Cost-effective boiler system retrofits
Retrofits that directly reduce operating costs are readily available, in most cases with well-documented savings and quick paybacks.
The cost of energy is a significant portion of the operating expenses for any commercial building. Yet these same energy expenses are often the single largest controllable cost of operations, according to the U.S. Environmental Protection Agency . Because the old adage “a penny saved is a penny earned” applies, the boiler room is one key area to earn that penny—and then some.
Table 1 outlines several cost-saving retrofits proven to pay back quickly, making excellent investments for commercial boiler systems with inputs in the range of 300,000 Btu/hr to 12 MBtu/hr.
One of the best ways to ensure efficient boiler operation is though a high-efficiency tune-up, which is applicable to both steam and hot water boilers. The benefits of performing this sort of high-efficiency tune-up on certain types of boilers are well demonstrated. Savings average about 4% to 5% of heating energy use with paybacks generally less than one year.
To be most effective, the tune-up must go beyond the basic cleaning and combustion check most contractors perform to focus on measures that decrease excess air and reduce stack temperature: adjusting draft control, sealing a leaky combustion chamber, installing flue restrictors (when allowed), up-rating or de-rating input, checking the adequacy of combustion air, and optimizing modulating burner linkage settings (when applicable).
If the boiler fuel is gas, the gas meter should be clocked using a stopwatch (when only the boiler is operating) to determine if the boiler input is correct. For oil-fired equipment, the nozzle flow rate and angle should be verified and the oil pump pressure measured to confirm the correct input.
In a tune-up beyond the basic, the primary objective is to reduce excess air to the minimum, while leaving a safety margin to prevent incomplete combustion. A flue gas analysis test to assess the combustion efficiency and fuel-air ratio should be done initially to determine whether the boiler is a good candidate for a high-efficiency tune-up, and then repeated throughout the tune-up to guide the adjustment process.
However, a tune-up should be considered on dual fuel burners if the combustion analysis on the gas-side indicates high-O2and stack temperature readings ( Table 2 ). Boilers that have power burners also should be considered for tune-ups if they meet the same criteria as for dual fuel burners.
Packaged, gas-designed boilers with atmospheric burners generally do not make good tune-up candidates because modifications are often limited by code requirements and the physical characteristics of the equipment. One exception might be when the boiler is considerably under-fired. In that case, up-rating to the input on the boiler nameplate is recommended.
De-rating is an option for an over-fired boiler, usually the result of successive burner replacements, each with a larger firing rate due to multiple margins of safety added by inexperienced contractors. Over-firing causes short-cycling of the boiler, higher flue gas temperatures, and lower steady-state and seasonal efficiencies. Savings can be up to 5% more than the 4% to 5% usually expected for a high-efficiency tune-up.
It also is important to optimize modulating burner linkage settings when applicable. Because it is difficult to maintain the proper fuel/air mixture across the entire firing range of a modulating burner, estimate the part of the firing range at which the boiler will operate most frequently and adjust the linkage for maximum steady-state efficiency in that part of the firing range. This is a much less expensive and reliable option than installing expensive and hard-to-maintain oxygen trim controls.
For cast iron boilers in multi-zone hot water systems, reset controls (lower control in Figure 2) that directly vary water temperature as a function of outdoor temperature can outperform systems that operate on a constant temperature aquastat. Savings average 10% to 15% of heating energy use, yielding a payback of generally two years or less. Even in cases where an operator is practicing manual setback of boiler water temperature, field studies show average savings of 10% of heating energy use.
On steel fire tube boilers, resetting boiler water temperature directly carries greater risks of thermal shock and corrosion. A better strategy in this case is to reset the distribution water using a three-way mixing valve, although this is more expensive due to both the piping involved and the need for a control that can modulate the valve. Savings are also somewhat lower, resulting in paybacks of five years or more. An alternative is to install a reset control that has the option of setting a minimum water temperature. This is considerably less expensive, but energy savings are also considerably reduced. Savings for a reset with a minimum temperature are estimated at 5% to 10% with an expected payback of two to four years.
After specifying a reset, it is important to verify that the control is installed, adjusted, and working properly because malfunctioning controls are not uncommon and will reduce or eliminate any gains. In addition, even if a boiler has an existing reset control, it is always good practice to confirm operation as it may be set incorrectly or no longer functioning properly, suggesting the need to replace it.
When specifying a replacement boiler, a reset control always should be recommended. Some boilers come with integral reset controls. However, some integral controls may not optimize savings and comfort due to poor operating logic or inflexible setting options. As a result, an after-market reset control may be worth considering.
Reset controls also can be used on very high-efficiency condensing boilers. Reset controls also are available with additional functions such as: boiler staging in situations with multiple boilers or a single multiple-stage boiler, regulation of a service hot water pump or valve in situations where the boiler provides both heating and service hot water, or boiler cutout as described in the next section.
Because it is impractical for a building operator to constantly cycle the boiler on and off manually to accommodate temperature swings and heating demand during the swing seasons, most operators simply let the boiler run from start-up in the fall to shutdown in the spring. This results in wasted heat on days when the boiler is not needed to meet heating demand. For commercial buildings, this is usually when the outside temperature is 45 to 55 F.
An outdoor cutout control solves this problem by automatically turning the boiler off whenever the outdoor temperature reaches a preset limit (upper control in Figure 2). Such controls typically save in the range of 3% to 5% of heating energy use with paybacks of two to three years.
A cutout control is applicable to both steam and hot water boilers. For hot water boilers, a cutout option is often available as part of a reset control, making it convenient and often less expensive to specify both functions with one control (Figure 3 and Table 1 ).
Many steam boilers are controlled by a thermostat with a tight span setting, which causes the boiler to cycle on and off frequently in response to small deviations from the setpoint. Short cycling also can be caused by pressure controls with cut-ins set too high (for single pipe steam they should be set to 0) and cutouts set too low. Short cycling results in uneven heating, especially in the spring and fall, causes undue wear and tear on the boiler, and quickly degrades the seasonal efficiency of a steam boiler.
Solve this by installing a thermostat or other control with a widely adjustable dead band (or differential) that can be used to establish the appropriate boiler cycle length (Figure 4). Such control devices are available with night setback capability as well as with one or more remote sensors. This way the control itself can be installed out of harm’s way (for example, in the boiler room) and the remote sensor(s) can be mounted in an occupied space (Figure 5).
When a control has more than one remote sensor, it may operate the boiler based either on the average temperature of all remote sensors, or on the sensor with the coldest reading. In either case, sensor location is an important consideration. Locating a sensor too close to the boiler room may in itself result in the short-cycling you are trying to avoid. The sensor(s) must be located far enough from the boiler to keep the boiler firing long enough to get heat to the farthest parts of the building and fill all radiators with steam (or hot water in the case of a single-zone hot water system).
Although widely available, a steam control that operates the steam boiler based on outdoor temperature is typically not a good choice for most situations. This type of control tends to operate a boiler for shorter cycles in mild weather and correspondingly longer cycles as the weather gets colder. Intuitively, this may seem like the correct approach, but it is the opposite of what works best—in mild weather the boiler actually needs a longer cycle. Because a building requires less heat in mild weather, the length of time between cycles is typically longer, allowing the radiators and attached piping to cool down completely between cycles.
Conversely, in cold weather the radiators and attached piping stay warm/hot between cycles. A longer cycle is required to heat a system that is cold (the situation in mild weather) and a shorter cycle to heat one that is already warm/hot (the situation in cold weather). Therefore, a steam control that operates the boiler based on outdoor temperature often leads to more uneven heating and higher fuel costs.
Savings for installation of an appropriate cycle control are typically about 3% to 5% of heating energy use but can be much higher in cases of severe short-cycling. Paybacks can be immediate up to three years. This type of thermostat is also appropriate to use on a single-zone hot water heating system.
Main line air vents
Uneven heating in single pipe steam buildings causes major comfort issues, and substantial energy waste. Uneven heating is usually the result of short boiler cycles combined with large differences in steam arrival times. The solution involves two measures. The first is to install a properly located thermostat with an adjustable dead band. The second is to add very high capacity main line air vents to reduce the difference in steam arrival times to different radiators within the building. A vent with an orifice of 5/16 in. is needed on the longer main lines, while smaller capacity vents are fine for short lines (Figure 6).
Savings for adding larger main line air vents vary depending on how out-of-balance the building’s steam delivery actually is, but are generally in the range of 5% to 10% of heating energy use, with typical paybacks of one to two years.
Most two-pipe steam systems have steam traps located both on the condensate return lines and at the outlet side of each radiator (Figure 7). The purpose of the steam trap is to allow air (or other gases) to vent out of radiators and main distribution lines, and to prevent steam from going beyond its point of use. Unfortunately, steam traps fail frequently, typically in the open position.
A stuck-open radiator trap causes steam to escape into the condensate return line, and if traps on the return lines have also failed open, steam can escape into the atmosphere. Alternatively, in the stuck-closed position, a radiator becomes air-locked and steam cannot enter. In either case, failed traps lead to uneven heating, substantial energy losses, and much higher fuel bills.
Life expectancy for a typical mechanical trap in a heating system is three to five years. As a result, steam traps need to be checked routinely to ensure proper operation, and repaired or replaced as needed. Proper trap selection and sizing are important in this process. Diagnosing whether a trap is broken (especially radiator traps) requires a certain level of skill and appropriate testing equipment. As a result, some facilities find it most efficient to simply replace one-quarter of the facility’s traps annually so that at the end of four years every trap has been replaced. Then the replacement cycle starts over.
Savings for this retrofit are not well documented, but could be substantial. Paybacks are thought to be less than two years for line traps and somewhat longer for radiator traps. An alternative to steam trap replacement is discussed in the next section.
Steam trap maintenance is an expensive and recurring cost associated with operating a two-pipe steam system. In addition, diagnosing broken traps can be a difficult and time-consuming process even with experienced personnel and suitable equipment.
An alternative to continuous steam trap maintenance is to abandon steam traps in place and install an orifice (or restriction) across the inlet-side of the piping to each radiator. The purpose of this orifice is to limit the speed at which steam enters each radiator to the rate that matches the radiator’s actual heat output. The size of the orifice hole therefore depends on the heat output (or size) of the radiator it will be installed on; larger orifices are used for larger radiators and smaller orifices for smaller radiators.
Once properly sized orifices are installed on each radiator, steam trap maintenance is no longer required as the orifice will restrict the volume of steam that can enter each radiator to the amount that the specific radiator can condense. As a result, all the steam that enters the radiator will condense within the radiator itself and only condensate will reach the return side of the radiator.
Eliminating steam trap maintenance is a huge advantage. Other advantages are more even heating (and comfort) in the building and reduced fuel use. In most cases, the size of a replacement boiler for a building with radiator inlet orifices can be decreased because the peak load required from the boiler will more closely match the actual output of the attached radiation load, requiring less pickup factor to be built in.
Inlet orifices also can be used to downsize a specific radiator’s capacity. Installing a smaller orifice than the radiator’s actual heat output would require, effectively limits the amount of heat produced by that radiator. This may be useful in areas where the total output of a particular radiator is not needed.
Savings for this retrofit are not well documented but could be substantial. One facility has reported anecdotally that its fuel costs dropped by about one-third after installation of inlet orifices. Paybacks for this strategy are not known but are probably less than three years.
Boiler system retrofits that directly reduce operating costs are readily available, in most cases with well-documented savings and quick paybacks. Every dollar saved by reducing operating costs is one less dollar that has to be earned, and in this economy that’s a worthwhile investment. Many of the retrofits and upgrades discussed in this article require careful specification, detailed installation instructions, and follow-up to ensure the measure was properly implemented.
Table 1 : Fast payback central heating boiler system retrofits
|Retrofit||Application||Estimated cost||Estimated savings||Estimated payback||Hot water||One-pipe steam||Two-pipe steam|
|Applicable boiler system|
| Notes |
3Additional savings of up to 5% are quite possible if boiler is over-fired, or modulating burner linkage settings need to be adjusted.
4This type of control is applicable to multi-zone hot water systems but not to single-zone.
5This type of control is applicable to single-zone hot water systems but not to multi-zone systems.
6Bottom of range for parts only; top of range includes parts and typical contractor’s labor assuming multiple installations.
|High-efficiency boiler tune-up||$275 – $500||4% – 5% 3||0 – 1 year||X||X||X|
|Reset control||Cast iron boiler, water temperature reset directly||$400||10% – 15%||1 – 2 years||X4|
|Steel fire-tube boiler, water temperature in distribution system reset using mixing valve||$3,000 – $5,000||5% – 13%||5 years or more||X4|
|Steel fire-tube boiler, water temp reset directly with Minimum temp. setpoint||$800 – $900||5% — 10%||2 – 4 years||X4|
|Reset/cutout control||Cast Iron boiler, water temp. reset directly||$500 – $800||12% — 18%||1 -3 years||X|
|Cutout control||$300||3% – 5%||2 – 3 years||X||X||X|
|Control to reduce short-cycling||$500 – $800||3% – 5%||0 – 3 years||X5||X||X|
|Larger main line air Vents||$300 – $350 per vent||5% – 10%||0 – 2 years||X|
|Steam trap repair/replace||Main line traps||$100 – $500 6 per trap||unknown||& 2 years||X|
|Radiator traps||$40 – $1756per trap||unknown||3 – 5 years||X|
|Inlet orifices||$15 – $150 6 per radiator||unknown||probably & 3 years|
Table 2 : Candidates for high-efficiency tune-ups
|Type of boiler||Flue gas analysis O 2 reading equal to or greater than||Flue gas analysis stack temperature equal to or greater than|
|Coal-to-gas conversion burner||5%||400 F|
|Power burners||6%||450 F|
|Dual-fuel burners||6% on gas-side||450 F on gas-side|
|Lobenstein is a freelance energy consultant and writer in Minneapolis who has conducted energy evaluations, efficiency studies, field research, and technology assessments for more than 28 years, and has extensive first-hand experience in transferring successful technologies and strategies to the building sector. Hewett , director of research at the Center for Energy and Environment, has conducted research on many aspects of energy efficiency and IAQ, including field tests of a number of boiler system retrofit measures, and is currently the chair of ASHRAE Standard Project Committee 155P — “Method of Testing for Rating Commercial Space Heating Boiler Systems.” Katrakis, founder and president of J. T. Katrakis & Assocs. Inc., has more than 28 years of experience in the design and upgrade of buildings for enhanced energy and environmental performance for clients that include commercial, institutional, and industrial building owners and managers; architectural and engineering firms; energy service companies; local utilities; and government agencies.|
Rebates and incentives
Government, utilities, and/or nonprofit organizations currently offer incentives (often substantial) to building owners who complete energy-efficiency improvements such as the ones described in this article. Below is a list of useful resources for rebate information.
State and Local Energy Efficiency Programs Site is a resource for finding local and regional programs to help businesses become energy efficient: www.business.gov/expand/green-business/energy-efficiency/state-local/
Database of State Incentives for Renewables & Efficiency (DSIRE) is a comprehensive source of information on state, local, utility, and federal incentives and policies that promote renewable energy and energy efficiency. www.dsireusa.org/
Tax Incentives Assistance Project (TIAP) has information needed to make use of federal income tax incentives for energy-efficient products and technologies. https://energytaxincentives.org/
State Incentives and Resource Database is a repository of energy incentives, tools, and resources for commercial and industrial managers. Sources include federal, state, county, and local governments; utilities; private companies; and nonprofits. www1.eere.energy.gov/industry/states/state_activities/incentive_search.asp
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