Wireless Monitoring for Rooftop Packaged HVAC Systems
Applications of wireless radio frequency (RF) technology to collect data from packaged rooftop heating, ventilation, and air-conditioning (HVAC) units relaxes some of the demands imposed by in-building applications of wireless communication. Equipment can be physically located so direct lines of sight are preserved and obstructions minimized. By simply positioning antennas sufficiently above the roof, all transmitting antennas can “see” their corresponding receiving antenna. As a consequence, lower transmission power can be used, greater sources of interference can be tolerated, and communication protocols with less sophisticated means for ensuring reliable data transmission can be used. As a result, system and component costs are likely to be lower for rooftop wireless data acquisition than for in-building systems. Electrical power for the data collection equipment can generally be provided at the packaged unit by tapping into the electrical power supplied for operation of the HVAC unit, eliminating the need for batteries.
PNNL has tested commercial wireless technology for providing energy monitoring and diagnostic services. The tests involved using wireless communication to collect sensor data from several packaged rooftop units at a central point on the building roof. From there, data are sent to a desktop or laptop computer via a serial interface, wired Ethernet connection, or wireless networking (802.11) for processing. The results can then be used by operation staff in the building or by a service provider located remotely in operating and maintaining the HVAC systems. The data can be “picked up” by an energy service provider, who can process the data further to produce easily used information, and then use it in-house or securely post results on the world wide web for use by other HVAC service companies or building operators.
The purposes of the field tests were to design/build such a system, test the use of wireless technology for this purpose, evaluate the cost of such systems, and identify critical technological advancements required to make this technology more cost-effective.
Pacific Northwest National Laboratory (PNNL) demonstrated two approaches to wireless data collection from packaged rooftop units: 1) using generic, off-the-self, wireless products and 2) using an integrated wireless sensing/communication package designed specifically for monitoring the performance of rooftop HVAC units.
Generic Wireless Sensing for Data Collection
In 2002, a system based on generic wireless data acquisition components (manufactured by Quatech) was installed on a 10-ton packaged HVAC unit on a medium-size commercial building leased by PNNL.
System Costs
The table below shows the system costs for a wired base case and wireless systems configured from commercially-available components. The key cost differences between the wired system and the wireless systems are attributable to the communication components. For the wired case, cable and conduit must be installed to each HVAC unit. For the wireless case, the cable and conduit are replaced with RF transmitters and receivers. The system evaluated monitors three HVAC units, located as shown in the diagram below.
The results show that low-cost Quatech wireless data collection ($163 per point) has advantages over wired data collection ($316 per point); however, a high-cost wireless solution ($583 per point) based on more expensive generic wireless components is not competitive. Greater numbers of HVAC units will generally decrease the cost per point of both wired and wireless systems because distances to the units will generally decrease on average. In addition, the cost of the system receiver is allocated over more sensor points. Regardless, the low-cost wireless solution still maintains a cost advantage.
| Cost Component | Cost | |
| Monitoring System for Three Packaged HVAC Units | ||
| Wired Design | Wireless Design | |
| Sensors | $636 |
$636 |
| Wiring | $68[1] |
-- |
| Communication and signal-conditioning hardware | $1903 |
$1500-$5900 |
| Labor | $1179[2] |
$450 |
| Total cost | $3786 |
$1950-$7000 |
| Average cost per sensor | $316 |
$163-$583 |
[2] Including installation of conduit.
Photos
To view the larger image click on the thumbnails.
Schematic diagram of a representative wireless data acquisition system for packaged rooftop HVAC units. |
Quatech transmitter and signal conditioners in an open enclosure mounted on a rooftop packaged HVAC unit. |
 Close up view of Quatech signal conditioners in an open enclosure. |
Quatech receiving unit in an open NEMA enclosure. |
Integrated Wireless Sensing/Communication Package for Data Collection
In 2004, a second wireless system was custom designed and built specifically to collect data from packaged HVAC systems. The system was specified by PNNL and was built by AirSentry Inc. Costs of the system were evaluated and compared to wired data acquisition for the installation with 6 HVAC units shown in the figure below.
Costs for applications of the AirSentry wireless system and an equivalent wired system are shown in the tables below.
Cost of a Wired System for Monitoring Six Packaged HVAC Units| Description | Quantity |
Cost |
| Thermocouple sensors ($10 each) | 24 |
$240 |
| Current switches ($77 each) | 6 |
$462 |
| RS-485 to RS-232 converter | 1 |
$94 |
| Thermocouple input signal conditioners (199.00 each) | 6 |
$1194 |
| Digital I/O modules (99.00 each) | 6 |
$594 |
| Twisted pair wire (12.5 cents/ft) | 275 ft. |
$34 |
| Rigid metal conduit, ½” ($1.03/ft) | 275 ft. |
$283 |
| Labor for installing sensors (2 hr per HVAC unit @ $60/hr) | 12 hrs. |
$720 |
| Labor for wire and conduit (275 ft) | $7.00/ft. |
$1925 |
| Labor for wiring from receiver to computer ($60/hr) | 4 hrs. |
$240 |
| Total Cost | $5,786 |
|
| Average cost per sensor point | $193 |
Cost of a Low-Cost Wireless System for Monitoring Six Packaged HVAC Units Using Integrated Transmitter Units.
| Description | Quantity |
Cost |
| Transmitters (including sensors, signal processing, and radio transmitter; $187 each) | 6 |
$1,122 |
| Receiver ($195 each) | 1 |
$195 |
| Labor for installing transmitters, sensors, and receiver (2 hr. per HVAC unit + 1 hr. for receiver; $60/hr.) | 13 hrs. |
$780 |
| Labor for wiring from receiver to computer ($60/hr.) | 4 hrs. |
$240 |
| Total Cost | $2,337 |
|
| Average cost per sensor point | $78 |
In 2002, the costs of wired and wireless monitoring systems for package HVAC units were roughly comparable (see Quatech cost table above). The costs of electronics have decreased over the two years between analyses, and the costs for wireless sensing have decreased even more rapidly than the costs of the components for wired data acquisition systems. By carefully matching capabilities and performance to the specific requirements for HVAC monitoring and packaging components (e.g., signal conditioning and radio transmitter together), the cost of wireless monitoring has been reduced even further. At $78 per sensor point, costs for the wireless system are just over 1/3 of the cost of the wired system (at $193 per point). For this retrofit application, wireless monitoring clearly possesses a cost advantage over installation of a wired system, and the cost may be sufficiently low to encourage careful monitoring of package units, which has not been done widely to date.
The cost of the wired system was estimated from the cost of various commercially-available individual components, while the wireless monitoring system was built as an integrated unit. The cost of hardware excluding labor and cost of wire for the wired system is approximately $2,600, and for the wireless-system hardware it is about $1,300. The question naturally arises, “Would the cost advantage of the wireless system persist, if we created an integrated unit for the wired system?” If an integrated wired data collection unit like the wireless one were made, the cost of hardware for the wired system would presumably be comparable to the wireless system ($1,300) and the cost per point for the wired system would drop to $147. Although lower than the cost for the wired system built from off-the-shelf components, because of the cost of wiring and labor for its installation, the wired system would still cost nearly twice as much as the wireless system. Therefore, wireless communication can be used more effectively to monitor packaged rooftop HVAC units than wired systems.
Rooftop HVAC unit with Air Sentry wireless equipment installed.
For larger picture click on image.
A question still remains concerning whether even with the lower costs provided by wireless sensing technology, is monitoring of packaged HVAC units itself cost effective. For the answer to this, we refer you to the analysis presented in:
S. Katipamula and M.R. Brambley. 2004. "Wireless Condition Monitoring and Maintenance for Rooftop Packaged Heating, Ventilation, and Air-Conditioning." In the Proceedings of the 2004 Summer Study on Energy Efficiency in Buildings, pp. 3-124 - 3-138. American Council for an Energy Efficient Economy, Washington, DC. PNNL-SA-41084. (PDF 1228KB).