Need some input and advice for placing tankless water heater outside. Need to move 40 gal water heater out of closet in bedroom upstairs. Plan is to build a new closet as well as a mechanical room outside of master bedroom(downstairs). Mechanical room to be accessed from outside and enough space for water filter and 1.5 ton HVAC. Thought was install indoor tankless on exterior wall and build temporary cover to protect heater so slab can be built later and mechanical room built around heater. I originally thought exterior tankless but dont think an exterior tankless can be inside a mechanical closet.