It's hard to find low BTU anything. On premium models, 60k BTU is the smallest. WTF! What if you have 2 systems and actually want the upstairs sized correctly. Forget 45k BTU, many homes might only need 25-30k BTU. You can get a fireplace that small, why not a 30k BTU 2 stage furnace with a nice even 18k BTU's on low fire, which is all many spaces need 3/4 of the time in winter or even a modulating version that goes down to almost 10k BTU's.