The challenge: Design a high-quality hvac system which is sensitive to aesthetics, has low operating costs, and minimizes construction costs. The client also requested that RNL Design meet these goals with a two-year-old construction budget. The Omni Interlocken Hotel is proof that these goals can be achieved through the application of innovative engineering.

The Interlocken Business Campus, located between Boulder and Denver, CO is an emerging location for telecommunications and software companies. Major occupants of the business park include Level 3 Communications and Sun Microsystems.

Prominently located at the main entrance of the Interlocken Business Park, the Omni Interlocken Hotel, a four diamond, luxury conference hotel, was designed to support the local business community and has been in operation since July 1999. The hotel offers over 400 first-class hotel rooms, meeting facilities, and a 27-hole championship golf course.

RNL Design was the selected architect/ engineer to design this $37 million hotel and conference center. Services provided by RNL Design included architecture, mechanical, electrical, and plumbing engineering as well as landscape architecture.

The evolution of the Omni project is an excellent case study of how energy efficiency and innovative engineering applications can enhance value to a building owner. RNL engineers decided to utilize thermal storage and low-temperature air distribution in a setting where the technology is not normally applied. This decision resulted in enhanced building aesthetics by removing prominent air-handling units (ahu) from the roof, increased ceiling heights, and an overall reduction in electrical costs. All of these goals were achieved without adverse impacts on construction costs.

The design criteria was subject to tight constraints and contradictory goals, including:

  • Keep within a strict budget.
  • Minimize the visual impacts of air conditioning equipment and maintain "clean" rooflines. The hotel is located adjacent to a major highway with high visibility.
  • Maximize ceiling heights within the public spaces with minimum impact on the exterior envelope height.
  • Keep operating costs (i.e., energy and equipment maintenance) to a minimum.
  • Provide redundant refrigeration plant capacity.

Figure 1. The interior lobby of the Omni Interlocken. In such top-end applications, aesthetics become an even higher priority than usual.

System Selection

In most luxury hotels, a four-pipe fancoil system is the preferred application for hotel rooms due to its ability to closely control the room temperature to the occupant's preference and maintain low noise levels. Omni corporate design standards confirmed this choice, and vertical fancoil units were chosen to minimize piping costs. For this reason, a central chilled-water refrigeration plant was required to produce the chilled water. The lowest first cost air-handling system for the public spaces would have been rooftop ahu's.

Additionally, rooftop units reduce the amount of interior space needed to house mechanical equipment. However, given the negative visual impact of roof-mounted air handlers, RNL Engineers chose to evaluate a unique combination of chiller plant and air-handling system designs.

Figure 2. The Omni's air-handling room. The unusual design of this ahu allowed for use of a fan room only about 40% as large as that required for more conventional air handlers.

Chiller Selection

The lowest first cost chiller system to support the fancoil system would have been air-cooled water chillers. The advantages of using air-cooled water chillers include a reduction in interior space for mechanical rooms and lower first cost for packaged air-cooled chillers compared to a water chiller/cooling tower configuration. However, there are many disadvantages to the air-cooled approach. Air-cooled water chillers are the least efficient chiller technology, with operating rates in the range of 1.4 kW/ton, or higher. In addition, air-cooled chillers are located outdoors and have much shorter predicted useful service lives than indoor water-cooled chiller equipment.

Colorado's climate, as well as that at the project location, also entered into the equipment selection process. The Rocky Mountains can be snowcovered from November through July; however, there are many days in winter when buildings such as the Omni Hotel, located in the plains (just a few miles east of the mountains), require cooling. This highly varying cooling load results from a combination of clear winter days and intense solar loads at the project's high altitude.

An air-cooled installation would have required the operation of compressors in the middle of winter to serve the chilled-water fancoil units of the hotel tower. By contrast, a water-cooled system can produce the needed chilled water in winter through a "free cooling" heat exchanger connected to cooling towers. Running through this list of pros and cons, RNL Design concluded that an air-cooled approach would have a negative impact on the aesthetic and low operating cost goals of the building's owner. RNL Design therefore decided to pursue a water-cooled chiller system being mindful that this could create further challenges with meeting the construction cost budget.

Figure 3. Chilled water distribution schematic.

Air Distribution System Selection

In order to reduce the construction cost of the indoor air-handling systems as much as possible, RNL engineers evaluated the use of low-temperature air. Conventional air-handling system designs generally employ conditioned air at 55°F. For the Omni project, RNL reduced the conditioned air to 47° for the public areas. This increased the supply air temperature rise from 18° to 26° and reduced the total building air quantity from 200,000 cfm to 110,000 cfm. An added benefit of the low-temperature air approach was the reduction in the number of ahu's required. With the exception of the health facility and bar, a single ahu could serve all of the public spaces.

In order to reduce fan room space requirements, a space-saving ahu was employed (Figure 2). The unusual design of this ahu allowed for use of a fan room only about 40% as large as that required for more conventional air handlers. Heating and cooling coils were arranged around a plenum box with a fan mounted on top with supply air drawn vertically out of the central part of the unit. A relatively small footprint for the room results, but the room is higher than required for conventional equipment.

Depressing the slab in the fan room solved the height requirement. Since the fan room serves as the mixed air plenum, no space is needed for connecting return air ducts to the ahu. This compact design allowed a fan room, normally sized for a 30,000-cfm air handler, to accommodate 60,000 cfm. Two other air handlers, sized at 20,000 and 30,000 cfm, use a similar "room as plenum" configuration.

The reduced supply air quantity of the low-temperature design yielded another important dividend: ceiling heights could be raised over a foot without any change in the floor-to-floor heights of the building. Higher ceiling heights did not reduce construction costs; however, the owner would receive greater value because the higher ceiling heights did not require a taller building.

In order to minimize drafts and the effects of cold air on office occupants, series fan-powered variable-air volume (vav) boxes were used to "reheat" the air using return air as the tempering medium. Most of the public spaces are meeting rooms with a high people density, resulting in high ventilation air quantities. Due to the high percentage of ventilation air, pinch down vav boxes were not used in these areas.

The Construction Cost Challenge

The remaining overriding question was where to find money in the budget to offset the additional cost of the water-cooled chillers and mechanical rooms required. It became necessary to find ways to reduce the mechanical system's cost without compromising the fundamental system selections made. RNL Design chose to pursue the idea that thermal storage, in addition to low-temperature conditioned air, could reduce chiller, cooling tower, duct, and fan sizes enough to free up the dollars required.

Refinement of the Thermal Storage Approach

One of the design challenges of thermal storage is to develop an accurate cooling load profile of the project. A conference hotel presents unique usage patterns. There are, in fact, three unique zones of occupancy patterns: the hotel tower, conference center, and public spaces. As part of the effort to minimize cooling plant size, it was necessary to predict, with some accuracy, the diversity of the peak-cooling loads, rather than use a "sum of the peak loads" approach. It was concluded that the sum of the peaks was approximately 700 tons. Through a usage patterns analysis of each zone of occupancy, it was estimated that the diversified peak-cooling load was 500 tons.

A conventional chiller system that provides partial redundancy would have required an installation of three 250-ton chillers for a total of 750 tons installed. However, with thermal storage, chiller redundancy can be obtained by both partially reducing the size of the chillers and using the ice storage bank as a virtual third water chiller. Since the ice bank provides cooling during peak load periods, the second water chiller can remain idle. In the event that the first chiller fails, the second chiller can maintain the building's cooling load.

By carefully calculating the cooling load pattern, the size of a thermal storage system can be minimized while maximizing the benefits of smaller chillers. A thermal storage system using ice to provide 1,400 ton hours of cooling was chosen. The ice bank was expected to reduce the peak-cooling load at the chillers from 500 tons to less than 300 tons, and to be used for approximately five hrs/day. Two rotary screw water chillers were chosen, one at 225 tons, the other at 175 tons (Figure 3). This uneven chiller capacity split was based on optimizing load and ice building chiller efficiencies. Even with efficient chillers operating at 0.45 kW/ton, thermal storage would reduce electrical demand 90 kW (200 tons x 0.45 kW/ton) over that of a conventional chiller plant.

Chilled-water distribution was designed to employ a primary/secondary pumping loop, which allows for simultaneous generation of brine (22°) for ice making and chilled water (39°) for the building distribution loop (Figure 4). The primary loop design includes the chillers, a "free cooling" heat exchanger and the ice bank. Each cooling device has a separate pump. A secondary piping loop serves the air-handling equipment using 39° water. The building piping system is reverse return, with two-way valves at the air handlers, two-way valves on eight hotel tower floors, and three-way valves on three tower floors to provide minimum flow. In order to maintain secondary system flow, the variable-flow secondary loop pump is controlled from a pressure sensor in the building return loop.

At night, when public spaces are unoccupied, the refrigeration system can perform two functions: cooling to the hotel rooms in the tower and production of ice. Experience has shown that ice production requires low-temperature brine at 22° to 24°. Omni's brine is produced at 22°. Chilled water for the building loop is mixed with the brine loop at valve V-2 to maintain a chilled water inlet temperature at valve V-1 of 39°. V-1 is, in turn, controlled by a temperature sensor in the building loop to maintain 39°.

Construction and Energy Cost Results

RNL Design's goal for the aesthetics of this project was to create additional mechanical space in an expanded basement and eliminate the need for rooftop ahu's. The size of this mechanical space was minimized through the use of low-temperature air and space-saving air handlers. The additional cost of the mechanical space was offset by the reduced mechanical equipment costs of the low temperature air and thermal storage designs. Low-temperature air yields a 30% reduction in air handler and duct system sizes, while thermal storage results in a 50% reduction in chiller sizes.

The actual construction costs proved these assumptions to be correct. The reduction in chiller capacity from 750 to 400 tons resulted in a net construction cost savings of $240,000, including the additional cost of the thermal storage equipment. Further, the reduced air distribution system size saved about $280,000 in construction costs. The final mechanical construction cost of approximately $5 million was nearly $500,000 less than the original budget developed using air-cooled chillers and rooftop air handlers. This savings more than offset the additional cost of the mechanical equipment rooms needed to house the indoor air-handling equipment.

The design also resulted in lower operating costs than that of a conventional system. By utilizing water-cooled chillers instead of less efficient air-cooled equipment, less electricity is needed, thus minimizing the building's drain on the utility power grid. Further, the reduction in air quantities due to low-temperature air saves fan energy costs, while thermal storage results in reduced electrical peak demand costs. The hotel is saving over $100,000 in energy costs annually. These savings are all the more impressive given the lack of any additional construction costs involved! How often is an instantaneous payback achieved on an energy conservation initiative?

Actual Operation

Eastern Colorado experienced one of the hottest summers on record during the summer of 1999. Additionally, the Omni Hotel has seen an occupancy rate of over 85% on a consistent basis. As a result of RNL's innovative design, the Omni Interlocken Hotel has only operated one of the two water chillers at full capacity when the ice bank was engaged. This proves the idea that thermal storage operates as a virtual third chiller.

The hvac design profession is understandably cautious to embrace a system concept that is not yet widely accepted. However, the thermal storage refrigeration plant combined with low-temperature air provides significant space and cost saving advantages. Although not appropriate for every application, in the case of the Omni project, the design solved many of the project's challenges and constraints. The design resulted in an affordable initial cost, lowering operating costs for cooling over many alternative designs. In addition, there is no visible hvac equipment to impact the aesthetic effects of the architecture.

"RNL demonstrated the highest degree of design ingenuity without compromising construction and operating costs or design aesthetics," praised John Brawner, chief engineer for Omni Interlocken Hotel. RNL Design provided a state-of-the-art hvac system on a fixed, low-cost budget and minimized the visual impact of air conditioning equipment by maintaining "clean" roof lines, thus reducing the hotel's operating costs. These advantages demonstrate to the engineering profession that the rewards for learning to apply this technology are substantial, particularly as they benefit the overall building design.


The Omni Interlocken Hotel's air conditioning system design is further evidence that innovative engineering can provide low-cost solutions that do not compromise the cost of construction, operating costs, or the building's exterior appearance. By evaluating the project budget as a unified whole rather than as the sum of each trade, it was possible to shift construction dollars from Division 15 (Mechanical) to other general construction areas, such as building an expanded basement. This extra effort was rewarded by greater value to the owner. It is a showcase for the cost-effective benefits of integrated architecture and engineering. ES