Confronting the Stack Effect
Building managers can save a lot of money by dealing with the flow of air through the structures they oversee. A key area of concern is the stack effect (also known as the chimney effect). It is a naturally occurring phenomenon that has the potential to create great efficiencies or, if ignored, literally send money up through the roof.
Unfortunately, it may be that not enough attention is being paid. “[The] stack effect has been well understood for decades,” Dr. John Straube, an associate professor at the University of Waterloo and co-author of a paper that covered the topic told Energy Manager Today. “Because it is being ignored we continue to do more and better research that quantitatively proves its impact. As we attempt to build (or retrofit) more energy efficient and airtight projects, the stack effect will matter more and more and will likely mean owner operators will be forced to pay more attention.”
Consultant Tim King does a good job of describing the chimney/stack effect at The Sunrise Guide, though the story focuses on houses and not buildings. The density and pressure of air differs according to its temperature. That is why hot air rises – it is less dense and is forced up by the colder air beneath. A fireplace and the chimney that expels the heat is an example. In that case, of course, the process is deliberate. That isn’t always the case, however:
Even if your home doesn’t have a fireplace, the same forces are still at work. Trouble is, unlike a fireplace, the chimney isn’t a single, easily identifiable chute. Instead, there are likely dozens of small, individual passages that are constantly pulling air through your house—from the basement to the attic – and back outside.
This uncontrolled and unmanaged process leads to waste and inefficiency. Last month, Green Lodging News reported on a study conducted by the Urban Green Council in partnership with Steven Winter Associates and the New York State Energy Research and Development Authority that looked at the impact of the chimney effect. The report found that buildings of 10 stories or more suffer the most impact from the effect if rooftop vents remain open. This allows the heat to flow unimpeded out of the top of the building.
The story says that the pressure – and thus the heat loss – is greater as the size of the building increases and as the wind outside the building accelerates. The study also points out that some cities require shaft vents to remain open as a fire precaution. While this is changing in New York City, the sense of the article is that it still is an issue. Steps can be taken, however, according to the story:
One solution is to cover two-thirds of the vent with annealed glass, and leave the rest open. The second solution is to install a motorized set of louvers that will remain completely closed until there is a fire, when the alarm system or a smoke detector will open them.
A link to the study, which is entitled “Spending Through the Roof,” is available at the Green Lodging News story.
The stack effect also is addressed in a long document from Rebuild America. Prepared by engineers from The Lawrence Livermore National Laboratory (LLNL), the theme is that many design and equipment processes can be introduced in apartment building retrofits that limit the stack effect. Since the inefficiencies are a factor of vertical air flow in structures, the best way to confront the problem simply is to obstruct that air movement:
By reducing vertical connections between floors (e.g., sealing where vertical plumbing cracks pass through floors and ceilings) it is possible to compartmentalize individual stories and minimize the stack effect.
There is a good deal of academic and industrial research that looks at how air flows through buildings. The paper co-written by Straube (along with RDH Building Engineering Ltd.’s Lorne Ricketts and Graham Finch) describes an in-depth case study of airflow through a building. The results confirm the idea that the stack effect is a key to efficiency:
The findings of this study clearly illustrate that stack effect and wind can frequently overwhelm pressure differences created by mechanical ventilation system resulting in unintended airflows into, out of, and within buildings. Furthermore, the airtightness of all building elements impact the distribution of pressure differences and consequently airflows. Overall, a holistic approach to building airflows should be taken when designing mechanical ventilation systems, including consideration of well understood airflow physics, to avoid designing poorly performing ventilation systems.
Clearly, there are myriad ways that air flows from one floor to another in a structure. Just as clearly, aggressively plugging these leaks will pays dividends. Easy gains can be made by learning from experience where these leaks likely will be and developing best practices to find and fix them.
Zehnder America’s Barry Stevens told Energy Manager Today that there is no single approach to reducing the stack effect — “It will differ,” he wrote – but there are some steps to consider. “The most common and impactful remedy is air sealing. Elevators are also a very significant contributor, and manufacturers are starting to address that. Building codes can limit remedies as well. There is still nothing like a good energy audit by a knowledgable auditor, who can identify the low hanging fruit of limiting stack effect and other factors.
The key is to pay attention. “This can become a big deal if you have a tall and leaky building,” Straube told Energy Manager Today. “Or it could be small, like 5 per cent of heating. It is often a bigger concern because of interfloor odour and pollutant transfer or top floors condensation issues and ground floor comfort complaints than energy.”
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