Ask the Expert Sept/Oct 2007 – Climate Control
 |
Eley |
Eley is an architect and mechanical engineer with nearly 25 years of experience in energy-efficient and sustainable design. He is the vice president of the Architectural Energy Corporation and serves as the technical editor for the CHPS Best Practices Manual on sustainable schools.
Q: I have a school project I’m working on that has a two-story classroom wing with windows facing east and west. The original building, constructed in 1959, had two-story curtainwalls on the east and west elevations. I want to return the building to its original design intent and provide daylight harvesting to the classrooms.
With a light shelf we can get up to 40 footcandles on the corridor wall desks with no glare near the window wall. My problem is convincing the school board to buy a light shelf. Do you know of any northeastern schools that get 75 inches of snow per winter and have a track record of using light shelves? The school board’s concerns are the structural snow loading on the light shelf, vandalism and attempts by students to climb on it.
Richard Pallone, AIA, LEED AP
Bell & Spina P.C.,
Syracuse , N.Y.
A: The impact of snow loads on light shelves can be minimized if the light shelf is protected by an overhang at the roof, however, snow can still accumulate when driven by the wind. Light snow on a light shelf will increase reflectance and improve performance, but heavy snow will simply block the aperture and the classrooms will receive no daylighting. In general, light shelves are not recommended when there can be significant snow accumulation. If a light shelf is used, you should make sure it is strong enough to carry the additional dead load and that when the snow melts it will not pond or create water pressure that could result in leakage through the wall or window. consider a louvered light shelf that allows the snow to fall through.
As an alternative to light shelves in snow country, you can consider interior mini blinds. Consider Alder Creek Elementary School in Tahoe, Calif., where the average snowfall is 216 inches a year, and the school district opted to avoid designing for the snow load and installed inverted mini blinds. The blinds are placed in between layers of window glass so they stay clean and are protected from vandalism. Blinds in clearstory windows are adjusted to bounce light onto the ceiling while the window blinds can control privacy, view and glare. This system does not collect dust, and it preserves views, has excellent SHGC and U-factor, and children cannot reach it.
Q: Do you think that integrating both stormwater management and energy conservation can generate greater savings and greener design? For example, in old buildings, roofs can be made green with pre-grown sedum panels to reduce the stormwater generation and energy use in the building by 35 percent. The roof life would be extended to more than 50 years. Does this concept make sense? Has this been done anywhere in the United States?
Ram Shrivastava
Larsen Engineers,
Rochester , N.Y.
A: In general, the more that building systems can be integrated, the better. You raise a lot of issues in your question. Let me try to address the issue of a vegetated roof.
This is somewhat personal and perhaps biased from living in California — where rains last about four to five months in the winter and the rest of the year is dry — but I have never been a fan of vegetated roofs. Their benefits are questionable because of added water consumption and additional structural load that requires more materials. If rainwater collection is the goal, a sloping metal roof would be preferable. If stormwater retention is the goal, swales, leaching fields and other techniques work well. From an energy perspective, a cool roof can extend the life of the membrane and reflect heat.
Q: Are there rule-of-thumb guidelines for calculating the payback period for a geothermal heating/cooling source without having to construct an energy model? If the estimated additional cost is $200,000 and the building is 35,000 square feet, including an 8,500-square-foot basement, and all other conditions are in the normal range, what might the payback period be?
Michael J. McKeon, AIA, LEED AP
Kaestle Boos Associates,
Foxborough , Mass.
A: The best way to assess the cost effectiveness of a ground source heat pump — I prefer this term to geothermal, which implies that the building is sitting on a geyser — is with an energy model. They are not that difficult to use and will better answer your questions.
On the cooling side, the benefits can be approximated by looking at the energy that would be used by the cooling tower and would be replaced by the ground source wells. On the heating side, shortcuts are more difficult, and this is where an energy model comes in handy.
Anytime you look at the cost effectiveness of an HVAC design option, it is necessary to determine the baseline. The most directly comparable system is a water source heat pump with a boiler and a cooling tower in the loop. The layout of heat pumps, ductwork and condenser water piping would be the same for both the basecase and ground source system. The differences would be that the basecase would have a boiler and a cooling tower that would be replaced by a system of heat exchanger wells. Other basecase systems can be used if they are common practice for your area.
In looking at cost effectiveness, it is important to consider differences in maintenance costs, as well as energy costs and initial construction costs. For this reason, comparing life-cycle cost is a better metric than simple payback.
Q: If our bond levy passes in November, our district is going to build a new 209,472-square-foot school for grades 6-12. We need the building to be as energy-efficient as possible, as operating costs for a new building are a concern within our community. What type of HVAC system would you recommend that is both energy-efficient and low-maintenance?
Kurt S. Kuffner
St. Marys City Schools,
St. Marys , Ohio
A: Climate is a factor in evaluating HVAC systems. There are a number of criteria to consider in determining the optimum HVAC system, including first cost, maintenance cost and durability, energy operating cost, acoustics, roof space required, thermal comfort, cooling effectiveness and heating effectiveness.
One of the systems that meets most of these criteria for Ohio schools is a displacement ventilation system, which provides air quality and cooling coupled with hydronic baseboards for heating. CHPS has guidelines for the design of this system in Volume II of the Best Practices Manual. The system is energy-efficient, quiet and has low maintenance costs. It can also have a low initial cost.