Information Brief - High Efficiency Boilers

This brief covers information about the optimal design and operation of high-efficiency boilers and associated heat distribution systems. Basic information on techniques to maximize the seasonal energy performance of condensing boilers will be discussed, including reducing return water temperature, using outdoor air reset, and the importance of operating units at part loads with tandem sequencing. In addition, common measures of boiler efficiency will be explained.

Common Measures of Boiler Efficiency
Combustion efficiency is a simple, though occasionally misleading, measure of a boiler's heating efficiency. It is equal to 100 percent of efficiency, minus the percentage of heat lost through the exhaust vent, which is called "flue loss." This means that combustion efficiency measures the amount of heat generated that stays inside the building. Combustion efficiency is measured once the boiler has reached a full and steady state of fire. Therefore, combustion efficiency is commonly higher than the efficiency of the boiler throughout a season of operation.

The thermal efficiency of a boiler is equal to the combustion efficiency, minus the jacket loss. Thus, thermal efficiency accounts for radiation and convection losses through a boiler's shell. This means that thermal efficiency measures the amount of heat that is delivered to the distribution system. Thermal efficiency, also known as "boiler efficiency" or "overall efficiency," is a boiler's energy output, divided by its energy input, and measured at full-fire and steady-state conditions; this measure of efficiency is also commonly higher than the actual efficiency of the boiler throughout a season of operation. Thermal efficiency is frequently quoted by boiler manufacturers and is based on the Z21.13 efficiency test.

AFUE is the measure of a boiler's seasonal or annual efficiency. It takes into account the cyclic on/off operation and associated energy losses of a heating unit as it responds to changes in heating loads, weather, and occupant controls. Therefore, AFUE is a more accurate measure of the actual energy efficiency of a boiler over the entire heating season. AFUE is a laboratory test, with procedures specified in the U.S. Department of Energy's Title 10, Code of Federal Regulations, Part 430, Energy Conservation Program for Consumer Products.

How High Efficiency Boilers Work
Standard boilers reach a maximum thermal efficiency around 80%. This means that close to 20% of the heat generated by standard boilers is lost up the flue. Much of this heat is in the form of water that has been vaporized during combustion. By condensing this water vapor on the boiler's heat exchanger, the extra energy carried by the vapor is released back to the boiler as useful heat. This heat is then transferred through the heat exchanger to the circulating liquid (normally water) that distributes heat throughout the building. Condensing boilers commonly achieve high thermal efficiencies of 90-98%.

To withstand the corrosive effects of acidic condensate, high efficiency boilers must use stainless steel or other corrosion-resistant heat exchangers. In addition, since the flue gas has lost much of its heat, high efficiency boilers cannot rely on rising warm air to draft exhaust gases up the flue. Instead, condensing boilers must have a fan to push the exhaust gas out the flue.

Maximizing Efficiency

Return-water Temperature
To achieve maximum efficiency, condensing boilers must be able to achieve condensation of exhaust gas on the heat exchanger. To do this, the circulating water's temperature upon return to the boiler must be as cool as possible. The cooler the returning water, the greater the rate of condensation and the higher the efficiency of the boiler. Peak efficiency is achieved with return water temperatures as low as 75 - 80°F. This demands low-temperature applications such as in-floor radiant heat. Efficiency declines dramatically as return water temperatures rise beyond 130°F because little condensation is possible at these high temperatures.

Figure 1 - Condensing boiler efficiency curves as a function of return water temp.

From "Maximizing Small Boiler Efficiency" by Peter D'Antonio, PCD Engineering Services

Outdoor Air Reset
Lowering the return water's temperature requires careful coordination with the building's heat load, which is primarily driven by the outdoor temperature. When outdoor temperatures are low, the building heat load is high and the boiler needs to supply more heat with higher-temperature circulation water. When outdoor temperatures are moderate, the building heat load is smaller and the boiler's supply water temperature must be reduced. Otherwise, the circulating water cannot give up enough heat in the distribution system and its return temperature will be too hot to allow condensation at the heat exchanger. For this reason, condensing boilers utilize an outdoor air temperature reset curve, which controls the temperature of the circulating water according to the outdoor air temperature. In this manner, the circulating water delivers an adequate amount of heat as its temperature drops back into correct range for condensing operation.

Figure 2 - Outdoor air reset curve for small condensing boilers

From "Maximizing Small Boiler Efficiency" by Peter D'Antonio, PCD Engineering Services

Part-load Operation and Tandem Sequencing
To control the circulating water temperature, condensing boilers must have the ability to modulate, or adjust, their rate of fire. As their rate of fire increases, the amount of exhaust gas increases and some of it cannot be condensed on the heat exchanger. In turn, this reduces the efficiency of the boiler. Looking back at figure 1, the efficiency of a condensing boiler at a full rate of fire (100% input) is nearly 8% less than the efficiency at a quarter rate of fire (25% input). This relationship implies that, unlike standard boilers, high-efficiency condensing boilers operate most efficiently at part load, rather than full load.

To take full advantage of this fact, high-efficiency boilers are often paired so that they can operate at part load most of the time. Rather than ramping up one boiler's rate of fire to 100% before turning on the second boiler (individual sequencing), both boilers are ramped up together (tandem sequencing) so that neither operates at a full rate of fire before it's absolutely necessary. By maximizing each boiler's part-load operation, the overall efficiency of the system is increased. The overall efficiency benefit of tandem sequencing is illustrated in the comparison chart below.

Figure 3 - Efficiency benefit of tandem sequencing for paired boilers

Individual Sequencing Tandem Sequencing
Total load (BTU/hr) Boiler1 Boiler2 Overall efficiency Boiler1 Boiler2 Overall efficiency Efficiency gain
100,000 100,000 92% off - 92% 50,000 95% 50,000 95% 95% 3%
200,000 200,000 88% off - 88% 100,000 92% 100,000 92% 92% 4%
300,000 200,000 88% 100,000 92% 89.3% 150,000 90% 150,000 90% 90% 1%
400,000 200,000 88% 200,000 88% 89.3% 200,000 88% 200,000 88% 88% 0%

From "Maximizing Small Boiler Efficiency" by Peter D'Antonio, PCD Engineering Services

Recommendations
Achieving maximum efficiency with condensing boilers comes down to maximizing the condensation of exhaust gas. This should be done by lowering return water temperatures as much as possible through the use of outdoor-air reset curves and by matching condensing boilers to distribution systems that don't require high water temperatures. Common baseboard radiators are not a good match for condensing boilers, but in-floor radiant heat is an excellent match because the circulation water temperature needs to be kept low to minimize discomfort and the sensation of hot feet. In a retrofit situation, if a condensing boiler is installed with existing baseboard radiators, it may be necessary to install additional baseboard units to achieve the expected boiler efficiencies. By installing additional radiators, the circulating water can be kept at lower temperatures, while still delivering the required amount of heat to indoor spaces. In new construction, baseboard radiators could be used with condensing boilers if engineers plan for additional baseboard units to deliver the required heat, while using low temperature water.

In addition to minimizing return water temperatures, maximum condensation of exhaust gas is achieved through part load operation. Maximizing part load operating hours with multiple boilers and tandem sequencing/controls will boost efficiencies and ensure a maximum return on equipment investment.

On average over the entire heating season, the efficiencies of condensing boilers may be 4% to 5% lower than the rated thermal efficiency. This drop can be due to a number of factors, but is more dramatic if the boiler is not controlled properly to maintain partial loads or is paired with the wrong type of heating distribution system, such as a baseboard radiator that requires high water temperatures.

Other Resources

Publications

D'Antonio, Peter C. "Maximizing Small Boiler Efficiency." Penton Media Inc. August 2006.