For years, HVAC engineers have been trained to provide “reactive” designs. After the architect determines a building's layout, the HVAC engineer designs a comfort system to fit. The result has been that the HVAC engineer has had little opportunity to influence overall building design to reduce HVAC and energy requirements and increase indoor comfort.

This process has to change if the HVAC building-design industry has any hope of creating a real collaborative integrated design process (IDP), by which heating and cooling loads are lowered as far as possible before passive systems that use the natural local energy around a building (i.e., natural ventilation, daylighting, passive solar heating, solar-gain control, etc.) are designed. HVAC engineers/designers must understand how building skins and passive systems work from the beginning. Also, they must be trained on the IDP, which some feel is an architect's responsibility. (For more on integrated design, see the author's article “High-Performance Buildings Through Integrated Design” in the February 2007 issue of HPAC Engineering.)

THE EDUCATION GAP

One of the biggest challenges facing the building-design industry is the lack of practical building-science/physics education aimed at mechanical engineers, HVAC designers, and some architects. Many advanced building-science courses are available to architects, but virtually no university-level core courses that focus on building-systems design and building science exist for engineers. There are various technical-school/college courses and programs that teach HVAC designers how HVAC systems work, but these classes do not educate beyond rudimentary heating and cooling calculations used to size HVAC systems and plants.

Few HVAC-design engineers have taken a formal university-level course on building science/physics and HVAC design. Additionally, few university-level engineering courses in North America focus on building-system design. Those that are provided are offered as third- or fourth-year “elective”courses. Therefore, many of the people who design the building systems that utilize more than 40 percent of the total energy used in North America have learned their trade through “on-the-job training” alone.

Technical schools provide graduates who are able to size pipes, ducts, and basic “all-air” heating and cooling systems and the science behind them, but are relatively uneducated on the harvesting of natural energy, other types of HVAC systems, building science, building-skin thermal performance, and the life-cycle cost of building systems. HVAC engineers/designers must be able to articulate the consequences of design decisions regarding building-occupant comfort, mechanical-plant first costs, ongoing operation for the life of a building, and environmental impacts. HVAC engineers/designers must attend to this as early as possible in a project's design process.

For the IDP to work properly and result in a truly high-performance, comfortable, low-energy building, entire design teams must understand building physics and passive building design before applying high-tech solutions. The next trick is to model building energy and envelope performance accurately and determine capital-cost trade-offs and life-cycle costs of various building-system options. This requires more effort during the schematic-design and design-development stages, before construction documents are started. The traditional engineering/design fee-management process does not allow for this level of up-front effort.

LOAD CALCULATIONS

A recent study¹ focused on architects' building-envelope details and their impact on HVAC designers' heating- and cooling-load calculations. Four identically insulated and glazed building models were set up with R-19 wall insulation, but with variations in window frames, wall detailing, and wall construction. The models had the same window areas and center-of-glass thermal performance. The overall wall thermal resistance varied from a low of R-4.8 to R-12.6. (This included adding the R values of the glazing and frames to the wall thermal resistance to get an “overall” R value).

Thermal bridging caused by accepting cheaper/faster construction products and details can degrade wall and roof thermal transmission by more than 25 percent below insulation values. For example, R-20 wall insulation, coupled with poor thermal-bridging details, may provide an actual R-13 to R-14 thermal-transmission barrier.

Three key questions need to be asked when HVAC loads are first calculated: Has thermal bridging been accounted for in the wall and roof R values? Has overall glazing performance been used? What do the architect's envelope air-infiltration-control and performance specifications look like?

After a client's goals are set, one of the first steps that must occur in the IDP is to generate a building energy model using a sophisticated software package. This will give the design team a chance to apply some basic building-envelope and orientation configurations and passive-design elements to reduce building energy loads. This is the point at which daylighting, natural-ventilation paths, and solar-load control designs can be tested and defined.

Once a building's design, shape, size, and general envelope elements are determined, envelope details can be modeled using software to minimize thermal bridging and optimize practical cost-effective window-framing and wall-, and roof-construction details. (Lawrence Berkeley National Laboratory's Therm software is available as a free download at http://windows.lbl.gov/software/therm/therm.html.) This helps reduce building thermal loads and provides accurate thermal-performance values for comfort-system load calculations and designs. Also, it can allow the consideration of radiant-heating and cooling loads inside of a building's perimeter zones and the proper application of ANSI/ASHRAE Standard 55, Thermal Environmental Conditions for Human Occupancy.