Sorting out size of stove needed, advice welcome

[DanTwin Sisters]

I find myself unsure whether i should be looking at a mid-size or large stove. I appreciate the wisdom and experience I've gleaned so far on this forum ... so, here's my situation:

Colorado mountains, at 8'000 feet elevation near nederland. I am building a large open addition to an existing cabin. New space is 450 square feet, basically 20' x 24', vaulted ceilings (13' at peak) lots of glass and otherwise super-insulated.
This space joins a 20' x 22' lower ceiling space, old but being rebuilt into super insulated, few windows. The space between these spaces is essentially open, no wall.

Total of about 950 square feet.

I heat primarily with ponderosa pine and have lots of it available.

I have looked at the midsize like that seem to fall in the 50,000 btu range (Lopi Rockport, Hearthstone Heritage or Green Mountain 60, Jotul 445, etc) and then wonder if I am on the border needing to next size up, which seem like 70- 80,000 btu size. (Hearthstone Manchester, Green Mountain 80, etc) I've read here about benefits of over-sizing rather than under.

It will be my main source of heat, and would love some decent burn into the night.

Opinions and experiences?

 

[John Lehet]

If it’s truly super insulated what you want is a small amount of heat spread out over time. I had a super insulated house in Vermont, about 1400 square feet and with south glass (which we covered at night) and on an insulated slab for thermal mass. I could heat it, though not super cozy, with a 120V resistance space heater. I built a masonry stove, which could spread out the heat from a burn of about 25 pounds of wood over 24 hours. It was perfect.
I have friends here in Vermont with a small super insulated house, which they heat with a heat pump most of the time. But it’s a second home. They have a wood stove just to bring it up to temperature on the weekend (it’s a second house) and then the heat pump does the rest.

I would think you’d either want the smallest stove you could get, or a stove with some thermal mass you could build small fires in and then coast.

I know it gets cold there, so you will have periods where you do need some heat. But if truly super insulated, it shouldn’t be very much!

 

[EatenByLimestone]

The super insulation changes an awful lot of variables.

Has your contractor done a manual J calculation? This works up the expected heat loss for your low temp.

You may find you will be losing very little heat. You should probably look into cat stoves as they can burn low and slow.

Find out how much heat you need and size the stove to that.

 

[DanTwin Sisters]

Thanks. I'm building myself. When I say "super insulated" it still has to be considered that the new half of the structure has a high vaulted ceiling and nearly half the wall space is glass. So I'd rather err on the side of more...

 

[begreen]

Err toward a larger stove in the 2.4 to 3.0 cu ft range.

 

[John Lehet]

The vaulted ceiling gives you more volume, but super insulated buildings behave differently regarding such details. In a theoretically true superinsulated building it would be the same temperature an inch off the floor and an inch from the top of the vaulted ceiling. You wouldn't get the "all the heat goes up there" situation that I get in my 1870s building. And of course if your glass is south facing, you'll get a lot of heat gain from that in the daytime. And then of course loss at night if you don't cover it. In mine I covered big glass with insulated panels.

In my superinsulated house with south glass with the masonry stove, I'd skip a fire if the forecast called for sun on a day, at least in February or March. Other winter months I'd make a smaller than normal fire on a sunny day vs cloudy.

The way I treated the superinsulated house was to think of it as a huge flywheel to which I had to give a small nudge now and then.

I know heat pumps are a bit less efficient at high altitude, but if I had a superinsulated house I would definitely be thinking of a heat pump instead of a wood stove. In the very coldest weather the heat pump will be working hard and be less efficient than a good woodstove, but the rest of the time it will be cheaper and greener to run.

Or a masonry stove. Expensive but perfect. Spread out a small amount of heat over a long period.

Sort of a trade off of a superinsulated house is that you are comfortable everywhere, but the trade off is that you don't get to have a hot-spot to bask, like a hot woodstove to sit around and be super toasty. That hot woodstove in a small superinsulated house would make too much heat.

 

[John Lehet]

Seems to me if you go forward with wood and can’t afford a masonry stove, if it were me based on my super insulated experience I’d go one of two ways. Either the smallest stove you can find — something like a jotul 601 — and you make a very small fire once or twice a day. (I had a 601 once, and it burns clean quickly and can also put out a fair bit of heat if you need it. I used it in an office in an old farmhouse.) The house is a flywheel, nudge it.

Then the other way to go — and I do not yet have experience with a stone wood stove, just the 3500 pound masonry one — a Woodstock or Hearthstone stove and you would use the stove as a heat store/modulator. Again, no experience here, but I would think a non-cat stove and small fires. If your house is super insulated you would only want a big load and the cat rolling along at 10,000 BTUs on the very coldest days. The rest of the time you’d want very small fires, warm up the stove a little and then coast along on that heat. I don’t know about this approach, because the stove is still very small compared to the thermal mass of the house itself.

 

[Todd]

Woodstock has the 2.4cu ft Absolute Steel stove that would probably be a good fit. Whatever stove you decide I would get a more radiant type stove with that open floor and high ceiling.

 

[EbS-P]

Lots of glass tells me you will need more than you think. But again how much glass?

2.0 cu ft is an 8 hour reload cycle on low. If I had ponderosa and wanted to cover all bases I’d be looking at a Blaze King 3 Cu ft stove.

 

[BKVP]

What is your expected overall chimney length and configuration?

BKVP

 

[John Lehet]

I had lots of glass in my superinsulated house (but covered it with insulated panels every night, as I said). My experience in a decade + of wood burning in that house in Vermont was that at worst I burned 20 to 25 pounds of softwood every cloudy cold 24 hours, skipping fires altogether on sunny or milder days. I burned a lot of poplar, just because there were always poplar trees growing up and getting ugly where they were better removed, and in the masonry stove a quick hot fire was what I wanted anyway. It worked great.

So if your house is similar and you want to heat with wood and not cook yourself, you need a way to burn 20 to 25 pounds of wood over 24 hours and get that heat evenly, not all at once.