A furnace installation is a $5,000 to $12,000 investment that will operate for 15 to 25 years. The difference between an installation that lasts the full warranty period without problems and one that leaks combustion gases, short-cycles, or fails prematurely is a checklist of roughly two dozen items — most of which take the installer 5 to 10 minutes each to verify, and most of which a homeowner can confirm by looking, listening, and asking the right questions on installation day. This checklist covers the pre-installation preparation, the installation-day work, and the post-installation startup verification.
Pre-Installation Checklist: Before the Crew Arrives
Pre-Install Verification
- Manual J load calculation completed. The U.S. Department of Energy advises that homeowners should “have a heating contractor size your furnace” (energy.gov). A Manual J calculation is the only accepted method. If the contractor quotes a furnace size without performing one — by using square footage or the existing furnace’s BTU rating — the sizing is a guess.
- Permit pulled. Most jurisdictions require a mechanical permit for furnace replacement. The permit ensures an inspector will verify the installation. The contractor should pull the permit, not the homeowner.
- Existing ductwork assessed for size and condition. A new high-efficiency furnace may require more airflow than the old furnace. A duct system that was marginal with the old 80% furnace may be undersized for the new 95% furnace.
- Gas line capacity verified. A new furnace may require a larger gas line than the old furnace, particularly if other gas appliances were added since the original furnace was installed.
- Electrical circuit verified. The new furnace requires a dedicated 120-volt circuit. An ECM blower motor may draw less running current but requires clean power — a circuit shared with a freezer or a sump pump is not acceptable.
Installation-Day Checklist: What the Crew Should Do
Equipment and Placement
- Correct furnace model and size delivered. Verify the model number on the box matches the quote before installation begins.
- Furnace is level. A furnace that is not level produces uneven condensate drainage in the heat exchanger, uneven burner flame patterns, and vibration noise. Check with a bubble level on top of the cabinet.
- Adequate clearances maintained. The furnace requires at least 24 to 30 inches of clearance in front for service access. Side clearances are specified in the installation manual and vary by model.
Venting (Condensing Furnace)
- PVC pipe is Schedule 40 solid-core, not cellular-core. Cellular-core PVC is for drains, not exhaust vents.
- Exhaust pipe slopes toward furnace at 1/4 inch per foot. Condensate must drain back to the furnace, not pool in the pipe.
- Intake and exhaust terminate on same wall, same height. Both pipes must be in the same pressure zone to prevent wind-induced pressure switch trips.
- Termination clearances met. Exhaust at least 12 inches from any operable window or door, 3 feet from a gas meter, 12 inches above anticipated snow line.
- Pipes supported every 4 feet. Unsupported PVC sags, pools condensate, and freezes.
Condensate Drain
- Drain line is trapped. The condensate drain must have a trap to prevent combustion gases from escaping through the drain. Most condensing furnaces have an internal trap; verify it is installed or connected per the manual.
- Drain line slopes downward to the drain or pump. No sagging sections, no uphill runs.
- Condensate pump (if used) is level and plugged in. Test the pump by pouring water into the reservoir.
Gas Connection
- Gas shutoff valve installed within 6 feet of the furnace. Required by code in the same room.
- Drip leg (sediment trap) installed on the gas line. A 3-inch nipple with a cap that catches pipe scale and debris before it enters the gas valve.
- All gas connections leak-tested. The installer should apply leak-detection solution to every joint and show you that no bubbles appear.
Electrical
- Furnace has a dedicated service switch within sight. A light-switch-style shutoff on or near the furnace, required by code.
- Furnace is properly grounded. An ungrounded furnace can cause erratic ECM blower behavior and control board failures.
- Thermostat wiring is correct and terminals are tight. Loose thermostat connections cause intermittent operation that is difficult to diagnose.
Duct Connections
- Supply and return plenums sealed with mastic or foil tape. Not cloth duct tape — it dries and falls off.
- Filter rack installed with a filter in place. The furnace should never be operated without a filter, even during startup testing.
Post-Installation Startup Checklist: Verify the System Works Correctly
Startup Testing
- Gas pressure checked at the manifold. The installer should measure the manifold gas pressure with a manometer and confirm it matches the furnace rating plate — typically 3.5 inches of water column for natural gas.
- Temperature rise measured. The difference between the return air temperature and the supply air temperature should match the range on the furnace rating plate — typically 35°F to 65°F. Too low = too much airflow. Too high = not enough airflow or a problem with the gas pressure.
- Combustion analysis performed. A combustion analyzer measures the oxygen, carbon monoxide, and stack temperature in the exhaust. A properly tuned condensing furnace should produce less than 50 PPM of CO in the exhaust, typically single digits. The DOE recommends that a technician “perform a combustion-efficiency test” when considering replacement (energy.gov).
- All safety controls tested. The installer should test the limit switch by temporarily blocking the return air, the pressure switch by blocking the intake pipe, and the flame sensor by shutting off the gas during operation.
- Thermostat cycles the furnace on and off correctly. Test heating mode, fan-only mode, and — if applicable — two-stage or modulating operation.
Final Walkthrough
- Installer demonstrates operation to the homeowner. How to change the filter, how to read the thermostat, what normal sounds are, what error codes look like.
- Warranty registration completed. Many manufacturers require registration within 60 to 90 days of installation for the full warranty to apply. The contractor should handle this.
- Copy of the permit, the Manual J calculation, and the combustion analysis provided. These documents are your proof that the installation was done correctly.
Red Flags: What to Watch For on Installation Day
- Contractor cannot provide a Manual J calculation when asked.
- Old flue pipe or chimney is left in place with no explanation of how it will be sealed or removed.
- Exhaust pipe is metal instead of PVC on a high-efficiency furnace.
- No combustion analysis is performed.
- Contractor refuses to provide a copy of the permit.
- Furnace cycles on and off rapidly (less than 5 minutes per cycle) during testing.
- Gas odor is present after installation. Stop the crew immediately and call the gas company.
CO detector requirement: The DOE advises that “every home have a working carbon monoxide alarm” (energy.gov). Before the furnace is installed, verify that there is a working CO detector on every floor of the house, including the basement where the furnace is located. If the installer does not ask about CO detectors, install them yourself. They cost $20 to $40 and are the last line of defense against a venting failure.
FAQ: Common Questions About Furnace Installation
How long should a furnace installation take?
A standard furnace replacement — removing the old furnace and installing a new one in the same location, reusing the existing ductwork — takes 4 to 8 hours for a crew of 2. A more complex installation — relocating the furnace, installing new ductwork, converting from an 80% to a 95% furnace that requires new venting — takes 1 to 2 days. An installation that is completed in 2 hours was rushed, and items on this checklist were almost certainly skipped.
What should the temperature rise be after installation?
The temperature rise — the difference between the return air entering the furnace and the supply air leaving it — should be within the range printed on the furnace rating plate. A typical range is 35°F to 65°F. If the return air is 68°F, the supply air should be 103°F to 133°F. A temperature rise below the range means the blower speed is too high — the air is moving too fast and is not being heated adequately. A temperature rise above the range means the blower speed is too low or the gas pressure is too high — the air is being overheated, which can crack the heat exchanger.
The 24 Items on This Checklist Are the Difference Between an Installation and a Correct Installation
A furnace installation is a mechanical and combustion safety project, not a plug-and-play appliance swap. The 24 items on this checklist cover the permit, the sizing, the venting, the condensate drain, the gas connection, the electrical connections, the duct sealing, and the startup verification. Most of these items are code requirements. All of them are good practice.
Print this checklist. Hand it to the contractor on installation day. If they roll their eyes, they were not going to do half of these things. If they nod and work through the list, they are a professional. The checklist is the filter. The contractor’s reaction to the checklist tells you everything you need to know.
