Fireplace Inserts Explained: Wood, Gas and Pellet

Chimney Academy · How it works · Updated October 2026

What an insert actually is

A traditional masonry fireplace is built to look good and to move smoke out. It is not built to heat. Air rushes through the opening and up the flue, carrying warmth with it. The U.S. Department of Energy says plainly that traditional open fireplaces are energy losers and should not be considered heating devices.

An insert changes the equation. It is a metal firebox, usually with a glass front, that slides into the fireplace opening. Fire burns inside it, a fan may push heated air into the room, and the exhaust leaves through a pipe or liner instead of the open throat.

According to the Department of Energy, inserts fit into existing masonry fireplaces and use the current chimney, and well-fitted inserts can nearly match wood stove efficiency. The words “well-fitted” carry weight there, and they are what the rest of this guide is about.

The three types

For pellet stoves, the Department of Energy describes them as the cleanest solid-fuel residential heating appliance, notes that they need electricity so a power outage stops heat, and says they require weekly homeowner cleaning and annual professional service. Its figures for EPA-certified pellet stoves are 70 to 83 percent efficiency. Insert models differ, so use the specific unit’s certified rating.

Wood inserts: the details that matter

Wood inserts are the hardest on a chimney, because they make smoke, particles and creosote. They work best with dry, seasoned wood. See dry firewood: how to season, test and store it for how to tell, and what not to burn for the rest.

EPA guidance says newer certified appliances lower emissions, and that higher-efficiency units usually emit less and produce fewer creosote deposits. The agency also notes that oversized appliances encourage smoldering, which wastes fuel and raises pollution. That means more heat output is not always better. A unit sized to the room it serves tends to burn hotter and cleaner.

Burn habits matter as much as hardware. The EPA recommends burning dry, seasoned wood and keeping the fire hot enough to burn cleanly. A smoldering fire in a closed insert can coat the liner with creosote faster than a lively open fire coats a fireplace flue, so an insert owner should treat the annual inspection as part of owning the appliance.

The Department of Energy suggests making inserts as airtight as possible and filling gaps only with approved materials. It also notes that a clean-out door above the connection can reduce how often the insert needs to be moved for chimney cleaning. Ask about this when selecting a model, because insert removal for sweeping is a real chore.

Gas and pellet inserts

A gas insert needs a gas supply and a correct venting arrangement. Energy Trust explains that direct-vent units draw outside air and exhaust through sealed pipes, while natural-vent units pull room air and use a single flue. Both can be used in an existing masonry fireplace when installed as an insert. Our guide to gas logs versus gas inserts compares them with the open alternative.

A pellet insert uses a fan-driven system and a small-diameter exhaust. It is popular because the flame is steady and the fuel is uniform, yet it depends on power. The Department of Energy notes that many pellet stoves can be direct-vented, which is a point to raise with the installer if your chimney is difficult.

With either type, a qualified installer reads the manufacturer’s venting requirements, not general rules of thumb. Different models have different pipe, clearance and termination requirements.

Liners, sizing and clearance

Wood insert exhaust is hot and corrosive. The Chimney Safety Institute of America states that a masonry chimney without a liner needs one for any new appliance, whatever the fuel. It also says that when switching from wood to gas, the existing flue may be too large and need a smaller liner.

The Department of Energy notes that the flue size should match the appliance outlet, since an oversized chimney slows exhaust and increases creosote buildup. In other words, the same oversizing problem that affects water heaters in old flues can affect inserts. A liner connected to the insert outlet is the usual remedy, and its diameter follows the appliance specification. See chimney liner sizing for the logic.

Clearances to combustible materials, mantel height, hearth extension and trim are set by the insert’s listing and the manufacturer’s manual. EPA’s installation guidance stresses that installation errors by non-professionals may not be visible, and problems may not appear until a fire occurs. Check clearances with a professional and your local code official rather than estimating them.

Before you buy: a short checklist

1. Get the chimney inspected. CSIA recommends an inspection before installing a new hearth appliance, including when swapping a wood insert for a gas or pellet insert. It checks flue condition, size and height, plus fuel compatibility.
2. Have the flue cleaned. Creosote and debris should be addressed regardless of appliance type.
3. Ask for a written analysis. A written report documents the condition and what modifications the new appliance needs.
4. Read the appliance’s listing and manual. Venting, clearances and any permit requirements follow from it. For permit questions, see permits and contractor documents for chimney work.
5. Choose the installer carefully. EPA notes that the National Fireplace Institute certifies installers and technicians, and that CSIA can help find a certified sweep.
6. Plan the upkeep. Fire officials recommend professional inspection of the appliance, chimney and vent each year.

What this guide cannot do, and when to call

This guide cannot tell you which insert your fireplace can accept. That depends on the firebox dimensions, the chimney’s height and condition, the home’s airtightness and local requirements. A professional measures and inspects before recommending anything.

Stop using the appliance and get it checked if smoke enters the room, if you smell exhaust, if a carbon monoxide alarm sounds, or if the glass or surrounding trim shows damage. AM provides chimney inspection, cleaning and liner installation, and explains findings and options before any work begins. Check the appliance manual and your local code official for the rules that apply to your installation.

Frequently asked questions

Related reading

• Fireplace vs insert vs wood stove: how each one vents
• Wood, gas or electric fireplace: how to choose
• Chimney liner installation and replacement
• Chimney inspection
• Converting a wood-burning fireplace to gas