need new stand-by boiler

George_19

Have replaced primary downstairs 5 ton gaspack with a new package heat pump. The old 4800 sf home has 100 year old hot water radiator system which we use only a few weeks per year when it's below freezing but it costs between $6-$8 per hour to run @ $2.15 -$2.50 per therm NG. I need a more efficient boiler. I calculate 175,000+ BTUH is needed. What is best given little use (aluminum?)
and could I get away with a smaller size (100,000 BTU) given the limited use. No point wasting bucks. Thank you