Pellet Stove Venting

Why pellet venting looks different

A wood stove relies on the natural draft of a hot chimney. A pellet stove has a combustion blower, an auger that feeds pellets, and electronic controls. The Department of Energy notes that pellet stoves need electricity to run fans, controls and pellet feeders, and that their exhaust leaves through a small flue pipe that can go out a sidewall or up through the roof.

Because the blower pushes the exhaust, the vent does not need to be tall to work while the stove runs, and short horizontal runs out a wall are common. The trade-off is that the vent must be tight and properly sized for the blower, and it must still be able to clear smoke when the blower stops.

The pipe: listed pellet vent

Pellet stoves use pipe made and listed for pellet appliances, commonly called PL vent. The Harman P-series installation manual, used here as an example throughout, states that only PL vent pipe, wall pass-throughs and fire stops should be used where the vent passes through combustible materials. Single-wall stovepipe and Type B gas vent are different products.

Diameter is usually 3 or 4 inches. In the Harman manual, 3-inch pipe is standard, but 4-inch pipe is required when the system has more than 15 feet of vertical pipe, when the equivalent vent length would exceed what 3-inch pipe allows, and at higher altitudes. Joints are sealed as the vent maker specifies.

Working out equivalent vent length

Manuals rate a vent by its equivalent vent length (EVL), which converts every fitting and run into one number. The Harman manual counts:

• each 90-degree elbow or tee as 5 feet,
• each 45-degree elbow as 3 feet,
• each foot of horizontal pipe as 1 foot, and
• each foot of vertical pipe as half a foot.

For its stoves, the maximum is 20 feet of EVL with 3-inch pipe and 30 feet with 4-inch pipe, and horizontal runs are limited to 4 feet.

The limits exist because the blower is sized for a certain amount of resistance. A run with excess length or extra fittings can leave the stove short of draft.

Power outages and the case for vertical rise

When the power goes out, the combustion blower stops but pellets already in the burn pot can keep smoldering. With nothing pushing the exhaust, it has to rise on its own. A vent that goes straight out a wall at stove height gives it little help.

The Harman manual recommends a minimum vertical run of five feet, preferably ending above the roof line, to reduce the chance of reverse drafting during a power failure or shutdown. A carbon monoxide alarm near the stove and on each sleeping level is basic protection with any fuel-burning appliance. See carbon monoxide alarms.

Where the vent can end

A wall termination has to sit clear of anything that could pull exhaust back inside or be damaged by it. A few of the distances in the Harman manual show how specific these tables are:

The same table covers soffits, corners, gas regulator vents and mechanical air inlets. It also notes that the vent should project far enough from the wall that deposits do not build up on the siding. Other stoves use different numbers, and local code can add requirements. Snow lines matter too: a terminal that sits 12 inches above a lawn can end up buried in a drift.

Using an existing chimney

Running a pellet vent up a masonry chimney puts the termination high above the roof and keeps the outside wall clean. The Harman manual allows several chimney methods and recommends a full-length liner to the top, 4 inches in diameter. It calls for a stainless steel liner when the chimney has no clay tile liner or the existing liner is cracked or flaking, notes that some jurisdictions require the vent to extend all the way to the top of the chimney, and describes sealing the damper area with a steel plate and noncombustible insulation.

The manual also says flatly that this stove may not share a flue with any other appliance. A chimney that also serves a boiler or a fireplace needs a separate flue for the pellet stove. Before a stove goes on a chimney, have the chimney swept and inspected; see inspection before a new appliance and chimney liners.

A pellet insert set into a fireplace opening uses the same approach: its vent connects at the insert's outlet and runs up the chimney as a liner to a cap, with the damper area sealed around it. The insert's own manual sets the liner size and method. See fireplace inserts explained.

Outside air and house pressure

A pellet stove needs combustion air, and a house that is short of air can pull exhaust back through the stove. The Harman manual recommends outside combustion air in all installations, especially on lower and main floors, and lists exhaust fans, range hoods, clothes dryers and unbalanced heating systems as causes of negative pressure that can lead to spillage of fumes and soot. It places the outside air inlet more than 12 inches from the flue outlet. It also says not to install a flue damper in the exhaust vent of that appliance.

Keeping the vent clean

Ash and soot collect in a pellet vent over time, and a partly blocked vent leaves the blower pushing against a restriction. The Department of Energy advises cleaning the flue vent regularly to prevent soot buildup and removing unused pellets from the hopper and feed system at the end of the season. It also recommends a yearly inspection of the system by a professional, preferably before the heating season.

Signs that the vent needs attention include a smell of smoke near the stove, soot on the wall around the terminal, or a flame that looks lazy and dark. Stop using the stove if smoke enters the room. See fireplace and wood stove cleaning.

Federal certification and local permits

Pellet stoves manufactured or sold since May 15, 2015 fall under EPA's emission standards for new residential wood heaters (40 CFR 60, subpart AAA), along with adjustable and single burn rate wood stoves. Stoves sold to burn corn only fall outside the limits when the maker's advertising and warranty rule out wood. See EPA-certified wood stoves for what the certification label means. Ask your municipal construction office about permits before a stove or vent is installed, and keep the manual with the stove for the inspector and for future service.

Questions homeowners ask

Sources

1. Harman (Hearth & Home Technologies): Installation Manual for P43-C, P61-C and P68-C Freestanding Pellet Stoves (8390-044I, R3, 2018)
2. U.S. Department of Energy, Energy Saver: Wood and Pellet Heating
3. 40 CFR 60.530, Who is affected by this subpart? (Standards of Performance for New Residential Wood Heaters), eCFR

This guide is general information. It can't diagnose a specific chimney; an inspection can.