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Is a high-efficiency boiler right for your facility?

When a boiler needs to be replaced, you have two basic choices: a conventional boiler or a high-efficiency "condensing" boiler. The path you choose is dictated by the boiler's function, the facility's heating requirements, and its infrastructure.

Conventional boiler. A conventional or non-condensing boiler typically heats water to temperatures ranging from 150 degrees to 180 degrees F or higher. Generally, current models of conventional boilers operate up to 85% efficiency, which refers to the amount of fuel ultimately converted into heat.

Condensing boiler. This high-efficiency 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. It operates at 90% or greater efficiency, and typically heats water to 120 to 140 degrees F. They 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 water vapor produced during combustion is "condensed out."

The decision about which type of boiler to choose must be based on actual boiler use and the water-temperature requirements needed to provide sufficient heat throughout the facility and its processes. Facilities that use many individual unit heaters will most likely require temperatures from 150 to 180 degrees F to provide sufficient building heat. Heating systems that require water temperatures this high can only use a traditional or non-condensing boiler. This is because the dew point of flue gas is about 140 degrees F for natural gas, so in systems where the return water temperature is higher than 140 degrees F, conditions are too warm to allow the flue gas to condense. If condensation is not possible, purchasing a condensing boiler would not make economic sense.

If your facility does not rely on a network of individual unit heaters, and the boiler can meet building and process needs by supplying water from 120 to 140 degrees F, a condensing boiler can be an option.

Condensing boilers are manufactured with a larger heat exchanger than what is used 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, but the condensate is acidic, with a pH level of 4 to 5, making it corrosive. This condensate must be collected and disposed of according to local regulations.

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 this 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, note that it can cost up to twice as much as a conventional boiler. The added cost is largely due to the higher price of stainless steel and other materials needed to resist the corrosive condensate. Condensing boilers also require special stainless steel venting to resist the corrosive condensate found in wet flue gases. By contrast, a conventional boiler can be vented in any tile-lined chimney or standard vent space.

Nonetheless, condensing boilers make economic sense as fossil fuels continue to rise in price. Consider that, historically, a manufacturing company would purchase a commercial boiler designed to meet its maximum needs, plus 10% to 20% more. For example, if a company needed 1 million Btus of heat, it might install two 600,000 Btu boilers to guarantee both the 1 million Btu, plus a bit more.

When you overlay the added cost of extra fuel and lower performance efficiency to this model, however, a more economical option enters the picture. If the 1 million Btu requirement is necessary only part of the time, the plant might be better served by having two smaller boilers, as before, but one could be a condensing boiler. In this scenario, one might be a 700,000 Btu traditional boiler, the other a 300,000 Btu high-efficiency or condensing boiler. The condensing boiler would operate most of the time, and the standard boiler would kick in only when extra heat was required.

Though condensing boilers are more expensive, the bigger the fuel bill, the faster the payback. Note that condensing boilers also reduce carbon-dioxide emissions, and are eligible for utility-company rebates in many areas.

Mestek, Inc.

Use InfoLINK 069-80501-104 or Call 800-441-6180

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Thomas E. Neill, Senior Application Engineer, Mestek, Inc., Westfield, MA
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Title Annotation:TECH CHECK
Author:Neill, Thomas E.
Publication:Industrial Maintenance & Plant Operation
Date:May 1, 2006
Words:750
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