Laboratory evaluation of aftermarket boiler control system.
Gas-fired boilers provide heating for over 30% of commercial heated floor space in the U.S. and have median lifetimes of 25-35 years. (1), (2) A retrofit solution that can provide energy savings and reduce [CO.sub.2] emissions would be a cost-effective option for existing commercial boilers relative to a replacement upgrade. Aftermarket boiler control systems are designed to be installed on existing boilers with minor interruption to boiler operation. By measuring the temperature at the boiler return and supply, or the boiler return only, these systems monitor heating loads at the boiler and adjust boiler cycling through the aquastat controls to prevent unnecessary and inefficient operation. The control device evaluated in this report incorporated an algorithm to evaluate prior demand patterns, current demand ramp up, and other factors to delay firing during periods of perceived low demand and release it to fire during significant heating demands. Aftermarket boiler control systems are seeing initial market and technical success and are now candidates for utility-sponsored energy efficiency programs. Commercially available aftermarket boiler control systems have reported energy savings up to 30% in some cases, while other field evaluations of these devices have shown marginal reductions in gas consumption. (3), (4)
The goal of this study is to evaluate an aftermarket boiler controller system in a controlled laboratory setting to evaluate its performance relative to baseline operation during typical building heating load patterns. The laboratory test stand also provides a means to assess the variability of these gains relative to load profile and temperature settings in order to determine the parameters for optimal performance and identify the best applications for maximum energy savings relative to climate and building type.
LABORATORY TEST SETUP
An automated laboratory test stand was developed to evaluate the performance of a hot water boiler with aftermarket controls and to quantify the range of energy savings that could potentially be achieved in field installations. The laboratory test stand was used for steady state testing and to simulate the heating demand from a typical building. The test stand, shown in Figure 1, consists of a 195 MBH (57 kW) residential boiler installed in a primary/secondary configuration. While the single-stage residential sized hot water boiler is a fraction of the size of typical boilers this device is installed on, the test stand was operated in a manner representative of typical installations, in agreement with the controls manufacturer. The boiler circulator pump and the secondary loop pump operate continuously throughout the tests at approximately equal flow rates, equivalent to a hydronic system simple circuit configuration.
[FIGURE 1 OMITTED]
The laboratory test rig was constructed to reproduce a given heating load pattern from the perspective of the test boiler. The secondary loop includes a 60 gallon (227 L) storage tank to simulate the total pipe volume in a typical hydronic installation based on recommendations from the manufacturer. The heating load at the boiler is generated by a cooling water loop circulating through a heat exchanger in the secondary loop. The cooling loop incorporates an electric chiller, storage, and a variable speed pump. A second heat exchanger added to the cooling loop provides the higher heating loads required for the winter heating demand profile and pipe heat loss. A real-time data acquisition system collects natural gas pressure, gas volume flow rates, water flow rates, water and gas temperatures every second during steady state testing and every ten seconds during 24-hour tests. Water temperatures are recorded at the boiler supply/return and throughout the secondary loop. The boiler heat output is calculated based on the temperature difference at the boiler supply and return, and the flow rate of the boiler circulator pump.
The heating load at the boiler was controlled by the flow rate in the cooling loop using a variable voltage controller on the pump. Data collected during calibration test runs was used to determine the relationship between the input voltage to the pump controller and the corresponding heat demand measured at the boiler. Automating the pump controls in the cooling loop allows the test stand to simulate variations in heating load at the boiler to match a given 24-hr heating load profile of typically building.
ENERGY PLUS MODELING OF HEATING LOAD
Heating profiles were determined using Energy Plus models based on DOE reference buildings for a secondary school, post 1980, using climate data for Chicago using TMY3 data (5). The Energy Plus model boiler capacity was based on a scale factor of 1.2 peak load and the 24 hour secondary school heating load was scaled for the test setup using the ratio of the reference building boiler capacity to the capacity of the test boiler
Twenty-four hour heating profiles were selected based on average outdoor temperatures. A winter heating profile, scaled for the test setup, is shown in Figure 2. During the 24-hour period, the default building energy management system turns on the HVAC for 1 minute every 20 minutes during the morning (hours 1-6) and evening (hours 21-24) while the building is unoccupied. At 6:00 am the HVAC is turned on, creating a rapid increase in demand which then reduces to a slightly lower level during the remainder of the day. The HVAC system returns to the night cycle settings at 21:00.
[FIGURE 2 OMITTED]
The aftermarket boiler controller was provided and installed in the test stand by the manufacturer. The boiler control system consists of a small electronics box which cuts into the boiler aquastat wire to control the boiler. Two surface-mounted temperature sensors are installed on the primary loop tubing at the boiler supply and return. Installation was completed in less than 2 hours with no disruption to the heating system. The manufacturer also participated in a design review of the test stand and testing protocol.
The test matrix for this project is shown in Table 1. Thirty minute steady-state tests and 24-hr heating profiles based on Energy Plus models were conducted using the test stand with the gas-fired boiler. Baseline tests were conducted with the aftermarket controller powered off. For both the baseline and controller test runs, the boiler aquastat setpoint was 175F (79C) with a differential of 5F (3C), corresponding to a high temperature limit of 175F (79C) and a low limit of 170F (77C). In addition, a third test run was conducted for each test condition to compare the performance of the aftermarket controller with simply increasing the boiler differential from 5F (3C) to 15F (8C), corresponding to an aquastat low limit of 160F (71C). In actual practice, boiler differentials can be higher or lower than 15F (8C) depending on the installation. Based on the manufacturer specifications for this aftermarket controller, it is only effective on boilers with aquastat differentials less than 15F (8C).
Table 1. Proposed Boiler Control Test Matrix DOE Reference Heating Load Test Condition Aquastat Setpoint Building ([degrees]F/[degrees]C) Steady State Low Heating Baseline 175/79 Demand Testing (120 MBH [35 Aftermarket 175/79 kW]) Controller Baseline with 175/79 15F (8C) Differential High Heating Baseline 175/79 Demand (159 MBH [47 Aftermarket 175/79 kW]) Controller Baseline with 175/79 15F (8C) Differential Secondary Average Baseline 175/79 Winter School, Aftermarket 175/79 Chicago, Controller Post-1980 Baseline with 175/79 15F (8C) Differential Shoulder Baseline 175/79 Season Aftermarket 175/79 Controller Baseline with 175/79 15F (8C) Differential DOE Reference Aquastat Differential Test Duration Building ([degrees]F[degrees]C) (hr) Steady State 5/3 0.5 Testing 5/3 0.5 15/8 0.5 5/3 0.5 5/3 0.5 15/8 0.5 Secondary 5/3 24 School, 5/3 24 Chicago, Post-1980 15/8 24 5/3 24 5/3 24 15/8 24
Steady State Testing
A summary of the steady state test results are shown in Table 2. The aftermarket controller reduced the number of boiler cycles by almost half, as compared to baseline tests. Fuel consumption was also reduced during both high and low heating demand by 9.0% and 3.4%, respectively. In baseline tests, the boiler cycles approximately same amount under low or high demand (10.6 vs. 11 cycles), but since the total amount of natural gas consumed is significantly less with a lower heating demand, this reduces the opportunity for natural gas savings at lower heating demand.
Table 2. Summary of 30-Minute Steady State Testing Temp. Gas Usage Reduction in No. of Differential (cu. ft./ Gas Usage Cycles ([degrees]F/ cu. m.) [degrees]C) High Heating Demand (159 MBH 47 kW]) Baseline 5/3 56.1/1.6 11.0 Aftermarket 5/3 51.1/1.5 9.0% 6.0 Controller Baseline with 15/8 51.8/1.5 7.7% 7.0 15F (8C) Differential Low Heating Demand (110 MBH [35 kW]) Baseline 5/3 26.5/0.8 10.5 Aftermarket 5/3 24.6/0.7 3.4% 4.0 Controller Baseline with 15/8 23.5/0.7 5.4% 5.0 15F (8C) Differential Secondary Loop Temp. Decrease Temp ([degrees]F/ ([degrees]F/ [degrees]C) [degrees]C) High Heating Demand (159 MBH 47 kW]) Baseline 167.5/75.3 Aftermarket 162.6/72.6 4.8/2.7 Controller Baseline with 162.8/72.7 4.6/2.6 15F (8C) Differential Low Heating Demand (110 MBH [35 kW]) Baseline 169.8/76.6 Aftermarket 160.2/71.2 9.6/5.3 Controller Baseline with 162.2/72.3 7.6/4.2 15F (8C) Differential
During steady state tests, increasing the boiler aquastat differential from 5F (3C) to 15F (8C), i.e. changing the boiler lower limit from 170F (77C) to 160F (71C), also reduced the number of cycles and natural gas consumption. During the 30 minute test, the aftermarket controller reduced the number of cycles from 11 to 6, while the 15F (8C) differential reduced the cycles to 7. Similarly, increasing the differential to 15F reduced fuel consumption by 7.7% and 5.4% at high and low demand, respectively, compared to the energy savings produced by the aftermarket controller of 9.0% and 3.4%.
Another factor to consider is the lower secondary loop temperatures resulting from both the aftermarket controller and the increased aquastat differential. The aftermarket controller reduced the secondary loop average temperature by 4.8F (2.7C) at high demand and 9.6F (5.3C) at low demand, compared to the baseline. Increasing the boiler aquastat differential to 15F lowered the secondary loop temperature at low and high demand by 4.6F (2.6C) and 7.6F (4.2C), respectively. Lower secondary loop temperatures may potentially affect building room temperatures or occupants comfort, but comfort issues were not evaluated within this test scope.
Table 3 contains a summary of 24-hr winter heating profile test results. Reductions in natural gas consumption produced by the aftermarket controller ranged from 4.8% to 11.5% compared to the baseline case. As seen in the steady state tests, the aftermarket controller produced the highest energy savings during occupied hours with higher heating load than the unoccupied hours. In addition, the number of cycles was reduced by 57% compared to the baseline. Increasing the aquastat differential from 5F (3C) to 15F (8C) also reduced the number of boiler cycles by 45% and gas consumption by 4.2%.
Table 3. Heating Profile Evaluation Gas Usage Reduction in Number of Reduction in (therms/MJ) Gas Usage Cycles Cycles Series 1 Baseline 10.63/1,121 380 Aftermarket 9.4/992 11.5% 164 56.8% Controller Series 2 Baseline 11.96/1,262 388 Aftermarket 11.38/1,200 4.8% 184 52.6% Controller Baseline with 11.46/1,209 4.2% 212 45.4% Increased Differential
The laboratory boiler test stand developed for this study provides a useful and flexible tool to simulate different buildings and climate zones in order to evaluate boiler controls and performance. This test method can be used to identify building applications and climates where an aftermarket controller can provide the most energy savings. The researchers will continue to evaluate these aftermarket control devices using further variations in simulated building type and climate zone.
* Aftermarket controller can reduce energy consumption up to 11% for a 24 hr period and reduce boiler cycling by up to 57%.
* The largest energy savings by the aftermarket controller occurred during high demand, most likely due to the higher energy use resulting in a greater opportunity for reducing gas consumption.
* In laboratory tests, the boiler aquastat differential had a significant impact on the energy use and cycling of the boiler.
* The benefit of the aftermarket controller is very dependent on the existing boiler setpoints. It will not provide significant savings for boilers with a differential of 15F (8C) or more (in agreement with manufacturer specifications). This may explain some of the conflicting reports about the field performance of these units.
* Both the aftermarket controller and a larger aquastat differential decreased the test stand secondary loop temperatures by as much as 10F (6C) below baseline testing. Field testing is needed to understand the effect of lower temperatures on building comfort and condensation issues.
* Although these tests have demonstrated the potential for fuel savings by increasing the aquastat differential, i.e., widening the primary loop temperature band, there are potential risks associated increasing the variance that may compromise the integrity of the boiler. Depending on the boiler setpoint temperature, increasing the variance of the primary loop may increase the potential for condensation within the flue and increase the magnitude of thermal cycling the heat exchanger.
(1.) Energy Information Administration, 1995 Commercial Buildings Energy Consumption Survey.
(2.) Building Energy Data Book, Table 5.3.9 Major Commercial HVAC Equipment Lifetimes and Ages, http://buildingsdatabook.eren.doe.gov/docs/xls_pdf/5.3.9.pdf.
(3.) Manufacturer's website, Greffen Systems, M2G Product Specifications, http://greffensys.com/products.php, January 2012.
(4.) Cove, G., Hammer, J. and Butcher, T., A Technology Demonstration and Validation Project for Intellidyne Energy Saving Controls, New York State Energy Research and Development Authority (NYSERDA).
(5.) Wilcox, S. and W. Marion. 2008. User's Manual for TMY3 Data Sets, NREL/TP-581-43156. April 2008. Golden, Colorado: National Renewable Energy Laboratory.
Paul Glanville, PE
Associate Member ASHRAE
Patricia Rowley is a Senior Engineer at the Gas Technology Institute, Des Plaines, IL. Paul Glanville, PE is a Principal Engineer at the Gas Technology Institute, Des Plaines, IL
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|Author:||Rowley, Patricia; Glanville, Paul|
|Date:||Jul 1, 2012|
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