Furnace Installation Checklist: What Your Contractor Should Do

Buying the right furnace is only half the equation. A bad installation can rob you of 20–30% of your system's efficiency, void your warranty, and create safety hazards that put your family at risk. The problem? Most homeowners have no idea what a proper installation looks like, so they can't tell the difference between a quality job and a hack job.

At Furnace Direct, we sell Goodman furnaces at factory-direct pricing to homeowners who hire their own installers. That means our customers need to know exactly what to expect — and what to demand — from their HVAC contractor. Here's the complete installation checklist.

Before Installation Begins

1. Manual J Load Calculation

This is the single most important step, and the one most contractors skip. A Manual J calculation determines exactly how many BTUs your home needs based on square footage, insulation levels, window types, climate zone, and other factors. Without it, your contractor is guessing at furnace size.

• Oversized furnace: Short-cycles (turns on and off rapidly), creates hot and cold spots, wastes gas, and wears out faster
• Undersized furnace: Runs constantly during cold snaps, can't maintain temperature, higher bills
• Right-sized furnace: Runs in longer, more efficient cycles, even temperatures throughout the home

If your contractor quotes a furnace size without performing a load calculation — or says "we'll just match what you have" — that's a red flag. The previous furnace may have been wrong too.

2. Ductwork Assessment

Your installer should inspect existing ductwork for leaks, damage, improper sizing, and disconnected runs. Even a perfectly sized furnace can't overcome duct problems. Key things to check:

• Visible gaps or disconnections at joints
• Crushed or kinked flex duct
• Missing or damaged insulation on ducts in unconditioned spaces (attic, crawlspace)
• Adequate return air pathways
• Static pressure measurement (should be under 0.5" WC for most systems)

3. Gas Line Sizing Verification

High-efficiency furnaces may have different gas pressure requirements than your old unit. Your installer should verify that the gas line diameter and pressure are adequate for the new furnace's input BTU rating. An undersized gas line can cause poor performance and premature failure.

4. Electrical Requirements Check

Verify the existing electrical circuit can handle the new furnace. While most furnaces use a standard 120V/15A circuit, high-efficiency models with ECM blower motors may have different requirements. The circuit should be dedicated (not shared with other equipment).

During Installation

5. Proper Unit Placement and Clearances

Your furnace needs specific clearances for safety and maintenance access. Manufacturer specifications typically require:

• Minimum 1" clearance on sides (3" recommended for service access)
• 24–30" clearance in front for filter access and service
• Proper clearance from combustible materials (varies by model)
• Level installation on a solid surface

6. Venting Installation

This is where safety is critical. Improper venting can lead to carbon monoxide poisoning.

For 90%+ AFUE furnaces in Minnesota, the exhaust termination must be high enough above expected snow levels. A buried exhaust pipe is a carbon monoxide risk. Most codes require a minimum 12" above expected snow line.

7. Condensate Drain Installation (90%+ AFUE Only)

High-efficiency furnaces produce condensate — acidic water that must be properly drained. Your installer should:

• Run a condensate drain to a floor drain, utility sink, or condensate pump
• Use proper PVC or CPVC piping (not copper, which the acidic condensate will corrode)
• Install a trap in the drain line per manufacturer specifications
• Ensure the drain has adequate slope and won't freeze in unheated areas
• Consider a condensate neutralizer kit if required by local code

8. Gas Line Connection

The gas connection must use approved materials (typically black iron pipe or CSST with proper bonding), include a manual shutoff valve within 6 feet of the furnace, include a drip leg (sediment trap) to catch debris, be tested for leaks with a pressure test or approved leak detection solution, and meet all local gas code requirements.

9. Thermostat Wiring

Modern high-efficiency furnaces often require more thermostat wires than older models. If your existing thermostat cable doesn't have enough conductors, your installer needs to run new wire. A two-stage furnace with an ECM blower typically needs at least 5 conductors (R, W1, W2, G, Y, C). A communicating system may need a proprietary cable.

10. Filter Access and Rack

The installer should ensure easy filter access. A properly installed filter rack should be airtight (no air bypassing around the filter), sized for the recommended filter dimensions, accessible without tools, and oriented correctly for airflow direction.

After Installation: Commissioning

11. Combustion Analysis

A professional installer should perform a combustion analysis after the furnace is running. This involves inserting a probe into the flue to measure:

• CO (carbon monoxide) levels: Should be under 100 ppm in the flue (under 25 ppm is ideal)
• CO2 levels: Should match manufacturer specifications (typically 8.5–9.5% for natural gas)
• Stack temperature: Verifies the heat exchanger is working correctly
• Gas pressure: Both manifold and supply pressure should match specs

If your contractor doesn't own a combustion analyzer, that tells you something about their quality standards.

12. Temperature Rise Verification

The installer should measure the temperature difference between the return air and supply air. This "temperature rise" must fall within the range printed on the furnace's data plate (typically 35–65°F for gas furnaces). If it's outside this range, something is wrong — incorrect gas pressure, wrong blower speed, or ductwork issues.

13. Static Pressure Check

Total external static pressure should be measured at the supply and return plenums. For most residential systems, total static pressure should be below 0.5" WC. High static pressure means the ductwork is restricting airflow, which reduces efficiency and can damage the blower motor over time.

14. Safety Switch Testing

The installer should verify that all safety switches are functioning:

• Pressure switch (inducer motor safety)
• High-limit switch (overheating protection)
• Flame rollout switch (heat exchanger safety)
• Door/blower compartment safety switch

15. Thermostat Operation Test

Run the system through a complete heating cycle from the thermostat. Verify proper staging (if two-stage or modulating), correct fan delays on startup and shutdown, proper operation in all modes (heat, cool, fan only), and emergency heat operation (if applicable for dual fuel).

Documentation You Should Receive

Before your installer leaves, you should have:

• Completed permit inspection (or scheduled date)
• Warranty registration confirmation
• Equipment manuals and data sheets
• Combustion analysis report
• Written record of static pressure, temperature rise, and gas pressure readings
• Filter size and replacement schedule
• Invoice with equipment model and serial numbers

Common Installation Shortcuts to Watch For

These are the corners most often cut on furnace installations:

1. No load calculation: "We'll just put in the same size" is not engineering — it's laziness
2. Reusing old flue pipe: When upgrading from 80% to 96% AFUE, the old metal flue pipe must be replaced with PVC. If they reuse the old metal flue, the acidic condensate will corrode it
3. No combustion analysis: If they don't test it, they don't know if it's running safely
4. Skipping the condensate drain trap: Without a trap, exhaust gases can escape through the drain line
5. Not adjusting gas pressure: Default factory gas pressure settings may not match your home's gas supply. Failure to adjust can cause efficiency loss or flame problems
6. Ignoring static pressure: If the ductwork restricts airflow, the furnace will overheat and short-cycle — but many installers never measure it

The Furnace Direct Advantage: Equipment Without the Markup

When you buy your furnace from Furnace Direct, you get the exact same Goodman equipment that contractors install — at factory-direct pricing. You choose your own installer and keep the cost of equipment and labor separate and transparent.

A typical contractor bundles equipment and labor into one inflated price, making it impossible to know what you're paying for the furnace versus the install. With Furnace Direct, you see the real equipment cost, and you can use this checklist to ensure your installer does the job right.

Same-day delivery available in the Twin Cities metro for orders before 3 PM CT. Full factory warranty included on every unit.