I replaced my oil burner (which also provided hot water).
In less than 3 years, it DID pay for itself. Gas water heater too.
My house has vinyl siding, over Styrofoam, over asbestos siding, over wood clapboard, with 6" of blown in insulation, wood lath, horsehair plaster... and some walls have drywall over that.
33 thermopane windows.
It was an ordeal.