Switching a furnace from natural gas to propane is more than swapping jets; it's a full fuel-system recalibration. The load (how many BTUs your home needs) does not change, but fuel energy density, orifice sizing, manifold pressure, and commissioning steps do. This guide distills field practices our techs use every week so HVAC pros and advanced homeowners can plan a safe, code-compliant conversion and nail the BTU math the first time.
Quick visual fuel fundamentals
Fuel |
BTU per cubic foot |
Typical manifold pressure (W.C.) |
Natural Gas |
1,030 |
~3.5” |
Propane (LP) |
2,516 |
~11.0” |
Energy Density & BTU Math: Why Propane “Feels” Hotter
Propane carries about 2.5× the BTU content per cubic foot compared to natural gas (2,516 vs. 1,030 BTU/ft³). In gallon terms, propane delivers ~91,452 BTU/gal, while natural gas is ~85,098 BTU/gal when normalized to equivalent volumes. Practically, this means you’ll move less propane to generate the same furnace input. For a 100,000 BTU/h input:
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Natural gas flow ≈ 100,000 ÷ 1,030 ≈ 97 ft³/h
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Propane flow ≈ 100,000 ÷ 2,516 ≈ 40 ft³/h
That ~2.4:1 reduction in gas volume is why the orifice and regulator strategy must change on conversion.
Keep your load calcs anchored to the house (Manual J). Fuel choice affects delivery, not the BTUs the structure needs. For equipment selection help, see our Sizing Guide.
Furnace Sizing: Same Load, Different Delivery
Your design load (say 100,000 BTU/h) doesn’t care what fuel you burn. The furnace still needs to deliver the same input to achieve the same output (accounting for AFUE). What changes is the fuel volume required and the distribution hardware needed to meter that fuel accurately at the burner.
Use established load calcs and climate bin data to confirm the target input range, then pick a furnace that matches. If you’re pairing with heat pumps or dual-fuel packages, coordinate balance points and staging so propane only carries during lower outdoor temps when it’s most cost-effective. Explore R-32 options for shoulder seasons: R-32 Heat Pumps.
Fuel Consumption & Metering: Planning for Storage and Runtime
Because propane is denser in energy, the gas volume through the manifold is lower for the same firing rate. For runtime planning: a 100,000 BTU/h input equates to roughly 1.09 gal of propane per hour at full fire (100,000 ÷ 91,452). Duty cycles and staging will bring this down in real operation, but this estimate helps right-size storage, delivery, and budget.
Runtime sketch
Input (BTU/h) ÷ 91,452 (BTU/gal) ≈ GPH of Propane
Example: 60,000 BTU/h ≈ 0.66 GPH
For multi-stage or modulating furnaces, log firing rates at each stage and confirm manifold pressure per stage post-conversion. Label data near the gas valve.
Need help mapping runtimes to equipment options? See Furnaces.
Orifices: The Heart of a Safe Conversion
Natural gas and propane require different orifice sizes due to pressure and BTU content. NG orifices are larger to pass more volume at ~3.5” W.C.; propane orifices are smaller (often around half the equivalent NG bore) because LP runs ~11” W.C. and packs more BTUs per unit volume. As an illustrative example from appliance burners: a 40,000 BTU cooktop might use about 0.125” (drill #35) for NG vs. 0.082” (drill #45) for LP—showing the directionally smaller LP jet.
Never “make it fit.” Verify with numbered twist drills, match the exact kit for your model and BTU rating, and apply the conversion label. Oversized LP orifices can cause overheating and dangerous fuel delivery.
For model-specific parts, visit Accessories.
Pressures & Regulators: From 3.5” to 11” W.C.
Expect three core changes:
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Gas valve spring (or conversion spring) suited to LP service
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Regulator adjustment for ~11.0” W.C. at the manifold (per OEM spec)
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Manifold verification at low and high fire (where applicable)
Commissioning visual
LP tank → 1st-stage reg → 2nd-stage reg → Shutoff → Sediment trap → Furnace gas valve (LP spring) → Burner orifices
↑ Check supply (~11" W.C.) ↑ Verify manifold per OEM
Use a calibrated manometer and capture readings at cold start, warm steady-state, and with multiple household loads active (clothes dryer, range). Document everything on the conversion label and job notes.
Confirm house piping and regulators can maintain setpoint under load. Low supply pressure will masquerade as ignition or flame-stability issues.
What’s in a Professional LP Conversion Kit
A quality, model-matched kit typically includes: correctly sized orifices, an LP gas-valve spring, sometimes pressure switches (high altitude variants), installation hardware, documentation, and conversion labels. The exact mix varies by manufacturer and furnace series.
Order by full model number and current BTU rating. Don’t assume interchangeability across a brand family. Cross-check OEM bulletins for any control board or ignition-system notes during LP conversion.
If your conversion prompts broader system updates (coil change, air handler, or dual-fuel planning), browse R-32 AC & Gas Furnaces.
Commissioning & Safety Testing: No Shortcuts
After hardware changes, perform a full start-up and safety sequence:
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Leak checks: Electronic detector + bubble solution at all joints
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Pressure verification: Supply and manifold at operating conditions
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Combustion analysis: O₂, CO, CO₂, excess air; compare to OEM targets
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Flame inspection: Stable blue flame, correct carryover, no lifting or rollout
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Shutoff tests: Verify safety controls and gas-valve closure
Record CO (air-free) and efficiency numbers on the data tag. If readings drift after 15–20 minutes, re-check manifold pressure and primary air. Repeat leak test after heat-soak.
Need a second set of eyes? Share photos and specs via our Quote by Photo and we’ll sanity-check the plan.
Altitude, Air, and Pressure Switch Considerations
At higher elevations, air density drops affecting combustion and pressure-switch operation. Many OEMs provide high-altitude pressure switches or derate tables for input adjustments. During a natural-gas-to-LP conversion, confirm whether your model calls for a different switch setpoint or board parameter at your site elevation.
Stable ignition depends on air + fuel balance. Post-conversion, verify flame signal and monitor for nuisance trips, especially on windy vent terminations or long vent runs. Consider a draft/combustion air audit if borderline.
If you’re pairing a converted furnace with high-SEER cooling or a high-static duct design, confirm blower tables and airflow targets. For matched components, see Coils.
Venting, Flame Quality, and Heat Exchanger Protection
Propane’s higher energy density can drive higher flame temperatures at the port if orifices or pressures are wrong. That’s a heat-exchanger risk. Keep inputs within OEM spec, verify stable flame without lifting, and confirm temperature rise across the heat exchanger is in range.
Visual check
Proper LP flame: tight, mostly blue, steady tips
Watch for: lifting (too much primary air/pressure), yellowing/shooting (poor air mix), delayed ignition
Log ΔT and compare it to the nameplate rise window. Inspect rollout switch status and verify no nuisance trips over a long run cycle. If adding or upgrading cooling simultaneously, ensure coil and cabinet cleanliness to protect airflow and exchanger life. For packaged alternatives, see Package Units.
Cost Planning & Efficiency Cross-Checks (SEO:
Budgeting propane usage starts with the math: GPH ≈ Input ÷ 91,452. Layer on duty cycle by climate and staging profile. If the home also runs high-efficiency cooling or a heat pump, optimize the dual-fuel switchover so propane handles only the colder bins where it wins on comfort and defrost dynamics.
Revisit envelope upgrades (air sealing/insulation) before resizing equipment. A smaller post-upgrade load can justify downsizing input saving fuel every hour for the life of the furnace.
If you’re evaluating broader efficiency options, browse Ductless Mini-Splits for zoned comfort in shoulder seasons.
DIY? Don’t. Why Licensed Technicians Are Non-Negotiable
Converting fuel types changes orifices, gas-valve springs, regulators, and requires combustion analysis and leak testing. Improper work risks fire, explosion, CO poisoning, equipment damage, code violations, and warranty voids. Manufacturers require qualified agencies to perform conversions per model-specific instructions, with proper documentation and labels affixed to the appliance.
Safety checklist
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Model-matched LP kit installed
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Orifice sizes verified with numbered drills
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Manifold pressure set/confirmed at each stage
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Leak-free joints verified electronically and with solution
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Combustion numbers recorded and within OEM limits
Need a vetted plan? Contact the Design Center for model-specific guidance.
Putting It All Together: A Sample Conversion Workflow
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Confirm load and current firing rate; photograph data tags.
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Order OEM LP conversion kit by full model number.
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Isolate gas, swap valve spring, change orifices, update labels.
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Set regulators for ~11” W.C. supply; verify sediment trap and shutoffs.
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Manifold pressure: set per OEM at low/high fire; log results.
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Leak checks (electronic + bubbles).
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Combustion analysis, ΔT, and flame quality verification.
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Pressure-switch/altitude checks; confirm venting.
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Document all readings; brief the homeowner on runtime and storage planning.
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Schedule a 30-day re-check.
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Save pre- and post-conversion readings in the service log. It shortens every future call.
For equipment refresh opportunities, see Scratch & Dent Deals.
When a Conversion Isn’t the Best Move
Some older furnaces lack approved LP kits or exhibit heat-exchanger wear, marginal venting, or control limitations that make conversion risky or uneconomical. In those cases, replacing with an LP-ready furnace or a dual-fuel packaged system may deliver better long-term safety and operating cost.
Decision cues
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No OEM-approved LP kit for your exact model
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Failing pressure switch/board with no LP spec support
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Aging exchanger or chronic rollout trips
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Major duct static issues that would remain post-conversion
Explore or coordinate a full system design via our Design Center.
Documentation: Labels, Readings, and Homeowner Hand-Off
A code-clean conversion ends with complete paperwork: conversion label, recorded manifold pressures, combustion numbers, ΔT, leak-test results, and any altitude or pressure-switch notes. Affix labels near the rating plate and in the blower compartment; leave a copy with the homeowner.
Add a QR code or link to your service record or the OEM conversion bulletin inside the cabinet. It speeds future diagnostics and warranty claims.
For ongoing education, point homeowners to our HVAC Tips.
Quick Reference: Numbers You’ll Use in the Field
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Natural gas: ~1,030 BTU/ft³; manifold ~3.5” W.C.
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Propane: ~2,516 BTU/ft³; manifold ~11.0” W.C.
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Propane GPH: Input ÷ 91,452
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NG CFH: Input ÷ 1,030
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LP CFH: Input ÷ 2,516
Visual pocket card
100k BTU/h → NG ≈ 97 CFH → LP ≈ 40 CFH → LP ≈ 1.09 GPH
Confirm with manometer + combustion analyzer every time.
Keep this alongside your drill index for fast orifice verification on site.
Ready to Convert or Evaluate Alternatives? (CTA)
Prefer to compare LP-ready options or dual-fuel packages? Browse Furnaces or reach out through the Design Center.