Ventless or “non-venting” gas logs are more efficient than venting gas logs, which makes them a popular choice among consumers. However, there are some points about this type of appliance that consumers are not always made aware of before their purchase. As service companies, chimney sweeps see many issues with ventless log sets after homeowners have a problem, and have first-hand knowledge of these issues. Members of the Midwest Chimney Safety Council made observations about these types of appliances and share them with the public.
According to the MCSC, vent-free gas-burning log sets are advertised as being 99% efficient, which means that virtually all of the heat produced is kept in the house, rather than going up chimney. However, this also means that the water vapor produced by the burning of gas also stays in the house- and this can be significant over a period of time. For this reason, it is the opinion of many chimney professionals that ventless logs should never be installed in a small room in a well-insulated home. Some homeowners have had mold issues caused by the excessive moisture.
The biggest complaint received is a strong odor when the logs are in use – at times so bad that the homeowner discontinues use of the fireplace. The source of the odor is off-gassing of items in the house such as furniture, carpet, cabinets, or chemicals like paint or bleach, which lurks in the air. In order to produce flame, the logs need oxygen, which is obtained from the air inside the house. After the chemicals are burned, they change composition and mild to strong odors may result. With venting gas logs, the odors are sent up the chimney, but with ventless logs, the odors remain in the house.
Another issue, especially with older ventless sets, is the level of Carbon Monoxide emissions. Currently, the acceptable level is 9 ppm (parts per million), which means that if a log set tests at 9 ppm or below, it is approved for installation in a home. Some of the sets tested in the field, however, are producing up to 24 ppm. Notable is Dr. David Penney’s opinion that no level of CO exposure is okay for anyone, even low levels under 9 ppm over a long period of time. According to Penney, it has been proven that low level exposure to CO can cause headaches, nausea, dizziness, and permanent brain damage. He suspects that CO may be a cause of Sudden Infant Death Syndrome.
With today’s tighter, well-insulated homes, the exposure to CO is a great concern. The Environmental Protection Agency recommends a minimum of six air exchanges per day, which does not occur in many newer homes. Another issue that is complicated by tight homes is negative pressure. Negative house pressure can cause back up of flue gases from venting and non-venting gas logs and wood-burning appliances, as well as furnaces and hot water heaters- which may go unnoticed by the homeowner. If testing shows negative pressure in the house a solution is to provide a whole house ventilator which when connected to the furnace supply, return air, and outside, provides the needed air for the home on demand whether the furnace is off or on.
A simple test for negative pressure is to tape some newspaper strips to the front face of the fireplace during cold weather. Make sure the doors of the house are closed and all fans are off. If the paper moves outward, into the room, there is a high possibility that negative pressure is an issue. This is always worse on the lower levels of the home. Now slowly open a window and watch the paper. At the point where the paper moves back into the fireplace stop and measure the opening of the window. This is the amount of air needed to add to the house to correct the problem for that appliance only. More air will likely be needed for other appliances to function properly. Note: adding an outside air source directly to the fireplace for combustion air is not the best solution since it dumps cold air on a fire and makes it less efficient.
Manufacturer and Code Requirements:
- If a non-venting gas log set is installed into a regular masonry chimney with a clay tile liner the entire chimney must be in good working order. So installation of a ventless gas log set is not a solution for a damaged chimney.
- In most jurisdictions, ventless logs are not allowed in bedrooms. Some cities do not allow them in any room.
- Most manufacturers state that when using non-venting logs a window must be opened, which defeats the purpose of warming the house.
- Most manufacturers state that their log sets may only be used for short periods of time, usually up to four hours.
- If a set of non-venting gas logs is already installed read the owner’s manual thoroughly and have the set serviced at least once annually by a qualified service technician to make sure they are in good working order and CO levels are low.
- If considering ventless gas logs, the MCSC suggest looking into Direct Vent gas inserts or freestanding stoves instead. This technology is a sealed system which uses outside air for combustion and exhausts toxic gases and moisture to the outside. Inserts can be installed into masonry chimneys and if the interior chimney is damaged it does not need to be repaired in most cases. DV inserts and stoves are high-efficiency units.
- Have the house checked for negative pressure and air exchanges, especially after insulating the home or adding thermal windows. If there is a problem, add a whole-house ventilator.
- If soot is noticed on the walls or baseboards, this is a possible indication that there is a problem with the appliance and it needs to be checked by a professional.