Let's do some math.

[karl]

I'm still trying to estimate how much wood i'm going to use this year. I looked at my gas bill from last year and I used 70 MCF of gas. I know that's not alot, but I kept my house at 60° except for the two rooms I heated with the space heater. 1MCF is a thousand cubic feet. I got that from the back of my gas bill. So I went online searching for BTU output of different types of wood. I found two charts. Let's use red oak for the calculations. Chimneysweeponline's chart says red oak has 24 million BTUs per cord. They define a cord as 85 cubic feet at 20% moisture. They take out 43 cubic from the cord as air space between the logs. Daviddarling's site has red oak as 24 million BTUs or 16.8 milllion recoverable. 16.8 million is 70% of 24 million. Am I correct in assuming he is counting recoverable as the amount of heat you get out of a 70% efficient stove? Anyway, my furnace is 45 years old and it was only 80% efficient to begin with and I doubt if it's that high now, plus I know I loose alot of heat from the uninsulated duct work that runs throught the unheated basement and crawl space. So my summit is 72.5% efficient and I'm guessing that's better than what I'm getting from my furnace. So let's go with the 24 million BTU number unless any of you on here disagree with it.

http://www.daviddarling.info/encyclopedia/W/AE_wood_heat_value_BTU.html

(broken link removed)


From looking online I came up with the BTU output of natural gas as being 1000 BTUs a cubic foot from one site and 1,031,000 BTUs from 1 MCF from another. So check my math here.


24 million divided by 1,031,000 = 23.78 MCF in a cord of red oak. Is this right?

70 MCF divided by 23.78 = 2.94 cords of wood to keep my house the same temperature as last year. Does this sound right?

I realize I'm going to be keeping the whole house warmer this year than last year, because I'm not going to be closing off two rooms and heating them with a space heater. I'm just trying to judge how much wood I will actually go through and I know I will still use the furnace some. My goal is to keep the house warmer, like 70° instead of 60° and still spend less on natural gas. I'm going to do some insluating this year as well but that's another thread.

Is my math correct and am I using the correct figures?

 

[begreen]

They define a cord of wood as 128 cu ft. Still checking the rest of the assumptions, but note that the gas consumption does not have a factor for the losses. Let's say 20% of the 80% gas furnace efficiency is lost to waste. So you might need only 42mcf of gas in a 100% efficient system. Use that as a basis.

The 16.8m btus for a cord sounds about right, figuring about 8,000 btus/lb. Let's say 16m for simple math. And for natural gas lets say 1000btus/cu.ft.

OK 42mcf = 42,000,000 btus. divided by 16.8m btus = 2.65 cords. But the stove isn't 100% efficient, and the house leaks heat, so that needs to be accounted for with a multiplier. Say the stove is 70% efficient, but house losses eat another 10% or 60% average efficiency. That brings total wood needed to 3.675 cords.

If my math is correct, I'd have at least four cords ready to go. If it's a mild winter, you have money in the bank. And remember, two winters are rarely alike.

 

[karl]

BeGreen he defined it as a stack 4x4x8 which is 128 cubic feet but with air gaps so he is going with 85 cubic feet of solid wood. That makes sense to me.

 

[begreen]

got it. did some back of the napkin calcs for you karl. see if they work.

 

[BrotherBart]

You need to apply the efficiency of the stove to that 24 million BTUs. Gives you about a mil seven per cord. Also with a stove it keeps cranking out BTUs. You can't turn it off when the joint gets warm like the furnace so there are stray BTUs flying around even when not needed.

Bottom line. Five cords of hardwood in West By God to keep the house warm. Three cords since you are gone a couple of weeks of the month so your numbers are close anyway.

 

[karl]

I don't understand your statement : "you about a mil seven per cord"

As for stove efficiency. That shouldn't come into play. I'm measure gas I bought from the gas company against wood stacked outback. The furnace is 80% and the stove is 72.5%. Plus the furnace is 45 years old. I doubt I'm getting 80% out of it now, and if I am I'm still sending 20% into the basement and crawl space. So honestly I think the stove will be more efficient. I do realize the it burns all the time thing. I should have 5 cords done pretty soon. Not all of it is good hardwood and It' probably won't be completely dry by this fall.

 

[Mr_Super-Hunky]

My head hurts!

 

[Gooserider]

I would agree wth the 3-5 cord estimates, but also keep in mind that wood keeps, and many folks will say it will heat better in the second year after being cut and split than it does in the first. Between these two factors, I'd suggest that you look at the estimate as a MINIMUM and actually try to lay in as much as you can - it won't be wasted.

Gooserider

 

[karl]

Goose,

That's what I'm doing. The problem is i'm about out of time for wood that will be dry enough to burn this year. I'm going to keep cutting and stacking all summer and fall. I'm even thinking about getting a load of slab wood. The local mill sells it for about a hundred bucks a dump truck load delivered and it's already cut to size. I can assure you that next year I will either have plenty of wood or I'll be selling a one year old insert on here. I will 5 cords cut and split by the end of the week. The rest I scrounge will be for next year.

 

[Todd]

Another thing to take into effect is breaking in a new stove or learning curve. You will burn more wood while figuring out the new stove. And then there's the greedy effect where you say you want it 70 degrees, but lets see what this stove can really do, and get the house up to 75-80. So I would say 5-6 cords the first season then you will settle down to around 4 after that.

 

[begreen]

Split it smaller to hasten drying. I think you'll be ok as long as the wood has plenty of air circulation and some sun on it. You may have to reject some splits to the 'needs more drying' pile, but by late winter they could be ready to burn.

 

[Stevebass4]

this will be my second year using an insert and while the original reason the inset was installed was to reduce my oil bill - i have found that even if the savings are minimal (oil vs wood and i am installing a new high efficient boiler next week) i truly love the heat that the wood stove provides – it is so much warmer in my home and fire is primal.

In any event this year I’ve been buying logs for 80.00 and I enjoy getting out there and bucking / splitting and stacking the wood (just noticed one of my stacks fell overnight). I have 4 cords that may or may not be ready this season but they will be ready next year and since I only burn nights and weekends I figure I’ll use two cords at most this year..

when folks drive by they either as if 1) the wood is for sale and 2) what do you know that i don't. my awnser to that one is that i don't forsee oil prices going down anytime soon

 

[wg_bent]

Another factor you need to consider is that with a wood stove, your temps in the house are not as perfectly consistent as they are with a furnace, and generally, the house is a bit warmer with a stove. Pre-wood stove we kept the house at 68-70. My wife was always cold and complaining. With the wood stove we keep it more like 75+. I'm always hot and complaining. How do you tell a happy wood burner?... he's got shorts and a t-shirt on in January. Point is that it takes more energy to get to higher temp. You'll need more than 3.654369276355542 cords of wood, but I'm guessing not more than 7.342294483. but That's a bit of a guess.

 

[Eric Johnson]

I don't understand your statement : "you about a mil seven per cord"

As for stove efficiency. That shouldn't come into play. I'm measure gas I bought from the gas company against wood stacked outback. The furnace is 80% and the stove is 72.5%. Plus the furnace is 45 years old. I doubt I'm getting 80% out of it now, and if I am I'm still sending 20% into the basement and crawl space. So honestly I think the stove will be more efficient. I do realize the it burns all the time thing. I should have 5 cords done pretty soon. Not all of it is good hardwood and It' probably won't be completely dry by this fall.

Not to confuse the issue, but I believe the 20% loss in efficiency on your gas burner is going up the stack, not into your house.

 

[carpniels]

Hi Guys,

It looks like after all this math, we basically have what we all know in our gut: for a normal winter, you need about 5 full cords of dried and stacked wood sitting around.

Carpniels

PS. someone out there has the better signature: you never have enough split and stacked wood.