Improving the Efficiency of an Older Wood Burner

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!



From Scott:

I have a 1981 pre-epa wood stove that has been heating our house faithfully since then.  I'm not alone, apparently, because I've read somewhere that about 70% of the stoves in use are pre-epa stoves.  A couple of years ago, I was looking into whether a newer epa stove would save us any wood over the 6 1/2 full cords we normally use.  I cut my own wood, so, other than wear and tear on my body, there was no real financial incentive.  I had read where some people had tried retrofitting their older stoves with some of the new burn technology, so I thought I'd give it a try.

Adding Air Supply for a Secondary Burn Area

I added a secondary burn system (see pictures just below) that consists of 3/4 black pipe, which superheats the air and feeds it in over the fire below the baffle in the stove.  I originally thought I'd be using the gate valves to control the air, but found they work best fully open.  the gate valves are important, though, as a way to shut the air down in case of a chimney fire.  The secondary air allowed the stove to burn a little hotter, cleaned up the exhaust gasses  a lot, and reduced our wood use by about 11% (when I compared 2 back to back years and compensated for differences in heating degree days between the 2).  Not bad for the price of some black pipe and a little labor.


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.

Retrofitting a Catalytic Combustor

I noticed that even though the secondaries cleaned the burn up a lot, that there were times that I could see noticeable smoke.  I thought that a catalytic combustor add-on might be able to turn that into some usable heat, as well as clean up the exhaust some more. The cost of the catalyst initially stopped me, but curiosity, coupled with the trend toward increasing wood stove emission regulations, caused me to try the idea this year.  There are only 1 or 2 companies that make a retrofit cat assembly these days.  It consists of a pregnant looking stove pipe section, where the cat is a damper that can be rotated vertically, out of the way.  What I didn't like about these, was that the cat was pretty high up in the pipe away from the stove.  I have a very short stove pipe, before it goes into an insulated chimney section, and wanted to get as much heat out of the cat as possible, and not just send it up the flu.  Also, they had to use a smaller, 1" high ceramic cat.  Originally, I toyed with the idea of putting the cat in the stove, but didn't want problems of an air leak where I would have to drill the stove for a bypass damper control.

I decided to fabricate a "cat box" to be mounted directly above the stove.  It would have a bypass damper, and a slight offset to it to extract as much heat as possible without causing problems with the draft of the flu  (see pictures just below).  I spent a little more money for a 2" high stainless steel cat.  These are less susceptible to thermal cracking and crumbling and light off at about 100 degrees lower than the ceramic catalysts. The box is fabricated from 1/8" sheet steel and the heavier 22 gage single wall stove pipe.  Luckily, I have a friend who can tig weld as I am welding-incompetent.

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


I've only had it in place since this weekend, but am impressed with how well it works.  

Measuring Efficiency

Prior to installing the cat box, I checked the stove's efficiency using the method 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 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.

Sources and Costs

Here's a 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.


Lastly, the disclaimer...One thing that I should mention is that the box gets as hot as a wood stove, so 36" clearances apply to it as if it were a wood stove.   Also, I'm making no claims that what I'm doing with my stove is safe for other people to do, and am definitely not "recommending" that anyone make modifications to their stoves. This is still a relatively untested work in progress.

Scott Smith

NEW  Here is a thread on discussing Scott's project....



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:

Spreadsheet for estimating efficiency after secondary air ....  (Excel)

Spreadsheet for estimating efficiency after catalytic combustor ...  (Excel)


Back to the main page on wood burners and biofuels...



Gary March 9, 2011