Vented or Vent-Free Gas Logs? An Honest Comparison

Vented gas logs sit in a working wood-burning fireplace and use the chimney the way wood does. The flame is tall and yellow, with a lot of movement, because it burns somewhat rich. The products of combustion, including any carbon monoxide, go up the flue. That is why the damper must be open whenever they run, and why many installations clamp it so it cannot close all the way.

Vent-free gas logs, sometimes called ventless, are built to burn so cleanly that their exhaust can stay in the room. The flame is smaller and bluer, and the log arrangement is tuned so the flame does not hit the logs and make soot. All vent-free heaters and log sets are certified to a national standard, ANSI Z21.11.2, which also caps their size.

This is the real choice. Vented logs make a fire that looks close to wood, but most of the heat goes up the chimney with the exhaust. In a drafty room they can even pull warm air out of the house. Think of them as decoration that gives a little warmth near the hearth.

Vent-free logs are efficient heaters. Nearly all the heat stays in the room, which is why people use them as backup heat when the power goes out. The tradeoff is the look, which is tidier and less like a real fire, and what else stays in the room with the heat.

If you want both a real-looking fire and real heat, a sealed direct-vent insert is the third option. It draws combustion air from outside and vents outside through a sealed pipe, behind glass.

Every vent-free set has an oxygen depletion sensor built into the pilot. If oxygen in the room starts to fall, the pilot flame changes and the sensor shuts the gas off. It is a good safety device, but it guards against low oxygen, not against every problem. A dirty burner or misplaced logs can still raise carbon monoxide.

Vent-free manuals require a CO alarm in the room. With vented logs the risk is a closed damper or a blocked flue, which turns a vented set into an unvented one that was never designed for it. Check the damper every time, and put a CO alarm in the room either way.

Burning natural gas makes water vapor. With vent-free logs that moisture stays in the house, and manufacturers warn it can cause condensation and moisture problems with long run times, especially in tight, newer houses. Some people also notice an odor or eye irritation, particularly when the set is new or dusty.

Size limits are part of the standard. Vent-free heaters are capped at 40,000 BTU an hour. The fuel gas code keeps unvented room heaters out of bedrooms and bathrooms, except for small wall-mounted heaters with an oxygen sensor in a room big enough for them, so vent-free logs in a bedroom fireplace are out. Every set also has a minimum room size for its BTU rating in the manual. Vent-free heat is supplemental and is not allowed as the only heat in a home.

Not every factory-built fireplace is approved for vent-free logs either. Check the label inside the firebox before buying a set for it.

Vent-free logs are designed as attended appliances. Their manuals say not to leave them burning while unattended or while anyone is asleep. Turn them off before bed. For vented logs, follow your manual, which typically says the same thing about unattended use, and never close the damper on logs that are still warm or have a pilot burning.

Both kinds need yearly attention before the cold weather. That means cleaning the burner and pilot, checking the logs for cracks and correct placement, leak-checking the connections and testing the safety devices. Vented logs also need a sound chimney and a working damper, so have the flue inspected as you would for wood.