Pellet Stove Theory and Repair

Pellet stoves have only a few primary components:

• Burn pot
• Auger
• Hopper
• Combustion fan
• Heat exchanger
• Exhaust pipe (chimney)
• Convection fan

Air is pulled through the burn pot by the combustion fan. The air supplies oxygen to the fire, and the fire in turn heats the air to a very high temperature. The air is then pulled through the heat exchanger, through the combustion fan itself, and blown up the chimney.

Lastly, the convection fan blows aviary (home) air around the stove and heat exchanger. It creates a hot breeze that you can feel when standing in front of the stove.

There are two basic auger setups: one that drops the pellets into the burn pot from above, and one that shoves the pellets into the pot from below. The latter was championed by a few manufacturers, all of whom appear to be extinct. The main problem with this design arises from the "clinkers" (un-burnable solids) that accumulate over a day or two of operation. When the pellets are shoved in from below, the clinkers accumulate on top of the fuel, and this naturally tends to put the fire out. In contrast, the top feed design adds fuel on top of the clinkers, which is much more sensible. Anyway, the main point here is that if a stove has the bottom-feed design, don't buy it.

All of the auger, combustion fan, and convection fan run on standard AC power. It is a simple matter to connect each to a power source to run full blast. However, it is more complicated to make them run "harmoniously." In this regard, the convection fan can run at whatever speed you please (faster is usually better for heat transfer). However, the combustion fan and auger must run at compatible speeds. If the auger is running very slow, then the fire will go out unless the combustion fan is also running very slow. If the auger is running very fast, then the combustion fan must also run very fast; otherwise, the new pellets added onto the fire will actually extinguish the fire.

Each pellet stove has an electronic controller that automatically synchronizes the auger and combustion fan (and the convection fan too). If this controller breaks, which does happen, then the stove will NOT work. This creates an opportunity where the cost-conscious can acquire otherwise perfectly good pellet stoves for cheap or free. You just say "Hey, it's broke. I'll give you $10."

So now you may be wondering what you're going to do with a broke stove. The answer is simply that you throw away the original control board and replace it with the controller diagrammed below. If you're devoid of electrical intelligence, then hopefully you can find someone else to build the controller. Otherwise, all you need are some extra wire, wire cutters, electrician's tape, a couple of screw drivers, maybe a soldering iron and solder, and some masking tape and a pen for labeling the wires. And maybe a drill. If, after trying again and again, you cannot build this controller, feel free to direct questions to the web master.

Note that this controller requires thinking by the user. This means that the user is responsible for synchronizing the auger and the combustion fan. However, this truly is not difficult. (Just a little twiddling and guess-work.)

The "speed knobs" noted above are simply dimmer switches. It's also possible to use a dimmer switch instead of the repeat cycle timer for the auger. However, it's best to maintain full voltage on the auger; otherwise, the auger will almost certainly bind on some "difficult" pellets, and the auger motor will fry.

Of the stuff shown above, the fans and auger are part of the stove. If you poke around a bit, you can figure out what is which. The Low-T switch is on the exhaust pipe. The "Low Combustion Fan Vaccuum Pressure Switch" can be identified by a hose that runs to a connection just upstream of the combustion fan. The High-T switch is typically somewhere on the hopper. The rest of the stuff you can mount on a control board (piece of plywood).

Note that the main difficulty in creating reality from an electrical drawing is simply that the drawing is nice and neat. In contrast, reality often has wires runnning all over the place because (for example) the Low-T Switch and the mechanical timer are not adjacent. The cure for this is simply to label all of your wires. Use masking tape and a pen.

Q from reader:
I read your article on the homemade Pellet stove controller and was incredibly impressed. One thing is different from the factory unit and that is that the combustion blower on factory units will keep running after the power is shut off to clear the burning pellets and then shut down after the unit is cool. Where would you add an additional circuit for that purpose?

Answer:
Ah. You don't need one of those. Instead, when you want to turn the stove off, just turn the ATC 422's "on-time" to zero. This will stop the flow of pellets. Then the fire will burn down, and eventually the Low-T switch turns everything off. (But I suppose if you REALLY want on off switch, the place to put it is in-line with the high-T switch. This will stop the auger, and all else will continue to run until the Low-T switch kills them.)

Also, you really don't need an "On" switch as shown in the diagram. I never touch mine - it's always in the On position. (For all practical purposes, the Low-T switch is the "real" On/Off switch.)

Side note: the ATC 422 is a mechanical switch, so it has a limited lifetime that depends on the amperage drawn. (The contacts will gradually burn off.) I think mine has been running for 3 winters w/o a problem.