I have a National-US model 99A-W-5 PA Propane water boiler.
AGA input 108000 BTU
AGA output 84600 BTU
Net IBR 74800 BTU
I believe it's about 30 years old. It's in a 1100 sq ft house. A couple of years ago we finished the basement and added registers on both ends of the basement and in the basement bathroom.
My question is what is available in a high efficiency boiler? What should I look for?