Installation Checklist: What to Expect When Replacing Your Old System with a 3.5 Ton AC and Furnace

🌟 Introduction: Why Knowing the Process Matters

Replacing an old HVAC system isn’t just a matter of swapping boxes — it’s a multi-step process that involves proper planning, safe removal, precise installation, and thorough testing.

A 3.5 ton AC with furnace combo is a big investment, and knowing exactly what to expect can save you from:

• Surprise costs

• Subpar performance

• Voided warranties

• Safety hazards

I’ve broken down the full journey from contractor selection to final inspection so you can be an informed homeowner from day one.

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📋 Pre-Installation Preparation

1. Choosing the Right Contractor

• Verify state licensing and insurance.

• Look for NATE-certified technicians.
  

• Get at least three written estimates for the same scope of work.

2. Manual J Load Calculation

Even though you’re targeting a 3.5 ton system, a good installer will run a Manual J load calculation to confirm it’s the right size.
Factors include:

• Square footage

• Insulation levels

• Window types and orientation

• Occupancy

• Local climate

📎 Reference: ACCA Manual J

3. Selecting Furnace Type

• Gas furnace for cold climates and lower heating bills where natural gas is available.

• Electric furnace for all-electric homes or mild climates.

4. Permits & Scheduling

• Most jurisdictions require a mechanical permit and sometimes an electrical or gas permit.

• Coordinate with the contractor to schedule installation when you can be home.

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🛠 Removal of Old Equipment

1. Refrigerant Recovery

• Federal law requires EPA-certified recovery of refrigerant from your old AC system.

• Your contractor must have Section 608 certification.

📎 Reference: EPA Refrigerant Regulations

2. Dismantling the Furnace

• Disconnect gas or electric supply.

• Remove flue pipe (gas models).

• Disconnect from plenum and return duct.

3. Ductwork Assessment

• Contractor should inspect for leaks, insulation gaps, and sizing issues.

• Some ducts may need resizing to handle the airflow of the new system.

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⚙ Installation Day – Step-by-Step

Here’s what happens on installation day for a 3.5 ton AC + furnace:

1. Delivery & Staging

• New furnace, evaporator coil, condenser, thermostat, and accessories are delivered.

• Tools, safety gear, and materials organized before starting.

2. Furnace & Coil Installation

• Furnace positioned to allow service access.

• Coil installed above (upflow) or below (downflow) furnace in the plenum.

• Proper sealing with mastic or UL-181 foil tape.

3. Outdoor Condenser Placement

• Pad leveled and vibration isolation pads used.

• Minimum clearance from walls and obstructions per manufacturer specs.

4. Refrigerant Line Set

• Replace old lines when possible.

• Properly insulated suction line to prevent condensation.

5. Electrical & Thermostat Connections

• Dedicated circuit for AC and furnace.

• Low-voltage control wiring connected to thermostat.

• Smart thermostat installed if included.

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🔍 Quality & Code Checks

Before startup, your contractor should run system integrity checks:

1. Leak Testing

   • Pressurize with nitrogen.

   • Use electronic leak detector.

2. Airflow Verification

   • Adjust blower speed for correct CFM (cubic feet per minute).

   • Check static pressure against manufacturer’s max rating.

3. Combustion Safety (Gas Furnaces)

   • Inspect gas connections for leaks.

   • Verify venting slope and termination.

   • Test CO levels.

4. Electrical Safety

   • Confirm breaker sizing and wire gauge.

   • Tighten all lugs and connections.

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🌡 System Startup & Testing

1. Refrigerant Charging

• Charge by superheat (fixed orifice) or subcooling (TXV) per manufacturer specs.

• Verify correct refrigerant type (R-410A or R-32 in new systems).

2. Temperature Split

• Measure return and supply air temperature.

• Target: 16–22°F difference for cooling.

3. SEER2 Performance Verification

• Check amperage draw and operating pressures to ensure efficiency rating is met.

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📄 Paperwork & Post-Install Steps

1. Warranty Registration

• Many manufacturers require online registration within 60–90 days.

2. Permit Inspection

• Local inspector checks installation for code compliance.

3. Homeowner Orientation

• How to change filters.

• Thermostat programming.

• Maintenance schedule.

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✅ Mark’s Pro Tips for a Smooth Install

• Be Present: Stay home during installation to approve major decisions.

• Document Everything: Take photos of new equipment model numbers and serial numbers.

• Ask for a Copy of Load Calculation: It’s your proof that system size is correct.

• Request a Commissioning Report: Includes test results and refrigerant readings.

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📎 Verified External References

1. Energy.gov – HVAC Installation Best Practices

2. EPA Section 608 Requirements

3. ACCA – Quality Installation Standard

4. International Mechanical Code

5. OSHA – HVAC Safety Guidelines

6. Energy Star – HVAC Installation Checklist

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✅ Conclusion

A 3.5 ton AC with furnace replacement is one of the most important home upgrades you’ll make. When you understand the installation process, you can:

• Hold contractors accountable

• Avoid costly mistakes

• Ensure your system runs at peak efficiency for years

If you follow this checklist, your installation day will be smooth, safe, and set up for long-term comfort.

In the next topic we will read more about: How to Maintain Your 3.5 Ton AC and Furnace for Maximum Lifespan