Recover the Dollars Going Up Your Boiler Stack

06/12/2013 | By Eric Woodroof, Ph.D., CEM, CRM

Energy waste can be harnessed to increase efficiency

The exhaust gases from most natural gas-burning boilers are vented to the outside – and quickly forgotten. But there is much value in the heat and water in that waste stream. Recovering these “assets” reduces energy costs and carbon footprint.

In 2012, U.S. industry and commercial buildings consumed approximately 10 trillion cubic feet of natural gas. However, 30 to 50% of this volume is wasted energy going up the flue. And although natural gas is cleaner than coal, every 1 million BTUs of energy recovered means a reduction of approximately 117 pounds of CO2 in the atmosphere.

How to Determine Your Boiler’s Waste
When performing annual boiler/burner tune ups, most maintenance provide a report showing combustion efficiency, which is basically a measure of the fuel-to-air mixture. Although combustion efficiency is important, it does not reflect a boiler's overall efficiency, which includes the amount of energy (post-combustion) that is NOT transferred within the boiler and goes up the flue stack. If you can identify the data points below, you can determine what you are wasting up the flue:

  • What is the exhaust temperature leaving your flue/chimney?
  • How many cubic feet of gas per hour goes into your burner?


Condensing flue gas heat recovery units or “condensing economizers” can help you recover waste heat and use it to preheat air or water going into the boiler or to preheat another fluid. This technology is designed to work with large natural gas appliances used in the commercial building industry as well as the process industry. Any facility with a large boiler, including hospitals and campuses, can benefit from condensing economizers, which typically have paybacks of 2 to 3 years.

In contrast to other exhaust gas heat recovery equipment, condensing equipment is designed to bring the exhaust temperature down below the condensation point, creating a slightly acidic/corrosive “rain” from the exhaust gas. Taking the temperature this low allows the equipment to recover the maximum BTUs. The upside is that natural gas appliances can operate at well over 90% energy efficiency, and even 98% efficiency in some applications. The downside is that the recovery unit requires anti-corrosive materials (stainless steel, aluminum, etc.) to handle the acidity. The condensate has a pH of approximately 4.5, which can easily be neutralized.

A flue gas condenser can reduce exhaust temperatures to well below 100 degrees F. Doing so creates a significant amount of condensate from the exhaust gas. The volume depends on the size of the natural gas appliance and the heat recovery process, but it can amount to as much as thousands of gallons per hour. To ensure that the water does not end up getting back inside the boiler, flue gas condenser units are installed alongside and below the original flue stack.

A neat aspect of this design is that most of the condensing action takes place where the cold water enters the heat recovery unit through the top rows of tubes. The condensate from the exhaust gas falls down from the top, washing the heat recovery surfaces of any residue. The condensate then falls into the base section where it drains away.

This recovered condensate, most of which is distilled water, is fairly clean and usable. With minimal treatment, the water can be used as boiler makeup or added to an industrial plant’s wash-down and/or used for make-up water in the cooling towers. In a commercial building, the water can be put into the sanitation water lines. If you don’t want to treat the water, you can store it in an underground container and use it for landscape irrigation. Note that sewer rates are typically 1.5 times more expensive than incoming water rates, and sewer volume is calculated based on your incoming water volume, so by using the water in the flue gas, you will reduce your water and sewer bill.

Every condensing recovery application is different due to differences in heating and cooling requirements at each facility. Some are “closed loop” (with respect to the fluid within the tubes of the heat exchanger), while others are “direct contact” designs where the exhaust gas actually touches the water that is being heated in the waste heat exchanger.

Not everyone can attain 98% energy efficiency, but it is worth reviewing your options because even a small improvement may have a good return on investment. In my opinion, condensing recovery units should be installed on most boilers as the “default” design. Our country would reduce its energy, water, and carbon expenses in a very cost-effective manner. 

I would like to thank Mr. Sid Abma of Sidel Systems for contributing to this article. For more information, feel free to contact Mr. Abma at:
(805) 462-1250
(800) 668-5003

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