How to Estimate Usable BTUs

[MidwestWoodBurner]

Hello all, I've recently discovered the forum and have spent a while reading articles on this site. I'm impressed by how much information is here. Thank you to all who have started this site and have contributed throughout the years.

My question is how to estimate the usable BTU output of a wood-burning appliance. I currently own a Heatilator Accelerator which boasts a 100,000 BTU output, but it doesn't take an expert to stand in front of it and tell you that that is not the usable heat output. I'm afraid that most of it is going up the flue (and hopefully not taking much furnace air up with it). With outside combustion air, gasketed (but not air-tight) doors, and circulating space between the firebox and the jacket aided by an internal fan, I figured it would put out more heat than it does. I am burning mostly sugar maple and honey locust at 12% or less, according to my $20 moisture meter.

I own an infrared thermometer if that would be of any use in the estimation. Any ideas?

 

[begreen]

Welcome. The Accelerator has a 100,000BTU input listed. And that would probably be stoking it with a major load of wood, or at peak rating. If the fireplace is 50% efficient then it would have a 50K BTU output, at maximum capacity. Normal running output would be considerably lower.

 

[Sprinter]

There are so many variables involved in actual heat output, that it's hard to quantify if at all. It involves the wood quality and moisture content, any secondary burning capability, user experience, etc.

It's good that you have outside air for combustion. That helps with the amount of heat that may be lost.

It's also good that you are trying to monitor the wood's moisture content. How are you using the meter? Please make sure you are making a new split from any splits you want to measure, and then stick the pins in the middle. A 12% reading is pretty hard to get except maybe in Phoenix or somewhere and your wood may actually be much more moist that you think. Not to be critical, but it's rather important and often overlooked.

I don't think it's practical to try to estimate actual BTU output. Just do the best you can with what you have....

 

[MidwestWoodBurner]

Welcome. The Accelerator has a 100,000BTU input listed. And that would probably be stoking it with a major load of wood, or at peak rating. If the fireplace is 50% efficient then it would have a 50K BTU output, at maximum capacity. Normal running output would be considerably lower.

Good catch on "input" versus "output". I did not notice that. Theoretically, they would only be the same if 100% efficient. Do you think 50% efficiency is reasonable for this particular unit, or was that just a hypothetical figure you threw out there?

There are so many variables involved in actual heat output, that it's hard to quantify if at all. It involves the wood quality and moisture content, any secondary burning capability, user experience, etc.

It's good that you have outside air for combustion. That helps with the amount of heat that may be lost.

Even with outside combustion air, I still notice that the wall outlets, leaky windows, and doors tend to let in more air when I have a raging fire going. That indicates to me that the outside combustion air inlet is providing insufficient air volume for hot fires in this unit, and the rest is getting in through the leaks in my gasketed (but not air-tight) fireplace door. It could also be that I observed the air infiltration on particularly cold windy days when that kind of thing tends to happen anyway. I'll have to pay closer attention.

It's also good that you are trying to monitor the wood's moisture content. How are you using the meter? Please make sure you are making a new split from any splits you want to measure, and then stick the pins in the middle. A 12% reading is pretty hard to get except maybe in Phoenix or somewhere and your wood may actually be much more moist that you think. Not to be critical, but it's rather important and often overlooked.

I don't think it's practical to try to estimate actual BTU output. Just do the best you can with what you have....

Also a good catch about the moisture test method. I was simply inserting the electrodes into the end grain of dried wood, which consistently reads between 9-13%. This morning, I halved a split that tested 11% on the end grain only to find out that when tested in the middle of the fresh split, it read 22%. That means I have two cords of not-quite-seasoned firewood. Would you still burn it if it's all you had, and this is only a supplementary heat source?

I use an unoccupied greenhouse as my firewood shed, and during the summer, with the windows open for ventilation, it still gets above 100*F and single-digit humidity, so very much like Phoenix! However, I overestimated how quickly split wood can dry under these conditions.

 

[nola mike]

You could weigh the wood, calculate the number of btus in the wood given the moisture content, and see how long it takes to burn down (btus/hour), multiplying that by the efficiency of your fire place if you can find it. That would still be a very rough estimate, but might get you in the ballpark.

 

[Sprinter]

Even with outside combustion air, I still notice that the wall outlets, leaky windows, and doors tend to let in more air when I have a raging fire going.

Frankly, I don't know much about your particular appliance, but it may very well be that your combustion air intake can't keep up with a vigorous fire. It's especially problematic with traditional open fireplaces which can draw over 500 CFM compared to about 50 for EPA stoves. But I don't know how your Heatilator compares with that.

Also a good catch about the moisture test method. I was simply inserting the electrodes into the end grain of dried wood, which consistently reads between 9-13%. This morning, I halved a split that tested 11% on the end grain only to find out that when tested in the middle of the fresh split, it read 22%. That means I have two cords of not-quite-seasoned firewood. Would you still burn it if it's all you had, and this is only a supplementary heat source?

Yup. Very common mistake. I wish they would put that front and center on the directions. If you don't do it right, the readings are worthless. I wouldn't worry about 22%. I get very concerned with anything over 25% which seems to be a tipping point (my experience anyway). Just think of wood as drying from the outside-in. It's inside that counts.

 

[begreen]

Good catch on "input" versus "output". I did not notice that. Theoretically, they would only be the same if 100% efficient. Do you think 50% efficiency is reasonable for this particular unit, or was that just a hypothetical figure you threw out there?

Hypothetical, but in the ballpark I think.

 

[MidwestWoodBurner]

Thank you, all.