Osburn 3500-I Wood Insert

The Osburn 3500-I Wood Insert is an EPA 2020 certified wood insert for 1,000 to 2,700 sq ft. It burns up to 10 hours on a load and takes a 6-inch (insert liner) chimney.

Who it's for

Right buyer. Owners of a large existing masonry fireplace who want serious primary heat output instead of a decorative open fire — single-family homes, well-insulated cabins, or open-plan spaces of roughly 1,500–2,400 sq ft, with reasonable insulation and access to seasoned hardwood at 15–20% moisture content.

Buyers who want comprehensive visual configurability: cast-iron arched door with Black or Brushed Nickel door overlay, Medium 32" × 44" or Large 32" × 50" faceplate, and matching faceplate trim in Black or Brushed Nickel — eight finished-look combinations to match home decor.

How it performs

The 110,000 BTU/hr figure is a maximum-output rating based on dry cordwood at high loading density and short reload intervals. It is real, but it is a peak rating, not the average output across a full burn cycle of normal operation.

Good to know

• The opening is the gating factor. The 26 1/4" × 31" × 18 3/8" minimum masonry opening is meaningfully larger than the smaller Osburn inserts.
• Non-catalytic burn time. A non-cat insert gives you simpler operation and no catalyst to replace, but you pay for it with shorter burns than a catalytic equivalent.
• Wood loading is a daily activity.

Dimensions

Minimum fireplace opening

Measure the inside of the firebox: width, height and depth. The insert needs at least this much, and the faceplate covers the rest of the old opening.

Dimensions from the specification sheet

Dimensions

Where it can go

The Osburn 3500-I is designed to slide into an existing code-compliant masonry fireplace with a connected and inspected masonry chimney. It is not approved for installation in a factory-built (prefab) metal fireplace, in a mobile home, in any outdoor or unconditioned space, or as a freestanding stove.

Clearances to combustibles

Floor protection

Floor protection requirements depend on whether the existing masonry hearth is raised 4 inches or more above the surrounding combustible floor. If the hearth is raised 4 inches or more, a non-combustible floor protector with no R-value is required and must extend at least 16 inches in front of the unit in the USA, 18 inches in Canada. If the hearth is raised less than 4 inches, a non-combustible floor protector with an R-value of at least 1.00 is required and must extend at least the same distance in front of the unit.

The manual includes a thermal-resistance lookup table for common floor protection materials (brick, cement board, ceramic tile, marble) to calculate the required R-value combination. Tile alone is not sufficient — the manual requires a continuous non-combustible sheet beneath any tile installation. Final hearth protection must be calculated from the manual's floor-protection worksheet and approved by the installer/AHJ.

Clearances

United States figures, in inches. The rating plate on the appliance overrides any published number; the installer confirms on site.

Documents

• Osburn 3500-I wood insert install & owner's manual (PDF - 8.8 MB)
• Osburn 3500-I Dimensions (PDF - 0.6 MB)
• Maximum recommended heating area by climate zone (PDF)
• Osburn wood products brochure, USA edition, with dimension drawings (PDF)
• Osburn limited lifetime warranty
• EPA test reports
• Replacement parts

Details

Warranty

A one-time replacement limit applies to all parts with lifetime coverage. Warranty is void if the unit is used to burn anything other than seasoned cordwood, or if it is not operated according to the owner's manual. Damage caused by misuse, improper installation, lack of maintenance, overfiring, downdrafts, venting problems, or under-estimated heating area is not covered. Improper installation by anyone other than an authorized qualified technician voids the warranty.

Warranty claims are made through your Osburn dealer and remain subject to SBI/Osburn inspection, approval, and the current written warranty. Kaminos is the retailer for this insert and supports buyers through purchase; final warranty approval rests with SBI as the manufacturer. SBI's parts network is well-stocked — replacement bricks, baffle, secondary tubes, glass, gaskets, and blower are openly available at fair prices through multiple parts vendors.

Osburn may require photos or returned parts to support a claim; repair work covered by warranty requires prior manufacturer approval.

Coverage details can change by component and warranty revision; the current Osburn warranty controls.

Heat

From the specification sheet

Choices and prices

Options

The unit price is Osburn's 2026 list price for the wood insert alone. Options sold on this page add to it; those marked on request we add to your order by hand.

Door overlay

Faceplate and trim

Fresh air

Other options

What arrives

• Osburn 3500-I wood insert (OB03510) with welded carbon-steel firebox and cast-iron arched glazed door
• Configured at order with required door overlay and faceplate/trim selections — see "Where it can go" for the options
• 144 CFM variable-speed tangential blower with included thermodisc heat sensor for automatic on/off
• Ash lip
• C-Cast baffle and stainless-steel secondary-air tube assembly
• Refractory firebrick lining
• Removable wood-grip air-control handle
• Liner fixation brackets and hardware
• Owner's manual and product documentation

Sold separately

• 6-inch continuous stainless-steel chimney liner kit (UL 1777, CAN/ULC-S635, or CAN/ULC-S640) — required for installation, sized to the chimney height
• Hearth extension or floor protector sized per manual specifications, with R-value of at least 1.00 if hearth is raised less than 4 inches
• Sheet-metal block-off plate, high-temp silicone, and mineral wool insulation — effectively required in practice for the insert to deliver rated performance
• Optional 5-inch fresh-air intake kit (SBI AC01298) — for code jurisdictions requiring outside combustion air or for tight new-construction homes
• Optional 5-inch × 4' insulated flex pipe for fresh-air intake (SBI AC02090) — HVAC type, ULC S110 or UL 181 class 0 or class 1
• Optional liner hook-up system (SBI AC02006) — simplifies liner-to-flue-collar connection
• Optional offset liner adapter (SBI AC01214) — for installations where the chimney throat geometry requires offset connection
• Optional rigid fire screen (SBI AC01281) — not permitted for open-door use in the United States or in Canadian provinces/jurisdictions with particulate-emission limits; never leave the insert unattended when used with a fire screen
• Optional tempered glass hearth pad 10mm 18" × 50" (SBI AC02760)
• Pin-type wood moisture meter — not optional in practice
• Flue thermometer — strongly recommended for insert installations since stove-top temperature isn't accessible
• Installation by an authorized qualified technician (WETT, NFI, or CSIA certified) — required for warranty coverage and often required by code, permit, or insurance

Installation

Chimney requirements from the specification sheet

The Osburn 3500-I requires a continuous 6-inch stainless-steel chimney liner extending from the insert flue collar to the top of the chimney. The liner must conform to ULC S635 or CAN/ULC-S640 (Canada) or UL 1777 (USA) and be rated for solid fuel. The minimum liner height is 15 feet, subject to installer verification, certification label, draft conditions, and local code. The chimney must extend at least 3 feet above the highest point of contact with the roof and at least 2 feet above any roof line or obstacle within 10 feet horizontally.

The insert is not approved for a "positive flue connection" to clay tile — a continuous stainless liner is required. If the existing fireplace's throat damper is to remain, it must be locked open to clear the liner; otherwise the damper plate must be removed. A sheet-metal block-off plate sealed at the damper level (or, in Canada per CSA B365, mineral wool packing around the liner) is recommended at the throat to reduce cold-air backdraft and to allow the insert to deliver rated performance — without it, jacket-recovered heat is lost into the smoke chamber. An insulated liner (or pre-insulated liner with a wrap) improves draft and reduces creosote, and is required by code in some jurisdictions for clearance reasons.

Outside air

An optional fresh-air intake kit is available for code jurisdictions requiring outside combustion air or for tight new-construction homes with mechanical ventilation. In conventional homes the manual identifies room air as the safest and most reliable supply for combustion; almost all houses have enough natural leakage to provide what the insert needs. A smoke detector and carbon monoxide detector are required in the room where the insert is installed.

Codes and approvals

Code compliance for any specific installation is determined by the local Authority Having Jurisdiction. Manufacturer listings cover what the insert is approved for; the AHJ approves what is permitted at your address. A WETT (Canada), NFI, or CSIA (USA) certified installer is strongly recommended and frequently required by code, permit, or insurance. In Canada the CSA B365 standard applies; in the USA, ANSI NFPA 211 applies. Confirm local requirements before purchasing — particularly in EPA non-attainment counties and in HOA jurisdictions where new wood-burning installs may be restricted.

California Proposition 65

This product can expose you to chemicals including carbon monoxide, which is known to the State of California to cause cancer, birth defects or other reproductive harm. For more information go to www.P65warnings.ca.gov.

Planning the install

• Check with your building department and insurer: wood appliances need a permit in most places and an inspection after installation.
• The chimney is the other half of the system. It is sold separately and quoted from your installer's plan.
• Clearances and floor protection come from the manual and depend on the pipe and shields you choose. Send us your plan and we will check it.
• Burn seasoned wood below 20 percent moisture. Wet wood is the cause of most complaints about any wood stove.
• Special-order goods ship by freight.