In addition to knowing what to look for in an energy-efficient gas fireplace, it is important to know what is not recommended. In the case of gas logs, ceramic "logs" with gas burners are placed directly into an existing wood fireplace to give the effect of a burning fire. These gas logs provide no real heat to the house and are essentially a waste of energy and money, as well as a potential source of pollutants, although they can provide an attractive fire.
Gas logs can also cause venting system problems, particularly in colder regions. This is primarily because wood fireplace chimneys are not designed to handle the low-temperature, low-flow, high-moisture flue gases. If a wood fireplace chimney is not relined to accommodate these gases, the chances of flue gas condensation and chimney deterioration are high. As well, if the fireplace is on an outside wall, there is a risk that the gas burners will not generate enough heat to create a good draft. Under these conditions, the house can become a better chimney than the chimney itself (see sidebar on page 20), disrupting the flame and drawing carbon monoxide and other combustion products back into the living area.
Source: Natural Resources Canada (NRCan) - Office of Energy Efficiency