By: Russell M. Keeler, PE
For most church administrators, the planning and construction of a new sanctuary or worship center is an exciting enterprise. It is probably one that has never been attempted by your congregation. There are many resources available to assist you in the architectural, space planning, sound system and financial aspects of the project. Little information, however, is available up-front to assist you in developing another critical part of your new facility; the air conditioning system. This article provides information on these systems so that air conditioning can enhance, rather than detract from, the worship experience.
The focus is on sanctuary rooms with over 1,000 seats, assuming that support facilities such as classrooms, offices and general purpose areas are part of the project. Such buildings can easily exceed 100,000 square feet, and generate significant amounts of heat, in the sanctuary and especially on the stage due to high lighting levels. Three elements are involved in the air conditioning system; air distribution, refrigeration and temperature controls. Each will be discussed separately below.
The single most important air conditioning system element in a satisfactory worship experience is the distribution of cooling air to the space. Cooling air needs to be delivered quietly, and without drafts. These apparently simple goals can be a challenge in a large, high ceiling room.
It is not unusual for these rooms to have ceiling heights in excess of 30 feet. In most of the churches that we have observed, cooling air is introduced at the ceiling, with supply ductwork routed above the ceiling. This approach is the norm for most air conditioning applications, but it presents several challenges.
First, the air quantities involved are considerable, with large ducts required to deliver the air to ceiling outlets. In one project, a 2,700 seat sanctuary, the main supply air duct is eight feet square! Large ducts can cause conflicts during construction if great care is not taken to coordinate conflicts between the structure and the ducts. The second problem with introducing the cooling air at the high ceiling is that most ceiling diffusers (Figure 1) are designed to spread air horizontally in spaces with much lower ceiling heights. Special outlets with downward air patterns are used for high ceiling spaces, but the required high downward air velocity can produce both noise and draft. An alternative is to use exposed ducts with horizontal outlets (Figure 2), but this approach tends to create drafts.
The ideal approach is to install an under floor air distribution system to introduce the air at floor level at very low velocity. This concept recognizes that the only part of the room that needs to be cooled is the 7 to 8 feet above the floor, the zone that is occupied by people. As the cooling air warms, the air rises, and increases in temperature until exhausted at the roof. Benefits include improved indoor air quality because all of the filtered air is delivered in the occupied area, lack of drafts, elimination of air generated noise and reduced air handler size.
To achieve the under floor concept, a structural floor to support the people and furniture is installed with flush mounted outlets under the seats (Figure 3). An air flow space is created below the structural floor using fire rated materials to create a fire barrier between the sanctuary and basement. Air is introduced to this air space and distributed above via the floor outlets. To make sure occupants do not feel drafts, the air is provided at near room temperature. Because no attempt is made to cool the large upper volume of the sanctuary, less cooling is required. Higher air temperatures at the ceiling due to lighting, etc., are directly exhausted from the building. A significant energy conservation feature of the system is that mechanical cooling is not needed until the outdoor temperature rises to above 65 F. In other areas of the building, with lower ceiling heights, conventional overhead systems can be used satisfactorily.
Refrigeration systems fall into two categories; air-cooled and water-cooled systems. Air cooled systems are lower in first cost and ideal for small cooling loads, but are expensive to operate. Water cooled systems have higher first costs, are generally applied to larger buildings and are by far the most efficient, with operating cost about half those of air-cooled systems. These systems have an expected have an expected service life of over 25 years, as opposed to the air cooled units, lasting about 15 years.
Air cooled systems employ direct expansion (DX) refrigerant based cooling coils. This is the system used in most home air conditioning systems. In larger applications, such as small commercial and small to medium sized office buildings, the refrigeration system is packaged with fans, filters and temperature controls to create a “rooftop” air conditioner. These are the large boxes that can be seen on the roofs of many buildings (Figure 4).
Air cooled rooftop systems have the advantage of low first cost, simplicity of operation and maintenance and do not need dedicated indoor rooms to house the equipment. Disadvantages of rooftop systems include short equipment life, high operating cost, and relatively poor performance when exposed to wide variations of cooling load. Accurate controls characteristics are especially important in a sanctuary, where occupancy can range from over 100% occupancy during at Easter to 10-20% for a midweek service. The “on-off” nature of the controls can cause wide temperature fluctuations during light occupancy.
Location of the equipment is also of concern. A rooftop unit is designed to be located on the roof! This seeming economical solution can be the cause of severe noise problems in a sanctuary with long span structural members. If rooftop DX equipment is selected, expect your contractor to argue that mounting the equipment on the roof presents the lowest cost approach. Beware! Long span trusses and rotating machinery create the potential for a drum like condition. Low frequency vibrations created by this equipment can be almost impossible to eliminate. After the building is built, solutions can be costly and/or even ineffective. When using rooftops, it is best to place the equipment on the ground.
Air conditioning capacity is rated in tons (a typical home air conditioning system ranges from 2-4 tons). Above about 150 tons of refrigeration , water cooled systems become a cost effective choice. Water cooled systems can use either refrigerant or chilled water at the cooling coil, but reject heat to the outdoors using the evaporation of water.
Water cooled systems employ water chillers (Figure 5) and cooling towers or DX cooling coils with cooling towers. Separate air handling units with cooling coils provide the conditioned air to the spaces. A hybrid system uses DX cooling coils with evaporative condensers packaged in a rooftop configuration, obtaining the benefits of both packaged and water cooled systems.
Advantages of water cooled systems include:
- Air handling can be decentralized using piped chilled water to local coils for individual zones of occupancy and temperature requirements.
- Control sequences closely track the occupancy loads of each space.
- Lower supply air temperatures, resulting in smaller, less expensive air handlers and ducts, can be used.
- Refrigeration equipment is twice as efficient as air cooled equipment, lowering operating costs.
- For facilities with extensive weekday programs, operating strategies can be employed that reduce electrical costs through off-peak equipment use, such as thermal storage.
Thermal storage is a new-old technology. It was first used in the 1920’s to cool milk on dairy farms, and later in churches! The technology has been updated and is increasingly being used in air conditioning systems with weekday operation with little nighttime load. During the night, when electric rates are low, very low temperature chilled water is used to make ice in storage vessels (Figure 6). During the day, when cooling loads are high, the ice is melted to augment undersized refrigeration machines. Equipment sizes can be reduced by over 50% and operating costs are reduced by as much as one third.
Today’s temperature control systems are generally electronic, using direct digital controls (DDC). DDC systems offer much more accurate control of equipment and temperature than earlier electro-mechanical devices. The systems can easily be programmed to provide only the amount of heating or cooling that is needed in any one space. Different occupancy schedules are easily accommodated, for example, if Thursday night’s choir practice is cancelled, the temperature for the choir practice room can be adjusted to a more energy efficient setting.
Depending on how elaborate the system choice, detailed diagnostics of systems operation and local zone temperatures are easily displayed on a computer screen for adjustment by facilities engineers. Remote alarms are even available to provide alerts of system failures to off-site operators.
Project Delivery Process
Depending on your situation, you may choose to employ either a design/bid/build or a design/build process for project delivery.
In design/bid/build, you will select an architect to develop your design as well as create construction documents. The architect will employ engineering consultants to create construction documents for the structural, HVAC, plumbing and electrical systems. During the design phase, it is important that the topics discussed above are fully addressed by your engineering consultants. Be an informed client, and request detailed explanations as to how the design answers the noise and draft concerns.
In design/build, an architect creates the design for the building appearance, but may not fully develop construction drawings for the building systems. Design/build relies on the general contractor to obtain pricing from building systems subcontractors that include designs developed by the contractor, and which generally reflect the lowest cost, not necessarily the best solutions. If this process is used, it is highly recommended that you retain an HVAC consultant to help you and your architect create a conceptual description of the recommended HVAC approach.
A wide variety of options for air conditioning systems are available for the church planning a building project. Satisfaction with your new facility depends on much more than the selection of a site, façade materials and the quality of the sound system. Interior comfort, noise free operation and operating economies over the years will be determined in large part by the decisions made in the planning stages concerning the types of cooling and air distribution systems selected, as well as the careful application of those systems to deliver optimum performance.
Russell M. Keeler, PE, LEED is a register professional engineer with 35 years experience in diagnostics of air conditioning system problems, design of corrective measures and design of new building systems and renovations. His career has been highlighted by innovative designs for both conventional buildings and energy efficient systems. During the last five years he has concentrated his practice in the design of large church projects. He can be reached at firstname.lastname@example.org.
Figure 1: Ceiling supply air diffuser
Figure 2: Horizontal supply air register