Ours the heater itself is considerabley smaller than the space it occupied or used. The heater box fits thru a 7 3/4 X 17" hole to ~19" deep. As install instructions state minimum clearances are 1/2", the actuall space you might reclaim could be considerably smaller than what I gained. But if removing for weight-storage wouldnt be a consideration. Other than the Wave heater that replaced our furnace & wood to refit, all other items that are stored in this area are redistributed stuff we already carried. Still I guestimate 25# or better savings.
Just FYI, got the heater out of storage, sat on scale, heater complete with the duct hoses, registers, fittings, gas line, 2x2 block-everything except the wires & thermostat & 1 of the registers (thats covering hole in cabinet) weight just a tad over 37 pounds.
Heavier than I remember. Its an Atwood 8520-IV-DCLP- output rating of 1600BTU.