Case Studies

When Envision Mechanical Engineering was hired to design the heating and DHW systems for the Cheyenne Mountain Re-Entry Center, there were two major stipulations. “We needed to implement an energy-efficient solution, and it had to easily tie into the facility’s Invensys Building Automation System (BAS),” said Nick Sauer, president of Envision Mechanical Engineering. “Specifying AERCO Benchmark boilers for space heating and AERCO KC1000 water heaters for DHW allowed us to meet those objectives and helped to keep initial construction costs down for the 2003 construction project.”


Domestic Hot Water for a Dormitory-Style Setting
Located in Colorado Springs, Colorado, the Cheyenne Mountain Re-Entry Center is a 128,000 sq. ft., four-story building designed to house 780 inmates as they prepare to re-enter society through government-sponsored work programs. “Because the building has a dorm-style setting, with full-service showers, a full-service kitchen and a full-service laundry, its domestic hot water needs are quite large,” said Sauer. “To meet these needs, we chose three gas-fired AERCO KC1000 1.0 million BTU/hr. water heaters to supply 140°F water to the DHW recirculating loop. Tempering valves were added for shower temperature control. Storage tanks were not used.”

The KC1000 units deliver 93%-99% thermal efficiency under variable flow conditions while maintaining precise +/- 4°F temperature control. Designed specifically to operate in condensing mode, the heaters were piped to accept 40°F city water directly without temperature blending. Each KC1000 unit is capable of 14:1 burner turndown to precisely meet the facility’s fluctuating domestic hot water demands. The three heaters will support 1.4 gpm to 55 gpm loads without cycling.

In addition to minimizing wasteful cycling, the water heaters actually operate more efficiently at part load. As the firing rate drops from 1 million BTU/hr. to 70,000 BTU/hr., thermal transfer across the surface of the heat exchanger increases to maximize operating efficiency. This is crucial because the majority of each water heater’s operating hours will be spent running at part load.

Modulation is Key to Efficient Space Heating
The Envision team anchored the space-heating system with two high-efficiency, gas-fired AERCO Benchmark boilers. Each 2.0 million BTU/hr. condensing boiler offers 20:1 burner turndown. As a result, the Cheyenne Mountain Re-Entry Center’s boiler plant can fluctuate from 100,000 BTU/hr. to 4.0 million BTU/hr. without cycling the boilers on and off.

Just as with the KC1000 water heater, the Benchmark boilers also operate more efficiently at part load. To fully exploit the boiler plant’s inverse efficiency profile, Envision used the AERCO Boiler Management System (BMS) to coordinate operation of the boilers to achieve the highest seasonal efficiency.

The AERCO BMS maximizes plant efficiency under any part-load condition. For example, if the building requires the plant to fire at 400,000 BTU/hr., the BMS will split the load evenly between the two boilers rather than having one unit bear the full load. The two units, each running at lower firing rate, will require less fuel to meet the demand than if only boiler were employed.

Condensing Operations Increase Efficiency & Cut Costs
In their overall design of the space-heating system, the Envision engineers set the supply and return temperatures on the heating loop at a 40°F ∆T, ensuring that the boilers will be in condensing mode approximately 90% of the time. This design approach boosts efficiency 11%-12% while reducing upfront equipment and installation costs.

Bringing low-temperature (less than 135°F) return water to the boiler has a cooling effect on the heat exchanger. When water vapor, found in the combustion chamber gases, is exposed to this relatively cool surface, it changes into a liquid state. The change of state releases approximately 1,000 BTUs of heat for every pound of liquid created, turning more of the fuel into usable heat. This recaptured energy would otherwise be lost up the flue.

Because AERCO heat exchangers are manufactured from superior materials and are designed to drain freely, they can withstand years of condensing operation. This enabled the Envision team to place the boilers directly onto the loop without the piping, mixing valves and other temperature-averaging components used to protect conventional boiler plants from cool return-water temperatures. And the high ∆T design helped the engineers to cut construction costs beyond the mechanical room as well. The system’s low flow rate allowed for smaller pipes, valves and pumps with less electrical draw.

Easy Integration With Invensys Building Automation System
The AERCO BMS seamlessly integrates the boiler plant with the facility’s BAS via a Modbus interface. This allows facility managers to control overall plant operations and monitor up to 35 aspects of each boiler’s operation via a single connection between the Invensys BAS and the AERCO BMS.

“Controlling the heating plant through the Invensys system simplifies facility management. We can easily coordinate the operation of many of the building’s electric and mechanical systems. And the Benchmark’s advanced control systems enable facility managers to collect extensive data from each boiler and periodically evaluate how the plant is operating,” said Sauer.

Small Units for a Cozy Space
The added bonus of the AERCO Benchmark and KC1000 units was their small size, which allowed Envision to easily overcome the limitations of a small, 1,200 sq. ft. mechanical room. With a footprint of 12 sq. ft. and zero sidewall clearance for each Benchmark boiler, and measurements of a 22” wide x 57” deep footprint for each KC1000 water heater, both plants fit with room to spare.

“The relatively small size of the AERCO equipment, combined with its energy efficiency, flexibility and reliability, made it the ideal choice of our energy-efficient puzzle,” said Sauer. “We were able to build flexible, highly efficient, simple systems that are maintenance-friendly and cheaper to install than standard solutions. The building is set to open in August of 2005, and we expect very happy customers.”