Heating With Poor Insulation
If the room you're trying to heat has poor insulation, such as a garage or workshop, the area a 22,500 BTU gas fireplace can heat will be fairly limited. In colder regions, such as the northern U.S., such a heater can comfortably heat a 750 square foot building. In the south that area expands to roughly 950 square feet.
Heating With Average Insulation
In a better-insulated building, such as your average home, a 22,500 BTU fireplace is adequate for an area of 950 square feet in the northern states. If you live further south, such as in Tennessee or Virginia, a heater of this strength can heat a house of 1,150 square feet.
Heating in a Well-Insulated House
If you take special care to insulate your home well, a less powerful heater is required to maintain a much larger area at a comfortable temperature. In a house with superior insulation, a 22,500 BTU gas fireplace can warm an area of 1,250 square feet or up to as much as 1,650 square feet in the southern states.
Other Considerations
The stated dimensions should not be taken as strict rules. The area a given fireplace can heat also varies somewhat depending on elevation and air circulation. Under most circumstances, a 22,500 BTU gas fireplace can be used to heat an area within the ballpark of a couple hundred square feet larger or smaller than stated.