HVAC in Philadelphia is usually a boiler and radiators in a rowhouse, with window units doing the humid summer. Some houses have been converted to ducts. Some larger buildings share a plant. The basement under the stoop is where the boiler lives, often in a tight room that has been wet. No heat is urgent on a cold night. A third floor that will not cool is the August call. This is not a hill house with a condenser sitting on a wide pad. Space is the constraint.
Describe the symptom against the radiator. One cold unit and a hot house is not a dead boiler. Banging pipes, a radiator that leaks at the valve, and heat that never reaches the top floor are the ordinary complaints. A window unit that drips into the sash or trips a circuit is a different trade conversation from the boiler. If you have no ducts, say so immediately. Filters and returns are a distraction in a radiator house. If you do have ducts, say where the air handler sits.
The row limits the equipment. There may be nowhere for an outdoor condenser except a roof that already ponds or a rear yard that is an alley. A mini-split on the brick has to miss the cornice, the neighbor's windows, and the fire escape. Combustion air and the flue matter for a boiler tucked in a low basement. If that basement floods, the equipment story and the water story are the same call. Say if the boiler has been standing in water.
For replacement, say the fuel and whether you are keeping radiators. Ask who services a shared plant if you are not the only household. Ask about the chimney. A no-heat call and a cleaning are not the same visit. Say whether you are cold now, and whether the top floor and the parlor disagree. Old sash makes boilers look weak. Mention drafts if the radiators are hot and the room is not.
Philadelphia heat is a row boiler, a stair full of uneven rooms, and a window unit. HVAC starts there, not on a ridge. Call the number on this page.