How to Choose a Fireplace Insert | Measure First, Buy Second

A fireplace insert can transform an inefficient, heat-wasting fireplace into a genuine heat source for your home. But the wrong insert—too large, mismatched to the fuel you want, or incompatible with your fireplace construction—is an expensive mistake. The first and most important step happens before you browse a single model: measure the existing opening and confirm your fireplace type.

Start With Your Fireplace Opening, Not a Brand

The single factor that determines which inserts fit your home is the fireplace opening itself. Inserts are built to slide into an existing masonry fireplace opening, and size is limited by that opening’s height, width, and depth. Measure at least these dimensions before visiting a dealer:

• Opening height and width at the front
• Opening depth at the top and bottom
• Rear wall width (which may be narrower than the front)
• Hearth depth extending in front of the fireplace
• Mantel height above the opening

Take photos of the fireplace and surrounding wall, and note the brand and model number if a unit is already installed. Most fireplace inserts are designed for masonry fireplaces only. Factory-built fireplaces often cannot accept an insert unless the original manufacturer explicitly approves it, so check that first.

Match Heat Output to Your Room Size

Fireplace inserts are rated in BTUs (British Thermal Units), and the right output depends on the room’s square footage. Rough guidelines from major manufacturers:

An insert that is too small won’t warm the space; one that is too large can overheat a room and cause uncomfortable temperature swings. Measure the room’s square footage before shopping.

Pick a Fuel Type That Fits Your Home and Lifestyle

Each fuel type has different costs, installation requirements, and maintenance levels. Here is what to expect for each:

A certified installer can usually complete the job in about half a day, though the gas-line accessibility matters. Permits and an inspection may be required, and the insert needs proper venting—either a direct-vent system or a stainless steel liner inside the existing chimney.

They require no venting, gas line, or chimney work—just a standard outlet. Many electric models are self-contained and may not need a surround to cover gaps. They are ideal for homeowners who want ambiance with zone heating rather than whole-room heat.

Look for EPA-certified models with secondary combustion or catalytic converters for maximum efficiency and lower emissions. An older chimney may need a stainless steel liner (commonly 6″ or 8″ diameter) to ensure proper draft and prevent leakage.

They need electricity to run the auger and fan, and the pellet hopper requires regular refilling. These are a middle ground between wood and gas for convenience and heat output.

Before buying any insert, schedule a chimney inspection. A damaged flue or a chimney that was previously unlined may require significant repair, which changes the total cost and timeline.

If you are considering an electric model that doubles as a decorative unit, our product roundup on the best fake fireplace inserts covers electric-only options with realistic flame effects.

Don’t Forget the Surround and the Installer

Even a perfectly sized insert often leaves a gap between the unit and the fireplace opening. That gap requires a surround or front—a metal panel that bridges the space and gives the install a finished look. Some electric inserts are self-contained and skip this step, but gas and wood models almost always need one, and the surround is sold separately in many cases.

Installation is not a weekend DIY project for gas or wood inserts. Use a specialty retailer and a certified professional installer. A proper install ensures safe venting, correct gas-line connection, and compliance with local building codes. The handful of hours an installer spends on the job are cheaper than the consequences of a mistake.

Common mistakes to avoid: buying before measuring the opening at top and back (which can differ by several inches), ignoring the room’s heating load, assuming any fireplace can take an insert, and forgetting to order the surround, leaving you with an insert that technically fits but looks unfinished.