Film (Water Vapor) From Use of Vent-Free Gas Log Sets?

QUESTION: Is it a normal result of the operation of a vent-free gas log set for the inside of the windows to get a film on them? This cleans off easily but is not long before it comes back. I was curious as to whether this is normal or whether some adjustment may be needed?

ANSWER: The “film” you refer to on the inside of your windows is an accumulation of water vapor. Water vapor is a natural by-product of the combustion of gas and occurs with all gas burning appliances. One ounce of water per 1,000 BTU/hour is inputted into the living space with a vent-free heater. This is more than a quart of water per hour when your set is operated on high.

In dry climates, many people welcome this increase in humidity. Some people have humidifiers to accomplish the same effect.

Moisture accumulation is an indication of a lack of fresh air input and exchange with the outside. If you wish to decrease the film on your windows, you must either vent some of the products of combustion from the gas log set up the chimney or dilute the water vapor indoors with fresh air from outside. Without an adequate supply of fresh air to dilute the water vapor and the means to allow it to pass through to outdoors, the water vapor will adhere to all surfaces in the home. The same is true of a shower or a kitchen range. A home of tight construction (vapor barriers, weather-stripping, etc.) with an inadequate supply of fresh air will trap moisture, odors and fumes in the house.

Ultimately, fresh air is the solution for all indoor air quality issues, including moisture accumulation.