It gets involved if you want to do it right. Get IBR book and fill out the heat loss calculation form. Know you widow type, insulation type, etc.. You also will need to do your calculations for cooling if you are going to have central air. Typically a home in 30x40 home in N.E will take a 100- 150 thousand btu boiler or furnace and a 2 1/2 ton a/c condensor. There are a lot of variables to sizing it properly.
