Install a Gas Furnace

Set with clearances, vent per category (never mix), size + drip-leg the gas line and leak-check, set manifold pressure, then verify temperature rise and run a CO/combustion test.

Approx. 300 min

Before you start

•Furnace category and fuel known (Cat I induced-draft on metal/B-vent vs Cat IV condensing on sealed PVC; NG vs LP — LP needs the conversion kit and different manifold pressure).
•Gas supply, electrical, and the venting path are available and the load/airflow (CFM, blower tap) match the equipment.
•Combustion-air provisions identified for the space (confined vs unconfined; two-pipe sealed combustion if condensing).

Tools & materials

Manometer (inches W.C.) + manometer ports/adapter · Pipe wrenches, gas-rated thread sealant, drip-leg fittings · Electronic gas leak detector + leak-check solution · Combustion analyzer (O2/CO/CO-air-free, stack temp) · Vent pipe + fittings per category (B-vent or PVC/CPVC primer+cement), drill/screws

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Set & clearances

Set with clearances to combustibles and service access
Set the furnace level on a non-combustible base in the correct orientation (upflow/downflow/horizontal). Honor the nameplate clearances to combustibles and leave room to pull the blower and burner assembly for service.
Target: Clearances to combustibles per nameplate (often 0–6 in sides, more at the front); ≥ 24 in service access at the burner/blower
⚠ A downflow furnace requires the listed combustible-floor base — do not omit it.

Vent per category

Vent to the appliance category — never mix systems
Cat I (atmospheric/induced-draft, non-condensing) vents hot flue gas up a metal/B-vent with continuous upward slope and proper rise; size and common-vent it per the NFPA 54 tables. Cat IV (condensing) uses sealed, gasketed PVC/CPVC sloped back to the furnace to drain condensate, with the maker's max equivalent length. Do not vent a Cat IV into a Cat I metal vent or vice versa.
Target: Cat I: B-vent/metal, slope ≥ 1/4 in per ft UP, sized per NFPA 54. Cat IV: PVC/CPVC, sloped ~1/4 in per ft back to the unit, within the maker's max equivalent vent length
⚠ Improper venting is a CO/spillage hazard. Maintain listed termination clearances from windows, soffits, and grade/snow line.

Gas piping & leak check

Size the gas line and add a sediment/drip trap
Size the branch for the furnace input BTU and the run length per the NFPA 54 capacity tables (no starving the burner). Install a sediment trap (drip leg) at the appliance ahead of the gas valve to catch debris/condensate, plus a manual shutoff and an approved connector or hard pipe.
Target: Pipe sized for input BTU/hr at the run length (NFPA 54 table); drip leg + manual shutoff within reach of the valve
Leak-check every joint
Pressure/soap-bubble or electronic-detect every threaded joint after assembly. NEVER check for gas leaks with a flame.
Tools: Electronic gas leak detector, Leak-check solution
Target: Zero bubbles / no detector alarm at every joint at operating pressure
⚠ Never use a flame to find a gas leak. Ventilate and re-check after any correction.

Combustion & condensate

Provide combustion air
An open-combustion furnace in a confined space needs the required free-air openings (high and low) to the outdoors or adjacent space; a condensing furnace ideally pulls combustion air from outside through the second pipe (sealed combustion).
Target: Confined-space free-air openings per NFPA 54, or two-pipe sealed combustion for condensing units
Trap and route condensate (condensing units)
A Cat IV furnace makes acidic condensate. Install/prime the condensate trap, route it to a legal drain with slope, and add a neutralizer where required by code or local plumbing rules.
Target: Condensate trap primed, sloped to drain; neutralizer where required

Start-up & verify

Set the manifold gas pressure
With the burners firing, read the manifold pressure on the manometer and adjust the gas valve regulator to spec for the fuel. Confirm the supply (inlet) pressure is in range too, and clock the gas meter to verify the firing rate.
Tools: Manometer
Target: Manifold ~3.5 in W.C. on NG, ~10–11 in W.C. on LP (verify nameplate); clock the meter to the rated input
⚠ Convert and re-set the regulator before firing on LP — NG settings overfire on propane.
Verify temperature rise within the nameplate range
Measure supply and return air temperature with the burners stable; the rise must land inside the nameplate range. Too low = overfired or too much airflow; too high = restricted airflow or underfired — adjust the blower tap/airflow, not the gas, to land it.
Target: Supply − return ΔT inside the nameplate temperature-rise range (e.g. 40–70°F)
Run a CO / combustion analysis
Put the analyzer in the flue and confirm clean combustion — CO air-free in spec, O2/CO2 reasonable, and verify proper draft/no spillage at the draft hood or sealed termination.
Tools: Combustion analyzer
Target: CO air-free within the maker/industry limit (commonly < 100 ppm air-free), good draft, no spillage
⚠ Do not leave a furnace producing high CO or spilling flue gas in service — red-tag it.