Insert showrooms sell flame views and door handles; insert satisfaction is decided by six less-photogenic specs. Shop these and the pretty parts take care of themselves:
Spec 1: fuel, decided before the showroom
Wood, gas, or pellet is a household-habits question, not a product question, the personality guide settles it in ten minutes and filters 70% of the showroom instantly.
Spec 2: the double sizing
An insert must fit your firebox opening (width, height, and depth, the dimension DIY measurers forget) and match your room’s heating load, oversized units idle and soot, undersized ones disappoint, the sizing guide. This spec is why the process starts with a tape measure at your hearth, not a brochure.
Spec 3: efficiency numbers, read honestly
Wood models: look for EPA-certified with HHV efficiency ratings (70%+ is the modern bar); gas: steady-state AND AFUE-style numbers, and note direct-vent sealed architecture as the baseline. Distrust brochure numbers without test-standard citations, the honest brands cite.
Spec 4: the blower (the livability spec)
The fan converts a hot box into a warm room, mechanism three, and blower quality is where budget units cut invisibly: listen to one running before buying (bearing whine is forever), check for variable speed and thermostatic control, and confirm it’s replaceable, it’s the part that wears first.
Spec 5: glass real estate
The flame view is the daily experience; compare actual visible glass dimensions between candidates, not unit widths, ceramic glass on wood models (it survives closed-door burning heat) comes standard on quality builds.