Is a High-Efficiency Boiler Right for Your Facility?

When a boiler needs to be replaced, plant and facility managers have basically two choices: a conventional boiler or a high-efficiency, “condensing,” boiler. The path you choose is dictated largely by the boiler’s function, the facility’s water temperature and heating requirements, and the infrastructure of the facility. Boiler basics A boiler is a piece of equipment designed to provide heat throughout your plant or facility by circulating hot water or steam through a network of pipes and baseboard or radiator-type heat exchangers. In industrial facilities, boilers are also used to provide hot water or steam for operating manufacturing processes and equipment.
Conventional boiler. A conventional or non-condensing boiler typically heats water to temperatures ranging from 150 degrees to 180 degrees F (65 degrees to 82 degrees C) or higher. Generally, such boilers operate at an efficiency level of up to 85%, where the efficiency refers to the amount of fuel that is ultimately converted into useful heat. This compares to a maximum operating efficiency of 75% for a typical 50-year-old boiler.
Condensing boiler. A high-efficiency or “condensing” boiler operates with lower flue-gas temperatures, lower flue-gas emissions and reduced fuel consumption by recovering heat that would otherwise be lost up the flue. High-efficiency boilers operate at efficiencies of 90% and greater, or about 10% to 15% better than traditional boilers. They typically heat water to a temperature range of 120 degrees to 140 degrees F (49 degrees to 60 degrees C). High-efficiency boilers are called “condensing” boilers because during the process of recovering heat from the burned fuel, the temperature of the flue gas is reduced to a point where the water vapor that is produced during combustion is “condensed out.”
For plant and facility engineers, the decision about which type of boiler to choose, regardless of the company’s desire to be “green” and have the most efficient equipment possible, must be based on actual boiler use, and the water-temperature requirements needed to provide sufficient heat throughout the facility and to whatever processes are connected to the heating system.
Industrial facilities that use many individual unit heaters throughout the facility will most likely require water temperatures in the 150 degrees F to 180 degrees F (65 degrees to 82 degrees C) range in order to provide sufficient heat for the building. During your analysis of the boiler use, if you find that the heating system requires the water to be 140 degrees F or higher, then a traditional or non-condensing boiler is the only replacement option. This is because the dew point of flue gas is about 140 degrees F (60 degrees C) for natural gas, so in heating systems where the return water temperature is higher than 140 degrees F (60 degrees C), the water temperature is too warm to allow the flue gas to condense. If the heating system does not allow flue gases to condense, then purchasing a condensing boiler would not make economic sense because the system would not be able to run at peak efficiency.
On the other hand, if your facility does not rely on a network of individual unit heaters, and the existing boiler can adequately heat the space and supply hot water for various manufacturing processes with water that is heated to a range of 120 degrees F to 140 degrees F (49 degrees to 60 degrees C), a condensing boiler can be an option.
Condensing boilers are manufactured with a larger heat exchanger than is found in a traditional boiler. Hot exhaust gases lose much of their energy by heating the water in the boiler system. When working at peak efficiency, the water vapor produced in the combustion process condenses. The condensate is acidic and has a pH level of 4 to 5, making it corrosive. This condensate must be collected and disposed of according to local regulations. In short, a condensing boiler reduces flue-stack temperatures and heat loss, and saves money and energy in the long-run.
It should also be noted that high-efficiency boilers rely on the larger heat exchanger to operate in a condensing mode. However, while such systems are most efficient when operating in the condensing mode, they will also operate in a non-condensing mode, with efficiencies a bit higher than a traditional boiler by virtue of the larger heat exchanger.
While there are many benefits to installing a high-efficiency boiler, it can cost up to twice as much as a conventional boiler. This added cost is largely due to the higher price of stainless steel and other materials needed to resist the corrosive condensate present in high-efficiency boilers. Additionally, condensing boilers require special venting, such as an AL29-4C stainless steel vent that resists the corrosive condensate found in wet flue gases. This is not only an added cost, but also a maintenance item in the long term. By contrast, a conventional boiler can be vented in a standard tile-lined chimney or standard vent space. The base-load option Despite the larger up-front price tag, condensing boilers make economic sense as fossil fuels continue to rise in price. Consider the following:
Historically, a manufacturing company would purchase a commercial boiler at a size designed to meet its maximum needs, with another 10% to 20% of wiggle room. For example, if a company needs 1 million Btus of heat from its boiler, conventional wisdom might suggest installing two 600,000 Btu boilers to ensure that the 1 million Btu output is available whenever it is required, plus a bit more. However, when you overlay the added cost of extra fuel and lower performance efficiency to this model, another more economical option enters the picture. The idea is that if the 1 million Btu requirements are necessary only part of the time, the plant might be better served by having two smaller boilers, as before, but one would be a condensing boiler.
In this scenario, you might have one 700,000 Btu boiler and one 300,000 Btu high-efficiency or condensing boiler. The condensing boiler would operate the bulk of the time, and the standard boiler would kick in during that part of the heating season where extra heat or peak usage is required. The total output would provide the required 1 million Btu, but offer a more economical solution.
Base loading is only an option in companies where the maximum need is intermittent and where the company can operate at least some of the time with water temperatures in the 120 degrees to 140 degrees F (49 degrees to 60 degrees C) range. Efficiency ratings For commercial boilers those larger than 300,000 Btu/hour the standard efficiency rate is approximately 80%. However, there are two types of efficiency ratings for commercial boilers, so in order to compare apples to apples, you need to understand the two options: combustion efficiency and thermal efficiency.
Combustion efficiency. This is a measure of the heating efficiency of the boiler, based on how well the fuel burns, or how much energy is extracted from the fuel source. A 100% efficiency rating is theoretically possible, but cannot realistically be achieved. The formula is 100% (of fuel supplied), minus the percent of heat lost up the flue (called flue or stack loss). Typically, the loss factor is about 15% in the combustion process.
Thermal efficiency. Thermal efficiency measures the amount of Btus that the boiler generates. Btu output (actual heat provided) divided by Btu input equals thermal efficiency.
In side-by-side comparisons, the combustion efficiency rating is typically higher than the thermal efficiency rating by 1% to 1.5%. This is due to heat loss from the jacket. Cost comparisons As noted, condensing boilers are approximately 50% more expensive than non-condensing boilers. The bigger the fuel bill, the faster the payback, however, when the system can operate at water temperatures below 140 degrees F (60 degrees C). Besides the actual savings in terms of fuel consumption, condensing boilers reduce carbon dioxide emissions, and therefore do not contribute as much to global warming. In addition, many gas and utility companies offer rebates as part of demand-side management programs that encourage the use of high-efficiency boilers. Check with your local utility company to see what rebates might apply.