Gas furnaces dominate the Charlotte metro region, our natural gas prices are relatively low and our winter climate is mild compared to other regions of the US. Gas furnaces are available in 80% efficient models or 90% + efficient models; typically 80% models are installed in attics and 90% attics are in crawl spaces and basements. The major difference is the byproduct of the 90% models is condensation; they burn so clean there is very little heat in the exhaust gasses. The flue pipe material on 90% furnaces is PVC and usually sticks out a sidewall of the house, making it easy to position a furnace in almost any configuration under the house. 80% furnaces require a steel 4” flue pipe that must be terminated out of a roof and have clearance away from all combustible material.
For every dollar of gas that an 80% furnace uses it provides $0.80 cents of heat.
For every dollar of gas that a 90% + furnace uses it can return up to $0.97 cents of heat.
Gas furnaces are available in almost any Btu requirement, blower size, efficiency and configuration for installation, one size does not fit all and bigger is not better.