Wood stove BTU numbers VS. The real world

Alan Gage said:

Thanks for the replies. I realize I'll have problems getting long burns from such a small firebox but I'm ok with that. If I have enough coals left over in the AM to light some small splits I'll be happy. I actually prefer a house cooler than most. 70 degrees and over I start getting uncomfortably warm.

BK - An average heat output of 1/3 or less from peak over a burn cycle would give me lots of room to play so that's good news. Even if it averaged 1/2 of peak output I should be sitting pretty.

Jimbo - Good point about the amount of time the outside temp actually stays at the design temp. It was colder than average here this winter but we never made it to -20. Lots of nights of 10-15 below and highs around 0. Last year we hit it a few times as well as setting a record of -32. But like you said, it doesn't stay there long. I've tried running the numbers with design temps of -10 and -15 and it certainly helps out.

You're right about calculating air loss, that will be the wild card. I'm going to be very careful about building a tight envelope. The plan is 2x6 walls full of cellulose insulation, OSB sheathing, housewrap, and at least 2" of rigid foam on the exterior (with taped seams). That will give me another R10 as well as creating a thermal break for the studs. Flat ceiling with at least R-60 blown in. I've never lived in a well insulated and tight house so I'm really interested (excited actually) to see how well it holds heat.

I'm also worried about the stove being too hot for the majority of the burning season. I'm used to burning 24/7 from December through the end of February. I'm afraid the house will hold heat so well that except in the coldest weather the coals will be burned down to nothing before it's time to reload. Which will mean starting a fire from scratch most of the time, which is a pain. That's one of the reasons for wanting to place it by some windows, to let some of that heat get away. But I don't want to lose too much of it and run out of BTUs when it gets really cold. I suppose I can always break down and turn on the backup heat if that's the case.

I guess there are worse problems to have....like trying to heat my current house with the wood stove when it's -10 and the wind is howling.

Thanks for all the great info everyone!

Alan

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I'm not an expert but I've always heard cellulose with settle significantly over time. The builder insisted on using some in the ceiling and we finally agreed on R19 fiberglass with R30 equivalent of cellulose on top. I think I will regret not demanding R38 or higher fiberglass. I actually installed R38 in my pole barn and I think the builder concerns with leakage between the batts was silly. My walls were R19 (2x6) fiberglass. Now I see Owens Corning has this ATTI CAT type. Basically it is shredded fiberglass that blows in with a machine but it not supposed to settle and not as dusty to install. My local mom & pop lumber yard is advertising it now too. If you put cellulose in your wall cavities I think you can expect it to settle several feet over 10 years causing great heat loss. Research this a little more, sounds like you have a great plan for a nice well insulated home that will be easy to heat with your woodstove. I just wouldn't want you to be disappointed with the performance due to settling of the cellulose.

huskers said:

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I'm not an expert but I've always heard cellulose with settle significantly over time.

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Old cellulose definitely did. When I opened up my wall cavities the stuff was down at least 2 feet from the top plates.

That's not nearly as much of an issue in a ceiling application though, because there's much less thickness. Eight feet of cellulose will compress a lot more than one foot (due to gravity). In a ceiling, the cellulose will fill odd shaped spaces much better than fiberglass, too.

Supposedly the new cellulose, properly installed with a wet blower into open stud bays will not compress. I wonder though.

thechimneysweep said:

Or, how about a formula that takes into account the efficiency rating of your specific stove?

(( firebox size in cu. in.) x ( 0.015 ) x ( 6200 ) x ( stove efficiency )) / ( burn time )

To get the firebox size in cubic inches, we multipy the manufacturer's stated cubic foot measurement by 1728.

The 0.015 is the weight of the load per cubic inch. To get this number, we used an average of the top 60 species from our firewood comparison chart, and adjusted to compensate for airspace between pieces, using the ratio that a 128 cubic foot woodpile only contains about 85 cubic feet of wood. This creates that we're not using the weight of X cu.ft. of wood, we're using the weight of the wood that will fit in a X cubic foot firebox.

The 6200 is the available BTU (heat) content per pound of fuelwood at 20% moisture content.

For stove efficiency, we use the manufacturer's tested Low Heat Value rating.

For burn time we use 8 hours, although smaller numbers can be used for smaller fireboxes.

More at http://www.chimneysweeponline.com/wscomp8.htm

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Spot on.

Tom, I just want to say that your site contains the most accurate (IMHO) info on wood burning that I have encountered so far on the Internet. Kudos to you and your staff for providing a virtual treasure trove of well-vetted facts and general info. Keep up the good work. There is lots a good info on Hearth.com, don't get me wrong, but there are some wacky notions as well... all part of any public forum where anyone can voice an opinion while having neither a technical background nor real-world experience as a prerequisite.

Alan, if you get your house built as described, you'll have roughly R30 walls and that R60 ceiling. If you pay attention to air sealing all around the envelope, for a very tight house, you'll be pleasantly surprised at how slowly it cools down overnight. While your planned house is smaller than mine, your experience ought to be similar. Briefly, I am nearing completion on a superinsulated house. Walls are R40, attic floor is R60, windows are triple pane, basement walls R20, slab is R20. I have been heating it for now with a small stove, Quadrafire Millenium 2100, with output on the tag of 11-28K BTU/hr. The house design heat loss is 21,000 BTU/hr, for 4,000 sqft of conditioned space. It doesn't heat up or cool down very fast at all. With the outside temp getting down to zero for a while overnight, I lose a degree or two inside after the stove burns out around 12-1am. I don't rush over to restart it, as I know the temp won't have dropped drastically. But neither do I worry about the stove overheating the place during the day. The mass of the house absorbs the heat easily.

Getting the air sealing is very important, however. Heat loss due to air leakage costs more than many think. But for any amount of leakage you can easily calculate the heat loss due to it. A tight house also means you ought to have a small HRV to provide continuous makeup air for health and for keeping the inside humidity down. This seems strange to anyone who has no knowledge of or experience with superinsulated, tight houses. Also, tight means you really ought to design in an outside air duct for the stove, directly connected, as you would have the real possibility of backdrafting if someone turns on the range hood or dryer.

Alan, you did read correctly. I am indeed able to heat that 4,000 sq.ft. of conditioned space with the little QuadraFire Millenium 2100. The numbers said it could, and the observed result confirms it. On some days it has had a little extra help, in the form of three dehumidifiers running to get rid of humidity from the plastering and painting. That might be another 1.5 KW (about 5,000 BTU/hr) or more at times, and some temporary lighting, all of which is heat released inside the building envelope. I've been finding that it's more effective to open one door downstairs an inch or so and another up for a few hours, with the stove running to offset the heat loss. Later I close the doors and keep the stove running to regain the degree or two lost during the venting. As I said, it's a long, slow climb back up. The primary heating system will be a two-ton Climatemaster Tranquility 27 heat pump. That won't be in for another few weeks. I needed to have the blower door test done before specifying the size. I got a better than hoped for result, 0.65 ACH at the standard 50 pascals depressurization. That would be something like 0.03 ACH "natural."

Above around R30 in a wall, the easiest way to achieve a high R wall is with a double frame. I have a 12" cavity, with dense-packed cellulose blown to a density upwards of 3.0 lb/cu.ft. That's over twice the natural settled density of the stuff, so it always wants to expand, avoiding settling over time. That double frame, with insulation between inner and outer studs, eliminates nearly all of the thermal bridging. The attic is a loose blow, and around 10% settling over time is factored into the amount, so that right now it could be slightly higher than R60. A small change in around 2,400 BTU/hr in my case won't be noticed. I do have good windows, triple pane, low-E, argon fill, fiberglass frame; U is around 0.17. All are fixed glass or casements. Double hungs don't perform as well in a cold climate.

Bear in mind that a superinsulated house is an extreme end of the scale, with an old, drafty, barely or uninsulated house at the other end. There is everything in between, and even among superinsulated houses there is a matter of scale. Four times the conditioned area equates to roughly twice in each direction, so there is surface/mass ratio to consider as well as level of insulation. While the smaller but equally well insulated house will have a lower absolute rate of heat loss, the thermal mass will be lower as well, and the rate of temperature drop inside could be greater. Between our size differences and different insulation, where my house may drop two degrees over a cold night without heat, yours could conceivably drop 3-4 degrees. That would be hard to calculate, since the temperature profiles won't ever be in steady state. You'll just have to wait and observe what happens. But I really don't think you'd have to wake up to a house in the 50s if you don't have an overnight burner. Still, you may want a quick warmup in the morning, with the main heating system.

With a tight house, you certainly will need a means to provide fresh air. You simply can't design in a certain amount of "leakiness." All you'll know is that it's worst in bitter cold and windy weather, and practically nil in mild, windless weather. Further, you'd have absolutely no control over it, other than by opening windows. Some have argued that exhaust via bathroom fans on a low setting, with makeup elsewhere via passive air inlets (such as by Aldes), are sufficient. Others argue that for the sake of limiting heat loss an HRV or ERV is appropriate. You really don't want to try to use the woodstove as an exhaust device, as the range hood or clothes dryer could easily backdraft the chimney if the stove air inlet is open to the room, even if you have passive air inlets. A side benefit to having outside air ducted directly to the stove is that when the fire burns out you won't have cold air flooding the room. That's one reason to make sure that the selected stove allows direct connection of an inlet air duct.

I haven't really participated in this forum until recently, although I've browsed the posts from time to time. Now, with the observations on the house and woodstove in hand, I thought I could offer a few things to say. I guess the big thing is that as a new house approaches the superinsulated end of the scale, old rules of thumb as to how big a stove is needed and even the idea that an overnight burner is essential don't apply. One really needs to have a good calculation of the house's heat loss at design conditions.

This isn't really the sort of forum to get much into building science, as it's more for stuff relating to wood burning equipment. If you haven't already done so yet, go over to Green Building Advisor (http://www.greenbuildingadvisor.com/) and Building Science (www.buildingscience.com). You can search there for all sorts of information on ways to achieve high energy efficiency in new construction.