This is my first year with an RSF Opel II. After I bought it, I often wondered if I should have bought the Kozyheat Z42 instead since the Opel mandates 7" ICC chimney =$$$$, but overall I like it very much. I am heating roughly 2200 sq ft. (my wife has most of the other rooms filled with crap and hence the doors are closed on most rooms
When temps drop to 5-15 f, I agree with bgood--it struggles, but I think that is due to my less than seasoned wood. I have finally managed to get some good overnight burns, following the advice on these forums of buying a unit with a large firebox (Opel II = 3.6 cu ft)
The best advice I can give you is to avoid ANY venting options until you get it installed and to see what the unit does in your environment (just like my dealer warned). Wait and see how the unit behaves in your home. I feel like I have totally NEUTERED my stove by installing a gravity venting kit on it
I replaced a heatilator with the Opel in an existing internal chase, and figured a vent on the side would be nice to spread heat around the room (and also cover the hole where the previous vent was).
Since having done that, however, my unit often now only gets secondary flames on the right side at times, with consistent foggy glass on the left. This coincides with the vent being on the left hand side above the doghouse, so by nature I would guess the firebox is colder on that side. Not sure if design flaw or just mother nature...
Come spring, I am ripping out the gravity vent and going back to plain stock heat out the front louvre!
After many soul-searching days of staring at the expensive 7" stack rising above my roof, I am now content...lol. BUT...spending hours trimming up the free and oversized wood I get my from my father in law, I still sometimes wonder how the 22" log capacity of the Kozyheat z42 and 6 " chimney would have fared. Alas, I have accepted the 18" max log size for the Opel II...
With 2300 sq ft, I would recommend OPEL II or III from RSF (same exact units except II has 2 doors). Then, also compare with the Kozyheat as BeGreen mentioned.
If it were ever possible, I would love to see a 'shootout' between the RSF and Kozyheat units based upon total price of stove+chimney and some scientific formula of btu's. Either way, they both seem like good choices!
As for me, I can honestly recommend the RSF Opel II with the 18" and venting caveats...
Also, FWIW, the Opel II + III are NOT EPA certified without the cat option, but they have a secondary burn tube, and I can attest that they produce little smoke with the right wood and a good deal of heat, so they must be pretty darn close!
Good luck in your quest! I'm still a newb, but I've learned a GREAT deal from this forum and from many hours spent sipping bourbon and enjoying (studying) the flames from my Opel II.