Osburn 3500 Wood-Burning Fireplace Insert
Osburn 3500 Wood Insert OB03510 – EPA-Certified Wood Fireplace Insert
The Osburn 3500 Wood Insert OB03510 is a high-capacity, non-catalytic wood-burning fireplace insert designed for compatible existing masonry fireplaces. It provides a recommended heating area of 1,000 to 2,700 sq. ft., a 3.5 cu. ft. firebox, capacity for logs up to 22 inches, up to a 10-hour maximum burn time, and a listed maximum dry-cordwood output of 110,000 BTU/h.
Model
OB03510
Fuel
Dry Cordwood
Heating Area
1,000–2,700 sq. ft.
Max Output
110,000 BTU/h
Firebox
3.5 cu. ft.
Max Burn Time
10 Hours
Max Log Length
22"
Blower
Up to 144 CFM
Required Components Are Not Included
The Osburn 3500 is shown with finishing components that are required to complete the unit but must be purchased separately.
- Black or brushed-nickel door overlay
- Medium faceplate kit – 32" × 44" – or Large faceplate kit – 32" × 50"
- Black or brushed-nickel faceplate trim
High-Capacity Wood Heating for Existing Masonry Fireplaces
The Osburn 3500 is built for homeowners who want substantial wood-heating capacity from an existing masonry fireplace.
Its 3.5 cu. ft. refractory-brick-lined firebox, stainless-steel secondary-air system, C-Cast baffle, ceramic-glass door, and glass air-wash system support efficient non-catalytic combustion and practical daily operation.
The included variable-speed blower is rated at up to 144 CFM to help move heated air from the insert into the room.
EPA 2020 approved: The supplied specifications list an average particulate emissions rate of 1.32 g/h.
Osburn 3500 OB03510 at a Glance
Product Type
Wood-Burning Fireplace Insert
Heating Area
1,000–2,700 sq. ft.
Maximum Output
110,000 BTU/h
Firebox Volume
3.5 cu. ft.
Maximum Burn Time
10 Hours
Blower
Included, Up to 144 CFM
Is the Osburn 3500 Right for Your Fireplace?
Best For
- Compatible existing masonry fireplaces.
- Homes needing a recommended 1,000–2,700 sq. ft. heating range.
- Buyers wanting a large 3.5 cu. ft. firebox.
- Users needing capacity for logs up to 22 inches.
- Installations using an appropriate 6-inch chimney liner.
- Buyers wanting a non-catalytic EPA 2020-approved insert.
- Homeowners wanting an included blower up to 144 CFM.
Consider Another Product If
- Your masonry fireplace does not meet the minimum opening dimensions.
- You need a mobile-home-approved fireplace insert.
- Your chimney cannot support the required liner system.
- You cannot meet the required combustible clearances.
- You do not want to purchase the required door overlay, faceplate, and trim separately.
- You need a freestanding wood stove rather than a fireplace insert.
Heating Performance & Efficiency
| Recommended Heating Area |
1,000–2,700 sq. ft. |
| Maximum Heat Output |
110,000 BTU/h (32.2 kW) |
| Overall Heat Output Rate |
18,100–49,500 BTU/h (5.3–14.5 kW) |
| Overall Firebox Volume |
3.5 cu. ft. |
| EPA Loading Volume |
2.84 cu. ft. |
| Maximum Burn Time |
Up to 10 Hours |
| Average Overall Efficiency – HHV |
71% |
| Average Overall Efficiency – LHV |
76% |
| Optimum Overall Efficiency |
77% |
| Optimum Heat Transfer Efficiency |
76% |
| Average Particulate Emissions |
1.32 g/h |
| Average CO |
92 g/h |
Maximum output vs. tested heat output: The 110,000 BTU/h figure is the listed maximum dry-cordwood output. The manufacturer separately lists an overall heat-output rate of 18,100–49,500 BTU/h.
Key Features
3.5 cu. ft. Firebox
A large firebox designed for substantial cordwood loads.
22" Maximum Log Length
Accepts logs up to 22 inches, with 16-inch logs recommended.
10-Hour Maximum Burn Time
Actual burn duration varies with wood, draft, climate, heat demand, and operating conditions.
110,000 BTU/h Maximum Output
Maximum listed dry-cordwood output under the specified conditions.
Up to 144 CFM Blower
Included variable-speed blower helps circulate heated air into the room.
Non-Catalytic Combustion
Uses a stainless-steel secondary-air system rather than a catalytic combustor.
Ceramic Glass
Single viewing door with cast-iron frame.
Glass Air-Wash System
Helps limit deposits on the glass during normal operation.
Refractory Firebox
Firebox is lined with refractory bricks.
Wood Fuel & Firebox Design
| Fuel Type |
Dry Cordwood |
| Recommended Log Length |
16" |
| Maximum Log Length |
22" |
| Log Positioning |
East-West / Loading Over the Width |
| Overall Firebox Volume |
3.5 cu. ft. |
| EPA Loading Volume |
2.84 cu. ft. |
| Firebox Lining |
Refractory Bricks |
| Secondary-Air System |
Stainless Steel |
| Baffle Material |
C-Cast |
Door, Glass & Construction
Door Type
Single glass door with cast-iron frame.
Glass Type
Ceramic glass.
Air-Wash System
Included.
Body Steel Thickness
3/16".
Top Steel Thickness
5/16".
Secondary-Air System
Stainless steel.
Required Finishing Components
Faceplate
- Medium – 32" × 44"
- Large – 32" × 50"
Required to complete the unit: A compatible door overlay, faceplate kit, and faceplate trim must be selected separately.
Osburn 3500 Dimensions
| Overall Height |
24 7/8" |
| Overall Width |
30 3/8" |
| Overall Depth |
29" |
| Door Opening |
17 1/2" W × 8 3/4" H |
| Firebox Dimensions |
22 7/8" W × 20 1/8" D × 12 7/8" H |
| Flue Outlet Diameter |
6" |
| Shipping Weight |
540 lb. (245 kg) |
Minimum Masonry Fireplace Opening
| Minimum Height |
26 1/4" (660 mm) |
| Minimum Width |
31" (787 mm) |
| Minimum Depth |
18 3/8" (457 mm) |
Fresh-air intake note: If a fresh-air intake is required, the installation manual recommends adding at least 4 inches to the width of the minimum fireplace opening.
Opening dimensions alone do not establish compatibility. Chimney condition, liner requirements, combustible clearances, floor protection, fresh-air requirements, and local installation conditions must also be verified.
Chimney & Venting Requirements
Recommended Liner Diameter
6"
Liner Standards
ULC-S635, CAN/ULC-S640, UL 1777
Minimum Chimney Height
15'
Mobile-Home Installation
Not Approved
Professional installation guidance: The supplied manual recommends installation and service by qualified professionals and directs owners to verify local building, fire, and inspection requirements.
Minimum Clearances to Combustibles
Canada
| Top from Platform |
84" |
| Side Mantel to Unit |
9" |
| Top Mantel to Unit |
27" |
| Side Wall to Unit |
19" |
| Top of Unit to Shelf |
27" |
United States
| Top from Platform |
84" |
| Side Mantel to Unit |
9" |
| Top Mantel to Unit |
27" |
| Side Wall to Unit |
16" |
| Top of Insert to Shelf |
27" |
Certification label priority: The information on the appliance certification label takes precedence over information published in manuals, catalogs, flyers, magazines, or websites.
Certifications & Standards
EPA Status
EPA 2020 Approved
Canadian Emissions
CSA B415.1-10
U.S. Safety
UL 1482 and UL 737
Testing / Listing
Accredited laboratory – CAN/USA
Complete Osburn 3500 OB03510 Specifications
| Brand |
Osburn |
| Model |
OB03510 |
| Product Type |
Wood-Burning Fireplace Insert |
| Fuel |
Dry Cordwood |
| Combustion Technology |
Non-Catalytic |
| EPA Status |
EPA 2020 Approved |
| Recommended Heating Area |
1,000–2,700 sq. ft. |
| Overall Firebox Volume |
3.5 cu. ft. |
| EPA Loading Volume |
2.84 cu. ft. |
| Maximum Burn Time |
10 Hours |
| Maximum Heat Output |
110,000 BTU/h (32.2 kW) |
| Overall Heat Output Rate |
18,100–49,500 BTU/h (5.3–14.5 kW) |
| Average Overall Efficiency |
71% HHV / 76% LHV |
| Optimum Overall Efficiency |
77% |
| Optimum Heat Transfer Efficiency |
76% |
| Average Particulate Emissions |
1.32 g/h |
| Average CO |
92 g/h |
| Recommended Log Length |
16" |
| Maximum Log Length |
22" |
| Log Positioning |
East-West / Loading Over the Width |
| Flue Outlet Diameter |
6" |
| Recommended Chimney / Liner Diameter |
6" |
| Minimum Chimney Height |
15' |
| Chimney / Liner Standards |
ULC-S635, CAN/ULC-S640, UL 1777 |
| Baffle Material |
C-Cast |
| Mobile Home Approved |
No |
| Door Type |
Single Glass Door with Cast-Iron Frame |
| Glass Type |
Ceramic Glass |
| Glass Air-Wash System |
Yes |
| Blower |
Included – Up to 144 CFM |
| Firebox Lining |
Refractory Bricks |
| Secondary-Air System |
Stainless Steel |
| Steel Thickness – Body |
3/16" |
| Steel Thickness – Top |
5/16" |
| Overall Dimensions |
30 3/8" W × 24 7/8" H × 29" D |
| Door Opening |
17 1/2" W × 8 3/4" H |
| Firebox Dimensions |
22 7/8" W × 20 1/8" D × 12 7/8" H |
| Minimum Fireplace Opening |
31" W × 26 1/4" H × 18 3/8" D |
| USA Emissions Standard |
EPA |
| Canadian Emissions Standard |
CSA B415.1-10 |
| USA Safety Standards |
UL 1482, UL 737 |
| Canadian Safety Standard |
ULC S628 |
| Testing / Listing |
Accredited Laboratory (CAN/USA) |
| Shipping Weight |
540 lb. (245 kg) |
| Warranty |
Limited Lifetime |
Flame Authority Expert Recommendation
Choose the Osburn 3500 OB03510 when you want a large-capacity non-catalytic wood insert for a compatible existing masonry fireplace and need a manufacturer-recommended heating area of 1,000–2,700 sq. ft.
Its 3.5 cu. ft. firebox, 22" maximum log capacity, 10-hour maximum burn time, 110,000 BTU/h maximum output, 1.32 g/h particulate emissions rate, refractory-brick lining, ceramic glass, stainless-steel secondary-air system, and included blower up to 144 CFM make it a strong fit for higher-capacity wood-heating applications.
The most important buying step is verifying the complete installation and finishing package. Confirm the 31" × 26 1/4" × 18 3/8" minimum masonry opening, 6" liner system, 15' minimum chimney height, required clearances, fresh-air conditions, door overlay, faceplate size, and faceplate trim before ordering.
Also Available with Liner Kit
Official Manufacturer Resources
Installation & Owner's Manual
Manufacturer installation, operation, chimney, liner, clearance, fresh-air, floor-protection, maintenance, and safety information for the OB03510.
Osburn 3500 OB03510 Specification Sheet
Manufacturer specifications covering heating performance, dimensions, firebox information, emissions, construction, blower, and technical data.
Osburn 3500 OB03510 FAQs
How much space can the Osburn 3500 Wood Insert heat?
The Osburn 3500 has a manufacturer-recommended heating area of 1,000 to 2,700 sq. ft. Actual performance varies with chimney draft, climate, heat loss, fuel, home layout, and installation conditions.
What fireplace opening does the Osburn 3500 require?
The minimum masonry fireplace opening is 31" wide × 26 1/4" high × 18 3/8" deep.
Does a fresh-air intake change the minimum opening?
If a fresh-air intake is required, the installation manual recommends adding at least 4 inches to the minimum fireplace-opening width.
Are the door overlay, faceplate, and trim included?
No. A compatible door overlay, faceplate kit, and faceplate trim are required to complete the OB03510 and must be purchased separately.
What faceplate sizes are available?
The supplied product information identifies a Medium 32" × 44" faceplate kit and a Large 32" × 50" faceplate kit.
What size chimney liner does the Osburn 3500 use?
The insert has a 6-inch flue outlet and is designed around a 6-inch chimney-liner connection.
What is the minimum chimney height?
The supplied installation specifications list a 15-foot minimum chimney height.
What is the maximum log length?
The Osburn 3500 accepts dry cordwood up to 22 inches long, while 16-inch logs are recommended.
How are logs positioned in the firebox?
Logs are positioned east-west, or across the width of the firebox.
Can the Osburn 3500 be installed in a mobile home?
No. The OB03510 is not approved for mobile-home or manufactured-home installation.
Does the Osburn 3500 include a blower?
Yes. It includes a variable-speed blower rated at up to 144 CFM.
Is the Osburn 3500 EPA 2020 approved?
Yes. It is EPA 2020 approved and has a listed average particulate emissions rate of 1.32 g/h.
What is the maximum heat output?
The maximum dry-cordwood output is 110,000 BTU/h. The separately listed overall heat-output rate is 18,100–49,500 BTU/h.
Is the Osburn 3500 catalytic or non-catalytic?
The Osburn 3500 uses non-catalytic combustion technology with a stainless-steel secondary-air system.
How long can the Osburn 3500 burn?
The listed maximum burn time is up to 10 hours. Actual burn time varies with fuel, chimney draft, heat demand, operating conditions, and installation variables.
What is the efficiency of the Osburn 3500?
The supplied specifications list average overall efficiency of 71% HHV / 76% LHV, with 77% optimum overall efficiency and 76% optimum heat-transfer efficiency.
What are the overall dimensions of the insert?
The OB03510 measures approximately 30 3/8" wide × 24 7/8" high × 29" deep.
What should I verify before ordering?
Confirm the masonry fireplace opening, liner diameter, chimney height, combustible clearances, any fresh-air requirements, required door overlay, correct faceplate size, faceplate trim, and local installation requirements before purchasing.
Before You Order
Confirm Before Purchase
- You specifically need the Osburn 3500 OB03510.
- Your masonry fireplace meets the 31" W × 26 1/4" H × 18 3/8" D minimum opening.
- Your chimney can accommodate the required 6" liner system.
- Your installation can meet the 15' minimum chimney-height requirement.
- You can maintain the required combustible clearances.
- You have selected the required door overlay.
- You have selected the correct Medium or Large faceplate.
- You have selected the required faceplate trim.
- You understand that the insert is not mobile-home approved.
- If fresh air is required, you have accounted for the additional opening-width recommendation.
Do Not Assume
- That the door overlay is included.
- That the faceplate is included.
- That the faceplate trim is included.
- That the 110,000 BTU maximum output is the same as the tested overall heat-output range.
- That fireplace opening dimensions alone establish installation compatibility.
- That the insert can be installed without a suitable chimney liner.
- That the OB03510 is approved for a mobile home.
- That all fresh-air installations use the same fireplace-opening width.
- That website clearance data overrides the appliance certification label.
Final planning check: Use the Osburn 3500 Installation & Owner's Manual and Specification Sheet to verify fireplace fit, chimney and liner requirements, clearances, fresh-air needs, finishing components, blower operation, emissions, and installation requirements before ordering.
