Burner Mechanics: Getting the Flame Right (and Why It Matters)
Gas enters each burner through the orifice at a set manifold pressure (commonly ~3.5" w.c. for natural gas). Venturi action pulls in primary air, mixing with gas before the flame ports; secondary air finishes combustion at the flame. A stable blue flame with defined cones indicates near-stoichiometric combustion and minimal CO formation. Lazy yellow flames point to restricted air or contaminated burners; lifting or roaring flames suggest excess primary air or high pressure.
After cleaning, verify manifold pressure and clock the meter on a call for heat to confirm actual input matches the nameplate.
Explore compatible units in our gas furnaces.
Electronic Ignition: Intermittent Pilot vs. Hot Surface Igniter
Modern furnaces replaced constant pilots with electronic ignition to cut standby losses and improve reliability.
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Intermittent Pilot (IP): A spark lights a small pilot only when there’s a heat call; once flame is proven, the main valve opens.
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Hot Surface Igniter (HSI): A silicon carbide or silicon nitride element heats to ~1,200–1,400°C and directly lights the main burners.
Both rely on flame rectification (microamp DC current through the flame) to prove ignition. Weak signal? Expect nuisance lockouts.
Quick visual
Call for heat → Inducer → Pressure switch proves → Ignition (IP or HSI) → Flame proven → Main gas
Measure flame sense current in series; < 1–2 µA typically signals grounding or rod-to-flame alignment issues.
See matched thermostats and parts in Accessories.
Flue & Venting: Categories, Draft, and Condensate Management
Flue design safely moves CO, water vapor, and trace byproducts outside. Most 80% AFUE units are Category I (negative pressure, non-condensing), vented into lined chimneys or B-vent. High-efficiency (90%+) units are Category IV (positive pressure, condensing), vented in PVC/CPVC/PP with a sealed combustion path and condensate drainage.
Quick visual
Cat I: HX → Draft hood/Inducer → B-Vent ↑
Cat IV: Sealed HX → PVC vent → Condensate trap → Drain
On condensing models, slope intake/vent back to the furnace ¼" per foot to manage condensate; trap, neutralize if required, and heat-tape in unconditioned spaces.
Considering packaged options? Review gas/electric and heat-pump package units.
Heat Exchangers, AFUE, and What “Efficiency” Really Means
AFUE expresses seasonal efficiency: 80% units exhaust hotter flue gases; 90–98% condensing units extract latent heat from water vapor via a secondary heat exchanger. The more heat you pull out, the cooler—and wetter—the exhaust becomes.
Quick visual
Primary HX (sensible heat) → Secondary HX (condensing/latent) → Cooler flue gas
Use a combustion analyzer to verify O₂/CO₂ and CO at steady state; adjust input and check for recirculation. AFUE is the label; your readings tell you how this install is performing.
Compare high-efficiency furnaces within our R-32 AC & Gas Furnace bundles.
Controls & Safeties: The Chain That Keeps You Safe
A standard sequence: thermostat call → IFC runs inducer → pressure switch proves draft → ignition → flame sensor proves → blower delay on. Safeties include high-limit (overheat), rollout (flame outside burner), auxiliary limits, and condensate overflow. Any open breaks the chain until reset or fault clears.
Quick visual (simplified)
Tstat → Inducer → Pressure switch ✔ → Ignition → Flame sensor ✔ → Blower
↑ HI-LIMIT / ROLLOUT / AUX LIMITS can open here
Log fault codes before cycling power. Intermittent pressure-switch trips during high winds? Add a properly sized concentric termination or wind-resistant caps per OEM.
Need help choosing controls? Visit our Help Center.
Airflow, Ducts, and Comfort: Avoiding Dryness and Hot Spots
Gas furnaces can deliver high supply temps (110–140°F), which magnifies duct deficiencies. Oversized equipment short-cycles, creating hot/cold swings and dry air complaints. Target ~400 CFM per ton of cooling and the blower airflow recommended by the furnace OEM for heating; verify external static pressure (ESP) against the blower table.
Quick visual
Return → Filter → Blower → HX → Supply → Branches → Rooms (balance dampers matter)
A high-MERV filter in a tight return can spike ESP and starve airflow.
Run a quick Manual J before selecting capacity. If ducts are marginal, consider lower-BTU, higher-AFUE with longer run times for even heat, or pair with zoning.
For ductless rooms or additions, see mini-splits.
Operating Cost & Climate Fit: Gas vs. Electric Heat Pumps
Pros of gas: rapid warm-up, high supply temps, strong performance in subfreezing climates, often lower per-BTU fuel cost. Trade-offs: combustion safety, more maintenance points, and gas-price exposure. In mild climates, variable-speed heat pumps can beat gas on operating cost, especially with favorable electricity rates. In cold regions, dual-fuel (heat pump above balance point, gas below) can optimize comfort and dollars.
Quick visual
Warmer climates → Heat pump
Colder climates → Gas furnace
Transitional → Dual-fuel
Don’t compare nameplate AFUE to a heat pump’s HSPF2 apples-to-apples; run a simple energy cost model for your rates.
Explore R-32 heat pump systems if electrification incentives exist in your area.
Maintenance That Pays Back: An Annual, No-Guess Checklist
Plan on annual service to keep combustion clean and safe:
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Inspect and clean burners, flame sensor, and HSI; verify microamps.
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Check manifold pressure; clock the meter for input.
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Test safeties (limits, rollout, pressure switch) under real heat load.
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Analyze combustion and draft; verify CO levels.
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Clear condensate traps/hoses; treat or replace tubing prone to kinks.
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Confirm blower wheel cleanliness and set proper heat-speed tap.
Keep common spares on hand (HSI, flame rod, fuses) and log readings each year to spot drift.
Order maintenance parts under Accessories when upgrading connected systems.
Field Troubleshooting: From “No Heat” to Root Cause
When a furnace won’t heat, follow the sequence and find what failed—not just the part.
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No inducer: check line voltage, door switch, and board output.
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Inducer runs but no ignition: confirm pressure switch closes; inspect venting for blockage, water in hoses, or termination icing.
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Ignites then drops: measure flame sense µA; clean/realign sensor, verify ground.
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High-limit trips: assess ESP, filter, blower speed, coil cleanliness, and gas input.
Decision sketch
Call? → Inducer? → Pressure switch? → Ignition? → Flame proven? → Blower on?
(stop where NO appears and diagnose there)
Photograph fault codes and data plate, then request a fast Quote by Photo if replacement is likely.
When a Gas Furnace Is the Right Call (and When It Isn’t)
Choose gas when you need high heat output, quick recovery, and proven performance in deep cold. Expect 10–20+ years with proper care. Trade-offs include combustion safety management, routine service, and potential dry air/hot spots if the system is oversized or ducts are undersized.
For rooftops or tight footprints, review packaged units and residential packaged systems.
