This is a project to increase the efficiency of an older but still very serviceable wood burning stove.
Scott added an air supply for secondary combustion and retrofitted a catalytic combustor.
Scott also goes through a method for actually measuring the efficiency of a wood stove using just a wind meter and a thermometer.
Thanks very much to Scott for sending this in!
Wood burner with the air supply pipes
for the secondary burn system.
The secondary burn air supply
pipes that fit inside the
Secondary burn air supply pipes
mounted inside the fire box.
Close ups of the Catalytic Combustor
Catalytic combustor with the bypass
damper fully closed.
With the bypass damper fully open
Pictures of the Catalytic Combustor mounted on the stove
Prior to installing the cat box, I checked the stove's efficiency using
detailed in this article...
This is basically a stack loss method of measuring efficiency, where you measure the heat lost up the chimney, and assume you get everything else delivered to the house. I used a conservative estimate (as the article suggested) of 6900 BTU per pound of wood for my 2-year seasoned beech that I'm burning. The initial test showed 72.3 % efficiency. Now, this test makes the assumption that there is 100% combustion efficiency, which just isn't true, but I thought it would serve as a good bench mark for the cat install. I was hoping that the cat would slow down the exhaust gas flow somewhat, and possibly reduce the flue gas temp some, as well as increase the combustion efficiency. So I repeated the test yesterday, during a snow fall (my neighbors must think I've really lost it). Both tests were done with 40 lb wood loads. With the cat box installed, I noticed immediately that flue temps were lower even though the cat was cranking out temps in excess of 1200 degrees at times. For most of the burn it ran 1000-1200F. The cat box surface temp was 400-600 degrees. Efficiency with the cat installed jumped up to 82.8 %. Now if you can believe the manufacturers spec on the cat of 93% combustion efficiency, if I'm getting the correct flow through it, the stove could theoretically have an overall efficiency of around 77%. Nothing short of amazing for an old air tight "smoke dragon". I also noticed the only thing coming out of the chimney was water vapor...you could practically breathe the exhaust, though I wouldn't recommend it. My clothes also didn't smell like wood smoke after the 6 hours of testing as they did with the previous (test with secondaries only). My only issue with the setup, so far, is that on a cold stove start, when there's more smoke, it's easy to get the cat temps up to 1700-1800 degrees, which is too hot (easily controlled by opening the bypass damper half way). So, at least until I get the hang of things, it's not a just set it and forget it kind of thing early in the burn. No problems with light off of the cat or the cat staying lit throughout the burn.
link to the catalytic combustor that I used...
I believe it's actually manufactured by this company...
This is a link to one of the premade add-on units...
If you are thinking about retrofitting a catalytic combustor, one thing to bear in mind, is that a catalyst needs a supply of oxygen to work. One of the problems with these older stoves is that they had poor secondary air flow, so I'm thinking that to get the most from an add on catalyst you need a source of preheated secondary air. Having done the secondary burn tubes first, made this easy to accomplish in my case.
Price wise, I had less than $100 into the secondary tubes. The catalyst was $176 plus shipping, the steel (precut by our local supplier) was $17 and misc fasteners were less than $15. I also had about $100 in stove pipe fittings, but only because I had to change my stove pipe configuration to fit the cat box in.
Some follow-on information from Scott:
One thing I noticed yesterday is that the stove can be run for very long times at a low rate. Yesterday, I made a fire at 6:30 am. My solar air heater was running so the house was still toasty at 6pm. Usually I would have to make another fire about that time or a little earlier to keep things warm even on a good solar day. At 6:30 I went to rake what I thought would be just a few coals forward to start a fire. I found wood chunks still in there! Raked them forward and opened up the draft some, and the cat went active again, stove top warmed up to around 300 degrees. Didn't actually reload the stove until 8:30 pm. That's a 14 hr burn out of a stove that's usually good for 7-8. (BTW, this is a fairly big stove...about 3.6 cu ft firebox...takes 24"wood.) Now that's a relatively low burn...400ish stove top temps. When it's really cold, I may actually have to open the damper part way to avoid overheating the cat, and run the stove hotter, using the secondary system mostly. Maybe not though, because the lower burn actually gets more heat into the house instead of sending it up the chimney. The beauty of this setup is that everything's controllable and you can see the direct effect of all the various tweaks. There will be a learning curve as I re-learn how best to burn this thing. I found out this am that the overheating cat on startup can easily be controlled by cutting the secondary flow down some.
One more picture to add. This is a typical picture of the chimney with a fire in the stove and the catalytic combustor engaged. As you can see, there's really nothing to see. I looked up at mine this afternoon after I noticed one of my neighbor's stoves smoking away.
Scott Also included spreadsheets he used to estimate efficiency after installing the secondary air supply and the catalytic combustor:
Back to the main page on wood burners and biofuels...
Gary March 9, 2011