Determine HVAC Needs For A Monolithic Dome: An Engineer’s Advice
Skeptic Turns Believer
Gordon Cuthbertson, owner of Cuthbertson Mechanical Engineers, of Mesa, Arizona and Ontario, Canada, was a skeptic. When Gordon first got involved with Monolithic Domes about four years ago, he, like so many others, had a hard time accepting and believing what the Monolithic Dome Institute (MDI) says about the thermal mass capability of its structures.
“The numbers and the discussions that came from David South (Monolithic’s president) and his enthusiasm made me skeptical,” Gordon admits. “However, as David continued talking, I realized that he was talking from practical experience because he built them, he’s been able to monitor them, and he has that nontechnical, front line experience.”
Gradually, David’s anecdotal evidence, such as the story of the Snyder’s dome in -30 degree Alaska, that went unheated for two days with no noticeable temperature change on the inside, convinced Gordon. He became a believer.
“This information, though not documented, technically is still very valuable. I decided that if David is finding this out, I have to figure out why that’s happening, technically. And I found that all the knowledge, mechanically speaking, on heating and air conditioning a dome are all rules of thumb,” Gordon says.
Monolithic Domes Are Different
Nevertheless, determining the size of the HVAC system for a Monolithic Dome differs from the process used for conventional construction, Gordon insists.
He says that for conventional structures, HVAC size specifications are usually determined by consulting ASHRAE, standards published by the American Society of Heating, Refrigeration and Air Conditioning Engineers. Another alternative often used by contractors and builders is a fill-in form published by major HVAC manufacturers, such as Carrier or Trane. Such forms ask all the questions pertinent to conventional construction, such as outside and inside temperatures, wall materials and the insulation’s R-value.
“But,” Gordon says, “these forms don’t deal with thermal mass. So, they simply don’t work for Monolithic Domes because the dome’s concrete, its thermal mass, has the ability to store heat and give up that heat at a later time.”
Six Factors For Determining Adequate HVAC Design
To solve that problem, Gordon suggests gathering the following information:
- Hour-by-hour temperatures, beginning with midnight, for the coldest and the hottest days of the year. The best starting point for this information is www.noaa.gov, maintained by the National Oceanic & Atmospheric Administration. Other sources include community and state websites with weather data.
- Outside surface area of dome, available through MDI.
- Volume or amount of air space inside the dome, available through MDI.
- Weight of concrete in the dome shell, available through MDI.
- R-value of insulation, available through MDI.
- Occupancy and use of the structure.
Factors 3 and 6: Large Monolithic Domes
Factors 3 and 6 are of particular importance for large Monolithic Domes, such as churches, schools, sports and commercial facilities, because they have to do with the amount of fresh air a dome can hold, the number of people it accommodates, and the activities taking place. Because humans produce carbon dioxide as they breathe, ASHRAE standards state that you need 15 cfm (cubic feet per minute), per person, of outside air when a structure is occupied. But, again, ASHRAE standards suit conventional construction, not Monolithic Domes whose volume or capacity for holding fresh air often exceeds that of conventional structures.
Gordon gives Living Word Bible Church, a Monolithic Dome facility in Mesa, Arizona, as an example. He says, “Living Word holds 2000 people and we have the outside air dampers set at 15% of the supply air. That is well below the 15 cfm per person. But Living Word can hold an entire church service, and the carbon dioxide levels will never reach the maximum threshold set by the government. What we’re saying is that there’s so much volume in a dome that bringing in 15 cfm per person is over ventilating the space.”
View an article on design criteria for an HVAC system in large Monolithic Domes.
Factors 4 and 5: All Monolithic Domes
Factors 4 and 5 are important for all Monolithic Domes, big and small. The weight of the concrete, together with the R-value of the insulation blanketing it, determines the shell’s potential for storing and releasing heat. Overlooking the Monolithic Dome’s ability to do this usually results in overestimating HVAC needs.
In turn, that usually means buying a system larger than what the dome requires. But that’s just the beginning of a string of costly results that can follow. Gordon says that a too-large system tends to click on and off more frequently, instead of running smoothly till the desired temperature is reached and then turning off and staying off while the dome’s thermal mass maintains the temperature. A larger system also requires more energy and maintenance.
Note: This article was originally published in October 2002.