efficiency assumptions when comparing fuel costs

[twd000]

I am putting together a spreadsheet to compare various alternative forms of fuel

I have propane forced hot-air furnace and a woodstove in the basement. Thinking about moving the stove to the first floor, or buying a new stove for the first floor. Trying to estimate the payback on that capital investment.

I can put in some assumptions about various fuel prices in the near-term and long-term. But I also need to account for the total distribution efficiency of those fuel sources.

Propane has 92,500 BTU/gallon, but how much of that transfers to the house? 80%? 90% Ducts are wrapped with 1" fiberglass insulation. Half the ducts are inside conditioned space (basement ceiling) and half are in unconditioned space (attic).

Similar for wood. Red oak has 22e6 BTU/cord, but how much of that makes it into the room? Can I trust stove manufacturer's 75-85% claims, or is that an optimistic laboratory number? What if I burn low and slow instead of hot and fast?

 

[Seasoned Oak]

I like to compare cost per Million BTUs . All heating sources have their inefficiencies.

 

[woodgeek]

1" fiberglass is not great, maybe R-3 or so and below current code, which I think is 2", R-6. The bigger concern is duct leakage in the attic.

If the ducts were well airsealed, you could bury them in cellulose or other blown insulation. I did mine that way.

To answer your question, look at snow on your roof....if it sticks and is even, you are prob <5% loss to the attic, if it has melted blobs etc, maybe 10% or more.

But most non-condensing furnaces have an upper limit for efficiency...the flue temp has to be hot enough to keep the flue gases from condensing. I would assume that is ~80% for a non-condensing unit with forced draft. And then shave another 5-10% off for the attic loss.

 

[twd000]

I like to compare cost per Million BTUs . All heating sources have their inefficiencies.

Propane is $23/MBTU gross, red oak firewood C/S/S is $12/MBTU gross. But they don't put the same # of BTUs into my living space; a condensing propane furnace burning a refined and uniform flow of gas is more efficient at extracting BTUs than a steel box full of chunks of wood. There's a reason why biomass power plants run on standardized pellet fuel. The difference is important.

 

[twd000]

1" fiberglass is not great, maybe R-3 or so and below current code, which I think is 2", R-6. The bigger concern is duct leakage in the attic.

If the ducts were well airsealed, you could bury them in cellulose or other blown insulation. I did mine that way.

To answer your question, look at snow on your roof....if it sticks and is even, you are prob <5% loss to the attic, if it has melted blobs etc, maybe 10% or more.

But most non-condensing furnaces have an upper limit for efficiency...the flue temp has to be hot enough to keep the flue gases from condensing. I would assume that is ~80% for a non-condensing unit with forced draft. And then shave another 5-10% off for the attic loss.

The snow on my roof sticks around fairly uniformly, with the exception of one spot that I plan to address. I also plan to air seal the attic top plates and add cellulose to what's already there. I'll assume 10% for duct losses. The Energyguide sticker on my furnace claims 92.5% AFUE, so I'll use 82.5% net efficiency assumption for propane.

Now what about the woodstove side? Do we have any real-world data on what a modern catalytic EPA stove achieves in typical usage? I assume the efficiency could be inferred by the exit temp of the stove pipe? How close is that to the manufacturer's ratings? I see this EPA rating sheet here: http://www.epa.gov/sites/production/files/2013-08/documents/certifiedwood.pdf
Blaze King claims ~80%, Jotul ~75%. Several manufacturers don't have any data on that sheet. I need to read up more on their test method and compare it to how I usually burn.

 

[maple1]

I think by the time you take a 92.5% AFUE for the furnace and try to estimate losses (the idea of heat ducts in attics makes me shiver), you would be close enough to the stove estimates that there may be no significant difference.

 

[Seasoned Oak]

Propane is $23/MBTU gross, red oak firewood C/S/S is $12/MBTU gross. But they don't put the same # of BTUs into my living space; a condensing propane furnace burning a refined and uniform flow of gas is more efficient at extracting BTUs than a steel box full of chunks of wood. There's a reason why biomass power plants run on standardized pellet fuel. The difference is important.

Theres lot more reasons to own and operate a wood stove than efficient heat production. If that were the only reason id probably never own one.
My number one reason is the beauty of the stove and the fire,heat production is just icing on the cake. One reason i probably will never buy a pellet stove.

 

[twd000]

Theres lot more reasons to own and operate a wood stove than efficient heat production. If that were the only reason id probably never own one.
My number one reason is the beauty of the stove and the fire,heat production is just icing on the cake. One reason i probably will never buy a pellet stove.

I agree; there are multiple dimensions for undertaking a decision. But for me, the financial dimension must make sense along with the others. What if wood heat cost MORE than other forms, and we were talking about spending money instead of saving money? How much would the beauty of the stove be worth to you? $100/year? $1000/year?

 

[Seasoned Oak]

I agree; there are multiple dimensions for undertaking a decision. But for me, the financial dimension must make sense along with the others. What if wood heat cost MORE than other forms, and we were talking about spending money instead of saving money? How much would the beauty of the stove be worth to you? $100/year? $1000/year?

Priceless

 

[Seasoned Oak]

People spend big money on fancy fake fireplaces with fake electric fires that use expensive electricity. $300 a pop for those miracle eden boxes.
I had a gas fireplace once ,and tore it out eventually. My point is you dont have to burn 24/7 to enjoy the beauty of a wood stove or wood burning fireplace. Im a part time burner,just dont have time to fiddle with it everyday for heat. I use coal in the dead of winter with an oil backup i use in spring and fall when i only need heat once or twice a day. Anyone could do the same with gas or propane. My woodstove saves me the cost of an emergency generator for heat even if i never use it.Some of those generators cost much more than a woodstove and are seldom if ever used. But i burn wood whenever i have time to enjoy the fire show an its always ready for emergency use as well. If you going gas or propane id recommend a condensing furnace ,good luck whatever you decide.

 

[maple1]

We have a wood burning fireplace in our living room. It gets used maybe twice a year.

 

[iamlucky13]

Oregon State University has a nice spreadsheet here that is a good template for your own calculations. Since parts of the spreadsheet are password protected, you may need to copy data and formulas to your own spreadsheet if you want to do more change numbers in the yellow cells:
(broken link removed to http://extension.oregonstate.edu/lincoln/sites/default/files/ec1643.xlsx)

Adjust figures in the spreadsheet per your local prices and assumptions. In particular, the gas furnace combustion efficiency needs adjustment. It should be at least 90% for any modern gas furnace. For the high end condensing models, it can be over 95%. Their BTU figures for wood might be a bit high, too.

You can incorporate duct loss in the same efficiency figure*, or add another column to make that adjustment. NREL has some estimates of duct efficiency here. They can be worse than you think:
http://www.nrel.gov/docs/fy05osti/30506.pdf

So figure just over 80% for well insulated, well sealed ducts in a cold climate in a crawlspace and as little as 63% for poorly sealed, poorly insulated ducting (but not entirely uninsulated and unsealed). Since you have moderate insulation and half of it runs in conditioned space, I'd probably figure on 75%-80%.

I've tried to find clear info on stove efficiency measurements. I found plenty on particulate emissions, but not efficiency. As high as they are for such simple heat transfer designs, I assume the efficiency ratings are for effectively steady state conditions and more or less the best efficiency you can get, not a realistic average. I'd drop 5-10 percentage points off to be conservative.

Keep in mind that although propane is relatively cheap this year, the long term price average is likely to be closer to what it's been over the last few years.

If you have regular power outages and don't have a way to power your furnace blower, consider the value you gain from the stove in those conditions.

Your whole season heat demand can be complicated to estimate. I approximated mine by using a heat loss calculator to get heat loss rate at a single temperature. I divided that by the difference between the indoor and outdoor design points (eg: 17,800 BTU/hour at 68 inside/30 outside, or 38 deg difference is 468 BTU per degree per hour), then multiplied 24 to get daily per degree demand. Then I searched online for a heating degree day estimate for my area (4697 average). Multiplying this by the daily per degree demand indicated I should need 53 million BTU annually.

Comparing to my actual energy bills (subtracting lowest summer bill x 12 from the sum of 12 monthly bills), this method seems give me a total in the right ballpark, but it's hard to verify, because I have an unknown ratio of 3 heat sources - my heat pump, the less efficient auxiliary heat strips, and my stove, used part time. I consider this a very rough estimate.

* Multiply combustion efficiency x duct efficiency. Don't just subtract duct loss from combustion efficiency or you're counting the duct efficiency from heating value of the fuel, not the heat that the furnace delivers to the upstream side of the ducting.

Eg: 93% combustion efficiency and 75% duct efficiency
Correct: 0.93 x 0.75 = 0.698 or 69.8% overall efficiency
Incorrect: 93% - 25% = 68% overall efficiency

EDIT - fixed obvious number mismatch in the example.

 

[iamlucky13]

A couple closing comments for my above technical post:

In your colder climate, I suspect you'll find that even at $2/gallon, propane costs add up pretty quickly, so unless you're buying all your wood, the cost of a new wood stove can be recovered in only a few years. The question then becomes whether you actually want to and will have the time to deal with processing the amount of wood it takes to significantly cut your propane use and keeping your stove going.

As far as whether burning wood could be more expensive than propane - on my own spreadsheet, with the efficiencies I've assumed, even paying $300/cord for split and delivered hardwood (I'm in a softwood area, so hardwood is expensive) vs. $2 per gallon for propane in a furnace, propane effectively costs twice as much as wood per million BTU.

 

[Seasoned Oak]

If you have regular power outages and don't have a way to power your furnace blower, consider the value you gain from the stove in those conditions.

Exactly , i have a friend who has several thousand tied up in a propane generator that almost never gets used for power failure ,but comes on once a month to self test and gets the tank topped off once a year for a hefty service charge. As i look around his neighborhood i see several of his neighbors doing the same. ID much rather a wood stove which can provide Heat, light (in the room its in) and cooking without any electric power whatsoever.

 

[Seasoned Oak]

The only type of heating appliance that i know that has the ability to use "free" fuel is wood,solar and waste oil heaters.

 

[iamlucky13]

ID much rather a wood stove which can provide Heat, light (in the room its in) and cooking without any electric power whatsoever.

I lean the same way. I might get one of those quiet, little Honda generators to keep my refrigerator, freezer, and stove blower running maybe part time during outages, but so far, I've managed without. The worst we had was some moderate freezer burn during an unusually warm weather fall outage. A whole house generator would add very little value to me.

 

[begreen]

We have a modest Yamaha generator, mainly to power refrigs and freezer. Our outages vary, but 12+ hrs is not uncommon. The worst have been for days. During the year we put up a lot of produce, so protecting that is the priority. That and a few lights to make it easier to cook at night are most of what we need. The generator runs on propane or gasoline so we have pretty good reserves if the outage is regional and the gas pumps are also out of power. This happened a few years back after a huge windstorm took down high-tension lines coming from the dams.

 

[Seasoned Oak]

In the In the 27 years iv lived in the same place , all the power outages combined wouldnt span 6 hours. half hour here ,an hour or so there. That said some folks get em all the time and for longer duration so it really depends on your location. No matter where you live up north you never can tell when you going to get one of those killer ice storms that will break all records and thats when my 6 month supply of wood will come in handy.

 

[twd000]

thanks for all the helpful comments

a couple more notes about my house - we just moved in this summer so I'm still getting familiar with it. The house came wired with a whole-house (14 kW) propane-powered generator. This area is famous for multiple multi-day power outages every year, winter/spring/summer/fall. I may not have opened my wallet to install such a backup system, but I am thankful we have it. I also agree that wood heat is one of the most resilient sources of fuel. Buy local/burn local means a lot to me.

@iamlucky13 thanks for technical info. The Oregon spreadsheet is helpful. I was surprised to see 60% efficiency for the woodstoves, hopefully that is a pessimistic assumption for the stoves I'm considering. I have corrected my duct loss calculation to 0.93 * 0.80 = 0.75 for the propane furnace.
My total space heating load is a crude estimate of 61 MBTU (I was told the prior homeowner consumed 1000 gallons propane/ year and I assumed 1/3 of that was for DHW).

Using those numbers, C/S/S delivered red oak is half the net $/MBTU of propane at current prices, which agrees with your calculations. Log-length is only 20% of the cost of propane. If I supplement 80% of my space-heating load with wood instead of propane, I'm looking at saving $1200/year at current rates. If&when propane goes back to $3/gallon, I will be saving $1800/year. So a $5000 Blaze King pays back in 3-5 years. Not too shabby.

 

[maple1]

Since you mentioned DHW there, just a quick note on that. I would be very surprised if 1/3 of that was for DHW. Or anything close to it. (Well, unless a LOT of DHW is used). We have an ordinary tank type electric water heater, and it only cost us about $20-25 month for DHW in the summer (at $0.18/kwh). Family of 5. (We're down to 3 now for 2/3 of the year). Doing some rough estimating on our electric backup boiler use, I figure that the winter time energy consumption ratio of space heating:DHW heating, is about 30:1. For us.

So, try to keep an eye on how much it does use for DHW. If it is anywhere close to 1/3, it might be worthwhile to consider an electric water heater - but you might be OK as is too. We used to do it with oil - now we don't even have an oil tank.

 

[twd000]

Since you mentioned DHW there, just a quick note on that. I would be very surprised if 1/3 of that was for DHW. Or anything close to it. (Well, unless a LOT of DHW is used). We have an ordinary tank type electric water heater, and it only cost us about $20-25 month for DHW in the summer (at $0.18/kwh). Family of 5. (We're down to 3 now for 2/3 of the year). Doing some rough estimating on our electric backup boiler use, I figure that the winter time energy consumption ratio of space heating:DHW heating, is about 30:1. For us.

So, try to keep an eye on how much it does use for DHW. If it is anywhere close to 1/3, it might be worthwhile to consider an electric water heater - but you might be OK as is too. We used to do it with oil - now we don't even have an oil tank.

I'm considering heat loads on a yearly average basis, and assuming DHW is constant throughout the year, while heating load of course is very seasonal. The ratio may very well be 30:1 in January, but I'm interested in the year-round average.
My estimate was based on articles like this:
https://www.eia.gov/todayinenergy/detail.cfm?id=10271
http://contractormag.com/plumbing_heating_and_cooling/future-space-heating-efficient-0610
Even if our DHW usage was extremely high (and I don't think it is), switching from propane to an electric resistance water heater wouldn't make any sense. Electric resistance heat @ 0.17/kWh = $50/MBTU, while propane @ $2.14/gallon & 95% efficiency = $24/MBTU.

I will definitely track my propane fill-ups from spring through fall to estimate my DHW consumption in the absence of space-heating load.

 

[twd000]

Since you mentioned DHW there, just a quick note on that. I would be very surprised if 1/3 of that was for DHW. Or anything close to it. (Well, unless a LOT of DHW is used). We have an ordinary tank type electric water heater, and it only cost us about $20-25 month for DHW in the summer (at $0.18/kwh). Family of 5. (We're down to 3 now for 2/3 of the year). Doing some rough estimating on our electric backup boiler use, I figure that the winter time energy consumption ratio of space heating:DHW heating, is about 30:1. For us.

So, try to keep an eye on how much it does use for DHW. If it is anywhere close to 1/3, it might be worthwhile to consider an electric water heater - but you might be OK as is too. We used to do it with oil - now we don't even have an oil tank.

$25/month at $0.18/kWh = 138 kWh/month for your DHW. I looked at the Energyguide sticker on my propane water heater. It states that it consumes 261 gallons of propane a year under "typical use" (in line with my 217 gallon estimate for 1/3 of total heating) That's 590 kWh/month. Am I mis-calculating something, or do you really use less than 1/4 the DHW of this "typical American"?

 

[iamlucky13]

I think 60% is probably pessimistic for a good, modern wood stove, but it could be about right as an average between modern and older stoves.

When I see figures like 81% efficiency advertised for the Blaze King Princess, I waver between being really impressed and skeptical. I'd imagine it can get that high for a full time burner using nice, uniform splits and managing their loads and air perfectly, but since performance can vary widely, I'd probably figure on 70-75% in your calculations.

I totally get why people in remote areas want generators. In our area, it can be nice to have, but longest outage I've seen in the last 4 years was less than 48 hours. During the last outage (~12 hours), I'd guess at least 1/3 of the houses around had one running, most of them the $500 ear splitters. They get really obnoxious when they leave them running over night, especially since a walk down the street suggests the reason they fired it up right away was so they could keep the TV's glowing.

 

[maple1]

All I know, is that's what we use. Typical household with showers every day, laundry once a week (maybe), dishwasher every 2 or 3 days.

I think I have seen those 'typical' numbers before - they seemed way over-estimated to me.

 

[twd000]

All I know, is that's what we use. Typical household with showers every day, laundry once a week (maybe), dishwasher every 2 or 3 days.

I think I have seen those 'typical' numbers before - they seemed way over-estimated to me.

OK, so our household is definitely higher than yours. Family of four 2-4 showers/baths a day, dishwasher 1-2x a day, laundry every 2-3 days.
How do you go three days between dishwasher loads? Do you not cook/eat at home?