Gas Logs – Vented and Ventless

DEAR TIM: I am considering purchasing a set of gas fireplace logs. My shopping trip left me overwhelmed with decisions. Do they really look like a real wood fire? Some log sets need to be vented, while others do not. How can this be? Can the logs be installed safely in an existing wood burning fireplace? Are there any other considerations? D. P.

DEAR D. P.: Three years ago, I installed a high quality set of vented gas logs for a customer. Just before lighting them, I made fun of their appearance. My comments quickly went up in smoke! I was entranced by the realism of the logs, flames, and the glowing embers. Generally speaking, the vented logs appear more realistic than the ventless gas logs.

The vented gas logs require a flue or a chimney to exhaust the toxic carbon monoxide they produce. Many of these log sets burn gas that passes through sand beneath the logs. Often vermiculite and clumps of rock wool are placed over the sand. These elements produce the highly realistic glowing embers. Vented gas logs also are capable of consuming large amounts of gas. Some can ingest up to 90,000 Btus per hour! Unfortunately, 85 - 90 percent of this energy goes up the chimney. Fireplace dampers must be locked or clamped in the open position at all times.

Ventless gas logs are technological marvels. They have special burner elements that burn gas with minimal carbon monoxide production. Vent free gas logs certified by the American Gas Association (AGA) are equipped with oxygen depletion sensors (ODS). These devices sense when oxygen in the room drops to a certain percentage. If this occurs, the sensor shuts off the supply of gas to the logs. Defective sensors fail in the safe mode. They will not allow gas to pass to the logs.

Ventless gas logs burn less gas than their big brothers. They are only permitted to burn a maximum of 40,000 BTU's per hour. However, since your fireplace flue is closed or non existent, all of this energy is released into your house. During a power outage, these appliances can be used as a temporary heat source.

All gas logs have the capability of producing huge amounts of water vapor. 1.5 gallons of water is produced for every 100,000 BTU's of gas burned. This massive amount of water can condense on cold chimney flue walls and drip into your fireplace. In the case of vent free logs, this water vapor can condense on windows and any other cold surfaces. Should you install these in a newer air-tight home, you may create severe moisture problems in your attic and in exterior wall cavities.

Certain states prohibit the sale and/or use of ventless gas logs: Alaska, California, Massachusetts, Minnesota, Montana, New York, Utah, and Colorado (30 counties permit their use). Certain other municipalities in other states may also prohibit their use. Check with your local building department prior to purchasing your set. Have your chimney professionally inspected and cleaned prior to any installation. Look for the AGA seal on all products. Follow manufacturer's instructions to the letter!