A new 93 percent efficient condensing boiler reduced natural gas consumption by 38 percent in 2011 and 35 percent in 2012 at Crosby Ironton's Cuyuna Elementray School.
Just because it says “high efficiency” on the label does not necessarily make it so.
Condensing boilers are very efficient because the water vapor produced as a byproduct of combustion is condensed into liquid water, releasing an enormous quantity of heat for productive use instead of sending it up the chimney. For facilities with condensing boilers and for buildings considering the installation of such a boiler, this article will help clarify how to achieve maximum efficiency for your system and get the most value from your investment.
The labeled boiler efficiency will be realized only briefly during the heating season. The actual efficiency will be somewhat lower most of the time. The key to keeping the efficiency as high as possible throughout the heating season is to keep the system reset temperature as low as possible at all times.
Condensing boilers can take advantage of the energy available in the water vapor of exhaust flue gases. This is accomplished by plumbing the return water through a heat exchanger that extracts heat from the exhaust flue gases and uses it to pre-heat the water entering the boiler. As long as the temperature of the boiler return water is below roughly 130°F (the dew point), the moisture will begin to condense out of the flue gas, releasing additional energy that can be utilized. This condensed water is usually drained into a convenient floor drain. If the condensing boiler is operating properly, the plastic condensate drain pipe will have a generous flow rate whenever the boiler is firing throughout the spring and fall. This is well illustrated in an information piece posted by the Center for Energy and Environment.
Condensing boilers achieve their renowned efficiency by removing as much heat as possible from the flue gases, which means lowering the flue gas temperature enough to allow vapor to be converted to water. However, the flue gas can only be cooled to a temperature slightly above the temperature of the circulating water being returned to the boiler. Therefore, return temperature must be kept as low as possible to take advantage of the high efficiencies possible in condensing boilers. (see Figure 1).
Figure 1. This graph shows the importance of maintaining low inlet water temperatures to achieve high boiler efficiency. (Chart adapted from ASHRAE.)
The outdoor temperature reset control is the critical piece of equipment to ensure that return water temperature is kept as low as possible (hence securing efficient operation of the condensing boiler). The “reset” schedule sets the temperature of the water supplied to the distribution system. It is a “schedule,” because as the outdoor temperature drops, the temperature of the water supplied to the distribution system needs to be increased to adequately heat the building. At moderate outdoor temperatures, the supply water does not need to be as hot. The reset schedule is different for each building because each building (even in the same climate) will have unique heating requirements based on the building’s levels of insulation, window performance, air leakage, mechanical ventilation needs, orientation, and other indoor conditions.
During the coldest part of the winter, the water temperature supplied by the boiler to the distribution system will typically be 30°F hotter than that returned to the boiler. At moderate outdoor conditions, the difference will ideally be just a few degrees. To function efficiently, the reset schedule of a condensing boiler needs to be completely different from that of the conventional boiler that was replaced. The reset schedule for a condensing boiler will be set up to supply water hot enough to just meet the building’s heating requirements, ideally resulting in return water temperatures below 130°F to allow for condensation of the flue gases to occur. Conversely, the reset schedule for a conventional boiler will be set up to supply water hot enough not only to meet the building’s heating requirements but also to provide an adequate safety margin to prevent condensation from occurring (typically 140°F or higher).
Figure 2 illustrates the difference between typical return water temperatures for a conventional boiler and a condensing boiler. At very cold outdoor conditions, both the condensing and conventional boilers have higher return water temperatures. As the figure illustrates, for condensing boilers this is typically well above the ideal 130°F for ensuring condensation of flue gases. However, if the temperature of return water to a condensing boiler is not reduced at warmer outdoor temperatures, it will operate in this noncondensing mode year-round. Note that the conventional boiler never operates in the condensing mode.
Figure 2. Typical return water temperature for condensing boilers (with proper reset control setting) and for conventional noncondensing boilers.
The reset control also needs to be programmed to facilitate morning warm-up after a night setback. During the warm-up period, the control is scheduled to bump up supply water temperature, resulting in return water temperature close to or perhaps even above the condensing threshold of 130°F. As space temperatures approach being satisfied, the supply (and concurrently the return) water temperature drops back down.
In boiler systems with control valves, there is an additional reason to keep supply and return water temperatures as low as possible. When control valves are trying to operate with higher water temperatures, the valves will throttle down, potentially to near closing. This will cause wire-draw damage, which occurs when water grooves a channel in the seating material of a valve that operates near the valve’s shut-off position for extended periods. Such grooves allow small water flows when the valve is in the closed position, leading to imprecise or even problematic system control. With proper reset control, the valves will operate closer to full open most of the time and avoid such wire-draw damage.
The boiler reset controller must have the capacity to adjust to the three distinct time periods of morning boost, day occupancy, and night setback in order to maintain comfort, maximize efficiency, and avoid wire-draw damage to valves.
Most boiler systems distribute heat to individual rooms through radiators. When a heating system is designed for a building, the radiators are sized to heat individual rooms properly at a certain operating water temperature range. However, if building envelope improvements have been made over the years (e.g., air leakage control, new windows, and additional insulation), then an originally installed radiator system will be able to heat spaces at considerably lower operating temperatures than the original design. As discussed above, those lower operating temperatures will result in higher efficiency for the condensing boiler.
In the fall, spring and even into winter, supply water temperatures of 100°F are not only possible but ideal for condensing boilers. However, heating a building with supply water temperatures of 100°F and lower is not easy. While the building’s heating load may have been reduced through envelope improvements, it will likely take a specialized radiator designed to function at lower operating temperatures to achieve maximum boiler efficiency. For example, if a building’s “radiator” is an in-floor heating system, then maintaining 100°F supply water temperature will be part of the design. Any warmer floors would be uncomfortable to occupants. Thus, a boiler serving an in-floor heating system can be run in very high efficiency condensing mode all winter.
Many low water temperature radiator products are now also available. They either have larger surface areas to shed heat at lower temperatures, or they incorporate fans to aid in carrying away heat. A web search for “low water temperature radiators” will locate many products. The Shakopee, Minn., Police Station has such a low water temperature radiator product (see Figure. 3).
Figure 3. Low water temperature radiators heat the Shakopee Police Station.
Condensing boilers were a popular efficiency improvement for recipients of the Energy Efficiency and Conservation Block Grant program administered by the Minnesota Department of Commerce from 2009-2012. At least 24 condensing boilers were installed by grant recipients. Several of these projects (Crosby-Ironton Schools, Sherburne County, City of Robbinsdale, and Chisago Lakes Schools) have been featured in case studies posted on the Commerce website. For example, Crosby-Ironton’s Cuyuna Range Elementary School installed a 93 percent efficient condensing boiler in 2010. The new boiler reduced natural gas consumption by 38 percent in 2011 and 25 percent in 2012.
“We implemented an aggressive hot water reset to take advantage of the condensing boiler,” said Bill Tollefson, business manager of Crosby-Ironton Schools. “Our return temperatures vary as the system also feeds air handling units. It has taken some trial and error, but the system has been functioning well.”
Steven Shadrick, facilities and safety manager for Beltrami County, said the Beltrami County Jail recently replaced the old 80 percent efficient boilers with new high efficiency boilers. “We reset the water temps based on outdoor ambient air temps, and there are times when we run 80 degree return temps during the spring and fall just to take the chill out of the building,” he said. Shadrick recently reported that “looking at the system today at 42 degrees outside temp, I have 113 degree supply water temp and 109 return temp.” Note how close these values are to the blue condensing-boiler line in Figure 2.
A correctly programmed outside temperature reset control is critical to achieving high efficiency with a condensing boiler. The reset control needs to adjust the boiler supply water temperature to be as cool as possible to provide adequate space heating through the range of outdoor air temperatures and operating conditions. If the building cannot be heated with the boiler in condensing mode, then additional investment to improve performance of the building envelope is needed to return the value from the considerable investment already made in the condensing boiler.
“Condensing Boilers” from the U.S. General Services Administration:
“Condensing boilers do not always operate in condensing mode, though high efficiency is achieved only when they do. This generally requires an entering water temperature of between 120°F and 130°F. It is important, therefore, to reduce the hot water return temperature whenever possible and to the maximum extent possible. For retrofit applications, this can be accomplished by reducing the hot water supply temperature, when the thermal load allows.”
“A Market Assessment for Condensing Boilers in Commercial Heating Applications” from the Consortium for Energy Efficiency:
“Technical requirements limit the suitability of condensing boilers in many commercial applications. The need for low return water temperatures and 2-pipe (minimum) hydronic distribution systems severely limits the penetration of condensing boilers into the large retrofit market. While existing terminal units can be used with lower than rated water temperatures, their heating output will be lowered, unless their temperature control systems can be adjusted.”
“Boilers” from Better Bricks:
“Both the system design and operating conditions are critical to the successful operation and performance of a condensing boiler. Return water temperatures below 130°F are typically required to get the rated efficiency out of a condensing boiler. Return water temperatures above 130°F prevent condensation of the flue gas and result in the boiler operating no more efficiently than a traditional boiler.”
This article was written by Bruce Nelson, P.E., who retired in March 2014 as senior engineer at the Minnesota Department of Commerce, Division of Energy Resources. For more information on energy efficiency, contact the Division of Energy Resources Energy Information Center.