A furnace changeout is a choreography: old unit out, new unit in, venting, gas, ductwork, condensate, electrical, and commissioning — in an order that does not trap you. This guide runs the sequence the way experienced installers do it, with the gas piping explicitly reserved for the licensed pro.
Most furnace installs are changeouts, not new construction: same location, same fuel, same ductwork. The job goes smoothly when the sizing is right, the venting is planned before the old unit comes out, and the gas connection is handled by the licensed pro while you handle everything around it.
This guide focuses on the installer-scope work. Gas line installation and modification is licensed-pro work in every jurisdiction — this guide does not teach it, and neither should anyone else's.
Gas piping — installation, modification, or reconnection — is licensed-pro work in every jurisdiction and is outside the scope of this guide. Refrigerant handling on a paired air conditioner requires EPA 608 certification. This guide covers the installer-scope changeout sequence; the gas and refrigerant portions belong to licensed, certified pros.
Furnace installs intersect the fuel-gas code (venting, combustion air, and gas piping per the IFGC as adopted locally), the mechanical code (IMC), and NEC Article 422 for the electrical connection. EPA 608 applies if refrigerant lines are opened on a paired AC. References are to current model codes — check your local amendments, since enforcement details vary.
Before ordering equipment, run a Manual J load calculation on the house: square footage, insulation, windows, orientation, and air leakage. Most existing furnaces are oversized — replacing like-for-like perpetuates short cycling, uneven heating, and early heat-exchanger failure. The right size runs longer, quieter cycles and lasts years longer. An hour of calculation beats a decade of wrong equipment.
Check the manufacturer's venting tables for the new unit: maximum equivalent length, allowed elbows, termination clearances, and pitch. Condensing (90%+) furnaces vent in PVC with specific slope back to the furnace for condensate; conventional units need proper metal vent and chimney condition. If the venting does not work in the existing location, that decision happens now — not with the old unit already on the truck.
Kill power at the disconnect and the breaker, verify dead, then have the licensed pro disconnect the gas line and cap it. Disconnect the venting, the ductwork, the condensate drain, and the thermostat wire, labeling the wire as you go. The old unit comes out in the reverse order it went in — and this is when you strip the blower motor, gas valve brass, and control wire for the salvage pile before the cabinet leaves the building.
Set the furnace on a solid, level base — an unlevel condensing furnace will not drain condensate properly. Build the supply and return transitions to fit the new cabinet footprint; it rarely matches the old one exactly. Seal every joint with mastic or approved tape, because duct leakage in an unconditioned space is energy straight out the window. Verify the filter rack is accessible and correctly sized.
Install the venting exactly per the manufacturer's tables: correct material, correct diameter, correct pitch, terminations with proper clearances from windows, doors, and grade. On condensing units, the intake and exhaust terminations have separation requirements — follow them. Improper venting causes condensate damage, pressure-switch faults, and in the worst case, combustion gas where it should not be.
Run the dedicated circuit per the nameplate (NEC 422), with a disconnect within sight of the unit. Land the thermostat wire on the correct terminals — R, W, Y, G, C — and run a common wire; modern thermostats and control boards need it. Double-check every low-voltage connection before powering up, because a miswired control board is an expensive lesson.
Condensing furnaces produce acidic condensate that must drain through a trap to an approved location — floor drain, condensate pump, or exterior where it will not freeze. Install a neutralizer where required by local code or good practice; raw condensate eats cast-iron drains and concrete. Route the drain so it cannot freeze, clog unnoticed, or back up into the furnace.
Fire the unit and measure temperature rise across the heat exchanger — it must land inside the manufacturer's rated range, adjusted via blower speed if needed. Check external static pressure against the blower's rated capacity; high static means the duct system is choking the new unit. Run combustion analysis to verify safe, efficient firing. Commissioning is what separates an installation from a box swap — do not skip it.
Every one of these has cost someone a callback. Learn them here instead of on the job.
The old furnace was probably oversized — most are. A Manual J takes an hour and gives you the right size; the nameplate gives you the last guy's guess.
Vent tables are engineering, not suggestions. Wrong vent length, diameter, or pitch causes condensate problems and safety issues.
Firing up without checking temperature rise and combustion means you never know if the unit is set up right. The analyzer earns its keep on every job.
High static pressure from undersized ductwork kills blowers and heat exchangers. Measure it — the number tells you whether the duct system can handle the new unit.
Condensing furnaces produce gallons of acidic water. A drain line that freezes, clogs, or was never installed floods the basement.
The furnace you are hauling out is carrying a blower motor with copper windings, a control board, gas valve brass, and feet of control wire — plus the copper line set if there is a paired AC coming out too. The blower motor alone is worth pulling: copper-wound motors are a steady, honest stream.
Strip the motors, brass, and copper off every changeout before the unit leaves the job. Most installers keep a motor pile in the shop. When it builds up, call Sonny at 312-880-8174 — and do not let the old unit ride to the dump with its copper still inside.
No minimums — every bucket counts. On bigger lots, we handle the freight so you don't lift a finger.
Work to code. Hire a licensed pro where required. This guide explains how the work is done — it does not replace training, apprenticeship, or a license.