Fireplace Inserts Explained

What an insert is

The US EPA describes a fireplace insert as a wood stove adapted to fit into an existing fireplace opening. Gas and pellet versions follow the same idea. The insert has its own firebox, a sealed or controlled air supply, and a surround panel that covers the gap around the fireplace opening.

The reason people install them is efficiency. EPA notes that most traditional open fireplaces lose over 90 percent of the fire's heat up the chimney. The Department of Energy says a well-fitted insert can perform nearly as efficiently as a wood stove.

Wood, gas and pellet inserts compared

Each manufacturer's installation manual is the controlling document for that model.

How wood inserts are vented

A wood insert sends its smoke into the chimney through a connection at the top of the insert. Older installations sometimes just slid the insert into the fireplace and let smoke find its way up the original large flue. Modern practice connects the insert directly to a liner.

Manuals spell out what is required. One wood insert manual requires connection to a listed high-temperature chimney or an approved masonry chimney with a clay tile or stainless liner, says the fireplace and chimney must be clean and undamaged with any damage repaired before installation, and shows the damper removed or wired open. Another stove manual requires a UL 1777 listed liner in masonry chimneys in the US. CSIA notes that stainless liners serving solid fuel appliances must be insulated. See liner options.

How gas inserts are vented

Most gas inserts are direct-vent: they draw combustion air from outside and send exhaust outside through a sealed system. In a chimney, this is usually done with two flexible aluminum or stainless pipes, called co-linear venting, running to a termination cap at the top.

As examples of what manuals require, one gas insert manual calls for the chimney to be verified clean and in good working order, lists minimum flue sizes for masonry and factory-built chimneys, sets a height range, and allows the damper to be blocked open or removed. Another manufacturer's venting specification states that the insert must not connect to a flue serving a separate solid-fuel appliance. For decorative gas options that are not inserts, see gas logs vs. gas inserts.

Inserts in factory-built fireplaces

A factory-built (prefab) fireplace is tested and listed as a system. EPA states that for factory-built fireplaces you must use an insert specifically designed and tested for that unit. One factory-built fireplace manual we reviewed prohibits installing a solid fuel or gas insert into that fireplace without written authorization from the manufacturer. If your fireplace is metal-lined with a metal chimney, check its manual before planning an insert. See factory-built fireplaces: care and limits.

What should be checked before an insert goes in

1. Identify the fireplaceMasonry or factory-built, its opening size, and any manufacturer restrictions.
2. Clean and inspect the chimneySweep the flue and look at the liner with a chimney camera when the flue allows. CSIA lists the addition or replacement of an appliance as a reason for a Level 2 inspection.
3. Repair what needs repairDamaged firebox, smoke chamber, crown, or flue problems are fixed first.
4. Match insert, liner and chimneyLiner type and size follow the insert manual; the chimney height must suit the insert.
5. Check hearth and clearancesFloor protection and clearance to mantels and trim follow the manual.
6. PermitsSome New Jersey towns list a fireplace insert as permit work. Check with your local construction office.

Choosing between wood, gas and pellet

• Wood suits households with a reliable supply of dry firewood and the time to load and tend a fire. It works without electricity, which some people value during outages. It also means regular ash removal and sweeping.
• Gas suits households that want heat at the push of a button with little cleanup. It needs a gas line at the fireplace as well as venting. We connect inserts to an existing line; if a new line is needed, a plumber or your gas utility runs it.
• Pellet suits households that want automatic feeding and steady output from a renewable fuel. Pellet inserts need electricity, pellet storage, and regular cleaning of the burn pot and vent.

EPA's Burn Wise program notes that the cleanest wood-burning appliances carry EPA certification labels. For wood or pellet inserts, the label tells you the unit meets federal emission standards.

Fit and sizing

An insert has to physically fit the fireplace: width, height, depth and the shape of the back wall all matter. The surround must cover the opening, and the insert's flue outlet has to line up with a path up through the damper area. Heat output should match the space it is meant to heat. An oversized wood insert run at low settings to avoid overheating a room tends to smolder and build creosote.

Living with an insert

A wood insert burns more efficiently than an open fire, but it still makes creosote, especially if it is run slowly with damp wood. Manuals set their own inspection intervals; one requires checking the connector and chimney at least twice a month during use. Annual inspection and sweeping as needed follow NFPA 211 as summarized by CSIA. Burning dry wood helps; see our firewood guide.

If you smell smoke, see smoke in the room, or a carbon monoxide alarm sounds, stop using the insert. If the alarm sounds, leave the house and call 911.

Next step

Considering an insert? Start with an assessment of the fireplace and chimney. Request a visit, and see our fireplace insert installation and inspection before a new appliance services.

Questions homeowners ask

Sources

1. US EPA Burn Wise: Choosing the Right Fireplace
2. US EPA Burn Wise: Choosing Wood-Burning Appliances
3. US Department of Energy, Energy Saver: Wood and Pellet Heating
4. Travis Industries Lopi Revere wood insert manual
5. Kozy Heat Nordik 34i gas insert installation manual
6. CSIA: Homeowner Resources and FAQ

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