Plumbing · Lesson 10 of 13Advanced7 min read
Gas Piping and Gas Appliances
Many plumbers also run the gas lines that feed water heaters, furnaces, ranges and dryers. Gas work is a different world from water: a leak isn’t a puddle, it’s a fire or explosion risk, and a bad vent can fill a house with carbon monoxide. This lesson explains how the system works and why it belongs to licensed pros. It is not a how-to.
StopIf you smell gas
The U.S. Consumer Product Safety Commission says that if you smell gas and suspect a leak:
- Leave the house immediately.
- Do not use your phone.
- Call your gas supplier, or dial 911, from a neighbor’s house.
- Do not light a match, turn on a light, or switch on anything electrical.
And if a carbon monoxide alarm sounds, CPSC says to get everyone outside to fresh air right away and call 911 or the fire department. Don’t go back in until the emergency responders say it’s safe.
How gas reaches an appliance
Natural gas comes from the utility’s main through a service line to a meter, usually outside the house. Propane comes from a tank on the property. From there, house gas piping carries it to each appliance. Near each appliance is a shutoff valve so that one appliance can be turned off for service; HUD’s manufactured home standard, as an example, requires an accessible shutoff within 3 feet of most appliances and within 6 feet of a cooking appliance. A short, flexible connector joins the valve to the appliance. Then the appliance burns the gas, and a vent carries the exhaust outdoors.
Gas piping is built from materials approved for gas. HUD’s standard lists steel or wrought-iron pipe, threaded brass, certain grades of copper tubing and corrugated stainless steel tubing (CSST) that is listed and installed to its standard. Joints use compound that resists the gas.
Low pressure, high stakes
Household gas runs at a very low pressure compared with water. Gas pressure is often measured in inches of water column. HUD’s manufactured home standard designs the gas system for no more than 14 inches of water column and no less than 7. NIST puts an inch of water at about 249 pascals, so that’s roughly a quarter to a half of a psi, about a hundred times less than typical water pressure.
Low pressure doesn’t make a leak harmless. CPSC warns that a gas leak can cause a fire or explosion. That’s why pros pressure-test new piping before connecting appliances. HUD’s standard, for example, requires the piping to hold 3 psi for at least 10 minutes with no drop, and connections checked with soapy water or bubble solution, never a flame.
Why gas work is licensed
The Bureau of Labor Statistics notes that most states require plumbers to be licensed, and some require a special license for gas line work. Gas piping has to be sized so every appliance gets enough gas at once, built from listed parts, supported, protected, pressure-tested, and often inspected before it’s used. Mistakes can kill. Installing, extending, repairing or disconnecting gas piping, and installing gas appliances, belong to a licensed professional, usually with a permit.
Even small parts matter. CPSC warns that some older uncoated brass connectors can separate from their end pieces and cause a leak, fire or explosion. Its advice: have only a qualified professional check and replace a connector; don’t try it yourself, and don’t even move the appliance to look, because moving it can cause a failure.
Venting and carbon monoxide
Burning gas makes hot exhaust that has to leave the house. When burning is incomplete, it makes carbon monoxide, which CPSC describes as a deadly, colorless, odorless, poisonous gas. CPSC lists malfunctioning fuel-burning appliances, such as furnaces, ranges, water heaters and room heaters, among its sources.
HUD’s manufactured home standard requires fuel-burning appliances to be vented to the outside, except ranges and ovens. Older water heaters and furnaces use natural draft: hot flue gas rises through a draft hood and a vent pipe on its own. If that vent is blocked or disconnected, exhaust can spill into the room. HUD’s inspection standard treats a blocked, misaligned or missing flue as life-threatening because of carbon monoxide, and HUD’s inspection guide says soot on top of a water heater, under the draft hood, points to a clogged flue. Newer direct-vent, sealed-combustion appliances draw their combustion air from outdoors and send exhaust out through sealed pipes; DOE says that design makes them unlikely to backdraft.
You canWhat you can do
- Know where your gas meter and appliance shutoff valves are, but leave the valves to the gas company and pros unless they instruct you otherwise.
- Follow CPSC’s advice: have heating systems professionally inspected and serviced every year, including chimneys and flues, and keep vents clear of debris.
- Install battery-operated CO alarms, or alarms with battery backup, on every level and outside sleeping areas, and test them (Test Smoke and CO Alarms).
- Learn what to do if you smell gas, and teach everyone in the house.
Earthquake country
In earthquake zones, a heavy water heater can tip over and break its gas line. HUD’s inspection guide says to check for seismic bracing that ties the water heater to the floor or wall framing, to check the gas service for vulnerability where it enters the building, and in the most active zones to look for an automatic emergency gas shutoff valve for the whole house. Local codes in those areas set the requirements.
| Part | What it does | Signs of trouble |
|---|---|---|
| Meter | Measures gas use; owned by the utility. | A smell of gas near it: leave and call the utility from away. |
| House piping | Carries gas to each appliance; built from materials listed for gas. | Any smell of gas; corrosion; damage. |
| Appliance shutoff valve | Turns off gas to one appliance. | A stiff or leaking valve is a pro’s job. |
| Connector | Flexible tube from the valve to the appliance. | Old uncoated brass connectors can fail: CPSC says have a pro replace them. |
| Draft hood and vent (natural draft) | Let room air mix in and carry exhaust up and out. | Soot, scorch marks, a disconnected or blocked vent. |
| Direct vent pipes | Bring outside air in and send exhaust out, sealed. | Blocked intake or exhaust (snow, nests, debris). |
Check yourself
Tap an answer to check it, or open “Show the answer.”
You smell gas in the house. What does CPSC say to do first?
Show the answer
Leave the house immediately. Leave right away, and call the gas supplier or 911 from a neighbor’s house.
Why shouldn’t you use your phone or a light switch inside if you smell gas?
Show the answer
A spark could ignite the gas. Electrical switches can spark.
HUD’s manufactured home standard designs gas piping for no more than how much pressure?
Show the answer
14 inches of water column (about half a psi). Up to 14 inches of water column, roughly half a psi.
What does a blocked natural-draft vent on a gas water heater risk?
Show the answer
Carbon monoxide spilling into the house. Exhaust, including carbon monoxide, can spill indoors. HUD treats it as life-threatening.
Who should check or replace an old brass gas connector, according to CPSC?
Show the answer
Only a qualified professional. Only a qualified professional, and don’t move the appliance to look.
Go deeper
- U.S. Consumer Product Safety Commission: Carbon Monoxide Fact Sheet CPSC: Carbon Monoxide Fact Sheet.
- U.S. Consumer Product Safety Commission: Certain Older Gas Connectors May Be Dangerous (Publication 5135) CPSC: older gas connectors and what to do if you smell gas.
- Being an Adult 101: Test Smoke and CO Alarms Test your smoke and CO alarms.
- Lesson 5: Water Heaters How gas water heaters are vented.
Sources
- U.S. Consumer Product Safety Commission: Certain Older Gas Connectors May Be Dangerous (Publication 5135)
- U.S. Consumer Product Safety Commission: Carbon Monoxide Fact Sheet
- 24 CFR 3280.705, Manufactured Home Standards: Gas Piping Systems (via Cornell LII)
- 24 CFR 3280.707, Manufactured Home Standards: Heat Producing Appliances (via Cornell LII)
- NIST Guide to the SI, Appendix B.8: Factors for Units Listed Alphabetically
- U.S. Bureau of Labor Statistics, Occupational Outlook Handbook: Plumbers, Pipefitters, and Steamfitters (last modified August 27, 2026)
- U.S. HUD: NSPIRE Standard: Water Heater (v2.1)
- U.S. HUD: Residential Rehabilitation Inspection Guide (2000), Chapter 6: Plumbing
- U.S. Department of Energy, Building Science Education: Gas Storage Water Heaters
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