Short definition
LPG (liquefied petroleum gas) is the umbrella term for propane and butane — both heavier-than-air hydrocarbon fuels stored as liquid under pressure and used as gas. In rural Washington — Olympic Peninsula, Cascades, eastern WA off-main-gas counties — LPG almost always means propane, delivered to a residential tank.
What it is
LPG covers two fuels: propane (C3H8, boiling point -42°C / -44°F) and butane (C4H10, boiling point -4°C / 25°F). Both are liquid at moderate pressure and vaporize as the pressure drops at the regulator. Both are heavier than air — propane has a specific gravity of about 1.5, butane about 2.0 — meaning leaked LPG accumulates at floor level in basements, crawlspaces, and below-grade meter pits.
For US residential heating, LPG means propane. Butane doesn’t reliably vaporize from a tank in a Washington winter (the Olympic foothills routinely see nights below 25°F, and butane needs warmer ambient temperatures to push through the regulator). Butane is for camping cylinders, not house heating.
LPG appliances run differently from natural-gas appliances. The orifices are smaller, the manifold pressure is higher (typically 11″ water column for LP versus 7″ for natural gas), and burner air-shutter settings differ. Switching an appliance between fuels requires a manufacturer conversion kit and a registered gas-piping contractor — not a DIY swap.
Like natural gas, propane is given a sulfur-based odorant (mercaptan) so leaks are detectable below the explosive range.
Why it matters to a homeowner
Two facts shape the homeowner experience of LPG in Washington.
First, LPG is heavier than air. A leak in a basement or crawlspace pools at floor level. Ignition sources at floor level — a water heater pilot, an electrical receptacle — are exactly where the gas accumulates. This makes basement and crawlspace LPG leaks more dangerous than equivalent natural-gas leaks at the same volume. Mercaptan-smell evacuation rules apply the same way (see gas leak hazard), and you should not assume “it’s just gas” at the floor — get out, call from outside.
Second, LPG depends on the tank, the regulator, and the delivery contract. Tank vapor lock in deep cold, regulator failure, frozen tank-shutoff valves in moisture-heavy air, and missed deliveries during PNW windstorms are the practical failure modes. The fix is usually preventive: oversize the tank for winter peak load, keep it ≥30% full at the start of winter, schedule annual leak tests, and know the supplier’s emergency line (it’s printed on the tank).
When you’ll encounter this term
- Buying a rural Olympic Peninsula or eastern WA home with an existing propane tank — confirm tank ownership (lease vs. owned), tank age (~30-year service life), regulator currency, and most-recent leak test
- Moving from a Seattle natural-gas home to a Cle Elum or Sequim propane home — appliances may need conversion kits or replacement
- Storm preparedness: keep the tank ≥30% full going into deep winter
- Adding a propane generator backup — calculate combined load (heat + DHW + range + generator) when sizing the tank
Common variants and disambiguation
- LPG / propane — used interchangeably in US residential contexts.
- LPG / butane — butane is portable / seasonal use only; not residential heating in WA winter.
- Propane vs. natural gas — see natural gas vs. LPG.
- HD-5 propane — the residential-grade specification (≤5% propylene); most US residential propane is HD-5.
- “Bottle gas” — UK term; US equivalent is “propane cylinder” or, for the small ones, “BBQ tank”.
Common failure modes
- Wrong-fuel appliance. An NG-listed appliance running on LP (or vice versa) without a conversion kit. Yellow flames, sooting, possibly elevated CO. See flame impingement.
- Regulator failure. First- or second-stage regulator stuck high or low. Flame too large, too small, or won’t sustain.
- Tank vapor lock in cold weather. Tank too small for the load; vapor pressure drops below the regulator setpoint; appliances flame out. Common with a 250-gallon tank during a windstorm in a peninsula home with full heating + DHW + range demand.
- Leak at tank manifold or buried supply line. Heavier-than-air gas pools at slab/grade level.
- Underground tank corrosion. Most WA installs are above-ground; below-ground tanks need cathodic protection.
Cost data
- 500-gal residential tank (purchased): $1,500–$3,500 hardware. Lease through a delivery company is typically free or low-monthly with a delivery contract.
- Residential propane price (WA, 2026): $3.00–$5.00 per gallon. Eastern WA is generally cheaper than the peninsula.
- Tank install (pad, line, regulator, first fill): $1,500–$5,000 turnkey.
- Conversion kit for water heater or furnace (NG ↔ LPG): $50–$200 part, $200–$600 labor.
Washington note
Federal NFPA 58 (LP-Gas Code) governs tank siting, separation distances, refilling, and labeling, and is adopted into Washington via the State Building Code Council (WAC 51-52 / 51-50). Standard residential tank siting (for tanks ≤500 gallons water capacity): 10 feet from the building, 10 feet from any ignition source, 10 feet from the property line — verify against the current NFPA 58 edition before relying on these distances for a permit.
WA Department of Labor & Industries licenses LPG transport and installation. The Utilities and Transportation Commission oversees bulk delivery rate practices. Above-ground residential tanks aren’t regulated as USTs by Ecology, but spills above the reportable quantity follow general hazardous-substance reporting.
For tank install, line repair, regulator service, and conversion kits: registered contractor (statewide) or licensed gas piping mechanic (Seattle). Not DIY.