Innovative PTAC Solution Provides 50 Percent Energy Savings
June 30, 2008
Building owners and investment firms continually balance the costs of renovation to remain competitive and retain tenants against the need to reduce operating costs and increase shareholder value. Washington Real Estate Investment Trust (WRIT), Rockville, Md., a publicly-traded investment firm, resolved the issue at one of its properties with a unique energy-saving HVAC system. Two imaginative building engineers came up with a solution that included 500 McQuay Enersaver™ packaged terminal air conditioning (PTAC) units that helped save over 50 percent in HVAC energy costs.
Glenn Anderson and George Kane, building engineers at 1600 Wilson Boulevard, a 157,000-square-foot building in Arlington, Va., could see the problem from a long way off. The existing through-the-wall PTAC units that had served individual offices for the past 27 years had reached the ends of their useful lives. The reciprocating compressors in the old units were far less efficient than the scroll compressors in the newer units. The foam gaskets that sealed the perimeters had become brittle and hard and were not serving their function. Furthermore, an overwhelming number of the units needed new compressors and fan motors.
Replacing the units was straightforward enough. Anderson and Kane just needed to choose an energy-efficient model that would operate reliably for many years like the ones they would replace. But they also wanted to install some energy-saving controls of their own design that would meet specific requirements for his building. To accomplish their goals, the engineers designed and built an electric circuit that could be added to the simple on-off and heat-a/c controls on a standard PTAC unit.
CUSTOM-DESIGNED CONTROL SYSTEM MEETS PROJECT GOALSAnderson and Kane developed five functions for the PTAC units - primarily load-shedding controls - that helped to increase the efficiency of the units and reduce energy costs for WRIT. The five functions are: 1) a start-stop control; 2) a morning warm-up; 3) a day reset control; 4) cooling lockout; and 5) heating lockout.
The start-stop control is the most basic function, and it’s simply designed to meet the terms of the lease; namely, that WRIT provides heating and cooling to tenants from 7 a.m. until 8 p.m. All of the building’s PTACs - 44 units per floor in 72 zones - are connected to a centralized energy management system (EMS) to accomplish this function.
However, starting all of the units at the same time in the morning would cause electric demand to peak. Dominion Virginia Power is the electric utility provider for the property. The current rate structure is demand-based meaning they are billed more for peak demand than consumption. The morning warm-up control takes advantage of the utility’s off-peak energy rates. “The magic moment is 7 a.m., Monday through Friday; that’s when the higher on-peak rates kick in,” said Anderson. Currently there is no off-peak demand charge making it extremely advantageous to pre-heat or cool the building during off-peak hours. “Previously, we always used the most electricity at about 8:30 a.m. when everyone arrived at work and turned on their units.” Accordingly, the engineers designed the circuitry so the EMS could start all units at a set time that is adjustable to the need and season. Currently the units start well before 7 a.m. when on-peak rates begin and stop at 6:45 a.m. That way, when workers arrive, their offices are already comfortable, and they won’t all simultaneously press their ‘on’ buttons.
The third control, the day reset button, is the most unique feature, according to Anderson. “Although the lease says WRIT provides heating or cooling until 8:00 in the evening, most people go home about 6:00. Keeping all those units running in empty offices for two hours was a waste of energy,” he said. “So we added a switch for tenants who work after 6:00. They simply push the reset button and the unit continues to run.” Tenants who remain in their offices after 8:00 still have heating or cooling, but the energy useage is charged to the tenant’s company rather than WRIT. The feature is also available for those tenants who work on Saturdays.
For further load-shedding efficiency, the engineers wanted to be able to lock out the compressor during the cooling season and the heating element during the heating season in any PTAC unit - or any chosen group of units - for a specified amount of time. “We can lock out the compressor or the heating element - but not the fan - for a few minutes at a time in any of the 72 zones. The fan continues to run, but the cooling or heating element is locked out of the circuit.” For example, on a moderately warm summer day, the north side of the building can be locked out, while air conditioning can still be made available to the south side, which is receiving direct sunlight.
ADDITIONAL CUSTOMIZATION REDUCES INSTALLATION TIMEBecause the gasketing around the wall sleeve had deteriorated, scraping it off the wall sleeve and replacing it would have significantly added to installation costs. Instead of replacing the gasketing of all 500 wall sleeves, Anderson and Kane designed a new component that allowed the PTAC with new gasketing to be dropped into place. They designed a metal flange that is attached to the PTAC unit, not the sleeve. As a result, they were able to attach the gasket to the PTAC unit and not the sleeve. When the new PTAC is inserted in the sleeve, the flange butts up against the wall sleeve and thereby seals the cavity.
RELIABILITY, SERVICEABILITY IMPORTANTOnce Anderson and Kane finalized their modifications, they turned their attention to choosing a new unit for their controls. They purchased one unit from three different manufacturers and conducted a side-by-side test. “We took each unit apart to find out which one was built the best and would perform reliably for the life of the equipment,” said Anderson. “We thought the McQuay unit was better engineered; it is built simply, which makes it easier to service. Serviceability is a big plus,” he said.
Anderson had worked on several previous projects over many years with Noyle Harding of Havtech Corporation, Washington, D.C. Havtech represents a full line of HVAC manufacturers, including McQuay. Harding worked closely with Anderson to help to retrofit the units to Anderson’s specifications.
Before installing all 500 units, Anderson wanted to test the efficiency of the new McQuay units against the old ones. He purchased 26 new units and installed them in a single riser. Then, 26 older units were installed in an identical riser located near the riser with the new units. The new units were connected to the building’s energy management system. Both risers were separately sub-metered for electricity. He monitored and compared the 52 units for one year. “The new McQuay units used 45 to 50 percent less energy than the old ones,” said Anderson. In part, this is because the new McQuay PTACs are about 20 percent more efficient than the old units they replaced. The rest is due to the innovative combination of controls to reduce energy use.
Finally, Anderson was ready to install all 500 new units. The final test was to determine whether McQuay could provide the flexibility and willingness to accommodate the prototype design and install it in 500 units. The answer turned out to be easy.
“After installing and testing the units in various parts of the building, Glenn came back to us and asked for our help to modify the entire 500 units with his controls,” said Harding. “We were happy to work with him and build a prototype to his specifications. We’re always looking for the best way to solve a customer’s problem. It meant close collaboration with the McQuay factory and engineers; it was a long process, but in the end we provided exactly what he needed, and that’s always the satisfying part of any project.”
With the new units and significantly improved energy efficiency, Anderson calculates that the new units will pay for themselves in six to seven years. And with reasonable maintenance, the units will last many years like the ones they replaced. All together, it’s an investment that increases tenant comfort, saves energy, and helps WRIT increase revenue and shareholder value.
For more information, visit www.mcquay.com.
Publication date: 06/30/2008