What heater do I use when the heater is installed “indoors”?

When the heater is installed indoors, it is most likely cost effective and logical to use the more expensive High Efficiency heaters. An indoor heater needs air for combustion and some way to discharge the exhaust. Those mechanical requirements can become costly with a standard heater, but the costs are very significantly reduced when the high efficiency heaters are used. Those heaters allow for direct plumbing for combustion air and exhaust, utilizing a much less expensive installation method and material.