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philsmith22

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Right off the bat, let me say that this question is strictly theoretical. The question is below, in bold. But first, some background.

I'm looking at the gas/oil heating cost comparison calculator at nwnaturalcompare.com.

The set of numbers I'm using is as follows:

• zip code: 97211
• electic provider: PGE
• square footage: 1800
• year built: 1970
• windows: average
• number of people: 4
• number of stories: 2
• current heating: oil - forced air - standard efficiency (89%)
• cooling: none
• compare to: natural gas furnace

The results I'm getting show an estimated usage of 613 gallons of oil (with the current oil furnace), 471 therms with a new 93% natural gas furnace, and 548 therms with a new 80% gas furnace. See attached screenshot.

With the new gas furnace, the overall heat load is 43.8 million btu per year, according to the calculator. (That's 471 therms at 93% efficiency, or 548 therms at 80% efficiency, assuming 100,000 btu per therm.)

For the old oil furnace, if we use that same heat load of 43.8 million btu's, and assume there's 138,000 btu of energy in a gallon of heating oil, I think that would mean 317 gallons of oil at 100% efficiency. Of course, oil furnaces don't operate at 100% efficiency, but to get to the 613 gallons that the comparison tool is coming up with, I believe the oil stove would need to be operating at 52% efficiency. (317/613 = 0.52)

(If I go back and select a high-efficiency 90% oil furnace as the current equipment, the oil usage goes down to 571 gallons - or an actual efficiency of only 56%.)

So here's my question: can anyone point out to me if my thinking/math is incorrect, in figuring out what efficiency is being assumed for the current oil furnace?

Ignore the dollar figures that are being generated - I'm interested in fuel usage, rather than overall cost (which fluctuates based on gas/oil prices).

Thank you in advance!

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user_12345a

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90% oil doesn't exist to my knowledge, normal efficiency would be something like 80% or lower if you have an older unit that hasn't been maintained.

To do comparisons, calculate cost per therm at various efficiencies and fuels.

For heating dominant climate do not get a 80% gas furnace.

Not sure about the tool you're using, but never rely on rough estimates for energy consumption. every house is different. Use your own bills with the right heating values.

If you have any interest in adding cooling and the climate is mild enough with cheap enough electricity, you can get a heatpump instead of a/c and save fossil fuel. The furnace takes over below a certain outdoor temp - when heatpump becomes uneconomic to run or can't keep up any more.

philsmith22

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I'm just trying to understand the calculations that this particular tool is making. Based on all the numbers above, am I correct that the tool is assuming that the oil furnace is operating at 52% efficiency?

Daniel Holzman

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One detail is important. The higher heat value of a fuel is the total heat energy in the fuel. Fuel oil has a higher heat value of about 138,000 BTU/gallon, as you noted. However, when you burn a hydrocarbon like oil or natural gas, the hydrogen in the fuel is converted to water, that goes up the chimney as steam. Unless you have a latent heat recovery system, which is only available in a natural gas unit, the latent heat in the steam is not recoverable. The lower heat value of a hydrocarbon fuel adjusts for this, and the lower heat value of fuel oil is about 129,000 BTU/gallon. For more information on this, see http://cta.ornl.gov/bedb/appendix_a...ting_Values_of_Gas_Liquid_and_Solid_Fuels.pdf.

So in your case, you should calculate the number of gallons per year of fuel oil used based on 129,000 BTU/gallon, which works out to about 340 gallons per year assuming 100% efficiency (excluding latent heat recovery). So your calculator shows 613 gallons per year, which works out to be 340/613 = 55%, which is about right for an older fuel oil furnace without modern controls. With a modern fuel oil furnace, you should be able to get 80 - 85% efficiency (lower heat value).

user_12345a

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how does the tool work?

do you punch in the square footage, age, location and in spits out the result?

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55% is low. wonder what makes these old oil burners inefficient.

in gas furnaces it's the pilot and draft hood which knock 10%+ off the steady state efficiency.

philsmith22

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One detail is important. The higher heat value of a fuel is the total heat energy in the fuel. Fuel oil has a higher heat value of about 138,000 BTU/gallon, as you noted. However, when you burn a hydrocarbon like oil or natural gas, the hydrogen in the fuel is converted to water, that goes up the chimney as steam. Unless you have a latent heat recovery system, which is only available in a natural gas unit, the latent heat in the steam is not recoverable. The lower heat value of a hydrocarbon fuel adjusts for this, and the lower heat value of fuel oil is about 129,000 BTU/gallon. For more information on this, see http://cta.ornl.gov/bedb/appendix_a...ting_Values_of_Gas_Liquid_and_Solid_Fuels.pdf.

So in your case, you should calculate the number of gallons per year of fuel oil used based on 129,000 BTU/gallon, which works out to about 340 gallons per year assuming 100% efficiency (excluding latent heat recovery). So your calculator shows 613 gallons per year, which works out to be 340/613 = 55%, which is about right for an older fuel oil furnace without modern controls. With a modern fuel oil furnace, you should be able to get 80 - 85% efficiency (lower heat value).
Ah, very interesting! Thank you - that's incredibly helpful. Honestly, I didn't know what a "condensing" furnace meant until reading that!

However, for purposes of the comparison/calculator - wouldn't using the 129,000 btu/gallon number mean we're then comparing apples to oranges? Getting 129,000 btu out of a gallon of oil would already put the furnace at 93% efficiency, right? (I know that's totally unrealistic - I'm just trying to work out the math.) That is, in determining efficiency, we're calculating against a theoretical maximum, not against a number that's already been reduced to account for inherent inefficiencies. A brand new oil furnace with a stated AFUE of 80% would get roughly 110,000 btu/gallon, not 103,000 btu/gallon. Or am I understanding that wrong?

(The 80% non-condensing gas furnace, for example, is 80% after taking into account the heat that's lost as water vapor.)

user_12345a

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afue includes the latent heat lose.

For gas i think the cut-off for non-condensing is in the high 80s but the mids don't go higher than 80% due to chimney condensation issues.

once you get above 90% it's mostly latent heat that's being condensed.

Daniel Holzman

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There is only 129,000 BTU per gallon of available heat in fuel oil, because the hydrogen in the fuel oil is converted to water during combustion, and the water is exhausted up the chimney as steam, with no recovery of the roughly 1000 BTU per pound it takes to evaporate the water. So if you got 129,000 BTUs per gallon of fuel oil out of a boiler or furnace, you would be at 100% efficiency. The only way to get more heat out of the fuel is with a condensing furnace, which condenses the water vapor (steam) back to liquid water, and recovers most of the 1000 BTUs per pound it took to evaporate the water. You can get recovery natural gas boilers and furnaces, but for reasons not clear to me, I don't think there are any recovery residential boilers or furnaces, so you are starting with less available heat per pound of fuel than with a natural gas unit.

user_12345a

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condensing oil furnace would be a service-nightmare. i'm betting the condensate would be contaminated with all sorts of nasties and foul up the secondary heat exchanger and drains.

philsmith22

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There is only 129,000 BTU per gallon of available heat in fuel oil, because the hydrogen in the fuel oil is converted to water during combustion, and the water is exhausted up the chimney as steam, with no recovery of the roughly 1000 BTU per pound it takes to evaporate the water. So if you got 129,000 BTUs per gallon of fuel oil out of a boiler or furnace, you would be at 100% efficiency. The only way to get more heat out of the fuel is with a condensing furnace, which condenses the water vapor (steam) back to liquid water, and recovers most of the 1000 BTUs per pound it took to evaporate the water. You can get recovery natural gas boilers and furnaces, but for reasons not clear to me, I don't think there are any recovery residential boilers or furnaces, so you are starting with less available heat per pound of fuel than with a natural gas unit.
I understand what you're saying about the latent heat recovery. However, I can't find anything that says that AFUE for an oil furnace is calculated relative to that 129,000 number instead of the 138,000 number. The spreadsheet/calculator at the energystar.gov website uses 138,690. (See box D225 on the 2nd worksheet. I'm using the excel spreadsheet available at www.energystar.gov/products/heating_cooling/furnaces, on the right where it says "Savings Calculator (Excel)".) If I follow the calculations on that spreadsheet, specifically in boxes G36 and G37 on the 1st worksheet, they're converting total heat load to gallons of oil using that 138,690 number, times whatever efficiency is given in box G25.

Again, I'm not 100% sure of my math/reasoning, which is why I posted my question to begin with, so please let me know if you think I'm getting something wrong!

*EDIT TO ADD*: I absolutely see your point about the possibility of measuring efficiency against that 129,000 number, and I now see that I need to be more specific when I say "52% efficiency". In the interest of keeping things comparable, I'm using that 138,000 number as the theoretical maximum (100% efficiency), since as far as I can tell a new oil furnace with a stated AFUE of 80% is referencing that 138,000 number (even though in practice only 129,000 of that 138,000 is accessible to a standard, non-condensing residential oil furnace).

beenthere

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Since the question is theoretical. Does the house actually exist? If so, how much oil did it really use last year.

Use either 138,000 or 140,000 BTUs for oil's gross heat content.

No oil furnace is 89% efficient, unless its a condensing oil furnace, look up Adams oil furnace(hate working on them, fortunately not many of them left in my area).

AFUE takes into account the latent heat lost, or regained from a furnace. Making up heat content numbers to allow for it a second time would throw off any calculation. Specially since AFUE=Annual Fuel Utilization Efficiency.

philsmith22

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Since the question is theoretical. Does the house actually exist? If so, how much oil did it really use last year.

Use either 138,000 or 140,000 BTUs for oil's gross heat content.

No oil furnace is 89% efficient, unless its a condensing oil furnace, look up Adams oil furnace(hate working on them, fortunately not many of them left in my area).

AFUE takes into account the latent heat lost, or regained from a furnace. Making up heat content numbers to allow for it a second time would throw off any calculation. Specially since AFUE=Annual Fuel Utilization Efficiency.
Strictly theoretical - all I'm trying to do is make sure I'm understanding how the calculator is working, and specifically what assumption it's making about oil furnace efficiency. (This has nothing to do with a specific installation at a specific house, and it's not an "Am I better off doing this than that" kind of question.)

This has all been very helpful; at this point, I think my question is answered, and I've learned a bit about condensing furnaces (both gas and oil, apparently) and AFUE in the process.

beenthere

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A 1970's hot air furnace would have an AFUE of around 70%. So 613 gallons of oil a year at 140,000 BTUs per gallon would give you annual delivered BTU of 60,074,000 BTUs.

A nat gas furnace at 95% AFUE would use 632 therms, or 632 CCF of nat gas.

Piedmont

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You're a lot like myself, having btu comparison calculation spreadsheets and I even have spreadsheets to calculate insulation savings etc.

I didn't know how cheap natural gas was until my father asked his neighbor how much he pays a year for heat. My dad heats with oil and wood, my neighbor natural gas. The neighbors house is 1/3rd bigger, half the insulation, 3x the glass, and built before plywood so his house is leaky planks my dad's house should kill that house in heating. The neighbors wife shows the excel spreadsheet and they're paying \$620/year to heat it. That's crazy... my Dad heats with 3 chords of wood as it's much cheaper than heating with oil and he pays \$680. That means natural gas is cheaper than chord wood, and a whole lot less work (I heat with wood and it takes an hour out of each day). The neighbor has a condensing gas unit, which has pretty much 0 standby loss and is extremely efficient heating domestic hot water. They also have baseboard forced hot water.

I ran the numbers over and over again and something does NOT add up with my oil. Square feet of my house, insulation values, my boiler efficiency, I even added Heating Degree Days into my spreadsheets. On paper my boiler says 82% efficient but over and over again it's more in the 50%-55% range. Then I came across this, read the American boiler vs. European from "the Shadow". That I believe is my situation, I have a big issue where once the boiler shuts off I have 50F basement air cycling through it as my boiler is pin style and doesn't prevent draught so it cools right down and cycles back on. Combined with the other issues the AFUE misses, like they test boilers at their maximum burn/flame efficiency but you can't install it in a house in that configuration, they don't account for standby losses, open flues from pin style boilers constantly cooling down the boiler, and the mode that they must be installed in results in several efficiency losses unaccounted for by AFUE.

After reading that guys post it makes me want a European oil boiler since in Europe if it says 89% efficient it's all aspects of the boiler including idle and whatnot and they must be field tested after the install in the person house that they're 89%+ efficient so they're getting what they paid for. I need a new oil boiler, and the boiler guy recommended I get the Pure Pro Trio euro design 3 pass cold start boiler. After reading what that user said about european designs I'm liking it. Anyway, if you're looking at an American design oil boiler I would expect it to be around 55% efficient in your spreadsheet. European design stick closer to 85%.

beenthere

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After reading that guys post it makes me want a European oil boiler since in Europe if it says 89% efficient it's all aspects of the boiler including idle and whatnot and they must be field tested after the install in the person house that they're 89%+ efficient so they're getting what they paid for. I need a new oil boiler, and the boiler guy recommended I get the Pure Pro Trio euro design 3 pass cold start boiler. After reading what that user said about european designs I'm liking it. Anyway, if you're looking at an American design oil boiler I would expect it to be around 55% efficient in your spreadsheet. European design stick closer to 85%.
LOL, that guy was a spammer.

A 1970s Bethlehem Dyna Therm would have had an AFUE of 75%.

Todays models have up to 90.5% steady state efficiency, and 88.5% AFUE.

Originally designed and made in PA back in 1937. It was high efficiency before anyone cared much about saving oil.

philsmith22

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Anyway, if you're looking at an American design oil boiler I would expect it to be around 55% efficient in your spreadsheet.
Is there data anywhere to back this statement up? I can't find anything that says that a reasonably-well-maintained 80+% American oil furnace would actually be operating at 55%.

Bud9051

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Hi Phil, I apologize for not going back and reading everything so I may be repeating some here. But an 80% furnace will be 80% less some delivery losses like leakage outside the envelope.

I like to compare my calculations to actual home usage and nothing ever indicates 55% for a 80% tested unit. Yes, there is moisture going up the chimney, but that is part of the 20% loss.

Bud

philsmith22

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I like to compare my calculations to actual home usage and nothing ever indicates 55% for a 80% tested unit. Yes, there is moisture going up the chimney, but that is part of the 20% loss.

Bud
Thanks, Bud. I had posted a question in another thread somewhere asking what residential HVAC professionals would expect the current operating efficiency of a 30-yr old oil furnace to be, if it was originally rated at 80%. From the replies I received, it sounded like a well-maintained furnace should (depending on one's definition of "well-maintained") continue to operate in the 70-80% range; a poorly-maintained furnace might drop down towards 60% or even towards 50%, if totally neglected.

philsmith22

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**UPDATE** The calculator at nwnaturalcompare.com is no longer giving the same results as what I've indicated in my original post; it's not even asking the same questions. So you won't be able to reproduce my numbers, or the screenshot.

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