Maryland's Union Mill Makes a High-Performance Comeback

December 29, 2013
January/February 2014
A version of this article appears in the January/February 2014 issue of Home Energy Magazine.
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In the 1880s, Baltimore’s hulking mill complexes sprang up along the Jones Falls stream, forming the industrial center of a bustling city until the early 1900s. Union Mill turned out cotton duck (commonly referred to as canvas), its 86,000 ft2 stone structure housing one of the largest such operations in the world. Now listed on the National Register of Historic Places, the mills are being repurposed, and their massive scale and thick masonry walls present both opportunities and challenges for today’s rigorous energy performance standards.

Vacant for several years, Union Mill reopened in 2011 after a $20 million mixed-use renovation that welcomes the city’s schoolteachers with 56 discounted apartments. It also includes 30,000 square feet of office space offered to local nonprofit education, health, and human services organizations, and a 1,500 ft2 café in the former boiler house building.


Built in 1866, historic Union Mill has been restored into a vibrant mixed-use complex of apartments and office space.


(Alan Gilbert Photography)


Union Mill, which was vacant for several years, reopened in 2011 after a $20 million mixed-use renovation. (Mitsubishi Electric US Cooling & Heating Division)


Union Mill welcomes the city’s schoolteachers with 56 discounted apartments. (Mitsubishi Electric US)


Mitsubishi Electric ducted systems provide this residential unit with individual zone control. The indoor units are concealed behind a traditional soffit. (Mitsubishi Electric US Cooling & Heating Division)

Seawall Development Company was behind the renovation, with its mission to transform defunct buildings in historic neighborhoods into modern, low-cost rental spaces for nonprofits and individuals working to improve Baltimore’s public schools. “The Baltimore school system recruits 700 to 1,000 new teachers each year,” says Evan Morville, a partner at Seawall, Baltimore. “We want to roll out the red carpet and provide them with affordable, smart housing close to the inner city.” To that end, the building also features shared amenities such as workout rooms with showers and teacher resource rooms for late-night lesson planning.

Remarkable Rebates

A key to tenant affordability, of course, is low operating costs, which Seawall takes seriously. The stately U-shaped Union Mill, built in 1866, was the first structure to be certified under Baltimore City Green Building Standards, established in 2010 for new or extensively remodeled commercial and multifamily buildings over 10,000 square feet. Aiming for LEED Silver equivalency, the project’s baseline modeling methodology was consistent with LEED and ASHRAE 90.1 protocols, and the proposed energy efficiencies resulted in a whopping $164,258 cash rebate from Baltimore Gas & Electric’s Smart Energy Savers program.

The rebate amount surprised even veteran energy engineer Mike Babcock, cofounder and managing partner of Sustainable Building Partners in Fairfax, Virginia. “A utility incentive of this size is extremely rare in our business,” he says. “What it all boils down to is this: Through our comprehensive whole-building hourly energy simulation model that compares standard packaged central units to the Mitsubishi Electric VRF zoning system, we were able to project for Baltimore Gas & Electric an overall savings of 450,000 kWh annually.”

The energy savings were driven by the synergies between the building’s HVAC system, high-performance makeup air (outdoor air circulated into the building to replace air loss), energy recovery ventilation, double-pane low-e windows and the thermal mass provided by the building’s stone walls, which are more than 2 feet thick. Further energy retrofits, such as insulating the stone walls, were not permitted by state and federal historic agencies, which provided tax credits for the project.

Sustainable Building Partners used DOE-2 energy simulation software—freeware from eQUEST—to model the Baltimore City Green Building Standards. “The utility vetted our analysis, so we got preapproval for the rebate before purchasing the HVAC equipment,” Babcock says. Upon project completion, “when they verify that everything was put in place, they give you the rebate.”

Choosing an HVAC System

In choosing a heating-and-cooling system, the project team, which consisted of architects, engineers, and contractors with extensive experience in sustainable development, entertained several options. The building’s pitched roofs would not support boilers, chillers, or condenser units. And the conventional choice—a split system with a heat pump and A/C unit packaged together—was off the table for aesthetic and acoustical reasons. The specifications would have called for 160 individual units, which are impossible to hide. And with so many points of entry through the stone walls, installation would have been cost prohibitive.

“We didn’t want to disrupt the historic nature of the mill,” Morville says. “And all the units would be underneath the apartment windows. We couldn’t have the living spaces affected by noise.” The team decided on the R2-Series Variable Refrigerant Flow (VRF) zoning system from Mitsubishi Electric US Cooling & Heating Division. During inspections of other projects where the VRF zoning units were installed, Morville noted their quiet operation. “We weren’t scientific about measuring decibels but took a commonsense approach to what was tolerable,” he says.

Energy Savings

In addition to the mechanicals’ unique ability to be maneuvered around a massive building, the VRF zoning system made sense for several reasons. One was its high-performance operating system, featuring 100% inverter-driven compressors, high-efficiency indoor units, and integrated energy recovery. The system’s design allowed for fewer and larger outdoor condensers. And unlike a split system, the VRF zoning system does not require costly electric-resistance auxiliary heating, which typically kicks in when temperatures reach 25°F. “This equipment is in heat pump mode all the time, so we’ll see lower heating bills,” Babcock says. “To the best of my knowledge, this operation [varying the heat pump’s speed to provide only the precise amount of cooling or heating needed] is unique in the HVAC industry.” The indoor units also come in different configurations, offering flexibility for installation in a variety of zones.

The VRF zoning system’s narrow ductwork was a good fit for the mill retrofit, Babcock says, because they were dealing with set ceiling heights and had little room to add extensive chases. The ventilation ducts in the building can be smaller in diameter because only the required amount of fresh air is delivered to the space, where it is then combined with air that is being recirculated at the zone. This is different from a rooftop unit, which combines the recirculated air and ventilation air at the unit, and uses large ducts to distribute the mixed air to several zones.

Here’s how it works: The outdoor condenser requires only one small wall penetration to connect two bundled refrigerant lines to the branch circuit controller, which regulates the coolant. From there, two pipes carry refrigerant to each indoor unit. One branch circuit controller can serve up to 16 zones. In the case of Union Mill, the branch circuit serves 8 apartments, while 7 outdoor units serve the project’s 56 apartments, and 16 more units are used to heat and cool the common areas. The roughly 2-foot x 3-foot air-handling units sit on a shelf above the water heater in a utility closet in the apartments; they’re ceiling mounted on the commercial side.

“There is an element of free energy exchange between residences when one wants more cooling than another,” Babcock says. “Each apartment gets only the refrigerant it needs, and the fan speeds up or slows down to maintain temperature efficiently.”

The HVAC team also liked the fact that the R2-Series simultaneously cools and heats different zones within the mill complex. With this energy-conserving function, heat energy is collected from the areas on cooling mode and then transferred to the zones that require warm air. “This system gives us the best option for being able to accommodate the cooling and heating requirements within larger spaces,” Morville says. “An 8,000 ft2 commercial space on the west side will need more cooling in the afternoon than a smaller office on the east side.”

A Mitsubishi Electric Lossnay energy recovery ventilator is integrated into the VRF zoning system’s controls on the residential side, providing fresh air and heat recovery. One feature of the ERV is that it not only reduces the temperature of the air (sensible heat transfer) that passes through it, but it also removes moisture (latent heat transfer), which keeps the air dry and comfortable. “We turn on the VRF zoning indoor units only ten to fifteen days per year,” Morville says. “We find that the ERV can satisfy the needs of the commercial spaces and common areas during the summer by itself with the heating and cooling unit set to fan mode. On those rare days that the temperature reaches at least 95 degrees, we will turn the VRF zoning systems to cooling mode.”

TG-2000 Tenant Billing Software

Seawall uses TG-2000 tenant billing software from Mitsubishi Electric to apportion HVAC costs. TG-2000 monitors the allocation of refrigerant flow (in kWh) to each tenant as a percentage of the total refrigerant in the system. At the end of each month, TG-2000 reports the respective kWh used for cooling and heating by each tenant. Since plug loads and hot water cannot be measured separately, the kWh used for plug loads and hot water are added to the HVAC usage.

In the two years since occupancy, a metering system on each apartment has confirmed the project team’s low-energy-use projections. Utility bills for the one- and two-bedroom units, which range in size from 600 square feet to 900 square feet and have 14-foot ceilings, are averaging $50 a month. Roughly 20% of that cost is for cooling and heating, Morville says. A recent summertime monthly average, for example, clocked in at $40.57, with $32.57 for the plug load, including the water heater, and $8 for heating and cooling.

Commercial metering, in fact, was a requirement of Seawall’s eligibility for the city’s rebate program. “One of the things we recognized was the rate difference between residential and commercial in the cost of electricity,” Morville says. “To accommodate that, the residential side’s electricity usage is submetered through Studebaker Submetering” in Alexandria, Virginia. “We get a printout each month of the exact HVAC usage for each outdoor compressor and for each apartment. The third-party company combines the heating and cooling with the electricity cost and creates the bill.”

Along with the TG-2000 software, a user-friendly touch screen mapping of the condenser locations allows Seawall to monitor and control them remotely. “If someone calls on a weekend and says, ‘We want to work today,’ we can turn on the heat or A/C in their space,” Morville says. The technology also reduces service calls. “We’re able to confirm reports that the heat is not working, and troubleshoot a bit before we have to go out and evaluate everything.”

These efficiencies are at the heart of a rehab that returned Union Mill to its former role as an urban landmark—not just physically but also economically and socially—and helped to deliver one of the largest utility rebates the owner has ever known. “The building itself embodies what sustainability is all about,” Babcock says, “repurposing an unused facility for new teachers and incubator businesses, done in an energy-efficient manner that results in an affordable monthly bill.”

Pamela Androff, PE, LEED AP, is product manager of commercial and product planning at Mitsubishi Electric US Cooling & Heating Division.

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