Design Criteria for HVAC in the Monolithic Dome
Monolithic Dome = Large, Thermal Storage
To the HVAC engineer, the Monolithic Dome presents some serious challenges. The number one challenge has to do with recognizing and understanding the thermodynamics of the Monolithic Dome. Unlike any structure built in the conventional world, the Monolithic dome is a very large thermal storage.
Properly designed, the equipment needed for the HVAC will be reduced by 60 to 75 per cent. This reduces the required electrical capacity significantly. Minimal heating is required and is generally provided by the crowds and the lights. When the HVAC is improperly designed, the customer gets “sand bagged” by the power bills. A very large church built here in the South has a “conventionally” designed HVAC system. In all but a couple of months, the demand charge is larger than the run charge for their electric power bill. This is not acceptable.
First and foremost all HVAC designs must include the thermal storage of the Monolithic Dome:
- The dome is insulated on the exterior of its shell. This creates a huge thermal mass that is an integral part of the heating and cooling system of the structure. This integration is similar to that of an ice storage in a conventional building. Each pound of concrete stores approximately one BTU for a five degree temperature range. A large Monolithic Dome weighs millions of pounds; therefore it can store and release millions of BTUs of heat. A fifty-foot Monolithic dome-home weighs approximately 200,000 pounds. A church with a 200-foot diameter weighs approximately 2.5 million pounds.
- Codes are designed to prevent sick building syndrome. Nobody wants people housed within their structures to become sick from air quality. But codes also are designed to allow the engineer to consider what is appropriate within the structure.
- At times the air outside of a building can be dirtier and sicker than air within a structure. During Ozone alerts, we are cautioned to keep children and those with lung problems inside. During those periods we may want to limit the “fresh” air intake.
- Fresh air requirements are tied to the number of occupants. That means that if you have a huge gymnasium or a sanctuary with only the janitor inside, there is no need to be drawing in millions of cubic feet of air for him to breathe. On the other hand, there is need for enormous quantities of air to be brought into the structure with a crowd and standing-room only.
- A HVAC engineer should separate the heating/cooling system from the fresh air system. Sometimes the same equipment can be used for fresh air, but the control system must be designed for the separation.
- Fresh air intake requirement should be engineered in such a way as to allow this requirement to be adjusted for number of occupants. A large sanctuary seldom has a full house. It has a partially full occupancy at times but is often used with a fraction of the available occupancy. The fresh air system should be so designed to take all of these conditions into account.
- In many parts of the country and at many times of the year, it is possible to cool the thermal mass of the shell by utilizing fresh air. Therefore the control system must be designed to pull in maximum amounts of fresh air to accomplish this task. In other words, if it is in the spring and the outside air temperature drops below the inside air temperature at night, cooling can be accomplished by using copious amounts of fresh air to cool the shell. In the heat of the day, this “cool” can be used to cool a crowd within the structure, without need for cooling other than possibly for dehumidification.
- There are times when the dehumidification needs to be disconnected from the heating and cooling cycle. In other words both need to be run to dehumidify (generally in one hour segments).
Some guidelines that should be considered:
- Air movement within the Monolithic Dome is important. Rarely should it be turned off. A continuous air movement is needed to load and unload the heat into the shell. The air movement should be of a volume about half equivalent to what is utilized in a conventional building under full load. Air should not be cut off from the empty spaces as they are part of the system. Monolithic Domes have a lot of space. There is no need to “push” the air fast or hard. Let the building move a lot of the air. It will, as it is a natural thermal siphon for the air flow.
- Except for small spaces, such as offices, the entire building should be kept at the same temperature at all times. It is not practical or even possible to change the temperature in a Monolithic Dome quickly. It will hold its temperature over long periods of time. Remember, the Monolithic Dome is a thermal storage. Air must have free access to the dome to allow the storage of heat. Never isolate the Monolithic Dome shell from the air flow, neither by ceilings, nor by acoustical insulation, nor by ducting. Areas above ceilings should be used as a plenum to increase the contact of the air to the dome.
- The primary temperature to control is the temperature of the concrete. It is the reservoir that the interior heat is drawn from. In the summer, it may be well to keep it at 68 degrees and in winter at 73 degrees. Even that must be tempered by crowd size. Most people are very comfortable in meetings and assemblies between the 68 to 73 degree mark. Therefore managing the temperature between these temperatures is most desirable.
- Fresh air should be introduced into the Monolithic Dome as needed and as measured by a CO2 sensor. In general, codes call for 15 to 20 cfm per person of fresh air to be introduced into the building during occupancy. Where smoke is not a factor, the requirement can generally be reduced to 7.5 cfm per person. Current practices are to size a system to the maximum required and then use it at all times. This is extremely inefficient. There is no reason to bring millions of cubic feet of air into a large sanctuary while the custodians clean it – nor that same amount of air for a small crowd. This is not practical, nor will it work very well in a Monolithic Dome.
- The very large volume of a Monolithic Dome allows for air dilution for a considerable time period. Until the air is “tainted,” outside air is not needed. CO2 is to be measured to determine when the “tainted” air is to be flushed by bringing in outside air. If the crowd is small, it may take a long time. If it is a “full house,” the time will be less. When indicated by the CO2 sensor, outside air should be introduced until a proper level is achieved. Thus the freshness of the air is maintained and the heat needed to process that air is minimized. A Monolithic Dome (thermal storage) can provide a major portion of the heat needed to temper the air properly. The use of the ERV-Energy Recovery Ventilator minimizes heat loss from fresh air being entered into the building. There is no excuse for the air within to not be fresh.
- Fresh air is only one reason for introducing outside air into a Monolithic Dome. A properly designed control system can also utilize the fresh air for heating or cooling the mass (heat sink) of the Monolithic Dome. The volume of fresh air to be introduced may need to be much more than for people. Proper design considers its use for heating or cooling. This varies for location and building use. Again, staging the amount of fresh air should be considered.
- For example: A Monolithic Dome high school in Idaho has no AC system. In the late spring it gets hot. To control the heat, cool fresh air is brought in at night cooling the shell. During the day, the air is continuously circulated keeping the building pleasant. These same conditions exist at times in nearly all locations. Free heating and cooling is always cheaper than fuel. Properly programmed controllers can take advantage of the free heating and cooling when it is available.
- The control system should be intelligent. It can be manual if run by a trained operator or it can be monitored and run by a programmed control system. Many decisions must be made for the system to run properly. Seasons of the year and the current weather should be factored into the control.
- Air conditioning condensers should be staged. Only the minimum amount should be used. Whenever possible, natural heating and cooling should be used. Great consideration should be given to designing a system with minimum condensers. Then leave a few of them off until proven they are actually needed. It is far better for smaller AC units to run longer periods of time.
- Humidity control is of utmost importance to human comfort. The control system should recognize high humidity and deal with it. This means that at times the AC and the heat must both be turned on simply to dehumidify. Care should be taken to monitor humidity and temperatures of the shell so as not to create condensation on the shell interior. This is a rare problem but should not be ignored.
Some “rules of thumb” may be in order:
- In Texas, one ton of air conditioning is all that is needed for 1000 square feet of space, such as an office or a lightly occupied area. A home only needs 1 ton per 1000 square feet. In general, two watts of electric heat per square foot is sufficient.
- In Idaho, air conditioning is rarely needed. Three to five watts per square foot of heat is sufficient.
- In Texas, one ton of AC per 1000 square feet is adequate for assembly space, such as in a church. More than that is wasteful or is not taking advantage of the thermal storage of a Monolithic Dome. This is predicated on assemblies lasting 3 hours or less with a 3 hour break to recool the storage. If the hall is to be used at full capacity for 12 hours a day, then the AC needs to be doubled to 500 square feet per ton. But rarely does that happen. If there is a suspicion of this kind of need, leave space to add more condensers as the need arises.
A paradigm shift in construction
Again, it must be reiterated that the Monolithic Dome represents a paradigm shift in construction. It is unlike any other structure. Its insulation is on the outside – doubling its effectiveness. It is one piece with no unplanned air infiltration. It is the strongest building that can be built at a reasonable price. It is also the most energy efficient because of the symbiotic relationship of the urethane foam and the thermal mass of the concrete.
A HVAC design must take into account the above properties. As the Monolithic Dome is not conventional, neither can the HVAC system be conventional and yet take advantage of its attributes. Please remember the power bill is forever. Properly designed, it will be 50 to 70 per cent less than conventional. This is especially true of the demand charges. Properly using the thermal storage eliminates heavy demand charges.