1) AFUE Ratings: The US method for establishing efficiency is the Annual Fuel Utilization Efficiency rating, or AFUE. This method has serious flaws as looks at only steady state combustion to determine the overall system efficiency. No consideration is given to system mass, boiler design and materials, off cycle cooling, jacket heat losses, the common method of installation, or the manner in which the boiler is operated. The major rating flaws are as follows:

1. The boiler combustion is typically tested at 12.5-13.0% CO2 which is the range where the equipment should operate for maximum heat release from the fuel. However, to reduce field problems due to poor boiler design and serviceability limitations, the US manufacturers instruct the installers to run the boilers at 10.5-11.0% CO2. As CO2 is actually a measurement of flame (chamber) temperature, dropping the CO2 reduces the temperatures by 250-300º, and, conversely, increases the stack temperature by 150-200º.
2. Most US boilers carry multiple boiler ratings for a single unit. The AFUE testing is typically done at the lowest firing rate, which drops the stack temperature, and enhances the testing. However, in actual installations, the higher firing rates are typically installed, increasing the flue temperatures to 450-550º, which reduces the actual field operating efficiencies.
3. Boiler testing is accomplished at "steady state", with the boiler operating between the aquastat set points. As all of the US cast iron boilers are made from brittle cast iron, the US manufacturers require that the boiler operate in a zone from 140º to 200º, which insures that the cast iron will not be damaged by low return water temperatures. Testing by this format alone totally ignores other, more efficient operating schemes, such as use of an outside reset control which sets boiler operating temperatures based on demand, or even the "cold start" method which allows the boiler temperature to run all the way down, until there is a call for heat.
4. During the AFUE testing, very little consideration is given to off cycle cooling. The standard "pin type" US boiler requires a chamber that is open to the chimney, and is subject to the negative draft created by the chimney. This creates a situation where the chimney basically vacuums the heat from the chamber, both during operation, creating high stack temperatures, and during the off cycle, creating continuous cooling of the appliance.
5. Also not considered during the qualification testing is boiler design or mass. The typical "pin type" boiler is a single pass design, with a mass weight of cast iron and water in excess of 650#, and a chamber flue pass age length of approximately 2.5’-3’. The bottom line is that boiler design is in reality simple physics, and this tells us that the larger the mass, the more Btu’s you need, and therefore, the more fuel consumed. Also, the shorter the flue passes, the less cross sectional area and time you have for heat absorption.
6. Finally, there are other issues not seriously considered in the AFUE rating system, such as jacket heat loss, which can exceed 5%of the available Btu’s, or the radiational cooling effect of the barometric damper, which is required for safe operation on single pass boilers.

In the final analysis, considering all the factors presented, although a conventional boiler can demonstrate an AFUE rating of 82-84%, the actual thermal efficiency of these units can be as low as 56%.

2) A Better Efficiency Model: The Europeans, who every day face energy costs 2-3 times higher than the US, have developed significantly more efficient equipment, and a much more accurate system for determining the actual efficiency. Their rating system looks at all aspects of thermal efficiency, and current efficiency requirements for boilers exceed 89% for the entire system. The major rating requirements are as follows:

1. All boilers must operate during testing and in the field at 13.0+ % Co2, which allows for maximum heat release from the fuel. This performance level is verified during the annual field efficiency test, which is mandated by the manufacture, and verified by law. The standard boiler design is very tight, creating a forced draft situation which compresses the flame in the chamber, and releases the maximum Btu’s.
2. European standards require a single firing rate for each boiler, which typically is the maximally efficient rate for that boiler. These rates are verified during testing, and, again, during the annual field efficiency testing.
3. It is mandatory that European cast iron boilers be manufactured from GG20 cast, which is flexible and allows for low return water temperatures without damaging the boiler. Control schemes such as cold start, and outside temperature reset are common, and their efficiencies are verified during testing. The testing procedures even include the total electrical draw of the burners, circulators, etc., so low amperage draw units are common.
4. The standard European boiler design is a multi-pass, forced draft unit, which is very tight, and is insulated from the chimney draw. The burner is required to force the flue gasses thru the system, and therefore, when the burner is off there is very little off cycle temperature loss, as the chimney cannot draw on the chamber. In addition, during the operating cycle, the flue temperatures range from 325-350º and consist of Btu’s not absorbed due to other system considerations such as chimney condensation, fuel constituents, etc.
5. These boilers are low mass, multi-pass units which weigh 50% less than conventional pin type boilers, and typically have water contents 50-60% lower. This yields a total mass considerably lower than pin boilers, and requires considerably less fuel to operate, which again, is simple physics. It is not uncommon to see a 100,000 Btu system with a total mass of 347#. In addition, as these units are at least three pass by design, the actual flue passage lengths are 6’-7’, which provides for a much longer flue gas contact time, and substantial heat absorption.
6. Serious consideration is given to issues such as jacket loss, and other installation requirements like barometric dampers, which rob heat from the system. The typical European boiler has a 3" insulation blanket, wrapped over the boiler block, which virtually eliminates jacket loss, and keeps the heat in the boiler where it belongs. The barometric damper is used only where other installation considerations, such as a low or high draft chimney, are present, or, in the case of natural gas, where a double acting barometric is required by code.

Taking all these facts into consideration, most European boilers end up with an AFUE rating exceeding 87%, and a real thermal efficiency in the range of 83-85%. The difference between the efficiency of pin type boilers, and low mass, multi-pass boilers can easily exceed an annual fuel savings of 30-40%. In addition, as there are low Nox units, which meet or exceed all North American standards, now available, we can clean up the environment, while saving a ton of money.

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