insulation,and air tight go a long way.address those first.600sq ft is not a lot of space to condition.too bad you dont know somone who is getting a new unit,and has the old one.even it was natural gas,a lp conversion kit is cheap.i know around hear i can buy a new 50,ooo btu gas furnace 90% for around $500.any elect. heat and watch that meter spin.gas is the only way to go or wood
