House Heat load and Stove

[DamienBricka]

My house heat load 25000 BTU/ hours with outside temperatures of minus 30F.

I have the CFM-FW240007 stove. It holds 1 cbf of wood.

Since a cubic foot of oak is 156000BTU. Since my stove with primary open at 1/4 is 12,112 BTU/h per home owners manual. Right now I need 250*50= 12500btu/Hours.

In theory I should be getting a burn time of 156000/12112 which is about 12.88 hours.

So why is it that my burn time is only 4 to 6 hours? I do not understand. Are they logical explanation?

Thanks for all your insights.

 

[Sons924]

i'm confused

 

[Rickb]

You have to take into account the heat going up the chimney, plus depending on the moisture of the wood the amount of BTU's till the wood is dry.

 

[DamienBricka]

Assuming the stove is 70 percent efficient. I know the wood has less then 20% humidity. Does this mean that to evaporate the humidity it takes about 30% of the wood calories.

 

[begreen]

How often does it get to minus 30F in Pittsburg?! I would expect a real world load of wood in a 1 cu ft stove to be more like 1/2 or 5/8s a cu ft at most. You need to account for the space between the wood splits. Where did the cu ft of oak = 156K btus come from? I get more like 172K btus for red oak and 188K btus for white oak. The stove's efficiency is not the same at all points in the burn cycle. It's worse than 70% at the beginning and end of the cycle.

 

[DamienBricka]

1 cu foot with oak = 156K. I came up with that number by assuming that a cord of white oak was 20 million BTU

 

[WarmBluthner]

Why dont't you fill up the stove, remove the wood and weigh it.

Take 80% to correct for moisture and then you've got the amount of pounds of fuel that fits in your stove.

 

[logger]

Any manual will tell you that a 1 cft stove will not get you an overnight burn. 4-6 sounds like the norm to me. All that maths gonna get you dizzy.

 

[oldspark]

One cubic foot wood stove gets an overnight burn, sign me up for that magic box.

 

[USMC80]

id say your doing good getting six hours

 

[Huntindog1]

House heat load? So how much insulation do you have to have to get that kind of number?

If it helps for comparison I heat 1900 square foot ranch upstairs from a 729 sq ft family room in the basement with a 2.12 cu ft wood stove. My setup is ideal to distribute the heat and my house is well insulated plus I have new energy efficient windows. Last night went to bed with the house temps 73 in the center of the house. Woke up with lots of coals in the stove and the center of the house was 71 degs. It was 9 hour cycle time as i had plenty of coals left to start another fire. Actually had to burn the coals down by throwing on some wood pellets. It got down to 6 degrees last night. I made some mods to my stove in the fall and its heating my house much better this winter. PM if you would like to know the mods I did.

 

[Backwoods Savage]

Sounds like over-thinking is confusing the issue. As someone else stated, you will not get overnight burns in that small of a firebox. And does Pittsburg really get to -30 degrees? If so, how often? I did not think that Pittsburg was that cold of a climate.

EDIT: I looked up Pittsburg and find the coldest average day temperature is reached on January 18-22 and that average temperature is 36-21. To reach -30 degrees that would mean a minimum of 51 degrees below the normal temperature. Not unheard of but certainly a rarity.

 

[DamienBricka]

I never said that Pittsburgh was getting -30F. I just used that number to calculate my heat load.
All I am trying to do is understand where all my BTU are going. Maybe I am worrying about something
that I should not worry about. All I know is that if I do not understand with numbers I get confused.
Just me.

 

[Backwoods Savage]

Damien, I understand your problem but do think you are trying to come up with some impossible numbers. With only a 1 cu ft firebox, you simply can not get that much heat from it. To get more heat the only way is to burn the very best wood in what you have but you will still have a short burning cycle. The other option is to installs something with a larger firebox. For overnight burns, 2 cu ft is about the absolute minimum size for most but even then, most find they need even larger firebox sizes. For sure if you are heating a large space, the larger mass of the stove will give you what you are searching for and that is more heat for a longer period of time.

While typing I kept thinking about the Woodstock line. We have the Fireview and 2 years ago they came out with the new Progress Hybrid stove. With the Fireview we typically top out around 650 degrees stove top. Others who have had the Fireview and then traded for a Progress tell us they get much more heat from the Progress running at much lower temperatures; like around 450 stove top. Of course they also can get much longer burn times; typically 2-4 hours more.

 

[DamienBricka]

Backwoods Savage, I understand what you are saying. My biggest concern with going to a bigger firebox is that I
will be burning more wood. Is that a true statement or am I way wrong?

 

[Backwoods Savage]

Yes, you no doubt will burn more wood but you will also be getting more heat. How much more wood is difficult to tell. Not sure what size of a space you are heating but to give you an example, we are heating around 1400 sq ft with a 2 cu ft firebox. We typically burn 3 cord of wood per year. We have no other heat source either so depend entirely on the wood. We do burn mostly ash at present because of the bugs that killed all our ash trees. But the total amount would not vary much if we didn't burn all ash as we'd then burn a mix of elm, cherry, oak, soft maple and a few other odds and ends. We do not have much oak here so we save that for the coldest part of winter; typically when the night temperatures are lower than 5 degrees or lower than zero. So far this year our lowest has been 2 degrees but no oak burned yet. So far, mostly all ash.

 

[DamienBricka]

The size of your house is similar to mine. My little stove heat the house very well. I am just tired of reloading the stove all the time.
I estimated that I would be burning about 3 cords this season. So our numbers are pretty close. That is a good sign.
I guess it is time to look for a bigger stove for next heating season.
Thanks, Backwood Savage

 

[begreen]

A larger stove will not necessarily burn more wood. Cat stoves can burn longer at a lower rate than a secondary tube stove. You could move up to a 2 cu ft cat stove and get longer burn times without more heat unless you desire to turn up the stove. Even a 2 cu ft non-cat with a lot of mass like the Alderlea T5 will not overheat the house, yet will burn longer. In milder weather you would just burn partial loads of fuel and let the fire die down.

 

[DamienBricka]

Begreen, What do you think of the Englander 13NC it is 1.8 cubic feet and it sells at a good price. Or do you think that I should look at other option? Is there a better time of the year to buy a stove?

 

[Sprinter]

Does this mean that to evaporate the humidity it takes about 30% of the wood calories.

I know this doesn't really answer your question, but it requires about 1100 BTU to heat and then evaporate one pound of water. You can extrapolate from there.

Real efficiency ratings of wood stoves is a slippery thing. Here's an article on that subject. (broken link removed to http://www.rural-energy.net/docs/efficiencyRating.php)

 

[DamienBricka]

Thanks, Sprinter. Greatly appreciated.