AFUE and Real Boiler Efficiency

To understand real boiler efficiency, go beyond AFUE.

When the government created AFUE (Annual Fuel Utilization Efficiency) ratings, they had a good plan. They intended to help consumers understand differences in the efficiency of heating systems with a rating system that showed the percentage of fuel that is used to heat a home.

However, AFUE ratings don’t help consumers make educated decisions about a hot water heating system’s efficiency because it only measures some chimney related losses. It’s like saying highway driving is the miles per gallon you’ll get all year when you know you’ll be doing most of your driving around town. Recent U.S. Department of Energy lab results identify AFUE as a rating system that lets consumers and heating professionals down.

What is the boiler efficiency calculation method and why can it overstate efficiency by up to 40%?

AFUE is the prescribed method for heat only boilers, although it is not much different than combustion efficiency which measures stack related losses. Typical boiler efficiency is routinely much lower than AFUE or combustion efficiency, especially for cast iron boilers or boilers with high “mass” and less than 2” of insulation, and for boilers that finish above 120°F when the thermostat is satisfied (AFUE rates boilers with a 120°F return temperature, but they typically run with a 160°F return and 180°F supply). Substantial losses occur because heating a 400 to 600 pound boiler to 180°F may take 8 to 15 minutes just to heat the boiler itself, and if that energy cannot be used before it dissipates, then the fuel used to heat the boiler is wasted. That energy left wasted in the boiler is called “idle loss,” and it can lower boiler annual efficiency by 15% to 40% below its AFUE rating.  Further losses drop the real efficiency even lower.  These occur from draft hoods and draft regulators that suck heat out of the building, heat left wasted in the distribution piping, and maintaining temperature by heating the boiler up to 180°F 3 to 5 times a day to replenish a hot water tank – even in the summer.

Efficient boilers perform much more closely to their rated AFUE.  Characteristics of efficient boilers include low mass and low water content (for example not cast iron), 2″ or more of boiler insulation, no draft regulator or draft hood, and an effectively integrated heat and hot water energy saving control.  The most effective energy saving control in the U.S. Department of Energy lab study is the thermal purge control, which must be combined with a low mass non-cast iron boiler to be effective and virtually eliminate idle losses.  Also look for combined heat and hot water systems with an external plate heat exchanger for hot water as these are more efficient and produce more hot water than tanks with coils.  Other characteristics to watch out for include boilers with coils that make hot water called tankless coil boilers as these maintain high temperatures and are the least efficient.


How to identify boiler savings using AFUE

Top 7 Things to look for to help to determine the real efficiency of boilers beyond AFUE.

Fortunately, the chart below shows types of boilers and annual efficiency from a Department of Energy laboratory study to help identify better performing heating systems. For example, the well insulated low mass boiler with thermal purge anticipates the end of the heat call and pumps the heat left in the boiler out to the hot water tank or a heating zone to capture energy that would otherwise be wasted. The result is very high annual efficiency, even in the spring, summer, and fall when boilers run much less frequently.

U.S. Department of Energy lab results1: AFUE is inaccurate, especially for boilers that make hot water.

Boilers and annual efficiency chart from Department of Energy study.


This chart shows types of boilers and annual efficiency from a Department of Energy laboratory study to help identify better performing heating systems.

This study shows Energy Kinetics design is more efficient than the 95 AFUE modulating condensing boiler and all other condensing and non-condensing boilers and systems tested.

Energy Kinetics Achieves the Most Efficient Boiler in the Department of Energy Lab Study1 and is also Recognized as Energy Star’s Most Efficient Oil and Gas Boiler

The above chart shows a department of energy laboratory study1 for annual efficiency of heat and hot water systems. AFUE much more closely represents peak efficiency, and the energy wasted when the boiler shuts off really cuts down the annual efficiency. Comparing the 83.5% AFUE tankless coil boiler at right, System 2000 will cut fuel bills by over 43%…although there is only a 4% difference in AFUE ratings. This means the tankless coil boiler will burn 77% more energy than System 2000.

All of our high efficiency boilers, including Accel CS and the non-condensing 90+ Resolute and System 2000 are more efficient than the other 95% AFUE modulating condensing boiler and all boilers in the study. Notice the very small difference between AFUE and annual efficiency for all of Energy Kinetics high efficiency oil boilers and high efficiency gas boilers. Compare that to losses for all other boilers which will have 14% to 73% higher fuel bills than their AFUE ratings indicate.

How is this possible? Energy Kinetics pioneered low mass technology with thermal purge, and has perfected it over 35 years. Our design heats up in just a few minutes, heats your home or hot water, and then runs a thermal purge that puts energy remaining in the boiler to work heating your home or hot water. There’s no energy left wasted in the boiler!

We specifically designed System 2000, 90+ Resolute, and Accel CS to virtually eliminate the energy losses found in all other boilers, losses that are NOT measured by AFUE. To make the highest efficiency boilers, we combine the best technology: our unique well insulated low mass boilers, our high performance hot water system, and our Hybrid Energy Recovery (thermal purge) control. This delivers unsurpassed comfort and the lowest fuel bills.

In fact, oilheat System 2000 had a higher annual efficiency than all other boilers tested, including 95 AFUE and all other condensing boilers! And now, 90+ Resolute and Accel CS achieve even higher annual efficiencies, squeezing more out of your energy dollars.

Considering a heating system upgrade?

Click here to estimate your savings from various heating systems.


The U.S. Department of Energy’s Brookhaven National Laboratory confirms: Energy Guide (AFUE) ratings miss significant areas of energy loss1

Savings with System 2000 are much greater than with comparably rated heat and hot water systems. System 2000 had the highest annual efficiency in the study – even better than the 95% AFUE boiler tested.

What many homeowners, business owners and heating professionals don’t know about AFUE increases heating bills and wastes energy. The bar graph below compares AFUE ratings to the real efficiency delivered for space heating and water heating over the course of a year. Be sure to take a look to get a better understanding of real efficiency to cut your fuel bills.

Real Fuel Efficiencies
Shown below are comparisons of fuel efficiencies among typical and top rated available home heat and hot water systems.


Lead ASHRAE Article Identifies AFUE Alone Fails to Indicate Performance and Deep Energy Savings Potential

The industry’s preeminent heating, air conditioning and refrigeration engineering society, ASHRAE, has published “Performance of Combination Hydronic Systems” by Thomas Butcher, PhD, as the December 2011 cover story in the ASHRAE Journal. This article highlights that the current practice of labeling heat and hot water appliances with AFUE alone fails to indicate the actual annual performance and to identify the deep energy saving potential possible with a system upgrade.

Energy Kinetics has always been focused on delivering the best efficiency, and System 2000’s performance stands out in the study with the highest annual efficiency and the lowest idle loss (0.15%). System 2000 significantly outperforms the 95 AFUE modulating condensing boiler and al other systems tested from around the globe. Our unique purge control strategy, low mass design, and high performance water heating system results in the top rating, and virtually eliminates the effects of oversizing. The next best system in the study had an idle loss of four times System 2000, which resulted in higher annual fuel use and significantly lower efficiency during hot water production.

This chart summarizes findings in U.S. Department of Energy’s Brookhaven National Labs studies.1The large yellow numbers indicate efficiencies.

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Nearly 40% average savings were achieved by installing System 2000 to replace cast iron boilers rated at 85%-86% AFUE.

In this multi-year study, the U.S. Coast Guard in Kodiak, Alaska, replaced (24) new 85% and 86% AFUE cast iron boilers of various makes, indirect water heaters, and electric water heaters with System 2000 in residences.

The tremendous savings led to the 2007 replacement of another (149) cast iron boilers as part of a Super Energy Saving Performance Contract (Super ESPC) which is run by the Federal Energy Management Program (FEMP). This project capitalizes on the expertise and financing of the ESPC contractor to assist the Coast Guard in complying with the recent Executive Order which requires all Government agencies to reduce their energy use by 20% by 2015. The Coast Guard also realized operations and maintenance savings with System 2000 over the cast iron models. The FEMP takes advantage of real efficiency over AFUE, and you can too!

[ Read Looking Ahead in First Regional Super ESPC: Success on Kodiak Island, Alaska]

Brookhaven National Laboratory Study

Brookhaven National Laboratory study uncovers a need to retool boiler energy efficiency rating programs.

[ View Summary] or [ View Report ] What are Department of Energy National Laboratories?
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Brookhaven National Laboratory study uncovers a need to retool boiler energy efficiency programs

AFUE (government efficiency) ratings can be off by 25% or more, an error that can underestimate fuel consumption by up to 47%.

The study shows that a consumer who purchases a home heating system based only on a comparison of AFUE ratings may actually spend as much as $800 per year more on fuel with one system, compared to a second system with a nearly identical AFUE rating. This difference in real annual efficiency is determined by a characteristic of all heating systems called idle loss, a performance factor which is not evaluated for AFUE on units which provide both heat and hot water. Idle loss includes two primary areas of heat loss on conventional home heating systems: standby loss, and jacket loss. Also almost entirely neglected are draft regulator, draft hood, and room air losses.

In the study, the reduction of idle losses is identified as a primary factor in improving real system efficiency.

Only when idle losses are reduced or eliminated is it possible for real efficiency to approach the AFUE rating of a heating product. In fact, significant idle losses may lead to actual efficiencies as low as 55%. This real annual efficiency rating may determine system performance even when the government AFUE rating measures in the 80-95 percentile range.

The study also reports that the cold start and cold finish operation of Energy Kinetics’ System 2000 has the lowest overall idle loss and the best annual efficiency performance among all home heating systems tested to date, including 95% AFUE condensing gas and oil systems.

The study revealed that outdoor reset controls may save 5% to 15%, but need trial and error adjustments over the course of a heating season. By comparison, using proven Hybrid Energy Recovery,® System 2000 automatically adapts to heating loads. The result is a home heating system that delivers the best documented efficiency. View our efficiency checklist to see how your heating system idle loss compares.

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Article: AFUE ratings do more harm than good…

AFUE ratings don’t help consumers make educated decisions about a boiler heating system’s efficiency.

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How do boiler controls save energy and effect annual efficiency?

There are several different kinds of boiler controls available. Here is a brief overview of the performance and efficiency benefits:

  • Thermal Purge (Post Purge or Hybrid Energy Recovery): Thermal post purge or Hybrid Energy Recovery is by far the most effective at saving energy because the boiler finishes cold and uses the heat energy that was left in the boiler a the end of a thermostat call. There’s virtually no energy left to waste. Thermal purge must be combined with a low mass boiler (not a cast iron boiler), so there’s not too much heat energy left in the boiler when the burner finishes running. This type of thermal purge anticipates the end of the heat or hot water thermostat call. Since the boiler is hot, the burner turns off and the circulator and last zone that called stay on and the heat remaining in the boiler is “purged” or pumped into the living spaces or hot water tank, saving energy every thermostat call. The combination of low mass, thick insulation, and thermal purge mean that the actual efficiency is very close to the AFUE rating in Energy Kinetics heating systems – remarkable considering other heat and hot water boilers are typically 15% to 35% lower efficiency than the respective AFUE rating.
  • Thermal Pre-Purge Control: It’s confusing, but there is a world of difference between a thermal pre-purge and a thermal post purge. When a thermostat call ends, conventional boilers typically finish hot. The heat left in the boiler dissipates, flowing up the chimney or into the boiler room and out the draft regulator. If there’s another thermostat call before the boiler is below 140ºF, the Thermal Pre-Purge control will simply prevent the burner from firing until the cast iron boiler drops to that temperature. It will then finish hot again and waste the heat energy left in the boiler until the next call. And just like temperature reset controls, these boilers typically have several hot water calls per day, keeping the boiler hot and continuously wasting energy.
  • Temperature Reset or Outdoor Reset Controls: These controls are designed to save energy by reducing the boiler operating temperature. While the burner is running, conventional boiler temperatures may run as low as 130ºF in warm weather, and 180ºF during colder weather. In this example, during warm weather, the boiler temperature is 50ºF lower and so the vent temperature could be about 50ºF lower, resulting in a combustion efficiency improvement of about 1%. The Department of Energy estimates this as a 0.6% savings annually for heat only boilers. If that doesn’t sound like much, consider that boilers that make hot water typically run up to 180ºF for each hot water thermostat call, virtually eliminating any efficiency gain. And if you look at condensing boilers that can run at lower temperatures, the Department of Energy estimates efficiency gains of a paltry 3% (again, for heat only boilers and less for boilers that make both heat and hot water).


Misleading AFUE Ratings

How do low-efficiency boiler controls artificially boost AFUE ratings and undermine efficiency?
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How much of your home’s energy is used for heat and hot water?

Find out more about conservation and about the impact of saving energy and about renewable energy.
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The chart at left is created from source data from the Energy Information Administration, Office of Energy Markets and End Use, Forms EIA-457 A-G of the 2001 Residential Energy Consumption Survey, New England and Mid-Atlantic states. Air conditioning percentage is an aggregate for the region and may not be representative of specific home usage.

1Data and conclusions are drawn from the report “Performance of Integrated Hydronic Heating Systems” (2007), Energy Resources Division, Department of Energy Sciences and Technology, Brookhaven National Laboratory, Under Contract No. DE-ACO2-98CH10886 with the United States Department of Energy by Dr. T. Butcher; and “Chimney Related Energy Losses in Residential Oil-Fired Heating Systems” (1990), with the addition of Accel CS and 90+ Resolute. Note: Higher operating temperatures reduce condensing boiler annual efficiency; the AFUE standard prescribes lower boiler temperatures than are commonly required for heating American homes during winter months.
[ View Paper ] 90+ Resolute Oil and Accel CS are certified as Most Efficient ENERGY STAR® products in 2017.