This is the question that decides everything, and it takes two minutes with a tape measure. Measure the width, height and depth of your existing firebox opening — and measure the width at both the front and the back, because most masonry fireplaces taper.

Then compare against the insert's actual dimensions, listed on every product page. As a rough guide, the IDV24 is our most compact at 24¼″ wide, and the IDV44 is the largest at 42⅞″ wide and 18″ deep. Some Clean View fronts also carry their own minimum opening requirement — the slim front on the IDV26, for instance, needs at least 31″ of width.

Not sure? Send us your three measurements and we'll tell you which sizes fit before you order.

A licensed installer removes the damper assembly, runs a flexible aluminium liner from the insert up through the existing chimney, connects a gas line to the firebox, and slides the unit into place. The front and surround then cover the gap between the insert and your opening.

Because the chimney is already there, an insert conversion is usually the shortest and least expensive route to a working gas fire — no framing, no new vent run through a wall or roof. Most installs are done in a day.

Full walkthrough: converting a wood-burning fireplace to gas.

If you already have a fireplace opening, an insert is almost always the right answer. If you're putting a fireplace where none exists — a blank wall, new construction, a remodel — you want a zero-clearance built-in instead, which frames into the wall and vents through it.

The comparison in full: zero-clearance vs. insert gas fireplaces. Or browse direct vent built-ins if that's your situation.

— A Clean View front — required on every insert. This is the visible face, and it also determines the minimum opening width.

— Media — a log set or glass media. The appliance ships without one, and it can't run without it.

— A surround — usually, to cover the gap between the insert and a larger opening.

— A chimney liner kit — supplied and fitted as part of the installation.

These aren't upsells; they're how the product is sold. Our guide to reading a gas fireplace quote breaks down what each component should cost, so you can check a bid line by line.

Fuel isn't a preference — it's whatever your home runs on. A meter means natural gas; a tank means propane. Converting after installation needs a kit and a licensed technician.

Ignition is a real choice. Millivolt keeps a standing pilot lit and is the simplest, least expensive option. IPI lights on demand so nothing burns while the fireplace is off. ProFlame 2 adds a remote with thermostat and flame control. Full comparison: millivolt vs IPI vs ProFlame 2.