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AFUE Efficiency
Radiant Wizard
Member Posts: 159
I found this article and found it to be very interesting. Thought maybe other would too.
Analyzing Efficiency Data
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:
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.
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.
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.
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.
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 Btus 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.
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 Btus, 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:
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 Btus.
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.
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.
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 Btus not absorbed due to other system considerations such as chimney condensation, fuel constituents, etc.
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.
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.
Analyzing Efficiency Data
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:
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.
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.
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.
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.
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 Btus 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.
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 Btus, 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:
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 Btus.
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.
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.
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 Btus not absorbed due to other system considerations such as chimney condensation, fuel constituents, etc.
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.
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.
0
Comments
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This article is
typical propaganda of the euro boiler makers - who have much to gain by suggesting any standard that does not favor their technology is "unfair."
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Also, Dan Holohan
stated in a week-ago post that the euros do NOT have higher heating fuel costs. I believe he stated in some instances, their heating fuel costs were at parity or even lower than ours.
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There are flaws in the U.S. method, but...
The AFUE test does have a shut-down and restart portion involved (only flue temperature is recorded, though). The jacket loss is considered zero, if the boiler is for indoor installation only. The water temps are held to exactly 120* F in / 140* F out for the steady-state portion of the test. The CO2 you refer to is for oil only, and the burner has to be able to achieve this with a trace of smoke (which should be achievable in the field, under proper conditions). We provide an AFUE for each firing rate that a different nozzle size would give on each model boiler (if it has various inputs). The European method is more true in it's reflection of thermal efficiency, but I think that they only test at steady-state, and a different heating value (metric equivalent of BTU/cubic foot) is inserted into parts of the calculation that gives a higher efficiency result. There are 5 "classes" (ranges of concentration) of NOx that a European boiler can be certified for - the higher ranges do not look too stringent to me.0 -
There are flaws in the U.S. method, but...
The AFUE test does have a shut-down and restart portion involved (only flue temperature is recorded, though). The jacket loss is considered zero, if the boiler is for indoor installation only. The water temps are held to exactly 120* F in / 140* F out for the steady-state portion of the test. The CO2 you refer to is for oil only, and the burner has to be able to achieve this with a trace of smoke (which should be achievable in the field, under proper conditions). We provide an AFUE for each firing rate that a different nozzle size would give on each model boiler (if it has various inputs).
The European method is more true in it's reflection of thermal efficiency, but I think that they only test at steady-state, and a different heating value (metric equivalent of BTU/cubic foot) is inserted into parts of the calculation that gives a higher efficiency result. There are 5 "classes" (ranges of concentration) of NOx that a European boiler can be certified for - the higher ranges do not look too stringent to me.0 -
These Methods...
...are both un-necessarily complex methods of determining efficiency. Like anything put together by a committee, they confuse honest people, and let less than honest types hide monkey business or spin things for their own reasons.
Net heat out divided by heat input X 100 equals thermal eff. Nice & simple, and no room for "interpretations". It's the boiler/furnace/unit heater equivalant of "miles per gallon". It's worked like a charm, for over a century, for batteries of industrial steam boilers bigger than large houses that can each fire more fuel in an hour than many entire towns will in a year.0 -
re:
This is the crux of my question of last week. How does one compare a buderus vs a brand X. They seem to have design differences that make sense, but what is the real difference.
I think a different spec wuld be helpful. An equivalent to the EPA mileage spec. It isn't really real world and we all know it, but at least you can compare one car to another.
I remember the old Steven Wright lin, " I bought a humidifier and a dehumidifier nd put them in the house to let them fight it out. Hook the boiler to a test setup and run it through a real world 'night' and see how much fuel it uses. Combined with an AFUE it would seem to give you a better idea0 -
This article was actually
written here in the US.0 -
agree
> This is the crux of my question of last week.
> How does one compare a buderus vs a brand X. They
> seem to have design differences that make sense,
> but what is the real difference.
This is my question too. There is no reason we should have only the AFUE. Let's have access to standardized European ratings as well, and we can make up our own minds which one is more relevant to our application. But it's only useful if we have these ratings for US-made boilers as well.
As pointed in a previous, rather long and heated thread ('snake oil'), maybe modulation, reset, low-mass etc. are better, but there needs to be a standard measure of how much better.
Any standardized test is limited, but one is better than nothing and two or three are better than one.0 -
Care to share?
The publication and / or credentials of the author?0 -
Xplain this?
Boilers are allowed to have flue dampers and jacket losses aren't counted. But furnaces aren't allowed to have dampers and air used in combustion is assumed to be outside even though most are installed within the house. Never seen an explanation on why the 2 heating appliances have such different treatment.0 -
In general...
For the same boiler, run under the different conditions specified by the ASHRAE Standard vs. the CE Standard, the steady-state efficiency using the European method would be found to be about 5 % to 10 % higher than that using the U.S. method. The AFUE (only used here in U.S.) is usually found to be 0.5 % to 3 % less than the steady-state efficiency, mostly depending on the off-cycle losses. The AFUE for modulating input boilers greatly favors the minimum input efficiency (which is usually higher than the maximum input efficiency, when the air-fuel ratio is maintained). This information comes from lab testing experience.0 -
Both residential furnaces and boilers...
are tested for efficiency to the specifications of the ASHRAE 103 Standard, as far as I know. Combustion air is taken from the ambient test area, regardless of whether the appliance is a boiler, furnace, atmospheric, sealed combustion, fan assisted, etc. The temperature of the combustion air is taken at the point where it would normally enter the appliance jacket opening or air intake terminal (but the terminal is still in the same room). I don't see how jacket losses are treated differently, either. No comment on not being allowed to use a vent damper on a furnace, maybe a safety concern?0 -
My thought is
that this article is in laymen terms talking about Btu's used vs Btu's produced and where those Btu's are going (ie, usable btu's to the heating space). To me, that what is most important?
What a boiler can produce based on what amount of fuel it consumes is irrelevant to me. I want to know how many btu's in that gallon of oil is actually going into the heating space and not up the chimmney or through the jacket.
With all the technlogy we have in this country today you cannot tell me that this can't be done. This to me is the only true test of efficiency.0 -
It's...
...simple to do, and this is the method that has been used for calculating efficiency of big industrial steam boilers the size of large houses for over 100 years. Net BTU out divided by BTU in, multiplied by 100. And the exact same method can be applied to hot water boilers, forced air furnaces, or unit heaters.0 -
several thoughts
Re: % CO2 (note numbers sited apply to #2 heating oil fired equipment ONLY, not natural gas or propane fired), While in an ideal world a boiler would be operated w/ nearly ZERO excess air (high CO2) in reality a margin of error (in the form ov some excess air and lower CO2) is required to actually achiebve complete compustion and to allow for outside perturbations to the system, e.g. nozzle wear, changes in air temperature, changes in draft (in spite of a draft regulator, etc. What is important for comparison purposes is that the CO2 level is standerdized whether at 13%, 11.5% or other is less important -- at it is lowered ALL boiler's AFUE numbers will become a little lower but by quite similar amounts.
Stack temperature: at a given firing rate and CO2 level this will be a function of the design of the heat exchanger. A better heat exchanger design will cause this to be lower. If the flue gas temperature is the same for differant boilers it does not matter whether the exchanger is single or multipass, pinned, finned, or other.
Firing rates: If boilers are going to offer the flexibility of being fired over a range of firing rate (even if the range varies by only say 20-25% from highest to lowest) indeed the efficiencies will be slightly better at the lowest rate, this is physics. The lowest rate will be limited by the need to prevent condensation (physics again) otherwise we'd install large boilers and severely underfire them. It should be noted that manufacturer generally publish min and max firing rate efficiencies.
High vs. low mass: high mass does not (as appears to be claimed) consume more energy than low, it merely stores and releases more energy as the system warms up and cools down., but the energy does not disappear.
Jacket losses: This could be viewed as a flaw in the US AFUE test procedure as some/all this heat may be lossed if the boiler is located outside the heated space (e.g. in an unheated cellar well insulated from the living space, or a boiler used for domestic hot water heating when space heating is not required.
Draft losses: Another flaw, as typical operation is not steady state at 100% burner duty cycle. Perhaps a 50% (for example) burner cycle and a fixed chimney draft imposed on the boiler under would be better as differances in burner off losses would effect the final numbers, but as as installed operating conditions can vary so greatly it would still be only an approximation.
Reset/low temperature operation: Boilers *capable* of low temperature operation can be operated at low temperatures (as seen w/ condensing gas boilers) and would show higher efficiencies due to the recovery of the heat of vaporization of the condensed flue water (physic) but this testing would make no sense for boiler not designed for condensing operation.
The article while raising some good points does seem to be biased toward selling a particular style of boiler and does in places make some questionable claims.
The AFUE tests offer a means of comparison, even if flawed, as long as the limitations are kept in mind they are a useful tool.
0 -
Ditto. Thank you.
I wish I could speak my thoughts as well as you speak my thoughts.
Noel Murdough
Slant/Fin Corp.0 -
ICS
From what I've read, gas furnace efficiency is based on isolated combustion. That's why the screwball Lennox Whisperheat with the huge draft divertor wasn't penalized for the oodles of dilution air sucked up the flue 24 hours a day. Also explains why these chimney vent kits are allowed even though they suck warm air out of the house 24/7.0 -
I can only go by what I read...
in the ASHRAE 103 Standard for testing AFUE - which seems to treat both residential furnaces and boilers the same way on the subjects that you refer to.
There are 10 systems listed for each that equate to the venting / air intake setup and burner type. If the appliance has a draft hood, it would not be tested as an isolated combustion, as far as I know.
During the steady-state portion of the test, readings (combustion and temperature) are taken both before and after the hood (in the flue and the stack) - these readings all go into the efficiency calculation, to reflect the dilution factor. How this is reflected in the heat sucked up through the hood from the house during the off cycles, I'm not sure of.0
This discussion has been closed.
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