So, sorry to zombie this thread...
I went ahead and built a version of the Heat Champion. It isn't ornate, but this was an experiment that all in is under 1/10th of an insert's cost and works extremely well IMO.
Attached are some before and after pictures. The steel is 3/16 hot rolled and the square tubes are 1/8 thick x 1 1/2 OD. The doors come off an older fireplace and cost me $20, all in, including new neoceram glass and gaskets (pays to look on Kijiji). I plumbed in cold air from the basement, mortared in all the joints and edges, used glass blankets where needed, and the gasket tape for other spots. All hidden from view.
Again, I use Trembling Aspen (white poplar) as it is abundant. I have groves of birch and tamarack but the value for sale is high so I'd rather take the income from sales, and burn the Aspen. On the ranch, I have more dead-fall in aspen than I could ever burn in a year on my older fireplace. I get about 3 hours of flame, and I have coals ready for a relight using the lower intake grate as a bellows for about another 6-8, and 12 if I use handheld bellows.
Cold air is drawn up from the basement through the old ash dump (never used as such) and into the heat exchanger. Right now, I have a 200 cfm fan pumping heat into the ash tube (8" x 12" x 6' tall) so I do have decent outlet flow. I also lined the top of the exchanger with 1 1/2 x 9 firebrick, leaving a 3" gap on either side for exhaust. Outlet temps are shown on the pictures, and are in Celcius.
Even though I had thoughts of this for awhile, this was installed in a bit of a rush as this late winter/early spring has surprised us with colder than average temps, by a lot. A few nights lows smashed records, reaching -23 C. In March, that is not a common occurrence, and for a couple weeks, we've been 20 degrees C colder than normal. We should be at 10 C for a high on average, and not freezing at night. In this time, I have only burned about 1/4 cord of Aspen, with a heavy burn given the temps. The house (3200 sq ft) is about 26 C in the fireplace, and 23 in the extremities. Ranch style bungalow, 100' x 35', and not a really open floorplan thanks to the 1974 build. A couple ceiling fans on medium and Bob's your Uncle.
I could probably get more heat out of the exchanger with a better fan. I am looking for an 8" duct fan with remote for the rheostat to get up to 600 cfm, or I have a creative solution to get there with some fans I have lying around.
After running it for 3 weeks of my current billing cycle, I received my bills. I am down $100 in gas, and $60 in power from last month. Most of that cost is fees based on consumption, but the interesting thing is that the addition of a Carbon Tax was on this bill, so with the Carbon Tax, I was still down.
I can't do a Y/Y comparison as the outside temps last year were in the +15 to 20 C range, so those bills were low, and running a fireplace would be stupid.
So no, it is not ornate, which is what Stoll would give you. Yes, I used a bit heavier steel. Yes, mine looks old which is the intent - it needed to look like it was an original install. Yes, it is not hermetically sealed, but I've done a few leak tests and achieved really amazing results (a small leak between the doors is it). The lower grate is a hurricane when opened full and I can light from the smallest of coal beds. My first couple loads were a little too wet, but I had more than enough airflow to make them burn well.
CONS:
You have to do all the work yourself, including the math on expansion, flow rates, etc.
No warranty.
You need to really understand your firebox.
Stoll will cost more, but as shown here, with the heat exchanger/blower option, it would do the same things IMO. I do still have concerns with the blower motor being over the exhaust vs mine being 6' below and staying nice and cool.
I was lucky. I had a pristine firebox to start with, lined with premium refractory brick from the bricklayer who also did Exxon's refineries, so it is overbuilt. The other 2 fireboxes in this stack are also overbuilt, including the indoor BBQ that is coal fired. My set up is the fire breathing equivalent of the cliche "built like a brick sh!thouse".
Would I do this on a heavily used firebox? It depends on its condition, but given some of the haphazard builds I've seen throughout North America where wannabe masons threw something together? No. For one, I took the time to calculate the expansion factor of the metal, and even then I was a bit tight. On a poorly built masonry firebox, it could be a problem.
Again, for what I paid, I couldn't be happier for this application. For a new build, or a firebox that is of questionable condition or quality, get an insert, or a standalone stove. I did this more to see what the reality was vs some of the educated opinions I saw on this thread, and I had the steel, the time, and the drawings.
Thanks for reading.