Stove size for superinsulated home

Hey folks, first time poster here (This site is excellent!) looking for some advice on sizing a stove for a unique/unusual situation that seems to be resulting in a lot of conflicting advice from dealers/reps.

My wife and I are doing the owner-builder thing and building a new barn-home for ourselves in southern Vermont. The structure will house our living space as well as a large workshop (I'm self employed). We are both fairly new to wood burning (She's brand new to it and I've only ever had a crappy fireplace) Our new location has 10+ acres of woods and excellent eastern and southern exposure with glazing designed to take advantage of it. We are hoping to use wood as our primary heat source with electric backup (No natural gas here.)

Where we are running into trouble is the following:

-We are building the home in the style of a monitor barn with a 40x60 main level (2,400 sq. ft.) and a 16x60 loft (960 sq. ft.). Its all post and beam construction with enormously high cathedral ceilings due to the barn/workshop thing. Our eave walls start at 10' tall and go up from there with the roof pitches to 16' tall. Our central bay underneath the loft is 12' from slab surface to the bottom of the loft floor. Our total volume is a cavernous ~40,000 cubic feet. Yes I know, we made it too tall.... figured that one out a bit too late.

- On the other hand, we have a super-insulated wrap and strap design that is extremely air-tight (HRV ventilation) with 6" and 8" of polyiso foam on the walls and roof, giving us R-39/R-52, plus a heavily insulated slab floor on the main level along with triple pane windows, home-made insulated doors, and nearly no thermal breaks or other combustion appliances competing for air.

-To our benefit is there will be very little in the way of interior partitions. Except for the bathrooms/mechanical room the loft is entirely open and the main level has one wall dividing the living space and workshop space.

- Our Chimney height is also a fairly lengthy 34' from stove to cap with one offset, enclosed almost entirely within the conditioned space. So yeah, draft.

- Our math gives us a heat loss at a -15 degree outside temperature of about 50kbtu/hr for the entire structure. Approx. 50% of that figure is heat loss due to air exchange/leakage (before factoring in what the HRV recovers). Our joke is that on a 50 degree day a hot shower might be able to heat the space for the day.

Our big fear is that any stove large enough to hold a burn overnight will have us running out into the snow in shorts trying to cool off. Complicating the situation is that the air in my workshop and our living space air shouldn't really be mixing. (I gave up trying to find a safe and practical way to share air and heat between them today. We either have two stoves or we will be sucking down fumes and dust at the dinner table). So we are inclined to air-seal the partition wall between those spaces and therefore are really trying to heat two zones, one being about 1440 sq. ft. in a big open space, the other being around 960 sq. ft. with a stairway heading up to another 960 sq. ft. in the loft. We have not built any interior partitions yet and can thus air-seal and insulate (or not) wherever necessary, including the loft floor.

We are leaning heavily towards soapstone stoves to try to mellow out heat from the stove and thus are looking at Hearthstone and Woodstock. I like how the soapstone will look with the acid stained concrete floor. My wife likes the traditional cast iron (Jotul castine/oslo were her favorites, with shelburne/manchester a close second).

The 2 local hearthstone/jotul dealers are suggesting smaller stoves like the castine, castleton, and heritage. We talked directly to a hearthstone rep who said ignore the BTU numbers and look at the volume of the space and is pushing us towards a manchester/mansfield. Woodstock stoves advised their new ideal steel stove for its control-ability but honestly the aesthetic is a deal-breaker for us. We both love the progress hybrid both in form and function but fear its too much stove for us. Unfortunately the other woodstock stoves have been vetoed on aesthetic grounds, at least for the living space. (She doesn't care whats in my shop, so the ideal steel is actually a possibility there, the excellent price point may help me get over its modern look in our rustic barn).

Both local hearthstone/jotul dealers gave us an excellent impression, but visiting the woodstock factory really has me hoping we can make a progress hybrid work. We want to hand our money to stove geeks rather than marketing departments and the woodstock stoves didn't have hearthstone's crappy hardware or jotul's lack of soapstone. The amount of extra thought and engineering/design in them was obvious. We want a woodstock-built stove that looks like a hearthstone/jotul, ideally. That means a progress hybrid. Does that mean keeping windows open all winter to avoid cooking ourselves?


Help?

Attached is our floor plan with possible stove locations. South is up. Stoves have to be within the central corridor (between the two rows of posts) otherwise chimneys will run on the exterior and look really stupid towering up above the loft like bunny ears. Anything within the central bay can be routed through the loft with varying difficulty or ease.

Unfortunately the other, smaller woodstock stoves have been vetoed on aesthetic grounds, so its kinda the progress hybrid or another manufacturer.

What is different about the BK stoves that allow them to burn a given weight/volume of wood over such a longer period of time (thus lowering btu/hr output)? Nearest BK dealer is an hour away. Not ideal, but do-able.

Believe me, I've tried to figure out how to share heat on the main level for a long while. I was even trying to figure out if I could put folding/sliding double doors between the living space and the shop to heat up the shop from the big stove in the living space overnight and then seal up when I start work in the morning but my wife keeps reminding me how miserable the rest of the house would smell in that case. Its a metalworking shop so you've got lots of grinding/sanding/polishing dust, welding fumes, forge fumes, kiln fumes, burnt oil, etc. It would make our heating so much easier, but the wife is right, it needs to be isolated from the living space for our safety and comfort. This is probably the thing that is making this the most difficult, the two zones divided oddly.

Also, the structure is built and sheathed with windows installed and we are currently insulating, so changing the dimensions of the loft might be a bit difficult

We're just really confused on whether to listen to the dealers who say to use 2ish CF fireboxes at the very largest to avoid overheating given our superinsulation or the manufacturers who say head towards 3 CF to handle the volume.

We acknowledge our aesthetic tastes knock a lot of well-performing stoves out of the running, but we didn't spend the extra time, energy and money constructing a post and beam structure vs. conventional stick building to plop a very large and expensive hunk of metal in the middle of that isn't as good looking as the rest of the building. I really really wanted to like the woodstock ideal steel hybrid on functional grounds, but its just too much of a mismatch with our preferred aesthetic style.

- The BK ashford got the aesthetic approval from the wife, so we're going to take a look at it asap. I don't *need* a 20 hour burn time, but if I can load up 40 lbs of wood in the thing and it can be adjusted to take twice as long to burn that 40 lbs of wood than another similarly sized stove, that will halve the BTU/hr output which will be a very good thing in our case. Because yeah, in the shoulder seasons our btu/hr needs to keep the space heated are miniscule.

- Pellet stoves aren't in consideration because we will be selectively harvesting 10 acres of woods on the property, so aside from my time and energy to log it and split it, its free fuel.

- Air source heat-pumps are in our future, hopefully in a few years we will be able to install them along with a PV array. Our initial plan was to do 3 mini-splits (shop, 1st floor living, loft) with one small stove in the "great room" for a cold winter boost and the pleasure of a wood fire, but the cost to install 2 larger stoves and chimneys is less than the 3 heat pumps and I don't have to pay money out of pocket to feed the stoves (at least as long as I'm relatively young with a good back that is) so we've decided to see if we can make wood as the primary heat work, with a small heat-pump down the line to provide some summer A/C and handle the 40-50 degree days before its cold enough to fire up the stove(s). The stoves will pay for themselves in saved electricity fairly quickly, even at mini-split efficiencies.

So, obviously a novice question here. It sounds like the BK stoves are "tuned", so to speak, to be able to have a very slow burning fire compared to other brands. If we were to purchase a large stove, for example the woodstock progress hybrid that we like so much. If for the sake of math we load 60 lbs of wood in there with chimneysweep's 6200 btu/s per pound average for firewood, over a 10 hour burn cycle that would give us an average of 37kbtu/hr. If we load 30 lbs of wood into that same large firebox, are we looking at a significantly reduced burn duration? For example if 30 lbs of wood would only give us a 6 hour burn instead of the 10 hours above, that would still be pumping out 31kbtu/hr, still likely more than enough to overheat us. However, if that partial load of wood can still be stretched to 8-10 hours, that gives us much more promising ~18-23kbtu/hr.

I guess the question is, does building a small fire in a big box just reduce the burn duration, giving more or less level heat with a full load on an hourly basis or does it stretch a smaller amount of heat energy in the wood over a similar duration, thus reducing the hourly heat output?

If so, then what I'm hearing is that the BK ashford by having a wider range of possible burn times would provide an even wider range of average hourly btu outputs, perhaps mitigating the need to be stingy on the amount of wood loaded to avoid overheating?

Dick, perhaps the disparity between our outcomes may be due to air exchange?

We insulated with 6" (walls) and 8" (roof) of polyiso foam board @ R-6.5/inch directly over T&G sheathing that is also our interior finish which is held on by 1x and 2x strapping giving us .75" and 1.5" air gaps on the walls and roofs respectively. So the overall R value of the assembly is probably 1-4 points higher than the value of just the foam, but we've just been using the foam itself for our math. We figure it offsets the de-rating polyiso experiences in its outer inch or so in very cold weather.

Heres our math (Delta T of 80 deg for -15 outdoor to 65 indoor)

2,800 sq. ft of cathedral ceiling @ R-52 = 4304 btu/hr
3,200 sq. ft. of wall surface @ R-39 = 6564 btu/hr
307 sq. ft. of windows @ R-7 (European triple pane) = 3,508 btu/hr
207 sq. ft. of doors (incl. large barn carriage door) @ R-5 = 3312 btu/hr

Foundation perimeter loss math is a bit wonky, as we found multiple ways to calculate it, but if we just treat it like a wall surface we get:
300 sq. ft. (200 linear feet x 16-18" exposed) @ R-20 (4" XPS on exterior) = 1,200 btu/ hr
We are assuming given extensive sub-slab insulation and lack of thermal bridges that our heat loss downward through the slab is negligible.

So our total loss for all surfaces is a mere 18,888 btu/hr.

Where our number gets inflated is when we try to take a guess at air infiltration. We've just been guesstimating a rough .5 air changes per hour for a worst case. With a 40,000 cubic foot structure that alone gives us around 30,000 btu/hr loss which brings us up to the ~50k btu/hr loss figure we've been using.

But, if we calculate that in an alternate manner via the HRV's, lets say we achieve balanced pressure and thus all or most air exchange is taking place through the HRV system. Our Ashrae 62.2 compared with other suggested formulas gives us a range of 54-100 CFM for the 1st floor (2,400 sq. ft.) and 30-50 CFM for the loft (960 sq. ft.) for a total of 84-150 CFM. 40,000 cubic feet divided by 84 and 150 cubic feet per minute exchange would give us a full air change every ~8 to 4.5 hours or .125 to ~.22 air changes per hour.

Looks like thats can be the issue right there. We are making a wild guess at a number for which the effects are greater than all our other factors combined.

If we had exhaust-only ventilation we'd be looking at a heat loss of 7,200 to 12,672 btu/hr.
With HRV's if we recover a conservative 60% of that energy thats 2,880 to 5,065 btu/hr.

Bringing our total heat loss to 21,768 to 23,953, which is much more in line with the numbers you are getting for your place, and given yours is actually built and heated and lived in, you have actually been able to confirm yours in the real-world whereas we have not!

So, if thats the case, then our heat loss is even tinier than we've been estimating (hooray?) but it makes utilizing a wood stove even more difficult! Ironically super-insulating may cost us more money in the long run as we'd need to use heat-pumps rather than the free fuel of a wood stove!

The shop space is 43% of the volume/square footage of the building so its wood stove would need to output a maximum of 10,300 btu/hr unless I was cranking up the ventilation.
The living space is 57% of the volume/square footage (assuming the heat goes upstairs to the loft reasonably well) so its stove would need to output a maximum of 13,653 btu/hr.

If its 25 degrees out rather than -15, thats 5,150btu/hr and 6,826 btu/hr respectively.

Dick, does that check out better with your math and experience?

So, to swing us back around, if my math in the previous reply is correct, now we are *really* stumped on what wood stove to get!

By the above math we'd be burning 1-2 pounds of wood per hour throughout the winter season at most and a catalytic stove seems like a very important thing to regulate that very low burn.

- So, we were looking at the hearthstone heritage with a 2.3 cu. ft. box, but with no cat that seems like it should be scratched.
- The woodstock progress hybrid remains our favorite but looking at these BTU numbers it seems insane to put such a large stove in there. Should we scratch this stove as being too large?
- The woodstock fireview is out due to aesthetics.
- I'm trying to warm up to the woodstock keystone's aesthetic, as is the wife. Might this be the stove we are after? Tiny 1.75 cu. ft. firebox, catalytic and soapstone to mellow things out? Can it manage an overnight burn or would someone need to wake up to refuel it to avoid re-starting the fire every morning?
- The blaze king ashford 20 is also a possibility, but the blaze king rep was very adamant about upsizing to the 30, which looking at these numbers doesn't make sense to me.

So, the $3,000 question is, can we take a large stove like the progress hybrid and only put 10-20 lbs of wood in it and still get an 8-10 hour burn? Or do we need to get a tiny stove to avoid overheating us no matter what?

P.S. We drove up to woodstock's factory to take a look at their stoves and I tried very hard to like the ideal steel, but my wife might divorce me if I brought it home on a pickup truck. I appreciate its engineering and design, but its not a looker. My wife likes the Vermont Castings/Jotul look.