Commercial Rooftop HVAC Energy Savings Research Program
Bench Test Report
June 2008
Prepared by:
Mark Cherniack – Senior Program Manager
Howard Reichmuth PE – Senior Engineer
Prepared for:
Northwest Power and Conservation Council
851 SW Sixth Avenue, Suite 11100
Portland, Oregon 97204
(503) 222-5161
Acknowledgements
Acknowledgements
The design and set up of the testing chamber, testing of the components/systems, and initial data
assessment was completed by David Robison, P.E., Stellar Processes, Bob Davis, Ecotope and
Dennis Landwehr, P.E. under subcontract to New Buildings Institute (NBI). NBI staff is
responsible for the final report and conclusions.
The following organizations provided funding to make this phase of the ongoing research
possible. They responded to an invitation to participate in this opportunity to further the potential
for cooperative research partnerships among interested participants nationwide on small
commercial HVAC system issues. Their contributions are much appreciated.
From the Pacific Northwest through the Northwest Power and Conservation Council
Avista Utilities
Bonneville Power Administration
Eugene (OR) Water and Electric Board
Energy Trust of Oregon
Idaho Power
PacifiCorp (non-Energy Trust area)
Puget Sound Energy
Snohomish Public Utility District
From the Northeast
Cape Light Compact (through Northeast Energy Efficiency Partnership-NEEP)
Connecticut Light & Power (NEEP)
Long Island Power Authority (NEEP)
The United Illuminating Company (NEEP)
Western Massachusetts Electric (NEEP)
Efficiency Maine
Efficiency Vermont
National Grid
New York State Energy Research and Development Authority
NSTAR
The Project Team also acknowledges the responsiveness of Honeywell Product Manager
Adrienne Thomle and her engineering staff for making recommendations that strengthened the
research, as well as responding with new product designs that will allow economizers to fully
function as intended in saving energy.
Executive Summary
This work has been done as part of the Commercial Rooftop HVAC Energy Savings Research
Program which includes four interdependent elements: 1) bench testing of economizer controls,
2) field testing of repair protocols, 3) devising an appropriate measurement and verification
(M&V) approach and 4) developing a savings prediction methodology based on prototypical
buildings. Taken together, these elements are intended to lead to the development of a reliable
field repair protocol with a higher level of confidence in the associated energy savings. This
document summarizes the results of only the first of the four elements, the bench testing of
economizer controls. This document is also an interim summary of results because the bench
testing capability is being retained and will be used further during the project.
The bench testing research was applied to the most typical type of dry bulb economizer
controller using controlled environmental chambers to determine the environmental and control
factors that influence the operation of the economizer. The principal findings are:
•
The sensors and economizer controller system exhibits an operational pattern (deadband)
that can significantly interfere with expected economizer operation by limiting the
economizer potential during seasons with warm nights. We refer to this as “hysteresis”
in this report.
•
The sensor and controller components tested exhibited a consistent low bias in
temperature. Environmental temperatures that are supposed to activate the controller do
not correspond to those specified by the manufacturer. The apparent wide sensor and
controller tolerance leads to loss of economizer energy savings potential.
•
The enthalpy sensors tested appear initially to be more accurate than the dry-bulb sensors
tested. However, this stage of research did not measure sensor response over the range of
humidity that would be necessary to fully test enthalpy sensors. Some “hysteresis” is
present with enthalpy sensors as was exhibited in the dry-bulb sensors tested, but the
magnitude of the deadband across a range of conditions is not yet bounded by the
available data. Additional testing of the enthalpy sensor is still underway.
•
A dual differential economizer strategy was tested and compared to the single changeover
strategy as a potential improvement. The differential control strategy used in conjunction
with a 2-stage thermostat has the potential for a more sophisticated control than a simple
single change point strategy. However, test results showed only modest improvement
from this control strategy. This strategy may be more complex to execute in a simplified
and consistent field procedure. It also requires a 2-stage thermostat, which is
recommended in any case for effective economizer performance.
•
A proposed “work-around” solution in the field would substitute an inexpensive
contractor’s thermostat (<$10) for the temperature sensor commonly in use. This
combination operates with very little hysteresis and is expected to increase the amount of
economizer operation. This work around is amenable to a simple and consistent field
procedure.
•
Honeywell personnel provided helpful feedback on the testing protocol. They do not
support the proposed work around due to concerns about high feed-in amps to the
controller, even though a resistor could be added the circuit.
-3-
•
The test apparatus was successful in providing an inexpensive set of controlled
environmental chambers.
Taken together these findings cast significant doubt on the capability of existing economizers,
specifically with the Honeywell C7450 dry bulb/temperature sensor, to perform according to
their potential. Given that economizers have been embodied in building codes on the assumption
of performance consistent with their specification, there is an urgency to apply corrective
measures. Accordingly, the immediate recommendations are:
•
A dialog has been opened with Honeywell, the controller/sensor manufacturer,
concerning the findings in this report and the proposed work-around. These findings
suggest that a significant group of existing economizer controls currently in operating
RTUs cannot access the full economizer potential. This is a major functionality problem
with significant kWh waste that needs addressing and it is important to continue to access
the knowledge of the equipment manufacturer in this regard.
•
Honeywell, in response to the bench test results that provided important ‘customer’ input
to the sensor/controller product manager, has developed a new, advanced dry-bulb sensor
that should resolve the field problem. Honeywell has committed to sending several beta
stage sensors for bench and field testing in July. If this new sensor works as expected,
further discussions about a field retrofit package for utility programs will be held with
Honeywell. Commercial availability is expected 3rd quarter of 2008.
•
Utilities should assess the impacts of identifying the economizer controller sensor
equipment described in the report, that may be installed in new RTUs that are receiving
financial incentives through current or planned utility energy efficiency/DSM programs.
If the particular Honeywell sensor product is present in these new units, a decision must
be considered about including a modification to the equipment at installation time so as to
not install the problem sensor. A field work-around employing the contractor’s
thermostat or snapdisk could be used in lieu of the problematic temperature sensor. The
work-around should be based on the single change point control and not use the
differential control. The implementation of the work around should be viewed as
temporary until the new Honeywell sensor is commercially available. Alternately, utility
high efficiency RTU incentive programs could at least flag those systems for a follow up
sensor retrofit.
•
The bench test has been expanded to include Honeywell enthalpy-based economizer
controls. It is could be expected that the hysteresis effects observed for dry bulb
temperature sensors would not be as significant in the case of enthalpy sensor otherwise,
the use of economizing in the more humid eastern US will also be limited relative to its
potential. At the time of this report, data is still too sparse from the enthalpy sensor tests
for reporting conclusions.
-4-
Introduction
The principal conservation benefit from using an economizer proceeds from using cooler outside
air for space cooling instead of air-conditioning when conditions permit. Unfortunately, detailed
monitoring revealed that economizing is often ineffective. One cause of this problem is due to a
known problem with the economizer control system, referred to here as “hysteresis.” In the
typical control mode, the controller must sense a sufficiently cold outdoor temperature before
economizing is allowed – this temperature is typically 10ºF cooler than the indoor temperature.
This required temperature is referred to as the nominal changeover temperature. Earlier
monitoring studies have shown that this “hysteresis” effect prevented economizing during warm
summer months in mild climates because the coldest nighttime temperatures were not cold
enough to allow economizing. The same problem was not observed in climates with a larger
diurnal temperature swing and colder night temperatures.
The purpose of the current investigation is to test a typical controller system, identifying the
extent to which hysteresis or poor sensor calibration might limit full operation, and to develop
and test a “work-around” solution as part of the development of the field service protocol that
will be implemented in Phase 3 of the overall project. This preliminary task has been limited to
testing the controller apparatus within a set of controlled environmental chambers in order to
quantify the problem and verify the potential solution. A future task, and part of the field testing
portion of the larger project, will be to implement the proposed solution in the field and to verify
the energy impact on HVAC equipment in actual service. This bench testing alone is not
sufficient to quantify the full energy impact of economizing because that is a complex function
of specific site characteristics and operations. The full energy impact of economizing requires
building modeling guided by these lab results and by the field test results. The necessary
modeling and field testing are a coordinated part of the larger project.
It is usually not necessary to bench test commercial equipment. However, the current status of
this type of controller has been shrouded in conflicting anecdotal observations and incomplete
knowledge of the inner logic of the control system. In the view of the research project oversight
committee, the intended research needed to be based on a precise understanding of the control
system performance that could not be assessed from existing research or from the specifications
provided by the manufacturer.
This initial report discusses observations regarding the dry-bulb economizer control operations
as observed in an indoor test chamber. This type of economizer operation is important in the
Pacific Northwest and Rocky Mountain areas where humidity is generally low. Enthalpy controls
for economizers will be assessed at a later time as part of the project since they are important for
utilities supporting this research in the Northeast, where humidity has a greater impact.
-5-
Previous investigations have attempted to quantify the savings from repair of existing packaged
HVAC units 1 . These HVAC units are numerous in small commercial buildings, but conservation
options have been difficult to justify due to the costs necessary to reach these small customers.
Demonstration of cost and benefits will assist agencies in designing conservation outreach
programs.
Background
A preliminary task to the bench test effort was to review available field experience to identify the
most commonly used control items for test. Accordingly, we reviewed available characteristics
data collected as part of Puget Sound Energy (PSE) Premium Service Rooftop program. To be
eligible for the program, an RTU must have an economizer. This represented a fairly large set of
data expected to be typical of installations in the Northwest. Out of 223 systems with
economizers, the following characteristics were noted:
•
•
•
•
70% had the controller that we are testing
41% used enthalpy sensors with unknown drift and calibration, although only drybulb sensing is necessary in our climate.
56% had only a single stage of cooling wired, which significantly curtailed
economizer use.
73% used single changeover point (one outdoor sensor and no return air sensor),
27% used a differential changeover strategy.
Taken together these statistics suggest that about 60% of the units were operating below par. The
main reason is the predominant use of single stage control. A secondary reason is the
unnecessary use of enthalpy sensors in this climate. One manufacturer (Honeywell) has provided
the basic controller (W7459) used throughout the last several decades by most HVAC
manufacturers – with about 85% market share. Recently, other companies have developed their
own products. However, this particular controller is ubiquitous among existing installations that
would be candidates for retrofit repairs. For that reason, we targeted this particular controller as
the subject of study. The particular items bench tested are itemized below.
Item/Function
Manufacturer
Model /Part #
Economizer Controller
Honeywell
W 7459
Dry bulb Temperature Sensor
Honeywell
C 7650
Contractor’s Thermostat,
(<$10)
Temp-Stat
TS-65
Snap Disc Thermostat (~$25)
Service First
SEN00235A Model
20602L4-B74
Table 1 – Items Tested
1
Small Commercial HVAC Pilot Program Market Progress Evaluation Report, No. 1,
http://www.nwalliance.org/research/reports/135.pdf
-6-
The initial understanding of the particular control logic was somewhat general and informed
principally by the manufacturer’s product cut sheets and application guide. At the outset,
controller operation is generally understood as follows: The important part of the controller is a
potentiometer that compares conditions and modulates the economizer dampers in response to
control parameters. Those parameters include 1) installer-adjusted set points for the outside air
temperature which triggers economizer operation (A, B, C, D settings), 2) a reference condition
that is usually a set resistor, but an indoor temperature sensor is used for differential control, and
a 3) a sequence to assure that supply temperature is not overly cold. When there is a call for
cooling and economizing is possible, the controller sends 24 volt current to the damper motor,
which simultaneously opens the outside air dampers and closes the return air dampers. An LED
light indicates this condition. When there is no current, the LED turns off and the motor resets by
spring action to a minimum amount of outside air needed to meet indoor air quality guidelines.
This example can be described as a single changeover strategy since operation change is
controlled by a single outside air sensor referenced to the controller set points. Notice that this
idealized description of the control logic does not mention any hysteresis effect in the cut sheets,
though it is mentioned in a note in the applications guide. After the detailed testing conducted
here, a precise apparent control logic diagram was devised and is shown in Figure 3 (pg.9).
Component Testing Results
The bench testing was done in a testing facility, described in Appendix A, set up specifically for
this purpose. The initial bench tests were intended to reveal the specific operation of the control
system on a limited number of controllers and sensors, hence the results are essentially anecdotal
and are not intended to be statistically rigorous samples.
Two controllers and four temperature sensors were purchased for testing purposes. Eugene
Water and Electric Board (EWEB) staff also provided a number of working but used sensors
retrieved from various repair jobs. The test equipment recorded the position of the economizer
actuator arm and the status of the LED indication light. Both are conditions that indicate the
controller is operating in economizer mode. Both of these conditions agreed closely with each
other – typically the actuator arm moved within a few seconds of the indicator light.
The manufacturer’s cut sheet for sensors 2 indicates that control operations and sensor ma output
are expected to follow a linear response to outdoor temperature. This manufacturer’s
specification is referred in this report as the “reference”. Of course, the reference range of
operation also depends on the installer-adjusted set points (A, B, C, D settings).
Initial tests of sensors and controllers were directed at the electrical properties of the components
compared to temperature. These initial tests revealed that the controller and temperature sensors
were biased toward low temperature readings. That is, sensors activated operation at
temperatures lower than actual temperature. The tests also showed that the temperature sensor
output exhibits sensitivity to the excitation voltage. However, measurements on the individual
components, such as current versus temperature in the temperature sensors, led to inconclusive
results due to lack of knowledge of the electronic details of the control approach. Accordingly,
an overall control test protocol was devised that treated the sensor and controllers as a single
component. This overall control test protocol is described and discussed in Appendix B.
2
Figure 3. C7650 Temperature Sensor Output Current of C7650 Sensor Manual 63-2499-1, 1996.
-7-
Implied Controller Temperature Points
100
90
80
Reference
DegF
70
60
7650 Test
Result
50
40
30
20
D
C
B/C
B
Figure 1 - Test of Controller Settings
D
C
B/C
B
ON
OFF
ON
OFF
ON
OFF
ON
OFF
Reference Observed
Deg F
Deg F
45.0
39.1
54.6
48.1
59.1
51.0
68.7
60.5
65.5
59.2
74.8
70.1
71.9
61.5
80.9
71.0
Table 2 - Reference and Observed Change Points
Figure 1 and Table 2 show the response of one typical sensor at different controller set points
under the testing protocol as described in Appendix B. For this sensor, all the tests results are
biased lower than the specified reference. For example, at setting B/C, economizing is expected
to occur within the range of 65 to 75ºF ambient temperature. In fact, the operation occurred
within a range of 59 to 70ºF. The result is a constraint on economizing operation. Using this
example at setting B/C, night temperatures will have to fall to below 59ºF so that economizing
will take place the next day. Obviously, this rules out economizing during much of the summer
in a mild climate. Thus, even such a small error can result in a serious reduction of economizing.
The test results agree with previous field monitoring that showed ineffective economizing in
locations with warm night temperatures. An evident difficulty is that the installer, relying on the
manufacturer’s reference documentation, will not be able to select an appropriate setpoint due to
the undocumented bias of the components, which appears to be variable among sensors.
-8-
Previous studies have observed that the controllers are typically shipped from the factory set at
setting D. Figure 1 demonstrates that this setting assures little or no economizing. As a result,
the repair programs have recommended that installers change the setting to B or C. Based on
PSE program data, installers are following that recommendation and typically adjust the setting
to midway between B and C. Accordingly, B/C was used as the typical setting for subsequent
testing.
Example of Controller Operation
Tem perature, degF
90
80
70
On
60
Off
50
11
:0
0
7:
40
4:
20
1:
00
11
:4
0
15
:0
0
18
:2
0
21
:4
0
8:
20
40
Tim e
Figure 2 - Example of Controller Operation
Figure 2 shows an example of typical test runs that illustrate the hysteresis effect. In this first
part of the example, the chamber starts at a temperature near 70ºF, so the controller quickly turns
economizing “off”. Economizing remains “off” until the temperature falls to 60ºF; then it turns
“on” again. As the temperature rises, economizing remains on until the temperature reaches
about 70ºF. The important point is that the controller does not initiate operation until the
temperature falls to the “reset” point of about 60ºF. Then, economizing continues until the
temperature rises again to the high limit. At that point, economizing is halted until the “reset”
temperature is again experienced. This operation continues through any number of similar
cycles. This example duplicates the problem observed in the field – economizing will not occur
unless night time temperatures fall to the low set point, which may not happen under milder
nighttime conditions.
There was concern that the installer may not do A, B, C, D settings consistently. The set
potentiometer is small and difficult to read in the field, and the potentiometer is continuously
variable, with no physical ratchet for the A, B, C, D points. So one question tested was: how
reproducible are the settings? Multiple attempts to set at the A, B, C, D settings were fairly
repeatable.
-9-
APPARENT ECONOMIZER LOGIC
Set reference temps, ref low and ref high,
with ABCD potentiometer
Is OSA
less than
ref low?
yes
Turn on reset
indicator
yes
yes
yes
Is supply
temp
greater
than 55°?
no
Is reset
indicator
on?
Is OSA
less than
ref high?
no
no
Turn off reset
indicator
no
No voltage to
actuator, LED is off
and damper returns
by spring to minimum
setting
24 volts to actuator LED
is on, damper is opened
full or controlled to
maintain 55F supply
Figure 3 - Control Logic Diagram
Based on these observations, Figure 3 shows a diagram of the control logic as determined from
the bench test results. Of course, the controller is only operational when there is a call for
cooling.
Both controllers tested performed identically. This suggests that it might be possible to set the
controller at a setpoint to compensate for the observed temperature bias in the sensors. However,
the sensors exhibited some variability in the amount of bias. Figure 4 and Table 3 show how
several sensors performed with the controller at the same B/C setting each time. Such variability
makes it difficult to define a standard offset that would compensate for the biased measurements.
More important, this compensation approach does not solve the hysteresis problem.
Figure 4 and Table 3, show the performance of the four new C7650 sensors and six older C7650
sensors. Initially, we were concerned that older sensors might drift off calibration and be less
accurate. Disassembly of the sensors in Figure 5 shows that, although the model number has not
changed, the manufacturing design has changed. The new sensors utilize different internal
components than the old ones. While our sample is small, it suggests that there is little difference
in performance between new and older sensors.
- 10 -
DegF
90
85
80
75
70
65
60
55
50
45
40
Implied Controller Temperature Points,
Setting B/C
Reference
New
Sensors
Used
Sensors
Contractor
Tstat
Snapdisk
Figure 4 - Comparison of Sensors
ON
OFF
ON
OFF
ON
OFF
ON
OFF
ON
OFF
ON
OFF
B/C
Reference
DegF
65.5
New Sensors
DegF
#2
59.2
Used Sensors
DegF
E#1
64.1
70.1
61.6
71.3
60.7
70.3
59.3
69.8
E#7
72.7
63.6
72.0
63.7
73.6
61.1
71.0
60.7
E#2
69.9
62.1
74.8
#1
T-statA
SnapdiskA
63.3
65.3
64.2
65.9
64.5
SnapdiskB
75.7
66.6
T-statB
#3
#4
77.2
E#3
E#4
E#6
70.3
Table 3 - Observed Change Points by Sensor
- 11 -
Figure 5 – New (not the 2008 replacement model) and Older Sensors
The C7650 sensors output a current signal in milliamps (mA) to the controller. This is an
advantage in control applications since resistance in a long wiring run has less effect on current
based sensor than a resistance based one. The internal components of the sensor are necessary to
generate the required current signal. It was suggested that there may be a field measurement of
resistance that installers could make to verify sensor accuracy. However, measurements found
that the sensors exhibit no measurable resistance. That is, the internal electronic components do
not pass current without the excitation voltage.
Recognizing that the built-in hysteresis (deadband) limits operation, testing focused on finding
and testing a work-around solution. The research team investigated the use of a simple
thermostat (close-on-fall thermostat) to substitute for the outdoor temperature sensor. Replacing
the usual sensor with an on/off closure has the effect of providing a satisfying temperature input
that overrides any control constraints. The closure switch is a possible workaround for the
imprecision of the controller and sensors. Essentially, it bypasses the A, B, C, D settings and
provides an on/off control instead.
We tested two snapdisks (cost ~ $25) and found them to be highly repeatable and close to
specifications. However, these are relatively expensive, and they still show a significant 10
degrees of hysteresis deadband.
Two low-cost “contractor’s thermostats” (cost <$10) were tested and found to be equally
accurate and repeatable. These thermostats are connected in place of the sensor and then bypass
the changeover logic. Figure 5 shows the temperature control points for these options as well.
Either the snapdisk or the thermostat provides a solution to the sensor temperature accuracy
problem. But the preferred choice is the contractor’s thermostat because it has a narrow
- 12 -
deadband and it is relatively inexpensive. It permits economizing when temperatures are 63ºF
and below and continues until temperatures rise to 65ºF – the narrow deadband of this thermostat
is preferred because it will increase the use of economizing. This thermostat is also available in
other temperature ranges. Figure 6 shows the snap disc on the left and contractors thermostat,
right, that were tested.
Figure 6. Snapdisk and Contactor's Thermostat
It was suggested by the manufacturer that enthalpy sensors are more accurate than the dry-bulb
sensors tested. We briefly checked an economizer using an enthalpy sensor with a single
changeover strategy (i.e. with the reference resistor in place instead of an enthalpy sensor in the
return air). Air in the environmental chamber was quite dry and it was necessary to apply control
setting D in order to have any control response at all from the enthalpy sensor. As shown in
Figure 7, this particular sensor demonstrated a better response -- with operation for outdoor air
Temperature at a range of 67 to 77ºF. This suggests the enthalpy sensor tested may have better
accuracy than the dry-bulb sensors tested. However, the same deadband hysteresis still occurs
with the enthalpy sensors. Also it is known that this sensor is responsive to humidity as well as
temperature, and we are reluctant to extrapolate this performance to field conditions without a
more thorough test of response to a wider variety of temperature/humidity conditions. Ongoing,
limited testing of the C7400 enthalpy sensor is continuing until there is sufficient test point data
to assess and report on.
- 13 -
Enthaply System, Setting D (dry air)
90
85
80
OAT, degF
75
70
LED On
65
LED OFF
60
55
50
45
40
Figure 7 - Operation with Enthalpy Sensor
Differential Control Strategy
A dual differential control was also tested. Under this strategy, the controller is set up to
compare outdoor air temperature (OAT) to the return air temperature (RAT). This strategy is
expected to provide for more economizer operation when the indoor temperature is warmer than
the temperature implied by the reference resistor. This strategy requires that the controller be set
to the D setting. A test of the differential control requires that the two control parameters, outside
air temperature, and return air temperature, be varied in an orderly way to test the control system
under its full range of conditions. For this test, we cycled the outdoor temperature repeatedly
while maintaining the return air temperature at a specific point. The test was then repeated with a
different temperature set for the return air sensor. Results are shown in Figure 8.
Implied Differential Controller Temperature Points
100
90
80
DegF
70
7650 Test
Result
60
50
40
30
20
65
72
75
79
Figure 8 - Differential Control Results
In general, one observes that the initiation temperature or “turn-on” point increases when return
air is warmer. This is consistent with allowing economizing to continue longer since the building
- 14 -
can benefit from cooling even at higher outdoor temperatures. However, the high limit of “turnoff” temperature does not appear to be affected. Test results in Figure 8 are somewhat variable
due to noise in the experimental measurements. The results of these tests are simplified in Figure
9 showing generalized economizer “turn-on” and “turn-off” temperatures as a function of return
air temperature.
Outdoor Tem perature,
degF
Generalized Differential Operation
80
70
On
Off
60
50
65
70
75
80
Return Tem perature, degF
Figure 9 - Generalized Differential Operation
Benefits from differential control are likely to be modest. First, because the decrease in the
“hysteresis” deadband is modest. Second, because the amount of economizer cooling from
outdoor air will be small when outdoor temperature is close to indoor temperature.
Note that the “single changeover” operation described earlier is a special case of the differential
control. With single changeover, the reference resistor results in a mA signal that would be the
equivalent of the RAT sensor reading a high indoor temperature. Thus, the constraints related to
RAT are “locked in.”
Progress with Honeywell
In 2006, prior to the start of the project’s research phase, NBI staff had communicated the
potential deadband problem that researchers in the Northwest had identified earlier to
Honeywell’s product manager for the economizer controller and sensor of interest. In response,
the product manager noted that the C7650 does indeed act as described and was not
recommended for use for dry bulb applications. It was suggested that the utility service programs
could simply replace the C7650 with the C7400 enthalpy sensor.
This recommendation to substitute enthalpy sensors was rejected by the research team for two
reasons: 1) a sensor based on humidity measurement is not compatible with the generally lower
humidity levels in the Northwest, and 2) there is anecdotal evidence indicating that enthalpy
sensors are not particularly accurate and may suffer from drift and/or calibration problems. A
study of humidity sensors conducted by the Iowa Energy Center indicated widespread low
accuracy in the sensors currently available in the market. Honeywell staff noted that new
automated calibration techniques will be instituted for enthalpy sensor production that will result
in higher accuracy sensors.
- 15 -
Naturally, there was interest from the research team as to how this sensor came to be so widely
used if the manufacturer now does not recommend its use. As was learned, Honeywell had
designed the controller to be paired with an enthalpy sensor. In responding to its customers, the
HVAC manufacturers, to provide a lower cost sensor solution, the enthalpy sensor was replaced
with the C7650 sensor. The unfortunate result has been a limitation on the availability of
economizer cooling in the Pacific Northwest and elsewhere in the country under temperature
conditions where nighttime summer temperatures are elevated and the deadband limits the
changeover point to activate the economizer damper. There is no estimate possible of the kWh
savings lost from the use of this sensor with the W7459 controller and potentially other
controllers across tens of thousands of rooftop units.
As a result of the developing relationship and ongoing communications with the Honeywell, the
research team was asked to provide recommendations to Honeywell on the parameters of a more
effective temperature sensor and economizer controller. Some of the recommendations
referenced the features defined in the California Energy Commission’s Public Interest Energy
Research project for the Advanced Rooftop Unit (ARTU). One of the ARTU features was a
controller sensor with a 2°F deadband.
The recommendations of the research team provided support to the Honeywell product manager
internally to accelerate the development of a new dry-bulb sensor and updated economizer
controller. A new controller design will include DCV input capability, a startup check test
sequence for the installer and other features that have not been disclosed.
In late May 2007, research team members including representatives from the Bonneville Power
Administration, Ecotope, New Buildings Institute, Northwest Energy Efficiency Alliance
Northwest Power and Conservation Council, Portland Energy Conservation, Inc., and Stellar
Processes, met with the product manager and engineering staff in Portland, OR to discuss the
design and features of the new sensor. The sensor is field retrofittable having the same form
factor of the C7650 and is connected by the existing wiring leads. The new sensor that was
presented had six optional temperature ranges, from 48°F to 78°F. The lowest temperature level
addresses market needs for providing adequate cooling for data centers/rooms. The settings are
activated by dipswitches. This setup provides the HVAC installer with a positive signal that they
have set the unit’s changeover range where they meant to. The sensor has onboard logic and will
actually control the controller. The A-B-C-D setting ‘pot’ in the W7459 will be inoperative.
Team members recommended 63°F for the factory default setting. Honeywell will be sending
beta samples of the sensor for the research team to bench and field test. There was also a
discussion of the development and potential large-scale field deployment of a retrofit package in
the Northwest. Detailed discussions will take place once commercial availability and pricing is
known. The research team will stay fully in touch with the Northeast project partners on this
developing activity.
Honeywell expects to obsolete the C7650 sensor as the means of removing it from market
availability and expects a commercial launch of the new sensor in October 2008. No information
has been provided yet on sensor pricing.
Significantly, the utilities are now being viewed by this Honeywell product manager, as an
important customer group that has specific product functionality needs, in addition to the
traditional Honeywell customer base consisting primarily of the HVAC manufacturers, whose
product value needs are not necessarily consistent with utility and ratepayer needs. This is a
significant opening for utilities and energy efficiency organizations to be involved with
- 16 -
fundamental energy efficiency-related product development and design. This relationship with
Honeywell needs to be actively maintained and supported further as appropriate. There are
other energy performance and hardware-related opportunities that must be explored with the
HVAC manufactures directly. These will be further elaborated to the utility partners involved in
this project.
- 17 -
Appendix A– Description of Test Facility
The test apparatus was required to meet the following specifications:
•
•
•
Three independent temperature controlled test chambers.
o Dry bulb temperature controllable in the range 40-80 ºF.
Equipment test bench
o Log on/off status of at least 8 digital variables.
o Log 8 analog sensor outputs, 0-5 V, and 4-20 mA,
o Log economizer actuator output signal.
Communication system integration sufficient to store and archive test results both analog
data and digital data as appropriate.
o Log test conditions for dry bulb temperature and relative humidity
The chambers consist of three insulated boxed installed in a residential freezer as shown in
Figure 10.
Figure 10 - Environmental Chambers
Each box contains a small air circulation fan to maintain a uniform temperature. A computer
control system senses the temperature in each box and adds heat (by turning on a small light
bulb) as needed to maintain the specified temperature regime, as shown in Figure 10. Each box
can be programmed for a specified temperature regime, such as a specified change rate and
temperature range. For example, Figure 12 shows a program that varies temperature from about
44 to 72 ºF and back at a rate of about 1 degree per minute. This is a typical program to represent
the “outdoor” temperature range or OAT. While there is some fluctuation in the controller
temperature, the system can maintains an average temperature within about 0.1 ºF.
- 18 -
Figure 11 - Inside of Chamber
The test protocol alluded to in this report consisted of following a sensor/controller system
through a range of OAT temperature changes similar to that shown in Figure 12. We then
recorded the temperature points at which the controller operation changed to either economize or
not economize. These control points represent the “turn on” or “turn off” points described in the
report such as in Figure 1 and Figure 8.
T e m p e ra tu re in C h a m b e r
75
DegF
70
65
60
55
50
45
- 19 -
00
6:
00
15
15
:2
1:
00
Figure 12 - Example of Environmental Chamber Temperatures
:1
6:
00
:5
14
14
:4
1:
00
00
6:
14
14
:2
1:
00
:1
6:
00
13
:5
1:
00
:4
13
13
:2
1:
6:
00
00
13
:1
6:
00
:5
1:
12
:4
12
12
:2
6:
00
40
With multiple chambers, it is possible to investigate alternative strategies. For example, we test
the differential strategy by programming one chamber to represent the “return air” temperature
while another chamber represented the “outdoor” one. In this case, the test protocol specifies that
the chamber that represents OAT to follow a similar program to that shown in as described
earlier. At the same time, another chamber that represents RAT operates through a temperature
range that would vary between something like 65 to 75 ºF. We then record the temperature points
at which the controller operation changed to either economize or not economize. This protocol
then allows documentation of the conditions when economizer operations start and stop as
shown in Figure 1.
We built the apparatus with capability to measure other parameters, such as the mA sensor
output. That is because, at the onset, we were not sure what measured parameters might turn out
to be important. While we recorded mA outputs, it turned out that the controller operations could
be easily described using temperature measurements. The equipment is also capable of
measuring relative humidity although current work has focused only on dry-bulb controls.
Future work will investigate enthalpy controls.
Figure 13 shows a schematic of the test apparatus. The three environmental chambers are
indicated by different colors.
NBI ECONOMIZER TESTING SET-UP
TEST CHAM BERS
DS Adapter
USB
Hub
To U’s
MCC USB – 1208FS
Laptop w/Energy
Answer
A/D *4
Digital 1/0
*16
Honeywell
W 7459
Controller
Analog
Inputs
25W
AC Relay
x4
Port A
O utputs
Port B
O utputs
To W 7459
Input 1
0123
Multiplexer
4 x 8 input
1
24v
AC
Relay
Inputs
0A – 0H
DC Volt
Inputs
Digital
Input
1A – 1N
DC-MA
Inputs
U - DS 18B20 Sensor
… - AD 590 Sensor
24V AC
24vac
2
To Bulbs
2A – 2H
‘D igital’
Inputs
U
…
Honeywell
7650
sensor
3A – 3H
AD590
Sensors
To MA Input
To …’s
LED
To 10V
DC
50
To
MA Input
Photo
Diode
10 V
DC
24 V
AC
20 V
DC
On
W 7415
Actuator
120V Plug Strip
Off
Arm
To 3 AC
Relays
U
…
1
U Freezer
120 V
… Ambient
Figure 13 - Test Apparatus Schematic Diagram
- 20 -
Appendix B - Overall Economizer Control Test Description and Discussion
Purpose - This test is intended to establish the outside air temperatures associated with
economizer open and close points. This test is intended to be used with dry bulb temperature
sensors. In general, the economizer control may have several components (temperature sensors,
controller, actuator) that interact in the overall control function. Measurements on the individual
components, such as mA vs. temperature for the temperature sensors, lead to inconclusive results
due to lack of knowledge of the electronic details of the control approach. Regardless of the
electronic specifics of the controls, (or any sub component), all economizer controls ultimately
do the same thing: they open and close dampers at certain outside air, return air, and supply air
temperatures. This test seeks to quantify the performance of the economizer control system as a
whole.
A principal complexity of existing economizer controls is that the control points may depend on
the direction of change of the outside air temperature, increasing or decreasing. The control also
differs depending on whether or not the outside temperature has reached a sufficiently low
temperature trigger point. This dependency on a low temperature trigger point is referred to as
control hysteresis. The control test must be capable of identifying the circumstances leading to
the control points.
General Method - In general, the testing method seeks to vary the principal control parameters in
an orderly way, sufficient to exercise and observe the control system over the full range of
control conditions it can be expected to encounter. Here this is referred to as traversing the
control domain.
Single outdoor dry bulb sensor control - The simplest case involves a single dry bulb outdoor
temperature control. In this case, traversing the control domain consists only of varying the
outdoor temperature in both directions between upper and lower limits. This test uses a test setup
with a temperature controlled test chamber. The outside air temperature is simulated in the test
chamber, enclosing the outside air temperature sensor. This simulated outside air temperature
will be cycled from a high temperature, higher than the highest control temperature for the
subject control (usually 80-90ºF), to a low temperature, lower than the lowest control
temperature (usually about 45ºF). As the outside air temperature is cycled, the status of the
control output is recorded, open dampers (LED on), or close dampers (LED off).
- 21 -
Thermal Delay in Sensors
Tem perature, degF
66
Environment
Temperature
65
Program
64
Sensor Response
63
62.80
18
:2
9
18
:2
3
18
:1
7
18
:1
1
18
:0
5
17
:5
9
17
:5
3
62
Figure 14 - Thermal Delay in Sensor Response
We found that the sensors exhibit a large time response in responding to temperature changes.
Figure 14 exhibits an example. The test is programmed for step temperature change as shown by
the black line. The actual temperature in the environment boxes follows closely as shown by the
green data points. However, the response from the sensor, shown by the red line, exhibits a long
thermal decay response in adjusting to the temperature change. (The sensor output is actually in
mA but has been calibrated here to match the temperature units shown.) In this example, it takes
about 12 minutes for the sensor to fully equilibrate to a 1ºF temperature change. Given this long
delay, we elected to operate the tests with a full hour to allow for thermal equilibrium. Our initial
tests failed to recognize the importance of allowing for full thermal equilibrium. Note that a long
response is not necessarily an error in controls – it avoids control “chatter” if there is rapid
temperature cycling around the control point.
Economizer Response Test -
differential control
Temperature, degF
Outside Air or Return Air
75
70
65
60
55
50
45
40
0
60
120
180
240
300
360
420
480
540
600
Increasing Time, minutes
ON
Figure 15 - Economizer Response Test
- 22 -
OFF
660
720
780
840
The real problem in testing a control system is that there is often a very wide range of control
possibilities and the purpose of the controls test is to examine a broad enough range of control
possibilities that the full operation of the control is well understood. The challenge is to find
simple ways of presenting the multiple test results.
For the simple case of a single outdoor dry bulb temperature control, the test results are worked
into a graphical format such as Figure 15. The advantage of Figure 15 is that it maps the results
of an automatic test sequence consisting of hundreds of individual tests conducted in an orderly
sequence and range of conditions. For example, this figure shows that the dampers open
whenever the temperature is below about 56ºF. It also shows that the dampers are open at outside
air temperatures of 56 to 72ºF, but only if the temperature has been increasing from a
temperature below 55 ºF. It also shows that the dampers close at about 72ºF, and that the
dampers will remain closed in the range of 56 to 72ºF until the temperature again reaches 55ºF or
below. For the simple single sensor test, there need be only a few temperature cycles, not the
many cycles shown in Figure 12.
The operationally useful outputs from Figure 15 are the low ON control point (about 56ºF), the
high OFF control point (about 72ºF), and the recognition of the hysteresis pattern such that a
temperature of 55ºF or lower must be encountered in order for the dampers to open.
Figure 15 shows a simple single change point situation. Ideally, the outside air temperature
should have been cycled to a higher temperature to show operation of the control at temperatures
above the upper control point. In early tests, to limit duration of the test, the test control limited
the upper outside air test points to the high control point.
Enthalpy Sensor Control – This is a potentially complex control system to test because it
involves varying the humidity as well as the dry bulb temperature. Traversing the control domain
for this situation will involve traversing the psychrometric chart in a manner that is
experimentally achievable to establish a performance surface. The resulting data will then need
to be interpolated and reassembled to express the control system performance compared with the
parameters of interest.
- 23 -