How many square feet does a 120000 BTU heater heat?

Answer 1

Square feet has nothing to do with how much an X BTU heater can heat, heat is calculated by determining how much heat LOSS there is to make up for. AC is the exact opposite, it's size is determined by how much heat GAIN there is.

In general, a 120,000 BTU boiler can heat an average well insulated 2 story house, no problem. There are many programs that calculate heat loss/gain but since there are people who do it for a living, try asking one.