Converting from an old oil boiler to a new high efficient gas boiler. I have 1900 square feet of living space, broken into 2 zones. How do I determine the number of btu's needed for the new gas boiler? Hank
call a HVAC contractor. The calculations are too complex to not be using a computer program to figure this correctly. Too small or too big and you have issues and/or energy waste. Basically you need to measure the cubic feet of each room. then the sq. ft of each exterior wall. Then add the individual k values of the materials in the wall to get the net U-Value. Then total the R-value of the existing insulation by type/thickness/installation quality and deduct the inverse of the u_value X 25% to get net wall R-Value. Do you really want the rest of this?