Since the 1980s, hot water condensing boilers (CB) have aimed to improve efficiency by utilizing the heat of water vapor condensation from flue gas. Modern CB continue to be notorious for corrosion and cold weather problems [ThisIsMoney, Guardian] and need thorough (and expensive) home insulation. Still, the market share of CB grows steadily, thanks to favorable incentives and government regulations.
The Annual Fuel Utilization Efficiency (AFUE) value for CB is estimated at 92-98%, compared to 82-86% for non-condensing boilers. “Many people today are very pleased to assert that the efficiencies of certain space-heating systems are nearly 100%. This way of thinking often projects an incorrect picture of a space-heating system” [Wall Goran].
AFUE is measured based on the First Law of Thermodynamics as a ratio of utilized heat (heat from fuel combustion minus heat loss through the chimney) to supplied heat (heat from fuel combustion). However, “boiler efficiency, as commonly defined, is very misleading. These efficiencies are normally of the order of 80 to 90%, whereas the second-law efficiency of Industrial boilers is generally 30 to 45%... Two boilers with identical first-law efficiencies may be vastly different from a second-law viewpoint; e.g., one generating steam at a high pressure (and therefore temperature) will be much more efficient than one delivering steam at a low p (and T)” [Energy Engineering]
Second-law efficiency is also known as exergy efficiency. A quick and simple explanation of the concept of exergy can be found here. Exergy deals with the energy quality and the potential to produce useful work from the viewpoint of the SLT. One Btu of heat has different potential for useful work at high and low temperatures, just as U.S. and Taiwan dollars have different purchasing power. Among other things, it means that maximum possible efficiency can never exceed 100%. It is not uncommon for members of environmentalist groups to circumvent this law. Reports of energy efficiency levels of 104% and even 109% have raised few, if any, doubts or objections.
The exergetic efficiency of a steam or hot water boiler can be calculated using the following general equation [Szargut Jan, eq. 2.33]:
ηB = (ηE/α) (1- To/Tm) (1)
Where ηB - boiler exergy efficiency,
ηE – boiler energy efficiency,
α – ratio of chemical exergy of fuel to its chemical energy (considered 1 hereafter)
To,K – ambient temperature,
Tm,K – mean thermodynamic temperature of the heated fluid during heat absorption (temperature of hot water supply).
Normally, the operating temperature of a hot water boiler is below 180oF/82.2oC. In a condensing boiler, this upper temperature bound is reduced to below 130oF/55oC in order to condense water from flue gas [Lehigh U, Engineering Toolbox]. The recommended temperature for water entering the condensing section (also called the boiler return temperature) is 100oF/37.8oC or lower. Assuming the typical temperature difference of 20oF throughout the hot water system, that corresponds to a 120oF temperature for the boiler’s water supply.
For estimation, a closed system that includes a boiler, radiators, and a house at To = 63oF/17oC is assumed in equilibrium with the environment. Heat loss from a house is proportional to the temperature difference from the outside temperature and is offset by heat from the radiators at temperature Tm. In Table1.pdf, the exergetic efficiencies of a hot water boiler, a hot water condensing boiler, and a steam boiler are calculated based on typical energetic efficiency AFUE and heating media temperature. Please note that these exergetic efficiencies are maximum values that can only be achieved ideally and are independent of boiler and distribution system design.
Exergetic analysis clearly indicates that although the AFUE of a CB is close to 100%, its relative exergetic efficiency is 39.7% less than a non-condensing boiler’s (9.4% versus 15.6% – Table 1). The estimated CB exergy efficiency falls within the data range reported in literature [Lohani -5%, Kilkis – 5%, CCS- 8%, Favrat -12%]. “This low efficiency is the result of the conversion of a high-quality fuel, natural gas, into low temperature heat.” A comparison of the typical energy and exergy flows of a CB is shown here.
What do these numbers mean? Essentially, if the boiler must use fuel, it makes more sense to use the heating media at higher temperatures. From an exergy point of view, a steam boiler is a better option than a hot water one, but the high mass, poor control, and uneven heat distribution in old steam systems wipes out this advantage.
This conclusion contradicts numerous reports on the energy savings achieved by CB. To clarify the discrepancy, consider the annual boiler load distribution. For illustration, DOE and CARB operational data on seasonal thermal efficiency of a CB are presented in Table2.pdf and Table3.pdf, respectively. Calculated values are shown in italics. An average room temperature of 65oF and supply water temperature are used as To and Tp, respectively.
An exergy analysis of each data set reveals the same trend: overall efficiency is attributed to a 6-7% increase in “condensing mode energy efficiency” achieved during 31- 35% of the heating load. Omitted from the picture is that the boiler operates at a 43-47% higher exergetic efficiency in non-condensing mode (during the other 65-69% of the time). As Richard P. Feynman said, “The first principle is that you must not fool yourself, and you are the easiest person to fool.”
According to traditional energy analysis, condensing mode usage (along with its allegedly mighty savings) is highest at warmer outside temperatures and lower temperatures of return water. But CB account for only 2% of all boilers in sunny Italy [LEAP], presumably because of lack of government incentives. Surprisingly, in the UK, the CB percentage is close to a hundred [UK market update, p. 17], thanks to regulations and the colder, more humid weather.
The presented exergy analysis demonstrates how and why the results of very thorough research produced erroneous conclusions. None of this is meant to reject the concept of CB in general; rather, I mean to clarify the inborn catch-22 of hot water systems. In fact, the concept can succeed in 2-pipe steam and vacuum systems, in which a significant temperature difference between the supply water vapor and liquid condensate return is achieved naturally because of a phase change in the radiators.