Short definition
Propane (C3H8) is a hydrocarbon fuel — heavier than air, stored as liquid under pressure, used as gas — that serves as the standard heating fuel for rural Washington homes off the natural-gas main. It’s the most common form of LPG in US residential use, and it works in cold climates because its boiling point (-42°C / -44°F) is well below any WA winter temperature.
What it is
Propane is a three-carbon hydrocarbon (C3H8), molecular weight 44, specific gravity about 1.5 (heavier than air). It’s stored as a liquid in a tank at moderate pressure and vaporizes at the regulator on its way to the appliance. Liquid energy density is high — about 91,500 BTU per gallon — and the fuel burns clean when the appliance is set up correctly.
The standard residential setup is a 500-gallon outdoor tank, an above-ground regulator, a buried supply line to the house, and indoor appliance shutoffs at each appliance. Smaller homes use 250- or 320-gallon tanks; larger or generator-equipped homes go to 1,000 gallons.
Propane has a low explosive limit (LEL) of 2.1% in air and an upper explosive limit (UEL) of 9.5%. Mercaptan (an added odorant) makes leaks detectable well below the explosive range — see gas leak hazard.
The residential grade is HD-5 (≤5% propylene). All US residential propane is HD-5; auto-propane and forklift propane are the same chemistry but different tank fittings.
Why it matters to a homeowner
In rural WA — Olympic Peninsula, Cascades, eastern WA off-gas counties — propane is the fuel that keeps the house warm and the water hot. Three practical things shape the homeowner experience:
Cold-weather vapor lock. Liquid propane vaporizes off the wetted wall area of the tank. In deep cold with high demand (heat + DHW + range), a 250-gallon tank can drop in pressure faster than vapor can be produced, and appliances flame out. PNW solution: oversize the tank for winter peak load, and keep the tank ≥30% full going into deep-freeze season.
Heavier than air. A propane leak in a basement or crawlspace pools at floor level, exactly where pilot lights and electrical receptacles tend to be. The mercaptan smell is the safety net, but the geometry of the risk is different from natural gas.
Tank ownership and supply. A leased tank is “free” with a delivery contract — but the contract often locks you to that supplier. An owned tank can be filled by any supplier and gives price flexibility. Confirm tank status at purchase.
When you’ll encounter this term
- Buying a rural property with an existing propane tank — confirm ownership status, tank age (~30-year service life), regulator currency, and last leak test
- Annual fall fill in mid-October — get the tank full before winter
- Generator-ready home plumbed for propane — calculate combined load (heat + DHW + range + generator) when sizing tank
- Switching to electric (HPWH + heat pump): leased tanks are typically removed at no cost; owned tanks may have salvage value
Cost data (WA, 2026)
- Residential propane price: $3.00–$5.00 per gallon, varies by region, contract, and season. Eastern WA generally cheaper than coastal/peninsula.
- 500-gal residential tank: $1,500–$3,500 owned; lease typically free with delivery contract.
- 100-lb cylinder (pre-fill / exchange): $40–$80 fill, $25–$50 owner-filled.
- 20-lb BBQ cylinder: $20–$30 exchange.
- Tank install (pad, line, regulator, first fill): $1,500–$5,000 turnkey.
Common failure modes
- Vapor lock in cold weather when tank is small and load is heavy. Pressure drops below appliance regulator setpoint; appliances flame out.
- Regulator failure. First- or second-stage stuck; flame too large or won’t sustain.
- Tank-shutoff valve frozen open in deep cold from moisture in the valve.
- Leak at tank manifold or buried supply line. Heavier-than-air gas pools at slab/grade level.
- Buried supply line damaged by settlement, roots, or excavation.
Common variants and disambiguation
- Propane vs. butane. Both LPG, but butane (boiling point -4°C) doesn’t reliably vaporize from an outdoor tank in WA winters. Butane is camping/portable use only.
- Propane vs. natural gas. See natural gas vs LPG.
- HD-5 propane. Residential-grade specification. All US residential propane is HD-5.
- Auto-propane / forklift propane. Same chemistry, different tank fittings. Not interchangeable hardware.
Washington note
NFPA 58 (LP-Gas Code) governs propane installations and is adopted in Washington via WAC 51-52. Tank siting for a typical residential 500-gallon tank: 10 feet from the building, 10 feet from any ignition source, 10 feet from the property line. Confirm current edition of NFPA 58 before relying on these distances for a permit.
WA Department of Labor & Industries licenses LPG transport and installation. The Utilities and Transportation Commission oversees aspects of bulk delivery. Above-ground residential tanks are not regulated as USTs by Ecology, but spills above the reportable quantity follow general hazardous-substance reporting.
For tank install, regulator service, line repair, or conversion kit work: registered contractor (statewide) or licensed gas piping mechanic (Seattle). Routine fills, gauge reading, and operating the manual tank shutoff are homeowner tasks. The supplier’s emergency number is on the tank — keep it accessible.
FAQ
How long does a 500-gallon propane tank last?
It depends on what’s running on it. A typical 2,000 sq ft WA home using propane for heat, DHW, and a range will burn through 500 gallons in 1 to 3 months of full winter operation, depending on insulation, climate zone, and thermostat behavior. Annual usage for a primary residence in the Olympic foothills typically runs 800–1,500 gallons. Schedule an automatic-fill contract — running out is bad for the regulator and bad for the schedule.
Why is my propane appliance flaming out during a cold snap?
Most likely cause: tank vapor lock. Liquid propane needs wetted tank wall area to vaporize at high demand; a small tank in deep cold with full appliance load can’t keep up. The fix is sizing — go from 250 gallons to 500, or 500 to 1,000. Short-term, reduce simultaneous demand (don’t run all appliances at once during the coldest hours).
Can I run my generator on propane?
Yes — propane is a common generator fuel in rural WA, and a propane generator avoids the gasoline storage problem. But size the tank for combined load: heat + DHW + range + generator runs may exceed what a 500-gallon tank can vaporize in a winter cold snap. A generator-equipped propane home often needs a 1,000-gallon tank.