Do I have enough wood? A calculation

[BK's_ghost]

Hello World,

I calculated the amount of fuel needed for the heating season based off usage so far. I wanted to post it here because I'm open to criticism, and it might help out a new burner who thinks a lot about if they have enough wood.

First, I tracked wood usage. I have been burring exclusively from a cord of Poplar/Aspen for this reason. It's just about gone on December 1st. I assumed a heat value of 18 million BTU for this cord of aspen.

I also tracked Propane use. My 500 gallon propane tank has dropped from 76% to 72% since the house needed heat and I lit the stove on September 24th, this includes our cooking.
20 Gallons of propane at 91,502 BTU/gallon would be 1.83 million BTU.

Combined wood and propane that's 19.8 million BTU used so far.

Find out how much of the heating season has gone by. I pulled heating degree day data from my state climatologist's website. 8000 hdd per season with 1600 hdd as of December 1st. 1600/8000=20%

Finally use this to calculate your season's needs. 19,800,000 BTU / .2 = 99,000,000 BTU.

Since i started the season with 104 million BTUs in my 5 cords of ready enough wood I think I should be able to slide by.
I like this method because it takes system efficiency and internal gains (both highly variable) out of the equation.
I had some plans to try and get some of my recently split wood drying inside, but I think I just convinced myself (to Mrs. BK's great joy) that I'm close enough.

 

[peakbagger]

There is an assumption somewhere that your stove efficiency is going to be steady state, meaning X amount of wood equals Y amount of heat going into the house. Most stoves are idled during "shoulder seasons" and my guess is the stove is less efficient over a 24 hour period compared to running it full bore during times of higher heating demand. The other big improvement would be to weigh your wood and go on a pounds burned basis. Cords are volume based and my guess is even if the wood is bone dry in a kiln the stacker and stacking method may be responsible for quite a bit of variation.

 

[stoveliker]

Though if his name indicates a blaze King, the low burns are not less efficient.

However, I think you assume the same efficiency of the propane heating appliance.and the wood stove. Is that correct?

 

[BK's_ghost]

Hmm, stoveliker you’re right, it does wrongly assume that propane burns with the same efficiency as the wood, artificially inflating the wood efficiency a bit. However there is going to be some propane burned every month, so it might still hold. Same thing for the stacking variations. Since I split and stacked all the ‘cords’ the same way generally they should be similar relative to each other. I did assume book value density differences between the species.

Also sorry, I thought my stove would show up in my signature. It’s a classic jotul 118. I generally did burn hot then burry the coals, so operation wise it should burn similarly as I burn more wood.

 

[stoveliker]

No need to be sorry.

It's not even the burning efficiency but BTU delivery to the rooms efficiency. (Ducting losses?)

Anyway "A" estimate is better than none.
Moreover HDDs are average. Get a colder winter and you'll need more energy.

 

[tebenhoh]

I like the idea of a “formula”, but yes, there are a lot of variables to consider, especially the weather. I suppose if you were to track your usage and the weather for a season, you’d have real data for next season. If you did the same over a few seasons, you may be able to eliminate some of the variables.

The closest I’ve come to this is: how full are the racks compared to what we burned last year! I would, however, be looking at your same info much closer if we burned 24/7, or had no other heat source. Good work!

 

[BK's_ghost]

Some good things to think about. I certainly can’t predict the weather. It’s essentially the “well, how much did I burn last year?” estimate for someone who doesn’t have a full year of burning. Before I had a few months of burning I wasn’t really sure if I was looking at a 30% or 70% efficient system, so there was a huge amount of variability. Now I know my guess is on track, so far at least. Thanks for the replies!

 

[Sawset]

I also used HDD degree days to correlate wood usage. But I waited until the season was done, and used it to allocate how and where to create seasonal wood storage. I have a current season supply outside, and some inside the garage. Outside leanto wood I don't mind getting to if it's mild out. Dec, Jan, Feb, supply is stored inside away from below zero wind chills. How much and what kind? Lbs (or cuft) per degree day solved the issue.

 

[brenndatomu]

Most people can heat their homes with 4 cords/year...that's just a common average...

 

[BillBurns]

I mainly burn left over wood from my buddies shop. Anything from a 1x2 to a 2x12. A lot of small pieces to help get the fire going. No clue as to how much I burnt last yearAbout 10 SUVs full. 10 miles round trip for free wood, im in. I guess it really comes down to the weather. period. Thats my 2 cents at least. Oh, and I meant the hatch part of the SUV, seats still up.

 

[kennyp2339]

I was going to say (5) 4ft wide x 4ft high x 8ft long stacks should get you through till about the end of April

 

[Poindexter]

I agree in the lower 48 (broad brush) 4 cords annually is a good average. Maybe a little higher in Wisconsin for sure. I dunno which species of aspen are local to WI. I am not disagreeing with the OP, but Alaska aspen comes in around 14M BTU/ cord. We call it "gopher wood" as in put a stick in the stove and gopher another.

The other thing is it is very easy to keep the house warmer with wood heat than you might keep the house with propane. Not sure why this is, but many of us do so. I sure do keep the joint a bit warmer.

Do NOT look up the HDD for Fairbanks. If this is a new to you home, keep up with your energy usage this winter for better focus on next winter. If you have owned the place more than 12 months, figure out how many BTUs or therms or kwh you used last year and then see how many of those you can offset with wood.

The main thing is to be a supplemental burner when you are talking to your homeowner's agent. If you try to claim wood as your primary heat the underwriter is thinking about frozen pipes the night you stay late at work, and you get to pay for that risk.

 

[peakbagger]

I think it comes down to its going to take one or two seasons to get your wood usage down and by that point you should try to be two seasons ahead on your wood. At that point if you run short one winter you can raid the next seasons stack late in the season and then work a little harder to build up inventory.

 

[Woodsplitter67]

When I started I figured it out by how large my stove is and loading it. For example 1 overnight burn is 2.3 cuft of wood. December, January, February are roughly 90 days 90x2.3= 207cuft or 1.61 cords.. and so on. It was extremely accurate the dead of winter here are January and February.. 60 days @ 2.5 loads per day 2.3 cuftx 2.5=5.75 per day x 60 = 345 cuft = 2.69 cords for the 2 months.

 

[Eman85]

I just keep cutting and splitting, surely not going to put a pencil to figure if I have enough wood, when it's winter already it's too late.

 

[Woodsplitter67]

I just keep cutting and splitting, surely not going to put a pencil to figure if I have enough wood, when it's winter already it's too late.

Its a good idea to figure out. I did it when I first started. It gave me an Idea of many things.. Like setting up permanent storage, space needed for storage and processing, equipment needed for scrounging and processing, and most of all securing places/people to get my free supply of that quality of wood year after year..

 

[qwee]

I don't usually sell firewood but I did this year. My last delivery was almost 2 cords of my best firewood - dry elm, honey locust, ash, and some fir/pine. Where I live this is good wood. I could tell the customer was new to wood burning.

He had stacked about a cord of wet poplar he cut and split. But he thought it was some other wood - but I could clearly see it was poplar (I didn't argue). It was his 1st score, no doubt. I could tell he thought that wood was wood - but this isn't true. The wood he bought from me was nearly 2X as dense as the poplar he had. So his 1 cord pile of poplar = 1/2 cord of hardwood in terms of energy for heating. So know you may go through 5 cords of poplar faster that say 5 cords of a wood like oak or ash.

 

[stoveliker]

I hope you told him it was dry. He'll see your wood was way better - and infer (hopefully) it is the seasoning that made it so. No need to explain BTUs per split in this particular case...

 

[BK's_ghost]

I also used HDD degree days to correlate wood usage. But I waited until the season was done, and used it to allocate how and where to create seasonal wood storage. I have a current season supply outside, and some inside the garage. Outside leanto wood I don't mind getting to if it's mild out. Dec, Jan, Feb, supply is stored inside away from below zero wind chills. How much and what kind? Lbs (or cuft) per degree day solved the issue.

I like the lbs/degree day, thanks.

 

[BK's_ghost]

I agree in the lower 48 (broad brush) 4 cords annually is a good average. Maybe a little higher in Wisconsin for sure. I dunno which species of aspen are local to WI. I am not disagreeing with the OP, but Alaska aspen comes in around 14M BTU/ cord. We call it "gopher wood" as in put a stick in the stove and gopher another.'

We have the gopher wood joke around here too. I have seen some different estimates for the aspen aspen, the 18 was from a montana state extension page i think. i went with that one becuase, it obviously isn't a dense hardwood like oak, but really does seem more substantial than the basswood or white/red pine we have around here.

Do NOT look up the HDD for Fairbanks. If this is a new to you home, keep up with your energy usage this winter for better focus on next winter. If you have owned the place more than 12 months, figure out how many BTUs or therms or kwh you used last year and then see how many of those you can offset with wood.

I did look up Farbanks, woof! I know how many BTUs of propane I burned last year, it will be interesting to compare. Thats the number i based my guess on wood for this year on.

 

[BK's_ghost]

So know you may go through 5 cords of poplar faster that say 5 cords of a wood like oak or ash.

Luckily my wood isn't all poplar, it's just the cord i just burned, and another one for the spring. I've got 3 of maple/ash/oak that I am breaking into now. The poplar does seem to make great sholder season wood though. Its easy to get around here and a joy to split.

 

[BK's_ghost]

It's not even the burning efficiency but BTU delivery to the rooms efficiency. (Ducting losses?)

what is a reasonable number for ducting losses? I know my condensing gas furnace in the basement combusts a whole lot more efficiently than my wood stove, but having the heat radiant and right in living space just seems to be more effective. The efficiency doesn't seem matter has much as I thought it would. Do any users have numbers comparing wood use to propane use in the same home?

 

[stoveliker]

I'm sorry,.I don't. @EbS-P might know as he studied ducted heat a bit.

 

[begreen]

what is a reasonable number for ducting losses? I know my condensing gas furnace in the basement combusts a whole lot more efficiently than my wood stove, but having the heat radiant and right in living space just seems to be more effective. The efficiency doesn't seem matter has much as I thought it would. Do any users have numbers comparing wood use to propane use in the same home?

Heat loss calculations depend on several variables like the velocity of the air, length of the run, ambient temp vs internal temp, insulation, sealing, etc. Typical forced air systems are leaky and poorly insulated. They can get away with this because the outpt temp in the supply plenum is around 145º so the losses are less noticeable. Hot air still comes out of the registers unless the run is quite long. Heat pump systems don't have this luxury. When I converted our system over from propane to the heatpump I sealed and insulated everything due to the lower output air temp of the HP. The difference is quite impressive.

Heat Loss from Ducts Equations and Calculator

Heat Loss from Ducts in a Building, basement or home Equation and Calculator. Include cost of energy loss.

www.engineersedge.com

 

[JRHAWK9]

Most people can heat their homes with 4 cords/year...that's just a common average...

4 cords of Cottonwood or Osage Orange? There's a BIG difference. (54 MBTU's vs 132 MBTU's)