Compact Cast Iron Wood Stove with TruHybrid™ Catalytic Combustion and a Soapstone-Lined Firebox

Traditional black cast iron construction with a soapstone-lined firebox, TruHybrid™ catalytic combustion, 79% HHV efficiency, and heating coverage of up to 1,400 square feet.

Key Features

• TruHybrid™ combustion combining primary, secondary, and catalytic tertiary stages for a clean, efficient burn
• 79% HHV efficiency with EPA-certified emissions of 1.07 g/hr
• Heats up to 1,400 sq. ft., depending on home layout, insulation, and operating conditions
• 1.32 cu. ft. soapstone-lined firebox accepting logs up to 16 inches
• Burn time of up to 16+ hours and HeatLife™ of up to 20 hours, per the current owner's manual
• Included rear heat shield for reduced rear clearances
• Large ceramic glass window with an air-wash system for fire viewing
• Mobile-home approved when installed per the owner's manual, with the outside air kit required

Product Features

TruHybrid™ Catalytic Combustion

The Green Mountain 40 pairs primary and secondary combustion with a tertiary catalytic stage. The stainless steel catalytic combustor becomes active at approximately 500°F and burns smoke and remaining particles that were not fully consumed in the firebox, so a well-established fire produces little to no visible smoke from the chimney. The stove typically operates in the 800°F to 1,300°F catalyst range.

Soapstone-Lined Firebox

The firebox is lined with natural soapstone blocks. Soapstone absorbs the high heat of the fire and then radiates gentle, even warmth into the room, extending comfortable heat well after the flames die down.

Cast Iron Construction

The GM40 is styled with clean lines in traditional black cast iron and is built from precision-made cast iron, soapstone, steel, and ceramic components. The 2026 HearthStone catalog identifies the Green Mountain family as EPA-certified room heaters made in Vermont.

Simple, Direct Controls

A single primary air control under the ash lip regulates the burn rate, a bypass handle routes combustion products through or around the catalyst, and a catalyst thermometer indicates when the stove is in the active catalytic range.

Optional Convection Blower with Reduced-Clearance Heat Shield

The optional Blower & Reduced Clearance Heatshield Combo (96-57400) moves air up and across the stove to circulate more heat into the room. A snap switch turns the blower on automatically, and a rheostat adjusts its speed. The kit's shield supports the reduced-clearance configurations published in the owner's manual.

Outside Air Capability

The optional Outside Air Starter (96-53400) connects the stove's air intake to a minimum 3-inch duct (ducting and termination supplied by others) so combustion air is drawn from outside the home. A 3-inch to 4-inch increaser (90-53308) is available when a 4-inch duct is needed. Outside air is required in all mobile-home installations, and Canadian outside-air installations require the stove to be attached to the floor and a damper allowing 100% closure.

Available Finish and Configuration

Specifications

Clearances and Hearth Requirements

Clearances to combustibles. Dimensions are measured from the body of the stove unless otherwise indicated; C, D, and F are chimney-connector reference dimensions. Refer to the current owner's manual for complete installation requirements.

Alcove installations. Alcove side, rear, and connector clearances (A through D) match the corresponding shielded configurations above; corner dimensions E and F in an alcove follow NFPA 211. The minimum unprotected alcove width is 51¼", and the minimum alcove ceiling height from the floor is 56" with either a single-wall or double-wall connector. For protected surfaces, refer to NFPA 211 and your local authority having jurisdiction.

Chimney connector clearances (through-wall, top flue exit). With either the included rear heat shield or the blower kit shield installed: single-wall connector — 13¾" from the appliance rear to the wall and 18" from the vertical connector to the wall; double-wall connector — 9¾" and 14" respectively. Single-wall connector pipe requires 18" to an unprotected ceiling or 9" to a protected ceiling; for double-wall pipe ceiling clearance, refer to the pipe manufacturer's specifications.

Hearth and floor protection. The Green Mountain 40 does not require an insulated hearth pad; noncombustible ember protection with no R-value requirement must cover the floor beneath and around the stove.

Luxury vinyl plank (LVP) or luxury vinyl tile (LVT) flooring requires extra consideration; some vinyl flooring begins to warp at 110°F. Consult the flooring manufacturer for maximum temperature recommendations, as additional shielding may be necessary.

Trim and mantel. The owner's manual publishes a 6" side trim clearance, a mantel height of 48" from the floor with a maximum mantel depth of 12", and a 4" trim thickness reference. The Green Mountain 40 is a freestanding room heater and is not to be installed in any factory-built fireplace.

Chimney and draft. The stove connects to a 6" listed Type HT metal chimney or a code-approved lined masonry chimney. The recommended minimum chimney height for a top-of-stove installation is 16 feet from the floor, and the recommended maximum chimney height is 30 feet. Proper chimney draft is required for safe, efficient operation; follow the current owner's manual and the chimney manufacturer's requirements, and note that installations with excessive draft may require a stovepipe damper.

Mobile-home installations. Install in accordance with 24 CFR Part 3280. The outside air kit (96-53400) is required, the stove must be permanently attached to the floor using the shipping clips supplied with the stove, and a Mobile Home Chimney & Connector Kit conforming to UL 103 or ULC S629 must be used, including a stainless spark-arrester cap, storm collar, and adjustable vented flashing. Do not install in a sleeping room, maintain the structural integrity of the home, burn wood only, keep the front door closed during operation except for startup and loading, remove the chimney when transporting the home, and preserve the exterior vapor barrier where the chimney penetrates the structure.

Included Equipment

• Green Mountain 40 Model 8640 woodstove with catalytic combustor, rear heat shield (5640-024), and catalyst thermometer (7200-520) installed
• Owner's manual
• Shipping clips (used for floor attachment in Canadian outside-air and mobile-home installations)

Compatible Accessories

Warranty

The Green Mountain 40 is covered by HearthStone's written limited warranty, under which the term Limited Lifetime is defined as 10 years from the start of coverage. Stone and cast iron components carry Limited Lifetime coverage, steel components and door handles and latches are covered for 5 years, and glass, accessories, and the enamel finish are covered for 1 year, with labor coverage during the first year for dealer-installed appliances. The catalytic combustor (Applied Ceramics model ACI-6M2) is warranted directly by Applied Ceramics, including no-charge replacement within the first two years and a prorated schedule thereafter. All coverage is subject to the terms, conditions, and exclusions of the current written warranty, and warranty registration is available at hearthstonestoves.com.

About HearthStone

Hearthstone Quality Home Heating Products, Inc. is based in Morrisville, Vermont, where the company builds soapstone and cast iron heating products designed for long, comfortable, even heat. Visit our showroom to see the Green Mountain 40 in person and talk through your installation with our team.