Warning, this is for your knowledge only!.
I do not assume any responsibility to you or the safety of your property.
By-passing any safety features and you and your home can become ash!
This is what I have discovered and repaired on my own pellet stove.
System for burning sawdust in a U.S. Stove American Harvest Model 1039; Arrived at after much trial and error.
1. Auger motor at 6 rpm to compensate for lower bulk density of sawdust.
2. Hopper agitator at 2rpm and 3 small vibrators to prevent bridging.
3. Triac in combustion blower circuit to control blower indepentent of auger.
4, Hopper extension to get 24 hour run time with lower density feed.
5. Repeat cycle timer to control burn pot agitator, hopper agitator and vibrators. This saves energy and prevents motors from overheating.
6. The 1rpm burn pot agitator motor was replaced with a 2rpm unit. This may not be required.
7. The U S Stove is well suited for this application because of the short horizontal feed auger which eliminates jamming by fines.
History:
My frirst purchase was a Corn Flame Stove to reduce heating cost for my house in Indiana. It worked well and I then purchased a Danson's Cheap Charlie for heating my house in the U.P. fo Michigan. When the price of corn went thru the roof, I started making pellets with an 8" mill. Because of the higher ash content of pellets made from a combination of sawdust, leaves and paper, the corn burner had to be cleaned every 8 hours. The mill that I have can not make pellets from straight sawdust. After seaching the web for pellet stoves suitable for burning biomass pellets, I decided to try a U.S. Stove(used model 6039) because it had a burn pot agitator.
This worked well with my home made pellets which I screened prior to burning to remove fines. Even with the screening when the hopper was nearly empty, there would be a lot of fines and smaller pellets. The auger would feed the finer material with no problem. The agaitator worked well to help the ash fall through the burn pot and running for a week was no problem.
I have now used this system for two winters burning about 25-30 gallons of sawdust a week. The sawdust is all screened through 1/4" mesh to remove larger wood chips screws.
Sawdust burning tests:
Because the fine material fed and burned well, I began seaching the web for ways to burn sawdust in a pellet stove. There are several stoves that will burn sawdust, but they are not pellet stoves. My first attemp with burning sawdust in the U.S. Stove was to empty the hopper of pellets an fill it with about 10 pounds of sawdust. Pellets were put in the burn pot and a fire started, the sawdust burned well. At the maximum auger setting there was a lot of fluidization of the sawdust and particles carried out the exhaust with sparks being visable. Bridging of the sawdust in the hopper was a problem.
Since the sawdust burned well, my next step was to evaluate the design of the 6039 stove and decide what changes were required for proper sawdust burning.
Here are the first set of changes that I made.
1. I added a agitator to the feed hopper, powered by a 2rpm auger gearmotor.
2. The bulk density of the sawdust that comes from a cabinet maker is about 1-3lbs/gal depending on the dust collector it comes from. Pellets are 5-6lbs/gal.
Based on this to get the same amount of heat, the the 4rpm auger motor would have to run at the maximum setting of 9 instead of the 3-4 that I had been running with pellets.
3.The burnpot agitator and the hopper agitator were put on a repeat cycle timer. 40 minute total cyle, 50 sec on 50 sec off, available on eBay.
This system was run for several hours. Bridging in the feed hopper was still a problem.
A vibrator taken out of a back massage pad was pop riveted to the outside of the hopper.
The vibrator was powered by the cycle timer. Sawdust flow was better, so two more vibrators were added to the other sides of the hopper. This solved the bridging problem. This system was run for about 8 hours. The air flow was still too high for the sawdust feed rate and fluidization was a problem.
A test was run as follows. The auger motor was powered directly by 110v to operate continuously.(not a good idea for a permanent setup because it by passes the safety shut down controls). Since on this stove the auger motor speed controller also controls the combustion blower speed, the setting was reduced until a minimum of fluidization occurred. This system was run for 8 hours with no problems.
The feed hopper was extended adapting a hopper extension I had for the Danson pellet burner. The lid was spilt and the agitator shaft extended. This allows filling with out removing the agitator. The agitator shaft is attached to the gearmotor shaft with clip that can be removed after the lid is raised only a few inches, the lid can then be fully opened for the removal of the shaft.
Next since the 4rpm auger running full time worked okay, ( through with the controller at maximum speed it only runs about 75%of the time) a 6rpm auger motor was installed .
To control the combustion blower a Triac speed controller was put in the line between the control panel and the blower.
This system is working well with little fluidization. More frequent cleaning of the exhaust duct is required. I clean it about once/month.
Some adjustment of the combustion blower is required based on the density of the sawdust being burned.
Further improvements
:Some type of auger speed feed contol
Feed of sawdust lower into burn pot
Almost thought about trying a Harmon, since it feeds at burn plate level. However reading about the double auger system with the slide plate and the jamming problem caused by fines changed my mind.
Cautions:
Sawdust is dust and it is difficult to keep the area clean.
My sawdust is screened through 1/4" mesh, this is necessay to prevent auger jams from foreign material. This was learned the hard way. You will be surprised at what you find.
Pellet stove is in the basement and the hopper is fed by a screw conveyor from the garage. Feed hopper has a shop vac connection for dust control when filling..
Hopper agitator: Uses 1/2" pvc pipe with screen door springs tipped with plastic hose to prevent hopper wear. Needs to be centered over the auger feed throat. Bottom bar of 1/4" plastic tube.
Springs must not be long enough to reach auger throat as they can bend down with the hopper full and can jam auger. Found out the hard way.