Osburn 3500-I EPA 2020 Wood Insert
110,000 BTU/hr of heat from dry cordwood, 1,000 to 2,700 square feet of coverage and burn times up to 10 hours from a 3.5 cu ft firebox that takes logs up to 22". The Osburn 3500-I is an EPA 2020 certified non-catalytic wood insert for an existing masonry fireplace, with a 144 CFM variable-speed blower included, a 19" x 11-3/4" ceramic-glass view, a 6" flue outlet, 77% optimum efficiency and 1.6 g/h particulate emissions. It starts from $3,819 for the insert alone — the door overlay and the faceplate with its trim, both chosen above, are required to complete it — and carries Osburn's limited lifetime warranty on the firebox welds and cast-iron door frame.
Choose the 3500-I if you want the most heat Osburn puts into a masonry fireplace: it has the largest firebox, the longest logs and the longest burn time of Osburn's inserts, sized for great rooms and open-plan homes up to 2,700 square feet. The blower is in the box and runs itself off its heat sensor, and the look is yours to set — a black or brushed nickel door overlay and a Medium or Large faceplate in black or brushed nickel trim.
It is not right for a factory-built fireplace, a mobile or manufactured home, or a bedroom — it installs only inside a masonry fireplace with a masonry chimney, and the opening must be at least 26-1/4" high, 31" wide and 18-3/8" deep. It is a zone heater rather than a furnace, and a conventional backup heating system is still recommended. In the United States it burns with the door closed: open-door burning behind a fire screen is not permitted, so if you want an open fire this is not the insert.
Specifications
The insert measures 24-7/8" high, 30-3/8" wide and 29" deep overall, and the masonry opening must be at least 26-1/4" high, 31" wide and 18-3/8" deep — add 4" to the width if you add a fresh air intake. Keep 19" from the insert to an adjacent side wall, 9" to a combustible side surround and 27" to a combustible top surround; a combustible mantel shelf up to 12" deep sits at least 27" above the insert, and the ceiling is at least 84" above the base of the opening. The maximum thickness of the combustible surrounds is in the table on page 36 of the installation manual. In the USA, floor protection runs 16" in front of the door opening and 8" to the side with no R-value when the fireplace floor is raised 6" or more; lower than that, it needs R 2.00 or better and 27" in front. Venting is a continuous 6" stainless steel liner to the top of a chimney at least 12 ft tall, and the blower plugs into a 115 V outlet.
How the 3500-I compares to the Osburn 3500 wood stove
The Osburn 3500 wood stove carries the same firebox and the same figures — 3.5 cu ft, 110,000 BTU/hr maximum, 1,000 to 2,700 square feet and burn times up to 10 hours — as a freestanding stove instead of an insert. This 3500-I starts from $3,819 and goes into an existing masonry fireplace; the 3500 wood stove starts from $3,839 and stands on its own with its own chimney connection. Pick the insert to turn a masonry fireplace into a heater; pick the stove where there is no fireplace to use.
Features
- EPA 2020 certified non-catalytic wood insert — 110,000 BTU/hr maximum on dry cordwood, 77% optimum efficiency and 1.6 g/h particulates, rated for 1,000 to 2,700 sq ft
- Osburn's largest insert firebox — 3.5 cu ft, logs up to 22" and burn times up to 10 hours
- Variable-speed blower up to 144 CFM included, with a heat sensor that starts and stops it on its own
- Cast-iron door with a 19" x 11-3/4" ceramic-glass view and an air-wash system, finished with a black or brushed nickel overlay
- Medium (32" x 44") or Large (32" x 50") faceplate with black or brushed nickel trim to close off your masonry opening
- Retrofits an existing masonry fireplace with a 6" stainless steel chimney liner
Answers to common questions
What venting does the 3500-I need, and is any of it included?
It needs a continuous 6" stainless steel chimney liner listed for solid fuel (UL 1777) running from the insert to the top of the masonry chimney, and none of the liner comes with the insert — our team puts the venting package together for your chimney. The chimney must be at least 12 ft tall, extend 3 ft above the roof and 2 ft above anything within 10 ft, and vent nothing else; at least 12" of rigid liner above the masonry is strongly recommended. The liner connects through a liner starter adapter, and an offset liner adapter is an option only when nothing else works and the system is at least 20 ft tall. A sheet-metal block-off plate at the throat is recommended, and a direct connection to the clay tile is not approved.
Will it fit my fireplace, and how close can a mantel or wall be?
It fits a masonry fireplace whose opening is at least 26-1/4" high, 31" wide and 18-3/8" deep (Osburn recommends at least 4" more width if you add a fresh air intake); it cannot go in a factory-built fireplace. From the insert, keep 19" to an adjacent side wall, 9" to a combustible side surround and 27" to a combustible top surround. A combustible mantel shelf up to 12" deep can sit 27" or more above the insert, and the ceiling must be at least 84" above the base of the opening. The surround thickness limits are in the table on page 36 of the installation manual.
What floor protection does it need in front?
Every 3500-I needs non-combustible floor protection in front, measured from the door opening. In the USA, when the fireplace floor sits 6" or more above the room floor, it extends 16" in front of the door opening and 8" to the side, with no R-value required. When the fireplace floor is raised less than 6", the protection added in front must have an R-value of 2.00 or more and extend 27" in front of the insert. The depth already in front of the insert counts toward the total.
What’s included, what’s required, and what’s optional?
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Included in the box: Variable-speed blower (up to 144 CFM) with an OFF / LO / HI rheostat and automatic heat sensor; Cast-iron door with 5/32" ceramic glass, air-wash system and adjustable latch; Firebrick-lined firebox, C-Cast baffle and four stainless steel secondary air tubes; Faceplate extension, three liner fixation brackets and the leveling bracket system; EPA 2020 certification, UL 1482 / UL 737 safety listing, and the Osburn limited lifetime warranty.
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Required to complete the installation, purchased separately: Door Overlay - the black or brushed nickel trim that finishes the cast-iron door frame (choose above); the insert ships without one; Faceplate - the Medium (32" x 44") or Large (32" x 50") faceplate with its matching trim, which closes the gap between the insert and your fireplace opening (choose above); Venting - a 6" listed stainless steel chimney liner running continuously from the insert to the top of the chimney (see pg 37 in the installation manual); Masonry Fireplace - an existing masonry fireplace and chimney with an opening at least 26-1/4" high, 31" wide and 18-3/8" deep; it cannot go in a factory-built fireplace; Floor Protection - non-combustible protection 16" in front of the door opening and 8" to the side when the fireplace floor is raised 6" or more; otherwise R 2.00 or better extending 27" in front; Electrical - a standard 115 V outlet within reach of the blower's 8 ft cord.
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Optional: The rigid fire screen, 5" fresh air intake kit and its insulated flex pipe, the offset liner adapter and the liner hook up system — the insert burns and heats without any of them.
Does it need an outside air supply?
Not by default — room air is the safest and most reliable combustion supply for any 3500-I, and the fresh air intake is optional. It matters in a house where a powerful exhaust fan, such as a kitchen range hood, can pull the pressure negative. The 5" fresh air intake kit fits on the right or left side of the insert and runs insulated HVAC flex pipe to an outside wall termination; Osburn recommends at least 4" more width in the masonry opening if you add it.
Can I burn it with the door open, like an open fireplace?
Not in the United States — every 3500-I burns with the door fully closed. It was tested for open-door use with the optional rigid fire screen, but in the US, and in Canadian provinces with a particulate emission limit, burning with the door open behind a rigid fire screen is prohibited.
Does buying online affect my Osburn warranty?
No. The warranty is Osburn's and applies to every 3500-I on this page; it runs from your dated purchase invoice, so keep it and register the insert online at osburn-mfg.com with the serial number from the certification label. The Torch Guys will help you with any claim. Coverage is a limited lifetime on the combustion chamber welds and cast-iron door frame (5 years labour), the ceramic glass and convector air-mate, 7 years on surrounds, trims, baffle and air tubes, 5 years on the handle, glass retainers and air control, 2 years on the blower and controls, and 1 year on paint, gaskets and insulation. It requires installation by an authorized qualified technician and covers the original purchaser only.
