The HVAC Factor: Wireless Thermostat Conversion
By Ron Wilkinson, PE, LEED AP, ASHRAE CPMP and David K. Roberts
From the June 2013 issue of Today’s Facility Manager
Early in 2011, the City of Oakland (CA), Pacific Gas and Electric (PG&E), and Quantum Energy Services & Technologies, Inc. (QuEST) established the Oakland Shines Program. This program, which deployed resources gleaned from the American Recovery and Reinvestment Act (ARRA) of 2009, represented an ambitious energy efficiency effort for the city. A five million dollar grant enabled QuEST to offer significant financial incentives and to leverage PG&E rebates and assistance.
The main goal of this program was to lower energy costs for commercial buildings by installing advanced energy efficiency technology. Through a projected overall energy reduction of 20% across the program area, Oakland was given the opportunity to create a “green showcase” of its commercial district.
When Oakland Shines team members went looking for the best projects to highlight new technologies, they talked with David Rojas, chief engineer at 2201 Broadway. Located in the Uptown District of Oakland, this 1931 vintage, eight story building has 193,000 square feet of rentable space used primarily for commercial offices. This building relied on pneumatic thermostats that sent individual signals to the HVAC system from each zone to address occupant comfort.
As energy efficiency standards grow stronger, legacy pneumatic controls such as those found at 2201 Broadway are often unable to meet the needs of facility operators. Disadvantages of legacy pneumatic thermostats include:
- no remote set point adjustment;
- no remote monitoring of temperature and control pressure;
- require frequent calibration; and
- cannot be individually programmed for schedule/setback.
As Rojas explains, “This outdated system could not be centrally controlled. Even worse, it was usually controlled by individual tenants, leaving energy management out of the equation.”
The HVAC system for 2201 Broadway was originally designed to have variable air volume (VAV) with inlet guide vanes but had since been converted to a variable frequency drive (VFD) VAV system. The building has five air handling units (AHUs), with two large units on the roof conditioning floors two through eight. Each is equipped with an air cooled chiller and chilled water coils. The other three AHUs, located on the second floor, are served by two direct expansion (D/X) chillers and condition the first floor and mezzanine level. For heating, each zone has perimeter reheat fed by a 4,000 MBtu boiler.
The Retrofit: Aiming For Fast And Unobtrusive
When an older building such as 2201 Broadway is considered for conversion from a pneumatically controlled HVAC to direct digital control (DDC), major concerns include: costs, time required to change out pneumatic piping for electrical wiring, and interrupting occupants for prolonged periods. In this scenario, wireless pneumatic thermostats (WPTs) can be an ideal solution. This emerging technology is a retrofit that gives a pneumatically controlled HVAC system the functionality of a DDC system at significantly less cost and installation time.
A full DDC retrofit at 2201 Broadway would require removing all of the pneumatic tubes and controllers and rewiring the system with digital controllers. The WPTs, however, could be installed quickly by removing the old pneumatic thermostat in the zone and connecting the branch and main line pressure tubes to the WPT.
In this case, 264 pneumatic thermostats controlled space temperature throughout the facility. The thermostats were standalone systems, and facility operators did not know what each thermostat was doing at any given time. Replacing the legacy units with the WPTs started in January 2012. To replace, the old unit was dismounted from the wall, tubing was disconnected, and the new unit was connected and mounted. This was done for all floors and all thermostats within three months.
Rojas notes, “All installation was completed during occupied hours. There was no disruption of occupant work, no dust or dirt, no complaints, and no overtime.”
Each WPT contains a pressure transducer that converts a pneumatic pressure signal (both branch and line pressures) to a digital signal. These signals are wirelessly relayed back to the facility’s energy management and control system (EMCS), or to a standalone operator workstation. The system uses a hybrid mesh wireless network that has wireless repeaters running through the facility. The backbone receives the signals from each of the new thermostats and transmits the data back to a BACnet gateway and then to the EMCS.
Along with the WPTs, programmable controllers were installed in existing HVAC control panels, and pressure transmitters and temperature sensors were installed in HVAC equipment. The final phases included the replacement of pneumatic thermostats with WPTs and the integration of the WPTs into the programmable controller system through a wireless network.
Reaping Energy Savings
By replacing each thermostat in the zones on every floor with a WPT, facility operators gained insight on how each floor is being run. With information on the temperature and actions of every zone, the following energy efficiency measures were put in place.
Occupancy override. Historically, 2201 Broadway has been about 8% vacant. WPTs in every zone allow the facility operator to shut the VAV boxes in every vacant zone, saving energy by not conditioning empty spaces.
Duct static pressure reset. With zone temperature and set point information from each thermostat, a duct static pressure (DSP) reset strategy can be implemented. Currently, the supply fans control to a constant DSP set point. A reset strategy will employ a trim and respond type of control loop. For example, if all the zones are satisfied, the DSP set point will be incrementally lowered until it either hits a minimum or the zones call for cooling. Once the zones call for cooling, the DSP is incrementally raised until the zones are satisfied again.
Supply air temperature reset. With zone temperature and set point information from each thermostat, a supply air temperature reset strategy was implemented. The cooling coil is controlled to provide a constant supply air temperature. The reset strategy will be a linear temperature change based on the zone.
Zone temperature reset. With digital thermostats, a zone temperature reset can be programmed as an inverse linear relationship to outside air, reducing zone reheat in the winter and cooling load in the summer.
Recent PG&E records for 2201 Broadway have verified notable savings since the retrofit. Compared to the same period in the previous year, gas consumption for heating decreased by more than 35%, power consumption decreased by nearly 15%, and power demand decreased by over 6%. These records reflect the readouts, supplied directly by the utility company, for the months of September through November 2012. (The Oakland Shines Program concluded in April 2012, according to parameters set at its launch.)
Rojas now receives detailed information on hot and cold spots throughout the facility. Since the WPTs became operational in the spring of 2012, complaints have been addressed quickly by checking VAV box parameters provided by WPT trend logs. As Rojas puts it, “My job is to talk with tenants daily. There is an overwhelming sense of improvement in the building environment since the installation of the WPTs.”
Wilkinson is chief engineer for sustainable buildings consultancy e4 inc. in New York, NY. He is the author of the first commissioning training program for the LEED for New Construction and Major Renovations rating system; the chair of the commissioning advisory committee of AIA Committee on the Environment; and the recording secretary for ASHRAE Guideline Project Committee 0.2/1.2, The Commissioning Process for Existing Building Systems and Assemblies/The Commissioning Process for Existing HVAC&R Systems.
Roberts is vice president of marketing for Cypress Envirosystems, a San Jose, CA manufacturer of retrofit technologies for facilities. He has worked in the energy field for more than 10 years, the last five focused on energy efficiency and auto demand response.