could potentially rob the lower floors of proper air exhaust. As for my opinion, I prefer to have a sheet metal duct liner. But in dry climates on hotels four floors or less, I allow the shaft to act as the duct.
The code also says that the airflow will be continuous in the subduct shaft. So what does continuous mean? First, we all agree it means the fan runs 24/7. But what about when there is a power failure? Some jurisdictions require subduct fans to be on emergency power. To take it a step further, some jurisdictions require airflow monitoriing with a trouble alert at the fire control panel.
Hotels with attics: This next comment is about subducts in regard to buildings with attics. With a flat roof, the subduct shaft terminates at a curb on the roof and the exhaust fan is installed at that curb. However, if there is an attic, where does the shaft terminate? It is not uncommon to see the shaft terminate at the upper floor ceiling with a fire damper installed at the attic floor. From that point unprotected ductwork is routed within the attic to a common exhaust fan. So here is my question: If the fire damper is activated, the exhaust air stops, is the subduct still code compliant? For this reason, I have always extended the subduct shaft through the attic to the exterior of the building. If you do not want to install the exhaust fans on the sloped roof, then simply install the exhaust fans inside the shaft in the attic with a fire rated access door. For the sake of the maintenance staff, remember to have the architect provide a catwalk to each fan.
Mushroom fans or Utility fans: There are two types of fans that can be used for a subduct exhaust. The first is a simple mushroom fan mounted on top of the roof curb. This is good if some type of acoustical treatment is included in the neck of the fan curb. A simple sound lined baffle is my favorite detail. Remember to have enough height on the curb that the subduct at the top floor bathroom does not stick up above the curb. The other type of fan is a small utility fan mounted a few feet away from the roof curb. This installation allows a horizontal section of ductwork which can contain vibration isolation and a sound trap. This is probably the best installation, but it costs more than a mushroom fan. And regarding direct drive vs. belt drive for the fans, I like belt drive for balancing, but direct drive is nice for maintenance. A typical hotel with belt drive exhaust fans generally has at least one exhaust stack not functioning due to a broken belt.
Tall Buildings: What about really tall buildings? A subduct cannot be balanced for more than about 20 floors. If the shaft is very tall, keep the shaft large and the airflow slow so it behaves as a plenum rather than a duct. If the building is more than 20 stories tall, install multiple sections of duct with no section of duct serving more than 20 stories. Another approach is to provide an exhaust fan in each guestroom as a “pusher” at each floor. This is a typcial Hyatt design standard. I have seen this done, but the code really does not address this approach. I can imagine a code reviewer arguing that the fan could force smoke from one floor to the next and defeat the subduct smoke control concept. Hyatt specifies that that pusher fan be relatively weak compared to the exhaust fan. So one could argue that the pusher fan would not push smoke from one guestroom to another. Since Hyatt does this as a standard approach, it appears plan reviewers are not chanllenging it.
I recently found a manufacturer of pre-fabricated subduct inserts that have a profile to minimize air resistance. See this product at http://www.subductriser.com/LSR060829.pdf