Gas log installation, vented or vent-free, in Lake Pickett

Gas logs in a metal factory-built fireplace

Factory-built fireplaces are tested as complete systems, and their manuals say what may be burned in them. One vented log set manual gives separate damper opening figures for factory-built fireplaces, which only matter when the fireplace's own manual allows a log set in the first place.

For vent-free sets the code is stricter. Section 621.7 keeps an unvented decorative heater out of a factory-built fireplace unless that fireplace system is tested, listed and labeled for the use under UL 127. Our pros read both labels before recommending anything.

Minimum firebox size for each burner

Every burner has a smallest fireplace it can go into. In one maker's table, the 18/20 inch burner calls for a firebox no narrower than 26 inches across the front and 20 across the back, at least 15 inches deep and 17 inches tall.

Its 24 inch burner needs 30 inches at the front and 24 at the back, and its 30 inch burner 36 and 29, with the same depth and height. Our crew measures all four dimensions in your firebox and compares them with the table for the set you want.

• Front width
• Rear width, measured at the burner's depth
• Depth from the opening
• Height of the opening

A screen in place while the set burns

One vented log set manual requires a fireplace screen to be in place whenever the system is burning. Unless combustion air is provided another way, the screen must have openings that let air in.

The same manual gives a simple test, there is enough combustion air when every flame bends inward, into the firebox and clear of the screen. Our technicians watch for exactly that during the first burn and adjust the setup when flames lean outward.

No wood fires once a vent-free set is in

A vent-free set turns a wood-burning fireplace into a gas appliance for good. One manual says solid fuels shall not be burned in a fireplace in which an unvented room heater is installed, and that the heater must not be used as a wood-burning heater.

The same manual says to use only the logs provided and never to place objects in the heater or on the logs. If you still want an occasional wood fire, our specialists explain what that means for the choice of set.

Gas pipe sized for the burner

Pipe that is too small or too long starves a burner. One maker says the gas piping must be sized to provide the minimum inlet pressure at the set's maximum flow, with a supply pipe of at least 1/2 inch inside diameter.

The same manual adds that a line longer than 20 feet may need a larger diameter and points to NFPA 54 for the details. Our pros look at the existing line and tell you whether it suits the set before anything is ordered.