Fuel Path Fundamentals Gas Line vs High-Amp Circuit
A gas furnace’s lifeline is its fuel piping. Most homes tap natural gas at 2–5 psi, stepped down to 0.5 psi at the appliance. Sizing the branch correctly prevents flame instability and nuisance lockouts. Add leak testing, drip legs, and a shut-off within 6 ft—all inspected under NFPA 54 rules.
Electric furnaces skip combustion completely. Instead, they demand a 240-volt supply sized for 125 % of continuous load (NEC 210.20). A 15 kW cabinet can pull 63 A, often triggering a sub-panel upgrade. Route conductors in EMT or MC cable, derate for attic temps, and keep the whip length under 6 ft to avoid voltage drop.
When upgrading service, add margin for future heat pumps such as the dual-fuel packaged units for seamless electrification later on.
Venting Realities Metal Stacks to No-Vent Freedom
Combustion creates CO, NOₓ, and water vapor that must exit the building envelope. Standard-efficiency furnaces rely on 4-in. B-vent; 90 %+ units use 2–3 in. PVC with a condensate trap. Keep rise-run ratios within label specs and terminate 12 in. above the snow line.
Electric furnaces remove the entire venting chapter. No flue, no clearance to windows, zero roof penetrations—an advantage in tight urban lots or historic homes.
On retrofit jobs, inspect existing chimney liners—many are undersized once the water heater is the lone draft appliance.
Combustion Air & Placement Avoiding Code Red Flags
Gas furnaces need two openings (upper/lower) or a direct intake pipe, supplying 1 in² per 4,000 Btu/h. Skip this and you invite flame roll-out and cracked exchangers. Bedrooms and closets are off-limits unless they open to an engineered combustion-air plenum.
Electric models are flexible: attic, crawl, or stacked in a utility closet. Just maintain 30 × 36 in. service clearance per NFPA 90B. Use the Design Center to verify cabinet dimensions before framing.
Wiring Nuances 120 V Controls vs 240 V Heat Banks
Gas furnaces run a 120 V blower motor and a 24 V control circuit. Confirm neutral integrity; a floating neutral can fry the IFC board. Low-voltage Class 2 wiring (<30 V) must stay 1 in. clear of line voltage conductors unless in listed cable.
Electric furnaces swap complexity for amperage. Sequencers stagger heating elements to reduce in-rush, but branch conductors still run hot—use 90 °C rated copper. Bond the cabinet and install a lockable disconnect within sight. For Wi-Fi stats, tap a 24 V transformer or pair with a line-set accessory kit that includes control wiring.
Permitting & Inspection Paperwork That Protects
Most jurisdictions issue separate permits for fuel gas and mechanical work. Gas piping drawings, vent sizing charts, and a combustion analysis report may be required at final inspection.
Electric furnace permits focus on load calcs and breaker sizing. Expect the inspector to meg-ohm test conductors and check torque specs on lugs. Always keep the UL- or ETL-listed installation manual on-site; inspectors reference it line by line.
Need local forms? Our Help Center links to permit checklists for all 50 states.
Cost & Time on the Job What Pros Quote vs DIY Reality
Adding a gas drop, vent stack, and combustion air chase typically doubles labor hours. Material adds up: black iron, shut-offs, roof jacks, and a condensate pump for 90 % models. Expect $2,000–$3,500 in install cost vs. $600–$1,500 for electric, excluding the furnace itself.
DIYers often underestimate panel upgrades. A 60 A feeder may require a new 200 A service budget another $1,500+. Use our Sizing Guide to calculate total amp draw before buying.
Safety Layers Sensors, Switches & Circuit Breakers
Gas units rely on roll-out sensors, flame rectification, high-limit switches, and pressure proving. Test flame signal (>1.5 µA) and verify inducer draft at 0.10–0.50 in. w.c. Electric furnaces depend on thermal cut-outs and sequencer timing; a stuck sequencer can overheat coils in minutes.
Replace high-limit switches in pairs and log trip temperature. A pattern of low trip temps signals airflow issues check filters and duct sizing. Stock replacements from our accessories aisle.
Efficiency & Operating Cost Beyond the Install
Gas furnaces hit 80–97 % AFUE, but fuel price swings matter. At $1.50/therm vs. $0.13/kWh, electric heat can cost 2× more per delivered Btu. However, pair an electric furnace with a heat pump for shoulder seasons and you slash kWh use. Browse dual-fuel packaged systems for blended savings.
A 95 % AFUE gas unit rated at 60,000 Btu/h consumes ~0.63 therm per hour; the equivalent 15 kW electric furnace draws 15 kWh.
When to Choose Hybrid or Heat-Pump Alternatives
Electrification incentives plus rising gas costs make hybrids attractive. A gas furnace handles sub-freezing peaks; a heat pump covers mild hours at 300 % efficiency. Check out our R-32 heat-pump systems for low-GWP refrigerant options.
For apartments or additions, ductless mini-splits in the DIY lineup can eliminate furnace installs completely while still meeting code.
Field-Pro Tips for a Code-Clean, Trouble-Free Install
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Photograph every hidden joint—helps during inspection and future service.
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Label disconnects with heat-load and breaker size.
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Slope PVC vent ¼ in./ft. back to the furnace to keep condensate out of the termination.
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Torque gas unions after pressure test, not before—threads relax under pressure.
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Balance airflow to 400 CFM per ton; under-flow trips high-limit switches.
Need parts fast? Our Furnaces Collection ships same-day from three U.S. warehouses.
Ready for Inspection Day?
Still weighing breaker upgrades against venting runs, or unsure which permits apply? Chat with a seasoned tech upload photos through our Quote-by-Photo tool or call the design desk. We’ll help you pick the right furnace, size the feeder or gas pipe, and assemble a code-compliant parts list so you breeze through inspection the first time.
