My firebox is narrower at the back than the front — which width do I use?
Use the rear width — the narrower number — as your limiting dimension. A firebox is almost always wider at the front and tapers toward the back, and any insert or log set has to clear that tightest point. Record both front and rear width, but size to the rear so the appliance actually seats without being forced into place.
Why the back is the number that governs
Masonry fireboxes are built to funnel smoke, with angled side and back walls that narrow toward the flue. That means the rear width — and the rear depth — are usually the real constraints, not the generous front opening. An insert is a rigid steel body: if the rear span is 24 inches, it cannot seat a unit wider than that no matter how open the 36-inch front looks. The tightest dimension wins, every time.
But keep both numbers on the sheet
For gas logs specifically, sizing charts list front width and rear width separately for a reason — the front sets the visible span you see when the fire is lit, and the rear determines whether the set even drops into the box. So write both down. The taper between them tells us how much clearance a set will actually have on each side once it's seated, which is the difference between a clean flame and one licking the sidewalls and sooting.
The safe habit: measure the tightest of everything
The rear-width rule is really one instance of a bigger principle — measure to the smallest point in every direction. Height is limited by the damper, not the top of the opening; depth is limited up high where the back wall slopes in, not down at the floor. Round down across the board and you'll never be caught short. Easier still, let us measure it on the free in-home assessment and match the unit ourselves. Call (828) 545-0525.
How we can help
Related questions
- How do I measure my firebox for gas logs?
- What do front width and rear width mean on a gas log sizing chart?
- How do I measure firebox depth correctly?
- How do I measure my fireplace for a gas insert?
- What measurements does a fireplace dealer need before quoting an insert or log set?
- How many BTUs per square foot do I need to heat a room with a fireplace?
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