The Whole Thing in One Sentence
An insert slides into a fireplace you already have. A zero-clearance unit is a complete fireplace you put where there isn't one, or where you're pulling the old one out and starting over.
Everything else is detail.
Why it Has to Have Burned Wood
An insert needs a masonry firebox around it. That brick is what safely contains the unit, which is why an insert goes into a wood-burning opening and not into an old sealed gas box.
If your fireplace has always been sealed gas, it gets replaced rather than fitted.
The Thirty-Second Test
Nothing but a wall? Zero-clearance. Existing fireplace that once burned wood—even if someone added gas logs later? Insert. Existing fireplace that was always sealed gas? Zero-clearance, as a replacement.
That covers nearly every Utah house.
What Changes the Answer in Utah
Older Wasatch Front homes often have a masonry fireplace and a chimney worth keeping, which makes an insert the cheaper and less disruptive route. Newer construction along the corridor frequently has a sealed unit from the nineties or two-thousands that has simply reached the end of its life—that is a replacement, not an insert.
Bring four measurements to the showroom either way: front width, front height, back width, and depth. A masonry firebox tapers, so the back is almost always narrower than the front.