HVAC Systems: Boilers
You can reduce heating costs by ten percent or more when you install a high-efficiency gas-fired boiler system. Calculate your potential energy savings |
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Overview
Space heating energy costs account for roughly 25-30 percent of the total energy costs for a typical commercial building in the upper Midwest. High-efficiency boilers can reduce heating costs by 10 percent and in many cases by as much as 20 to 30 percent.
In fact, a boiler replacement that reduces gas consumption by 20 percent could save about five to 10 cents per square foot annually.
Boilers are available in two efficiency ranges: around 80 percent for standard conventional boilers, and percentages in the 90s for energy-efficient condensing units.
The dividing line between these efficiency ranges is based on the boilers ability to withstand condensing flue gases. Condensing flue gases, which occur in high-efficiency boilers, require special design considerations to tolerate the corrosive effects of the condensate.
What are the options?
Size
Boiler size is a measure of boiler heat capacity in Btu per hour for smaller units, or boiler horsepower (bhp) for larger units. One boiler horsepower is equal to 33,475 Btu per hour.
The table below shows sizing ranges for different facility types:
Boiler Sizes | |
Typical Facility |
Size – Btu/hr (bhp) |
Small commercial or residential |
67,000-3,400,000 (2-100) |
Medium commercial or small industrial |
2,500,000-10,000,000 (100-300) |
Large commercial or large industrial |
10,000,000-33,500,000 (300-1000) |
A common rule of thumb is one bhp per 1,000 square feet.
Cautious designers often oversize boilers, sometimes two to three times the required heating capacity. The practice of oversizing boilers can decrease the efficiency and operating life of a boiler because of short cycling (frequent on-off cycles).
Energy efficiency
Boiler efficiency is defined as how much of the heating value of the fuel is being converted to useful heat.
- Condensing boilers absorb more heat from combustion gases, allowing the water vapor to condense and therefore providing increased efficiency. Any hydrocarbon fuel burned in a boiler, whether it is propane, natural gas, or fuel oil, produces water vapor during the combustion process.
- Conventional boilers are non-condensing boilers with materials that cannot tolerate the corrosive properties of condensing flue or stack gases. Conventional boilers operate around 80 percent efficiency, compared to over 90 percent efficiency for condensing efficient boilers.
Other factors also influence boiler efficiency, including boiler shell losses, piping losses, and cycling losses.
Fire tube and water tube
Boilers use water to absorb heat from a burned fuel/air mixture. Boilers can produce steam or hot water. The two most common types of boilers are fire tube and water tube.
- Fire tube boilers typically consist of a series of straight tubes that are housed inside a water-filled outer shell. As hot gas flows through the tubes it heats the water that surrounds the tubes.
- Water tube boilers are designed to circulate hot combustion gases around the outside of a large number of water-filled tubes. Newer boilers have tubes with complex and diverse bends and fins to maximize the heat transfer area.
Because the water/steam pressure is confined inside the tubes, water tube boilers can be fabricated in larger capacities than fire tube boilers and are often preferred for higher-pressure steam applications.
Steam or hot water
Boilers can be selected to supply hot water or steam, or both. Because they operate at lower temperatures, hot water boilers can operate at higher fuel conversion efficiencies than steam boilers.
Steam boilers typically range in efficiency from 75-83 percent, where a hot-water boiler ranges in efficiency from 80-93 percent.
In addition, the higher supply temperature of a steam system results in increased losses from the piping or distribution system and the shell of the boiler, compared to a hot water boiler.
Controls
Various methods are used to control the output of a boiler. Burner control, and supply water temperature control, or a combination of both, are often used to meet the heating load.
Hot water boilers control the burner based on a water temperature setpoint, typically 140-180F, where steam boilers used for space heating are controlled based on a pressure setpoint, typically 5-10 psi.
Efficient boiler control minimizes the number of on-off cycles. When a boiler is cycled on or off, a purging of the combustion chamber takes place before and after firing to prevent explosions of combustible gases that may have accumulated in the boiler.
This process wastes heat; in addition, frequent cycling of a boiler decreases its useful life because of the thermal stresses associated with each on-off cycle.
Most boiler burner combustion controls fall into 3 categories:
- On-off control is the least expensive but also typically the least efficient because of the losses associated with each cycle. Often boilers are oversized causing frequent on-off cycling. The associated pre and post-purge losses can be substantial.
- Low-high-low combustion control is also fairly inexpensive but more efficient and effective than on-off control. The boiler starts and stops with the typical pre and post purge and then cycles between a low-fire and max-fire condition to meet the heating load.
- Modulating control is generally more expensive but also the most efficient and most effective at handling varying heating loads. Modulating control turns down the firing rate of the burners to meet the heating load. Modulating control is specified by the turndown ratio, which is the ratio of the full firing output to low fire output. Most modulating control boilers will have a turndown ratio of 5 to 1 and up to 10 to 1 for expensive burners.
At low fire conditions the flow of combustion gases through the boiler is less and more heat can be captured before the combustion gases exit the boiler.
Another method to improve overall efficiency of a boiler is called hot water temperature reset. Hot water temperature reset decreases the temperature of the hot water depending on outdoor conditions.
Along with improving the overall efficiency of the boiler system, temperature reset also provides better temperature control and reduces other problems like overheating spaces.
A primary-secondary decoupled system can be used to allow for resetting the temperature in the secondary or distribution loop, while still maintaining the required return water temperature for the boiler.
Stack gas economizers and recuperators
Maximizing boiler efficiency means minimizing the waste heat energy losses in the stack gases. A 100F reduction in stack gas temperatures can result in a 2.5 percent increase in efficiency.
A stack gas economizer can be implemented on a conventional boiler to capture some of the heat of the exhausted combustion gases. Stack gas economizers are heat exchangers that capture heat from the exhausting gases and transfer it to the return water going into the boiler.
Recuperators increase the efficiency of a boiler by transferring some of the heat from the exhaust combustion gases to the intake combustion air.
A stack gas temperature greater than 400F on a larger boiler of 300hp or greater is typically required to make these technologies worthwhile.
O2 trim
Larger boiler systems will implement active flue gas monitoring to maintain an ideal flow of combustion air into the boiler.
This minimizes excess air and wasted heating of excess flue gases that go up the stack, as well as provides hotter combustion gases for greater heat transfer and combustion efficiency.
Circulating pumps for hot-water boilers
The circulating pumps in a hot water system distribute the water to spaces and processes where it is needed.
Typically, a hot water system is designed with a primary and secondary loop. The primary loop pump continuously circulates water through the boiler while the secondary loop pump circulates hot water to individual spaces and processes.
Along with using efficient pump motors, one can implement variable speed drives (VSDs) to decrease the speed of the motor driving the pump depending on the heating load or demand.
Savings for variable speed pumping will be greatly reduced if hot water temperature reset is implemented; reset should be provided first if the boiler and system allow for it.
Boiler tube turbulators
Boiler tube turbulators can be installed in fire tube boilers to increase the heat transfer of the combustion gases. Boiler tube turbulators are placed inside the fire tubes and may increase boiler efficiency by 1.5 percent.
Making the best choice
The table below gives general selection guidelines for implementing some of the energy efficiency options detailed below. Evaluating these options may require an engineer or consultant because there are multiple factors to consider.
Facility Requirements |
Condensing Boiler |
Stack Gas Economizers |
O2 Trim |
Supply Water Temp. Reset |
Small facility space heating (2-100hp) |
X |
X | ||
Medium facility space heating (100-300hp) |
X |
X |
X | |
Large facility space |
X |
X |
X | |
High pressure steam |
X |
X |
Hot water versus steam
Although hot water systems are typically more efficient than steam systems, there may be requirements that dictate the use of a steam system over a hot water system.
Most of the time steam is required because of its ability to provide vast amounts of heat quickly. This is beneficial in a facility that must heat 100 percent outside air for occupant ventilation or a hospital that needs to do sterilization and requires high temperatures.
Another application where a steam system can be a good selection is in large centralized heating plants for schools and campuses. For these applications, it is easier to circulate steam and handle large fluctuating heating demands with a steam system.
Steam systems require substantially more maintenance than hot water systems including water treatment for dissolved solids, PH control and oxygen scavenging and should not normally be considered for space heating of typical commercial or industrial facility.
Steam traps can often be a source of losses and are costly to maintain. The purpose of a steam trap is to maintain the steam in the heating element and to release the condensate. Steam traps are mechanical devices that are subject to wear, corrosion and eventual malfunction or failure.
A typical steam trap may open thousands of times a year, making wear and/or failure is inevitable. The most costly failure from an energy standpoint is when the steam trap fails open because steam will pass through the steam trap all of the time.
If you are looking at replacing a steam heating system with a hot water system, it is important to consider both energy and maintenance costs of the system when attempting to justify the change. In many cases, the maintenance savings will be larger than the energy savings.
Sizing
Proper sizing directly relates to overall boiler efficiency. Short-cycling (frequent on-off cycles) and the greater shell losses of an oversized boiler can dramatically decrease the efficiency of the heating plant.
It is always important to calculate heating loads within a facility in order to properly size a boiler especially for boiler replacements, where existing boilers may be 2-3 times oversized.
Designers often will select multiple smaller sized boilers to best accommodate fluctuating heating loads and provide redundancy. A single smaller boiler can be used to handle the lighter loads and minimize cycling of the boiler and additional boilers can be brought on-line during periods of higher heating loads.
Repair or replace
When considering whether to replace or repair an existing inefficient boiler you must understand where the losses are occurring and assess the overall condition of the boiler.
Start with the combustion efficiency. Combustion efficiency is simply the percent of heat content in a fuel that is converted to useable heat in a boiler. It is a good idea to have your boiler combustion analyzed with a flue gas analyzer at least once every year.
The stack temperature and flue gas oxygen (or carbon dioxide) concentrations are primary indicators of combustion efficiency. Often the combustion efficiency can be improved by tuning the amount of combustion air.
Another factor to investigate after combustion efficiency is scaling within the boiler tubes. Scaling will decrease the overall heat transfer ability of a boiler by creating an insulating effect on the tubes.
A good test is flue temperature: a high flue temperature can indicate that not enough heat is being transferred, which may be attributed to scaling within the boiler. Scaling can be chemically removed but this can be costly and requires draining the boiler.
Boiler failures are typically a slow progressive process and happen over a long period of time. Materials begin to fail with recurring cycling thermal stresses and the boiler tubes begin to leak or scaling buildup begins to prevent the boiler from meeting the heating load.
Efficiency
When looking to maximize boiler efficiency, consider a condensing boiler. Condensing boilers will capture more heat from the combustion gases than conventional boilers.
An important factor in maximizing condensing boiler efficiency is maintaining a relatively low return water temperature (85-100F) whenever possible. A lower return water temperature allows the boiler to extract more of the heat of combustion because of the larger temperature difference between the return water and the combustion gases in the boiler.
Running at a lower return water temperature can be accomplished by using supply water temperature reset to lower the supply water temperature. Condensing boilers are not typically available in larger sizes and are found in sizes from 500,000 to 2,000,000 Btu per hour (15-60bhp). To accommodate larger heating loads, multiple condensing boilers can be installed.
Modifications like stack gas economizers or recuperators can often be retrofitted to a current conventional boiler to increase the efficiency. Variable frequency drives can
On the horizon
Boilers are a mature stable technology. Look to see more active controls like O2 trim on smaller boilers in the future.
There is also a continuing trend to save electrical energy with variable speed drives on circulating pump motors.
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