A whole-system project covers everything a cooling installation does, plus the heating side and the combustion work that comes with it. The additions are what make it a different job rather than two jobs booked on the same day.
One Assessment, Two Loads
The load calculation runs for both seasons in a single exercise, and the two answers do not scale together. A well-insulated Chicago house frequently needs considerably more heating capacity than cooling capacity, and the equipment is selected so that neither side is compromised for the other. The blower has to satisfy the cooling airflow requirement while producing a temperature rise across the heat exchanger inside the range the furnace nameplate states.
What the Heating Side Adds
- Gas piping sized for the new input rating, with sediment trap and shutoff at the appliance.
- Venting rebuilt to suit the efficiency category, since a condensing furnace vents through plastic to a side wall while an older atmospheric unit used a masonry chimney.
- Combustion air provision, which changes when a mechanical room is tightened or a chimney is abandoned.
- Condensate drainage from the furnace itself on condensing equipment, with neutralisation where required.
- Consideration of any other appliance still using the chimney, because removing a furnace from a shared vent changes how a water heater draughts.
The Coil Cabinet Question
New coils and new furnaces do not always share dimensions with what they replace. The coil cabinet sits directly on the furnace and connects to the supply plenum, and fabrication is common on replacement work. Getting that transition sealed matters, because a gap there draws basement or attic air into conditioned supply air on every cycle.
Electrical Scope
The condenser needs its own circuit and disconnect. The furnace needs a dedicated supply and a service switch. Control wiring runs from the thermostat to the furnace board and out to the condenser, and modern staged or variable-speed equipment sometimes needs a communicating cable rather than conventional wiring. Older panels frequently need attention before any of that wiring can be run, which brings an electrician into the schedule.
Commissioning Both Halves
| Side | What gets measured and recorded |
|---|---|
| Cooling | Charge by superheat or subcool, airflow, temperature split, current draw on compressor and fans |
| Heating | Temperature rise across the heat exchanger against the nameplate range, manifold pressure, and combustion analysis |
| Air side | Total external static pressure, blower speed settings for each mode |
| Safety | Flame sensing, limit and pressure switch operation, condensate safety switches |
Permitting a Combined Project
Whole-system work is permitted in most Chicago-area municipalities, and the inspection is broader than for a cooling replacement because it covers gas, venting and combustion air alongside electrical and condensate. The permit is part of the project and the inspection is scheduled after startup.
What Is Outside the Scope
Duct replacement, chimney lining for other appliances, electrical service upgrades ahead of the panel and abatement are separate items quoted after the assessment. Where only the cooling half is being replaced the project belongs with central AC installation, and where only the heating half changes it belongs with heating installation and replacement. The combined project, covering both halves in one sizing pass, stays with whole-system HVAC installation.