Combination System Provides Efficient Alternative for Virginia Apartment Complex

March 21, 2006

When the 22-story Lincoln Towers residential apartment complex opened in 1990, five 1.76-million BTU/hour hot-water heaters and three 1.76-million BTU/hour boilers sat atop each of its twin towers to supply its 720 apartments with heat and hot water. However, the winters in Arlington, VA, ravaged the rooftop installation. From the very beginning, ice would continually develop on the boilers and water heaters at night and then melt in the warmer afternoon hours, causing the heating and hot water plants to break down on a regular basis.

In 2002, Lincoln Towers hired a team of engineers from Arlington, VA-based Consolidated Engineering Services Inc. in conjunction with Holman Boiler Repair of Springfield, VA, to remedy the problem by relocating the hot water and heating plants to a more hospitable location in the complex’s underground parking garage. The arrangement presented two large hurdles for the contractor and engineering team to overcome. Lincoln Towers set aside only five parking spots (just 600 square feet) as the space within which to raise walls and build the new heating plant. The new plant location was not only small, it was also underground and more than 150 feet away from one of the apartment towers.

“Given the small space we had to work with, a combination system using high-efficiency boilers to support both space-heating and domestic hot water requirements from a single plant was the best solution,” says Andrew Huck, who served as the senior engineer on the project. “This approach reduced the construction budget and offered the customer long-term fuel savings.”

Reduced Plant Capacity Minimizes Project Costs
The Consolidated team installed 10 AERCO 2-million BTU/hour Benchmark boilers in the new space to serve as the system’s anchor. “The original design, with separate boilers and water heaters atop each tower, provided more than 28-million BTU/hour total capacity,” says Huck. “By combining the space and domestic water-heating loads, and consolidating the plant in a single location, we were able to effectively support the complex with a 20 million BTU/hour capacity plant. This dramatically reduced the overall cost of the project.”

A combined system will always be a fraction of the size of independent space and domestic hot-water plants. Dedicated space-heating plants are engineered for the lowest possible outdoor temperatures, which may occur for just a few hours on one or two winter nights during an entire heating season. All the remaining days of the year, this extra capacity can be working to support the domestic heating load. Especially in an apartment building, demand for both space heat and hot water will virtually never peak simultaneously.

“This idea is most effectively demonstrated in a typical AERCO combined system installation, where one or more boiler units literally swing back and forth between dedicated support of the domestic water heating system and standby availability to support increased space-heating demand,” explains Huck. “But such an approach would have required separate piping loops - one for the space heating and one for the domestic hot water - to be run to each of the towers. Whatever savings we had reaped in terms of boiler equipment would have evaporated in the face of piping expenses. To keep costs low, we reverted to a more conventional design approach.”

The Consolidated Design
The engineering team decided to run one set of pipes to the pre-existing mechanical rooms that existed in each tower. The new boiler plant provides constant 160-degree F. supply water to support each building’s heat pump and domestic hot-water systems.

Each building employs a water source heat pump loop for space heating. Designed to run with 70-degree F. water, the space-heating loop uses temperature sensors, situated downstream of the boiler loop supply, to activate modulating two-way control valves to ensure constant temperatures. When loop temperatures rise beyond acceptable limits during warmer days, the water from the loop is routed to a cooling tower located on the roof of each building. If loop temperatures get too cool, the control valves open to inject the 160-degree F. boiler supply water into the heat pump loop as needed.

Each building employs a domestic-water, double-wall, plate-and-frame heat exchanger and a 200-gallon storage tank to support its domestic hot water (DHW) requirements. The 160-degree F. boiler feed water is routed as needed via a three-way control valve through the plate-and-frame unit to heat incoming 40-degree F. city water to 140 degrees F. for storage. When the tank aquastat drops to 120 degrees F., a pump is activated to re-circulate the stored water through the plate-and-frame heat exchanger until the tank set point of 140 degrees F. is reached.

Condensing Application Promotes Long-Term Fuel Savings
With AERCO boilers anchoring the system, the engineers automatically increased the energy efficiency of the DHW system. The best conventional hot-water heaters are rated at only 82-percent efficiency. At full firing rate in a non-condensing application, AERCO boilers deliver 86-percent efficiency. But to help Lincoln Tower really cut long-term fuel costs, Consolidated’s system design maximized opportunities for condensing operations.

“The more energy or heat given up to meet the space and DHW loads, the lower the return-water temperature coming back to the boilers,” says Huck. “Rather than go with the traditional 180 degrees F., we designed the boiler plant to deliver 160-degree F. supply water. We were still able to meet 140-degree F. DHW requirements, but starting with the lower supply-water set point increases our opportunities to operate the AERCO boilers in condensing mode. This translates to an 11- to 12-percent increase in the plant’s operating efficiency.”

Condensing occurs when water vapor, which exists in the boiler’s combustion gases, is forced into a liquid state. Releasing approximately 1,000 BTUs for every pound of liquid created, this change of state happens naturally as the water vapor contacts a heat exchanger surface that has been cooled by less than 135-degree F. return water entering the boiler. And, because AERCO units are constructed with the highest-quality materials, the corrosive condensate, a harmful byproduct of condensing, has no adverse effects on the heat exchanger.

Precise Boiler Modulation Increases Part-Load Efficiency
Each of the 10 Benchmark boilers features 20-to-1 burner turndown. The boiler plant can support any load between 100,000 to 20 million BTU/hour without temperature overshoot or wasteful cycling. And, more importantly, as each unit’s firing rate drops, its operating efficiency actually increases.

To maximize fuel savings, plant operation is coordinated by AERCO’s Boiler Management System (BMS). The BMS ensures that boiler plant output exactly matches system demand while maximizing the number of units - all operating at the lowest possible firing rate - in operation. AERCO’s BMS is also connected to the Lincoln Towers building automation system.

A Final Note
“The one major advantage of the underground plant was that we were able to easily vent the boiler plant directly into the parking garage’s pre-existing ventilation system,” says Huck. “Since the new plant went online 3 years ago at Lincoln Towers, the days of cold apartments and cold showers are gone.”

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