Gas Insert Buying Guide: Sizing, BTUs, and Choosing the Right Model

Author: Sean Summers, NFI Master Hearth Certified Technician – WoodstovePro.com

A gas insert is one of the smartest ways to transform an old, drafty fireplace into a dependable heat source. If you’re considering a fireplace upgrade, the biggest questions usually come down to sizing and heat output: What size insert do I need? How many BTUs are enough? Should I choose a direct vent model?

This guide breaks down gas insert sizing, explains BTUs in plain terms, and helps you choose the right insert for your home.

What is a gas insert (and why upgrade)?

A gas insert is a sealed firebox designed to fit inside an existing wood-burning masonry or factory-built fireplace. Most inserts are a direct vent gas insert, meaning they vent exhaust outdoors through a dedicated liner system while pulling combustion air from outside.

Benefits of a gas insert fireplace upgrade include:

• More usable heat than an open fireplace
• Less draftiness when the unit is off (sealed system)
• Convenient, controllable heat (thermostat/remote options)
• Cleaner operation and lower maintenance than wood fires

Step 1: Start with your existing fireplace opening (gas insert sizing basics)

Gas insert sizing begins with accurate measurements. You’ll typically need:

• Front width, back width (if tapered), and height of the opening
• Depth (from opening to back wall)
• Height of the rear wall (if the opening slopes)

Also note:

• Damper location and size
• Fireplace type (masonry vs factory-built)
• Whether the chimney is straight or has offsets

Pro tip: Measure twice. Many fireplaces are not perfectly square, and small differences can affect fit.

Step 2: Understand BTUs (your BTU guide for real-world comfort)

BTU stands for British Thermal Unit—a measure of heat output. A higher BTU rating generally means more potential heat, but it does not automatically mean better comfort. Your BTU guide should consider:

• Room size and ceiling height
• Insulation and window quality
• Open floor plan vs closed rooms
• Local climate and how often you’ll use the insert

General BTU ranges (very broad guidance):

• 20,000–30,000 BTUs: smaller rooms, supplemental heat
• 30,000–45,000 BTUs: medium spaces, frequent use
• 45,000+ BTUs: larger areas, open layouts, colder climates

Bigger isn’t always better. Oversizing can lead to an uncomfortably hot room and shorter run cycles.

Step 3: Direct vent vs other venting options

For most homeowners, a direct vent gas insert is the best and most common choice. Direct vent systems:

• Use a sealed combustion chamber
• Exhaust combustion gases outdoors
• Pull combustion air from outside
• Improve indoor air comfort and safety compared to non-sealed options

A direct vent insert typically uses two liners routed through your existing chimney—one for exhaust and one for intake. This helps create steady draft and safe operation.

Step 4: Choose features that match how you’ll use it

Once you’ve narrowed down fit and BTUs, focus on everyday convenience:

• Blower kit: improves heat circulation into the room
• Thermostat/remote: more consistent comfort
• Ignition type: standing pilot vs electronic ignition
• Viewing area: bigger glass for more flame presentation
• Log style and media options: aesthetic preference
• Efficiency rating: more heat output from the fuel you use

If your fireplace upgrade goal is real heat—not just ambiance—prioritize efficiency and blower options.

Step 5: Don’t skip professional installation considerations

Gas inserts must be installed to manufacturer specs and local codes. Key items include:

• Correct gas line sizing and shutoff location
• Proper vent liner sizing and termination cap
• Clearances to combustibles around the surround
• CO/smoke detector placement and safety checks
• Verification of draft and safe operation

Even the best insert won’t perform well if it isn’t installed correctly.

Final thoughts

A gas insert can turn an underperforming fireplace into a reliable, efficient heat source—one of the best fireplace upgrade moves you can make. Start with accurate measurements for gas insert sizing, use a realistic BTU guide for your space, and consider a direct vent gas insert for safe, steady heating.

Want help choosing the right model for your fireplace dimensions and heating goals?
Call our certified hearth professionals at 888-418-0005 or email info@woodstovepro.com.