Floyd County faced a unique set of challenges when tasked with the design of a climate control system for the brand-new, 274,952 sq. ft. Highland Hills Middle School in Floyds Knolls, Indiana. The county had a limited budget and no access to natural gas, and only 1,540 sq. ft. had been dedicated to the facility’s mechanical room. To overcome these obstacles, a hydronic two-pipe system operated on a propane-fired AERCO KC1000 boiler plant was implemented.

“Two-pipe systems cost less to build, take up less room, consume less energy and, because of their simplicity, are easier to maintain than the standard four-pipe system,” said Bill Wiseheart, director of facilities for Floyd County. “This is because they use half the piping, and half the number of pumps and actuators.”

The primary limitation of two-pipe systems is that heating and cooling equipment could not run simultaneously on the shared piping loop. Two-pipe designs require a switch from heating mode to cooling mode, and vice versa. In the past this could only be achieved via a difficult, labor-intensive process. But with today’s modern Building Automation Systems (BAS), the changeover can be both easy and fast.

“Our mechanical room contains eight 1.0 million BTU/hr. KC1000 propane-fired boilers, two chillers and two system pumps with variable-frequency drives,” said Wiseheart. “Seven of the AERCO boilers are connected through AERCO’s Boiler Management System (BMS), which is connected, along with everything else involved in the building’s climate system, to our BAS. The eighth boiler is used exclusively to control the temperature of the school’s swimming pool.”

Each classroom has heat-monitoring sensors, and teachers have the ability to adjust each room by a 3º-4ºF range. When the temperature either rises above or falls below the loosely established outside air temperature 60ºF changeover point, the BAS automatically switches the system over. The entire automated process takes only twenty minutes. The BAS will lock out the boiler system, actuate the single changeover valve from heating to cooling and enable the chiller plant. The BAS then monitors the water temperature as it cools. As it drops below room temperature, the BAS will change the HVAC equipment from heating to cooling.

“Despite the ease of the automated switch, the frequency of changeovers has not been excessive,” said Wiseheart. “ASHRAE standards maintain that the building must supply 15 cu. ft. of outside air per student. This means that if the building is running a little hot, instead of switching the entire climate control system over to cooling, we can bring the temperature down a bit by venting in some of the cool outside air while still running the hydronic plant in heating mode.”

Installing high-efficiency equipment ensures that the short-term cost savings of two-pipe designs deliver long-term fuel savings for the life of the building. When in heating mode, two-pipe systems generally deal with lower, wider-ranging water temperatures than four-pipe designs. With its condensing capability and resistance to thermal shock, the KC1000 boiler provided the perfect engine for the heating system.

“When the loop is in heating mode, it runs at a maximum temperature of 120ºF and a minimum temperature of 80ºF,” said Wiseheart. “The AERCO units are specifically designed to run at these low and frequently changing temperatures without any risk of thermal shock. This meant that the boilers could be installed directly onto the heating loop. And because the KC1000 boiler’s combustion chamber and heat exchanger are constructed specifically to operate in condensing mode, the low water temperature of the hydronic loop is an asset that enables the AERCO units to operate more efficiently, increasing fuel savings while lowering operating costs.”

By turning more of the boiler’s fuel into usable heat, condensing within the heat exchanger increases efficiency by as much as 12%. This process occurs naturally when the water vapor, created as a by-product of the combustion process, cools below its dew point (≈135°F). However, as the latent heat is extracted from the water vapor, a mild acid condensate remains on the surface of the heat exchanger. While this acid can severely damage most boilers, the KC1000 units are constructed with the highest-quality materials and are designed to drain freely.

The KC1000 boiler’s other major capability, its 14:1 turndown, contributes significantly to fuel savings. Each KC1000 can run with approximately 70-75,000 BTU/hr. – or just 7% input – and can gradually increase in precise 1% increments until firing at 100%. This eliminates burner cycling losses and wasteful overshoot, which stem from a strictly “on/off” burner operation. Furthermore, the lower the unit’s operating rate is, the higher its thermal efficiency will be. Through the control of the AERCO BMS, the units act as one system running all seven units in tandem. For example, each unit would run at approximately 30% to meet a 2 million BTU/hr. building load.

“The eighth AERCO boiler is used exclusively for the building’s swimming pool,” said Wiseheart. “The pool temperature is kept at a constant 82ºF range, which means that the heating loop is set at 140ºF. Unlike conventional boilers, no extra piping is needed to protect the AERCO boiler, and if there is a valve malfunction, 180ºF water won’t go rushing into the pool.”

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