Pre-selection: confirm what your home can support
Start by looking at what’s already in your house. If you have natural gas, a gas furnace is usually cheaper to run in cold climates. If you’re on electric only or propane, an electric furnace or heat pump can be smarter. Check your utility bills and service availability (natural gas vs. propane delivery). Walk the potential install area—a 30" x 30" level working space is a good minimum, with room to remove panels and swap filters. If the unit sits in a garage, plan for 18 inches of clearance above the floor. Think about maintenance access (filter changes, blower access, control board service). Finally, review your ductwork condition; if you’re replacing an older gas unit with electric—or vice versa—you may need transitions to match the new cabinet. If you’re starting from scratch or room-by-room, consider Ductless Mini Splits.
Electrical panel check (electric furnace specifics)
Electric furnaces draw real power. Most need a dedicated 240-V circuit at 60–80 amps. Pop your panel cover and confirm open breaker space and total ampacity. If your main service is already crowded, you may need a panel upgrade or subpanel before installation. Use the proper wire gauge (often 6–8 AWG for higher amperage runs), and install a lockable disconnect within sight of the unit. Ensure solid grounding and bonding and verify that other big loads (range, EV charger, electric water heater) won’t push you beyond your service rating when the strips kick on. Electric units don’t require venting or combustion air, which simplifies the mechanical work—but the electrical has to be right the first time. If you’re pairing an electric furnace with a new air handler or coil, shop compatible Air Handlers.
Gas line basics (gas furnace specifics)
For gas furnaces, verify gas service to the home and the existing gas line capacity. A typical furnace requires a properly sized supply based on the unit’s BTU input and pipe length (including fittings). Undersized lines cause poor combustion, nuisance lockouts, or noisy burners. If you’re adding a furnace to a system that already feeds a range, dryer, or water heater, recalculations are needed so everything has enough fuel. If you use propane, confirm tank size, regulator settings, and placement rules. Before startup, your installer should pressure test the line and perform leak detection with approved methods. Add a manual shutoff valve upstream of the unit and keep the flex connector short and protected. For venting, high-efficiency furnaces often use PVC with correct slope and termination clearances; standard-efficiency units typically use metal vent tied to a chimney or roof cap.
Space, clearances, and ventilation
Your installer needs room to work safely and service the equipment later. Plan at least a 30" x 30" clear workspace at the front of the unit, with vertical clearance to access filters, blower, and electronics. Keep combustibles away per code and the manufacturer’s instructions. If the furnace sits in a closet or mechanical room, provide adequate return air and combustion air (for gas) and confirm the door size and swing give you access for heat exchanger inspection. In garages, mount the furnace burner and ignition source 18 inches above the floor. Keep vent terminations away from windows/doors and at the required heights. A simple rule for gas combustion air: 1 square inch per 4,000 BTU/hr when using indoor air—your installer will size this precisely. Electric furnaces skip venting and combustion air entirely, but still need proper return/supply duct sizing and a straight 24 inches of discharge duct when possible for smooth airflow.
Get the size right (Manual J, BTUs, and home factors)
Skipping load calcs is how people end up with cold rooms or short cycling. Have a pro run a Manual J heat load using your climate zone, square footage, insulation, window type, ceiling height, and air sealing. Rough rules of thumb (30–60 BTU/ft²) only start the conversation. Typical ranges by total area: <1,200 ft²: 40–60k BTU, 1,200–2,000 ft²: 60–80k BTU, 2,000–3,000 ft²: 80–115k BTU, and >3,000 ft²: 115k+ BTU—but let the Manual J decide. Also consider duct static pressure, coil selection, and blower tables so the furnace actually delivers the target CFM to each room. If you’re unsure, our Sizing Guide can help you translate a room-by-room load into the right furnace and accessories.
Efficiency, performance, and operating cost
Gas furnaces range from 80% AFUE up to 98% for high-efficiency models. They heat fast and shine in cold climates, often with lower monthly costs where natural gas is affordable. Typical lifespan is 15–20 years. Electric furnaces convert electricity to heat at nearly 100% efficiency, run quiet, and are simple to install, with long lifespans of 20–30 years—but they can cost more to operate in cold regions because resistance heat is energy-intensive. If your winters are moderate, or you want lower electric bills, consider a heat pump paired with an electric furnace or air handler. Browse R-32 packaged and split options here: R-32 AC & Gas Furnaces. A dual-fuel setup (heat pump + gas backup) can optimize comfort and cost.
Permits and inspections (don’t skip this)
Most jurisdictions require permits before you start. For gas furnaces, you typically need gas, electrical, and building permits. For electric furnaces, you’ll need electrical and building permits. Expect inspections at key stages: rough-in (if lines or wiring are new), and final. Inspectors check clearances, wiring, venting, combustion air, and labeling. They’ll also look for proper disconnects, circuit breakers, and gas shutoff valves. Pulling permits protects you and keeps home insurance clean if there’s ever a claim. Keep your manufacturer instructions on-site inspectors reference them as part of code compliance. If you need help understanding requirements, our Help Center page can point you in the right direction.
Safety requirements: gas vs. electric
For gas furnaces, install carbon monoxide (CO) detectors near bedrooms (within about 5 meters) and at each floor served by the furnace. Verify combustion air sizing and keep the burner area clean. Plan a proper venting path with correct slope and termination clearances to prevent CO buildup. Pressure test the gas line, leak-check all joints, and confirm manifold pressure at startup. For electric furnaces, focus on grounding, breaker sizing (often 60–80A), and proper wire gauge. Both systems need smoke detectors per local rules. Label the service disconnect, and show everyone in the home how to use the emergency shutoff. Keep a copy of the warranty and register the unit promptly—missing registration can shorten coverage. You’ll find add-ons like filter racks, pads, and condensate solutions in our Accessories.
Infrastructure prep: gas venting, lines, and electric wiring
Gas: Use approved materials (black steel, CSST, or copper where permitted). Size the gas line to the furnace BTU input and longest run. Add a shutoff valve and drip leg as required. For high-efficiency units, run PVC intake/exhaust with the right pitch back to the unit to drain condensate; for conventional units, use metal vents to a chimney or roof cap.
Electric: Pull a dedicated 240-V circuit with the rated ampacity, install a within-sight disconnect, and confirm neutral/ground separation in subpanels. Avoid bundling high-amp furnace conductors with low-voltage thermostat wire in the same conduit. Electric means no venting, fewer wall penetrations, and typically a cleaner install. Stock up on line sets and related parts if you’re pairing cooling equipment.
Site prep: workspace, shutoffs, and ductwork
Clear a straight path for removing the old unit and bringing in the new one protect floors and nearby finishes with drop cloths. Shut off power at the main panel and gas at the appliance valve (if present). If you’re removing a boiler, cap water lines safely. Label and photograph old connections so you can compare during startup. Inspect ductwork for leaks, damage, and blockages. Seal joints with mastic or UL-listed tape, and aim for at least 24 inches of straight discharge duct before the first elbow to reduce turbulence. If switching fuel types or cabinet sizes, plan transition pieces and verify the return path is sized for new airflow. If you don’t have ducts or space is tight, consider Through-the-Wall Heat Pumps.
Hiring pros, timeline, and final checks
Choose a contractor with licenses for the work being done (gas and/or electrical), plus insurance and bonding. Check references and local ratings. A straightforward replacement usually takes 1–2 days; plan for temporary heat if it’s mid-winter. Schedule installs during mild weather if you can it lowers stress when the system is down. Coordinate utility connections and inspection dates early. Before signing off, walk the job: verify model/serials, breaker sizes, gas shutoff, drain routing, filter access, thermostat operation, and airflow to the farthest rooms. Register the warranty and keep a digital copy. If you need a quick estimate without a site visit, try our Quote by Photo.
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FAQs
How do I know if my panel can handle an electric furnace?
Add up existing large loads and check your main service rating. Most electric furnaces need a 240-V, 60–80A dedicated circuit. If you’re tight on capacity, ask an electrician about a panel upgrade.
Is a heat pump better than an electric furnace?
Often, yes especially in moderate climates. A heat pump moves heat instead of making it, so it’s usually cheaper to run. See our R-32 Heat Pump Systems.
Do I need CO detectors with an electric furnace?
No combustion, no CO—so not required for electric units. For gas furnaces, install CO detectors near bedrooms and on each level served by the system.
How long does installation take?
A typical straight swap is 1–2 days. Complex jobs (panel upgrades, new gas lines, or duct changes) can take longer. If timing is tight, consider scheduling during milder weather.
Can I use my existing ducts?
Usually, yes—but verify size and condition. Seal leaks, fix crushed runs, and confirm airflow. Major fuel or cabinet changes may need transitions or return upgrades.
Do gas furnaces require outside air?
They need proper combustion air. Rules vary, but a common guideline is 1 sq in per 4,000 BTU/hr when pulling from indoor spaces. High-efficiency sealed units often use dedicated intake piping.
What maintenance is essential after install?
Change filters every 1–3 months, keep the area clear, and schedule yearly service to check combustion (gas), wiring (electric), and airflow. Browse maintenance items in Accessories.
What if I don’t have room for a furnace?
Look at packaged or through-the-wall options, or go ductless. Start here Ductless Mini Splits.