How to size a woodstove for a superinsulated house?
Seems that sizing heatingunits in a superinsulated house is a real challenge! The 3000 sqf house I have built just outside Ottawa, Ontario, Canada, has R54 walls, R80 attic insulation, R25 slab insulation and triple-pane fibreglass windows. It is situated due south, and has a passive solar glazing area to floor area ratio of 11.5%. The heat load calculation determined a 33,000 btu heat load.
I am an experienced wood burner having burned wood all my life, and I am about to install a woodstove. This makes sense since we have about 20 acres of mixed hardwood bush we can manage. Given the above, how do I approach sizing the stove so as to use it as a primary heater, but not over-heat?
I have narrowed the choice down to three stoves produced by one manufactuer I respect. The stoves have the following outputs –
70,000 btus designed for a 2,000sqf conventional home
55,000 btus designed for a 1,600 sqf conventional home
42,000 btus designed for a1,300 sqf conventional home.
Any thoughts here would be appreciated.
John Scime
Ottawa, Canada
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Replies
I question the heat load calculation. My house in NH, CZ6, has nearly 4,000 (gross) square feet of conditioned space, R40/60 for walls/ceiling and R20 subslab (smaller R numbers than yours), and also triple pane windows, yet my own calculated heat load was only 22,000 BTU/hr. Of course my house is set on a hill, with half the lower level buried so that the heat loss is less than if those wall sections were above grade. On the other hand, I don't think I have as much window area as you do.
I picked a two-ton heat pump ("geothermal") for heating, and in the first winter last year I don't think the thing ever had to go to second stage. A builder of superinsulated houses in Maine told me that in his experience a superinsulated house tends to perform better than the heat loss model indicates. You ought to review the calculations carefully. Martin has had an article on this subject in the works, to be published in Fine Homebuilding soon, I think.
If you will be using the woodstove for primary heat, burning most of the time, you won't want a huge stove, or it will cook you out of the house, especially in the shoulder seasons. Keep in mind that those square footage numbers given by the stove manufacturers are for "ordinary" houses, which leak a lot heat to the outdoors in winter. Ignore those numbers. Also, a woodstove has a firing range, usually printed on a tag provided with the stove. In my case, I picked the smallest Quadrafire stove they make, with a firing range stated as 11-28,000 BTU/hr, bracketing my calculated heat loss. We actually heated the whole house with that during the previous winter when the crew was finishing the inside work, as the heat pump hadn't been installed. I ran it about half time. We kept the house in the mid 50s, just about right for the crew, they felt. In terms of crude numbers, the delta T, heat output, and firing time all seems to match up.
In your case, you'll find that the house loses heat so slowly that you can let the stove burn out overnight and not lose more than a couple of degrees by morning. On the other hand, with a heating system nicely matched to the heat loss of the house, and not grossly oversized, the house will take a long time to heat back up in the morning. The heat capacity of the house is large relative to the heat output of the stove. I find that with my stove, when I use it, the temperature comes up quite slowly if I leave open the door at the top of the stairway (the stove is on the lower level). I had feared that the stove would quickly overheat the lower level, but that doesn't happen. In your case, the south-facing glass will be a big help in winter. I hope it isn't overdone so that you cook in the shoulder seasons, but I imagine that has been thought out well.
As Dick points out the square footage should be ignored, and think only about the calculated heat load. (Good discussion, Dick!)
Most modern wood stoves will burn most efficiently at about a third to half the max BTU rating on the nameplate. The average midwinter heat load is also going to be about 2/3 to 3/4 of the 99% design condition heat load in Ottawa:
See: http://weatherspark.com/#!dashboard;q=ottawa%20canada
The 99% outside design temp for Ottawa is -8F, and using the cursor on the Weatherspark dataset the average January temp is about +15F, so assuming 68-70F interior temp, the average heat load based just on outdoor temp is about 70% of the load at -8F, give or take.
So oversizing the stove's max output by as much as 2x the design condition heat load won't lead to much in the way of overheating and it'll still burn cleanly in the 1/3-1/2 of max, but you probably don't want to go any bigger than that unless you've done the calc on the thermal mass in the house (which you probably have, if you did the passive solar design). With high thermal mass you can even undersize it slightly, but there's no advantage do doing so- you just need something that burns efficiently at ~20-25KBTU/hr.
Either the 42K or the 55K unit will get you there with reasonable efficiency, but the smaller stove may be somewhat more comfortable during the shoulder seasons when the loads are much much lighter. Or not, since the thermal mass is enough to soak up the "extra" heat for hours & hours, even with 20KBTU/hr of excess. The 70K unit would be fine in January, but would run less efficiently during the shoulder seasons since you'd have to throttle it way down.
FWIW: Just this afternoon I pulled the trigger on a ~50K woodstove for my house with a heat load only slightly higher than yours (lousier insulation, warmer design-temp :-) ). There was a smaller ~35K unit available that would have heated the place just fine, but:
A: It required smaller logs (the 50K unit can handle 21" logs, the smaller only 16".)
B: It's being installed partially inside a large 90 year old fireplace, and it would have looked a bit "lost" in there, whereas the 50K unit has a roughly-even ~5-6" clearance to edges of the fireplace opening on all 4 sides, for a more reasonable look.
Even without having a large passive solar thermal mass to help out it should do just fine. The manufacturer says "heats up to 1800 feet" and my house is more like 2400' (plus basement) but I'm intimately familiar with the heat load of this place, having spent more than a decade living in it and tightening it up. It's definitely not too little for the actual heat loads, its oversized by about 50%, which is fine.
Just curious... what brand of stoves are you folks liking?
I'd also ask on hearth.com.
I'm curious why I've never heard any mention here of central boilers when the subject is heating with wood. I've read a lot about various wood stoves and options, including masonry heaters like those used by Chlupp. I recently found out that a friend has been using a Central Boiler (CentralBoiler dot com), and I’m now intrigued. I’ve thought briefly of a list of pros and cons for central boilers, and the column with the Pros is pretty long. Which got me thinking: if it’s such a wonderful thing, why haven’t I read anything about it? Is it not that great, or not common, or am I reading the wrong websites? If it’s great, you’d think I would have read about in on GBA. :)
[Edit: not intending to hijack thread regarding sizing, but I think it might be relevant, and there are several wood-burners here.]
Andrew,
The GBA Encyclopedia discusses wood-fired boilers. The article notes:
"WOOD-FIRED BOILERS: Overcoming a dirty reputation
"All wood-fired boilers face the same technical hurdle: the efficient burning of wood requires high temperatures, and it's hard to reach high temperatures when the firebox is surrounded by water. That's why wood boilers have a reputation for inefficiency and incomplete combustion.
"But there are exceptions, such as the HS-Tarm Excel, a dual-fuel indoor boiler with a claimed efficiency of 80% with wood and 85% with oil. The device automatically switches from wood to oil when the wood fire burns down. Some manufacturers of boilers that burn wood pellets claim similar efficiency.
"Outdoor wood boilers, typically located some distance from the house, are usually inefficient and have been banned in some areas because of their high-particulate smoke. Responding to criticism, some manufacturers of outdoor wood-fired boilers have agreed to develop new designs that meet voluntary EPA guidelines. Although units that meet the new EPA guidelines (labeled with orange tags) have somewhat cleaner smoke than older units, they still may not get local approval."
One reason may be that talking about wood boilers is kind of like talking about geothermal. In a super insulated home they don't really make sense based on the high upfront costs.
David,
I have a Hearstone "Homestead".
I can't wait to fire it...
Andrew C: See hearth.com and read about INDOOR GASIFICATION boilers. I was going to put one in my new house (a Garn), but, like mentioned above, it would take 50 yrs to pay for it. Gasification boilers are not what many folks think about when you say "outdoor boiler". Put the thing in your house and keep all the standby losses and avoid losses to the ground whilst sending water in from afar. Besides, the convenience of loading the fire box in your briefs is appealing.