How Pellet Stoves Work

The idea behind how pellet stoves work is that wood pellets, produced to a strict standard of size, can be made to behave like a fluid and be taken to the stove's burn pot automatically as needed to maintain the fire's heat output at a consistent pre-set or thermostatically controlled level.

For more information about wood pellets, follow the link at the bottom of the page. Doctor Jerry Whitfield is credited with firstcoming up with this idea in the late 1970's and inventing a revolutionary form of home heating. The container these pellets are held in is called the 'hopper' and will usually be at the back of the stove, although some stoves have a feed system that draws the pellets from a remote hopper.

The reason that the pellets in the hopper don't ignite from the heat of the stove is that the air from the firebox is passed over a heat exchanger and then blown out into the room, which stops the body of the stove and the exhaust vent from getting dangerously hot.

Apart from that, the hopper is separated from the firebox by an air gap which is only bridged by an auger tube that feeds the burn pot with pellets. Inbuilt temperature sensor will shut the stove down if something goes wrong and it becomes dangerously hot.

The Auger

The mechanism that takes pellets from the hopper to the burn pot is called the auger which is in two parts: a metal screw something like a large corkscrew that rotates to draw the pellets along, and the tube that the screw runs inside. This screw mechanism is driven by an electric motor.

Gravity feed non electric pellet stoves are available, and becoming increasingly popular because with no moving parts or electronic controls there is virtually nothing to go wrong.

There are some variations on the auger system. Most pellet stoves are 'top fed' which means that the pellets are dropped into the burn pot down a chute from above.

Harman, Droletand Travis Industries AGP stove are the only manufacturers currently using a bottom feed system. The advantage of this is that as new pellets are pushed into the burn pot, ashes and othe debris are pushed out into the ash pan. This enables the Harman stoves, the Drolet Eco 65 and the AGP to burn cheaper low quality pellets that would clog the ash pan on other stoves.

Bixby take a different approach. Instead of an auger, Bixby stoves use a rotating drum which picks up pellets from the hopper at the bottom of the drum's cycle and drops them down a chute to the burn pot when pellets in the drum reach the top.

The Burn Pot

Unlike a wood stove, where the fire area is the whole of the stove, pellet stoves use a small burn pot where astonishingly efficient combustion of about 98% is achieved.

A pellet stove burn pot is usually made of cast iron or stainless steel and has small holes around the base to allow fan blown combustion air in. The size of the burn pot would be about six inches by four inches, and in stoves where the pot can easily be seen throught the viewing window, ceramic or steel fake logs are helpful to spread the flame and hide the workings, making the flame more pleasing.

In the centre of the burn pot, at the back, there will be a recess to allow access to hot air from the electronic igniter.

The Igniter

Early pellet stove, and some low cost no frills models today do not have an electronic igniter. Most corn stoves still don't because a higher temperature is needed to light corn than wood pellets.

Pellet stoves without an igniter are lit by using a handful of pellets with some starter gel applied and then lit using a flame. This type of stove will not cycle on and off with a thermostat because the stove won't re-light automatically. They will burn at low heat, and increase heat when more warmth is required.

An automatic elecronic igniter is a bit like a heavy duty car cigarette lighter. If a pellet stove has one of these, a thermostat will allow it to shut down completely when no heat is required, and then re-ignite when heat is demanded.

The Heat Exchanger

The combustion air is blown around a heat exchanger and then, with most of the heat removed, exits outside through the exhaust vent. The convection fan blows clean air over the heat exchanger and then out into the home.

Heat exchangers for pellet stoves come in two sorts: tubular or corrugated. Both are designed to maximise the heat transfer from combustion air to home heat. To clean them, a handle will be provided which is attached to a shaped scraper to remove ash deposits. If this tool, which is easy to use, is neglected, the efficiency of the heat exchanger will be reduced by a build up of ash.

Temperature Control

The primary means of regulating the temperature of a pellet stove is by adjusting the speed at which the auger feeds pellets to the burn pot. The combustion fan is regulated by the pellet feed to ensure that a given amount of pellets will be burnt effectively.

All stoves have a built in temperature regulator that can be set manually; most stoves can be connected to a wall thermostat to give more precise control of the room temperature.

The convection fan can usually be adjusted independently.

The Convection and The Combustion Fans

These two fans operate two completely independent systems. The combustion fan blows the air needed to feed the burn pot, while the convection fan passes air, (preferably taken from outside), over the heat exchanger and into the home.

Some pellet stoves have kits that enable both these fans to be fitted outside the home for increased quietness of operation.

Venting

Pellet stove venting has an internal and an external pipe with an insulating air space between them. Because most of the heat from the stove is removed by the heat exchanger, and because of the air gap between the inner and outer tubes, the venting pipe does not get hotter than warm to the touch.