What's the difference between natural gas and propane in a home?

Natural gas flows in continuously through a buried service line to a meter, and you pay the utility for what you use. There's no tank and nothing to refill. Propane works like a fuel delivery. The U.S. Energy Information Administration explains that propane is kept as a liquid because liquid propane takes up much less space than propane gas, "about 270 times less." Local dealers bring it in tank trucks called bobtails and fill a storage tank at the home. The EIA says "The average residential propane tank holds between 500 and 1,000 gallons of liquid propane, and a household may need to refill its tank several times a year."

The heat figures use different units on purpose. Natural gas is sold by volume as a gas, and propane by the gallon as a liquid. The EIA also notes that its conversion factors are recalculated each year "to account for changes in the quality and composition of the fuels," so treat them as close, not exact.

You can tell by what's outside and what's on the appliance labels. A home on natural gas has a gas meter, usually on an outside wall, with a pipe rising out of the ground into it, and a monthly bill from the utility. A home on propane has a storage tank on the property, above ground or buried with only a lid showing, and gets deliveries from a propane dealer instead of a utility bill. Some homes have both: natural gas for the house and a small propane cylinder for a grill or a patio heater.

Inside, check the rating plate on the water heater, furnace, or range. It names the gas the appliance was built for, for example "Natural gas" or "Propane (LP)". If you just bought a house, or you're replacing an appliance, confirm this before anything gets ordered.

No, not as it comes from the factory. Each gas appliance is built and adjusted for one gas, and its rating plate says which one. A.O. Smith's water heater manual puts it simply: "Use only gas shown on rating plate," and "Make sure the gas supplied is the same type listed on the model rating plate."

The difference is built into the parts. Propane burns with more heat per cubic foot, so natural gas and propane models use different burner openings, called orifices. In A.O. Smith's water heaters, the parts are made so they can't be mixed up by accident: "The orifice on Natural gas models has Right-hand thread, the orifice on Propane (LP) models has Left-hand thread." The same manual warns: "Have a qualified technician make sure that the L.P. gas operating pressure does not exceed 13" water column."

What that means when you're buying or moving appliances:

• Buy the version for the gas you have. Many ranges, dryers, water heaters, and grills are sold as a natural gas model and a propane model.
• Changing fuels is a technician's job. Any change to the gas type has to follow the manufacturer's instructions for that model, since the orifices and pressure settings are part of what the rating plate certifies.
• Read the rating plate, not the hookup. The plate names the gas the appliance was built for.

An appliance on the wrong gas won't burn right. Southwest Gas notes that carbon monoxide "is produced by the incomplete burning of fuel," so a mismatched appliance is a carbon monoxide risk, not just a performance problem.

Both are flammable, and both are handled safely every day, but they leak differently, and that changes where the danger collects. Natural gas rises. Southwest Gas says natural gas is "lighter than air, dissipates quickly in well-ventilated areas, and is non-toxic." Propane sinks. The CDC's National Institute for Occupational Safety and Health lists propane's relative gas density as 1.55, compared with air at 1.0, which means propane vapor is heavier than air and collects in low spots.

In practice:

• A propane leak can pool along the floor, in a low closet, a pit, or a drain area, and stay there. Check low spaces and never light a flame in one after a leak.
• A natural gas leak tends to rise and thin out, especially where there's airflow.
• Neither has a smell of its own. NIOSH describes propane as a "Colorless, odorless gas" and notes that "A foul-smelling odorant is often added when used for fuel purposes." Southwest Gas adds mercaptan to natural gas, which gives the rotten egg smell.

That's why water heater lighting labels tell you to smell near the floor before lighting. Our page on a water heater pilot that's out when you smell gas covers that step.

Get out first and call from outside. The steps are the same for both gases. Southwest Gas says: "Exit the area or building immediately," don't operate "any electric switches, thermostats, or appliance controls," and "From a safe place, call 911 and Southwest Gas at 877-860-6020, day or night." It also says to call "whether you have natural gas at your home or business or not."

The difference is who you call second:

• Natural gas: 911 and Southwest Gas.
• Propane: 911 and your propane supplier. Keep the supplier's number where you can find it outside the house.

Natural gas shuts off with a valve on the pipe at the meter, and your propane supplier can show you how its tank shuts off. Our page on how to turn off the gas to your house covers the meter valve, and what to do when the house smells like gas covers the leak steps in detail.

### Can a house have both?

Yes. A home on natural gas can still run a grill or a patio heater on a small propane cylinder. The two systems just have to stay separate, and each appliance has to match the gas it's connected to. Many owners on natural gas replace the grill cylinder with a fixed line off the house piping instead. Our page on a natural gas line for a grill covers that job.

HQ Plumbing & Air installs and repairs natural gas piping, appliance connections, and shutoff valves across the Phoenix area. See our gas line service.

Pipe and meter, or truck and tank. Rises, or settles. Match the appliance to the gas on its rating plate.