I calculate a furnace[quicky]average insolation ,no holes in wall's,etc,window's,looking,we'll insulated[weather stripped] 40btu's per square foot exampel 2000 squar foot liveing area multiplied by 40 btu's =80,000 btu furnace i would appreciate any feed back P.S yes a can read the cooling/heating load manuels,this has way save's me some time and it's worked good for ME[feed back wanted]