In the market for an add on hot air wood furnace

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Oct 6, 2014
Hancock, MI
I've currently got a generic 80's-90's Humble wood burner add on furnace downstream of an oil furnace in my heated basement. Cold return air comes into the top of the oil furnace (downflow) and out the bottom of the oil furnace then into the bottom of the wood burner, then it heads up into a large plenum that distributes heat to the house and spots in the basement I keep warm. Blower located in the top of the oil furnace is used for circulation for both, oil furnace is off and locked out except for 1-week when we go on vacation in the winter. The Humble is, well getting pretty humble and it has several cracks in it that I've had to weld up in-situ over the years to keep it going and I'm trying to decide if I should replace it, or pull it out and rebuild it, possibly adding some 2ndary air tubes. I burn on average 8 full cords of well seasoned hardwoods per year. 90% hard maple, the rest oak, ironwood, some ash. The chimney is 6" rigid stainless inside of 7" clay lined masonry, shared with the oil furnace and is an interior chimney. It's a little over 21' from the furnace connection to the cap and draws well. I also have a US stove S.S. water pre-heating tube that I installed into the humble and connected to a tempering tank that I must preserve the function of, but I'm not seeing this as an option on newer furnaces given the firebrick liner unless I plan on hacking up a brand new furnace. I also want a bigger door if possible, the humble only has an 11"x11" door on it, I'd really like something closer to 14"x14" like my Jensen indoor garage boiler has.

I like the idea of something that's eligible for the 30% tax credit, but not the end of the world. Seems like Kuuma (kind of out of budget, and a couple of bummer reviews) or Drolet Heat Commander are the only ones I see that tick the 2020 epa box? Any others I should be considering? Seems like the TSC/NT/L&M box store units aren't the way to go in my situation where this provides 99% of my heat.
Any others I should be considering?
Seems like Kuuma (kind of out of budget, and a couple of bummer reviews)
Really? Where did you see those? There have been a couple here over the years, but if you dig hard enough, in almost all those cases you'll find where the reviewer later recanted their complaints...or Lamppa just bought the furnace back because it was a bad application, or just one of those people that you will never make happy. If you spend the money for a VF100, have a proper chimney (any modern woodburner is the same) and your house is not just way too big/leaky, you will not regret spending the money, especially years down the road, when the new car smell is long gone and the furnace is just one of the things you can count on making you smile all winter long.
FYI, if you are using 8 cords/yr now, I can just about guarantee the VF would take that down to 5, or less...very likely 4 cords. It would be really (REALLY!) hard to run more than 6 cords/yr through a VF100.
I'm trying to decide if I should replace it, or pull it out and rebuild it, possibly adding some 2ndary air tubes.
The people that do add tubes, and actually get it to work (kinda, sorta) very seldom seem to stick with it (I didn't) it just so seldom works that well, especially on older wood furnaces, they are just such a bad firebox design for burning hot/clean.
Don't rule out the PSG furnaces, Drolet and Caddy. When we built in 2016 I was looking at the Kuuma but the price was pretty hard to swallow. Our Caddy is not the latest Caddy Advanced 2020 EPA model, but our 2016 version is still very clean and efficient. It is a great furnace, I'd buy another without question. And the glass door is really something I like. It might be a furnace, in the basement, but i sit down here all the time (like I am now) soaking in the warmth and watching the fire.
We go through about 3.5 cord, 4 max cord a year heating 1700 ft2 and the full basement.
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I don't think you would go wrong with the Kuuma. I run about 7-8 cords a year through mine, but I have a much longer season than almost anyone here. I'd run a lot more through if I wasn't happy with a house around 60 degrees year-round.

There are a couple of options to heat or preheat water with the Kuuma, both options now have to be diy now due to the EPA certifications. Neither affect the performance of the unit, they just don't fit into the EPA measurement methods, so they can no longer be. I heat all of my DHW with mine during the heating season - which can be all but 4 weeks some years. If you just want to pre-heat water probably duplicating the method that was offered as an option at one time by Kuuma with copper piping on the outside rear of the firebox would be the simplest. No real hacking required. I don't know if the original parts might be available as repair parts for a pre-EPA unit;). You might inquire... It wouldn't be too hard to build your own if not. If you go that route, I might be able to find some pictures that would help you. I think the pre-EPA units had a couple of studs welded to the outside rear of the firebox to hold the water heater. They would be pretty easy to add as long as you do the work prior to installing the unit.

If you search my posts you will find a lot on heating DHW with the Kuuma.

The Kuuma doesn't have a huge door (12w x 14h) or firebox and will require mostly smaller split wood. It's not a problem, but it's not a chunk burner. 1 chunk per load for me, if I need to, and that would then need smaller stuff all around it if I was to fill the firebox. I'm not sure if any of the modern furnaces are different. Being able to load it twice a day and forget it while burning less wood makes up for a little more splitting for me.

I'll second that the PSG products appear to be a good option as well, based on what I've read here over the years. I don't have firsthand experience with them.
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In 1982, when I built my home, we put in a Humble add on furnace, coupled to a propane furnace. The house is about 1400 sqft and well insulated so we burned about 3 cords silver maple, green ash, and a bit of swamp white oak per year. The Humble was a creosote monster, requiring me to be on the roof every 2-3 weeks cleaning the chimney. In spite of that we still had at least two dozen chimney fires over the life of the Humble. In 2015, we replaced the Humble with a Kuuma model 200 coupled to a new, high efficient propane furnace. The propane is only used in October and early May. The Kuuma uses 1/3 less wood to heat more effectively. Unfortunately for us, the existing ductwork was poorly designed, so that the gas furnace ECM fan would not distribute the Kuuma heated air effectively. To rectify this, I installed two ECM inline duct fans (Cloudline S8) in the Kuuma ductwork. The inline fans activate by snap disc controls and run when the Kuuma fan heats the two ducts to ~ 100 degrees F. The only downside with the smaller Kuuma is the firebox size - too short and limited capacity compared to the model 100. I can only hold fire for about 6-8 hours. Othewise, I clean the chimney once per year in the Spring with very little creosote collected. jsdyer