When the government created AFUE ratings, the goal was to help consumers understand differences in the efficiency of heating systems. The ratings show the percentage of fuel that is used to heat a home. Unfortunately, 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. We'll go over AFUE versus real boiler and combi boiler efficiency, and how Energy Kinetics boilers always come out on top when comparing true efficiency.
Basing boiler efficiency solely on AFUE rating is 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 AFUE?
AFUE, or Annual Fuel Utilization Efficiency, is a rating system for furnaces and boilers that determines how much energy goes toward providing your home with heat and how much is wasted.
For example, if you own a furnace or boiler with an 80 percent AFUE rating, it means 80 percent of the heat energy is in your home and 20 percent of it is wasted. If you own a heater with a 90 percent AFUE rating, 90 percent of the heat energy goes toward heating your home and only 10 percent is wasted. At first glance, it may seem straightforward: higher AFUE rating equals higher overall efficiency. But there's a lot that AFUE doesn't account for, and a lot of factors that make Energy Kinetics more efficient than its competition.
How is Boiler Efficiency Calculated & Why Can it Be Inaccurate?
AFUE is the standard rating method for heat only boilers. 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 two inches of insulation, and for boilers that finish above 120°F when the thermostat is satisfied. Energy Kinetics explains that "substantial losses occur because heating a 400 to 600 pound boiler to 180°F may take eight 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 advertised AFUE rating."
The chart below shows how much energy is left wasted in a hot boiler when the thermostat call ends. These numbers were taken from Burnham's spec sheets and are fully accurate.
When comparing the second boiler with the Energy Kinetics Ascent boiler on the right, you will see that they both have an 87 percent AUE rating. However after BTU loss, the Burnham boiler wastes $1,151 per year on lost energy, while the Ascent only wastes $390 per year -- clearly proving that AFUE is not a full representation of efficiency and energy savings.
Additional 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 three to five times a day to replenish a hot water tank – even in the summer.
Major energy losses that AFUE does not measure
Idle loss
Conventional boilers are hot at the end of a heat call and some even run continuously to keep hot water available, even when no one is actually using hot water. This "idle loss" is a huge waste of energy, but is common on standard hi-mass boilers that try to heat the equivalent of over 1,000 pounds of iron every time the system turns on. AFUE does not account for this massive waste of energy.
Jacket loss
Most boilers have poor insulation with only three quarters of an inch of fiberglass (only on the boiler jacket). Lack of insulation allows heat to escape constantly and causes a great amount of heat energy to be lost to your garage or basement. This jacket loss has no bearing on AFUE ratings.
Draft regulators, draft hoods, and room air loss
Hot water production
Draft regulators draw warm air out of your home in an effort to control chimney draft for the boiler, but AFUE does not measure this loss.
Energy Kinetics Oil and Gas Boilers Win in Real Efficiency
On the other hand, efficient boilers perform much more closely to their rated AFUE. Low mass and low water content (not cast iron, for example), two inches or more of boiler insulation, no draft regulator or draft hood, and an effectively integrated heat and hot water energy saving control are all important factors of an efficient boiler. 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 produce more hot water than tanks with coils. Watch out for boilers with coils that make hot water -- called tankless coil boilers-- as these maintain high temperatures and are the least efficient.
Energy Kinetics Boilers are truly efficient
The above chart shows a DOE laboratory study for annual efficiency of heat and hot water systems. AFUE more so represents peak efficiency, while the energy wasted when the boiler shuts off really cuts down the true annual efficiency. Compared to the 83.5 percent AFUE tankless coil boiler on the right, System 2000 will cut fuel bills by over 43 percent, even though there is only a four percent difference in the advertised AFUE ratings. This means the tankless coil boiler will burn 77 percent more energy than System 2000.
All of Energy Kinetics' high-efficiency boilers, including the Accel CS and the non-condensing 90+ Resolute and System 2000, are more efficient than the other 95 percent AFUE modulating condensing boilers and all boilers in the study.
How is this possible? Over 35 years ago, Energy Kinetics pioneered and perfected low mass technology with thermal purge. The advanced 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!
ECI Comfort is an authorized Energy Kinetics dealer. Looking to upgrade your home heating system? Take advantage of our experience and expertise and schedule a free estimate for a new Energy Kinetics oil or gas boiler. Click the button above or give us a call at 215-245-3200.
