Pittsburgh note
Pittsburgh design temperatures and humid summers mean both heating capacity and latent (humidity) loads deserve attention before equipment is chosen.
Load versus equipment catalog
The load tells you what the house needs. The equipment catalog tells you what machines can supply. Confusing those two steps is how homeowners end up with oversized cooling that short-cycles on sticky August nights.
Why older homes punish shortcuts
Additions, walkout basements, and brick walls change gains and losses room by room. A single whole-house guess often overserves easy rooms and abandons the stubborn ones.