Where should an unvented gas log heater not be installed?

An unvented gas log heater should not be installed in a factory-built fireplace because these fireplaces are designed specifically for vented operation. Unvented gas log heaters produce combustion gases that need to be expelled from the living space, and a factory-built fireplace typically includes a venting system to direct these gases to the outside. Installing an unvented heater in such a fireplace could lead to dangerous levels of carbon monoxide buildup and other hazardous conditions since the combustion products are not adequately vented.

In contrast, standard fireplaces and wood-burning stoves can have modifications to accommodate unvented gas heaters with proper local codes, and outdoor fireplaces are typically open-air structures that, by nature, do not retain harmful gases in the same way. Therefore, the most appropriate answer is that unvented gas log heaters should not be installed in a factory-built fireplace.