Gas log installation from a family-owned team in Riverside Acres

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

How tall the chimney must be for vented logs

A vented log set sends its exhaust up the chimney by draft, and short chimneys draw weakly. One maker lists a minimum chimney height of 15 feet from the hearth to the top of the chimney for its vented set.

Taller chimneys draw better, and the same manual lets a taller chimney get by with a smaller damper opening. Our pros estimate your chimney's height at the inspection and explain what it means for the set you are considering.

How far the damper must stay open

A clamped damper still has to leave enough open area for the exhaust. One maker's table sets a minimum permanent opening by burner size and chimney height. With a masonry fireplace and 15 feet of chimney, it asks for 27 square inches with the smallest burner, 35 with the 24 inch and 41 with the 30 inch.

Factory-built fireplaces and taller chimneys get smaller figures in the same table. Our crew measures the actual opening once the clamp is fitted and writes the number in your record.

Downdraft and the height of the chimney

Wind swirling around a roof can push exhaust back down a short chimney. One log set maker lists poor draft or downdraft as a cause of smoking and sooting and asks for the chimney to be at least 3 feet taller than anything within 10 feet of it in all directions.

The same manual suggests a chimney cap or a fan may help where that height is not met. Our specialists look at the chimney and its surroundings before a vented set goes in and explain the options in writing.

Choosing a burner size for the firebox

A bigger burner is not a better fire if the fireplace is too small for it. One maker lists a fireplace too small for the unit among the causes of a burner shutting down during operation, and its fix is to meet the minimum size requirements.

Our pros choose the burner by the firebox measurements first and the look second. When two sizes fit, we explain the difference in flame, gas use and damper opening so you can pick with the facts.