A customer calls, asking me for advice.
No heat loss calculation? No work for you!
Did any of the contractors measure your home to determine the actual heat loss on a room-by-room basis? Do you have a separate water heater or do you get the domestic hot water from a coil in your existing boiler? Do you have cast iron radiators, baseboard, or convectors either recessed into the walls or mounted up against the walls? Were you able to obtain desired indoor thermostat settings last year? Over-sized boilers have lower operating efficiency and can have a shortened lifespan too