Only electric patio heaters can be used indoors or in an enclosed area, and even then, that applies to just certain models. As always, check your owner’s manual to make sure that installation indoors or within an enclosure won’t void your warranty. Gas patio heaters, on the other hand, should never be installed in an enclosure or an area with poor ventilation. This is because they produce harmful byproducts of gas combustion that require airflow to be properly ventilated.