Gas Types + Pressures Reference
Why this matters
Wrong gas pressure means wrong appliance operation. Customer's furnace under-firing = no heat + safety lockout; over-firing = damage + risk. Different gases have different pressures, BTU values, + safety requirements. Mixing up natural gas + propane = catastrophic failure. This is the field card every gas-trade tech needs.
Natural gas vs propane
Natural gas (NG): methane (CH₄), delivered by utility via pipes, lower BTU/cu ft, lighter than air (rises if leak), slightly sweet odorant, lower pressure system. Propane (LP/LPG): C₃H₈, delivered in tanks, higher BTU/cu ft, heavier than air (sinks if leak), same odorant, different pressure system. Critical: appliances designed for one or other; converting requires orifice + gas valve change.
Operating pressures
Natural gas: utility service 0.25 - 7 PSI (typical 0.5 - 1 PSI residential); appliance manifold 3.5" WC; max 14" WC; install test pressure typically 10 PSI. Propane (LP): tank pressure variable (130 PSI at 90°F; 25 PSI at -20°F); regulator reduces to ~10 PSI; appliance manifold 10 - 11" WC. Both: regulators step down to appliance-suitable.
Pressure units: " WC (water column) for low pressures (appliances); PSI (pounds/sq in) for high (gas mains, propane). 1 PSI = 27.7" WC; 0.5 PSI = 13.85" WC. Manometer reads in " WC typically.
BTU values: NG ~1,000 BTU/cu ft; propane ~2,500 BTU/cu ft (2.5x more per volume). Per lb: NG ~22,000 BTU/lb; propane ~21,500 BTU/lb. Per gallon liquid propane: ~91,500 BTU.
Why orifice + valve differ
NG + propane appliances look same but orifice is sized for the gas's BTU + pressure. NG orifice: larger hole, lower pressure (3.5" WC), lower BTU/cu ft. Propane orifice: smaller hole, higher pressure (10" WC), higher BTU/cu ft. Conversion kit: orifice change + manifold pressure adjustment + gas valve + sometimes burner; must be done by professional. Wrong gas to wrong appliance: incomplete combustion + soot + CO + damage.
Gas line sizing (basic)
Pipe size required based on:
- Total BTU load
- Pipe length
- Gas type
- Pressure drop allowed
Typical residential gas line sizes:
| Total BTU | NG @ 0.5 PSI, < 50 ft |
|---|---|
| 50,000 | 1/2" pipe |
| 100,000 | 1/2" pipe |
| 150,000 | 3/4" pipe |
| 200,000 | 3/4" pipe |
| 300,000 | 1" pipe |
| 500,000 | 1-1/4" pipe |
Always verify per IFGC table + AHJ + manufacturer.
Common appliance BTU consumption
Natural gas: furnace 50K - 150K BTU/hr; water heater tank 30K - 80K; tankless water heater 150K - 200K; stove (4 burner + oven) 40K - 65K; dryer 22K; outdoor grill 25K - 60K; pool heater 100K - 400K; generator residential standby 200K - 700K. Propane same BTU output; different gas + pressure.
Tank sizing (propane)
Common propane tanks:
- 20-lb (BBQ tank): 18 gallons capacity, ~1.7 gal usable
- 100-lb tank: 23.6 gallons capacity
- 250-gallon tank: typical residential
- 500-gallon tank: large residential
- 1,000-gallon tank: large home or business
Tank fill:
- 80% rule: tank filled to 80% (expansion space)
- 500-gallon tank holds ~400 usable gallons
Tank temperature affects pressure:
- Cold: lower pressure (gas vaporizes slower)
- Hot: higher pressure (gas vaporizes faster)
- Below -20°F: propane doesn't vaporize well
CSST (corrugated stainless steel tubing)
Modern gas piping (since 1990s):
- Flexible (easier routing)
- Yellow OR black exterior jacket
- Manufacturer-specific fittings (Gastite, Tracpipe, OmegaFlex)
Critical: MUST be bonded to electrical ground (NEC 250.104).
Without bonding: lightning strike causes arc-puncture + fire.
Sizes: 3/8" - 1-1/4" CSST.
Pressure rating: typically 5 PSI residential.
Black iron (traditional)
Standard residential gas pipe:
- Schedule 40 black iron pipe
- Threaded NPT joints
- Pipe dope OR yellow PTFE tape for sealing
- Cheap + robust
Sizes common: 1/2", 3/4", 1", 1-1/4", 1-1/2", 2".
Pressure rating: 5 - 125 PSI (depending on schedule).
Gas leak detection
Smell: mercaptan odorant added; smells rotten egg / sulfur.
Bubble test: soap solution on joints; bubbles indicate leak.
Electronic gas detector: combustible gas indicator (CGI); senses ppm.
Pressure decay test: see Gas Line Pressure Test article.
Action on leak:
- Don't operate any electrical switches
- Open windows
- Evacuate
- Shut off gas at meter / tank
- Call utility OR gas service immediately
- DO NOT smoke OR light any flame
Combustion air
Atmospheric combustion appliances need combustion air:
- 1 sq inch free area per 1,000 BTU is rule of thumb
- Larger appliances may need direct outdoor air supply
- Modern sealed combustion uses outdoor air directly
Inadequate combustion air:
- Incomplete combustion → soot + CO
- Back-drafting (gas burner pulls air from other appliances)
- Customer's CO alarm sounds
Venting requirements
Atmospheric vent (Category I):
- Natural draft up flue
- Common older furnaces + water heaters
- Vertical flue typically
Power vent (Category III):
- Induced draft fan
- Horizontal OR vertical flue
- PVC OR stainless venting
Direct vent / sealed combustion (Category IV):
- Combustion air piped from outside
- Exhaust piped out
- Most efficient + safest
- PVC venting (high-efficiency)
Vent material:
- Atmospheric: Type B vent (double-wall) OR Class A chimney
- Power vent: Category III stainless OR PVC for cool exhaust
- Direct vent: PVC for condensing; Category III for non-condensing
CO (Carbon Monoxide) safety
Source: incomplete combustion (orange flame, sooting, blocked flue).
Warning signs:
- Headaches, dizziness, nausea (customer)
- Yellow OR orange burner flame (should be blue)
- Soot around appliance
- CO alarm sounding
- Repeat appliance failures
Combustion analysis (during service):
- CO at flue: < 100 PPM target
- CO in living space: < 9 PPM acceptable
- 35+ PPM in living space = problem
- Customer evacuates if 70+ PPM
Codes + regulations
IFGC (International Fuel Gas Code): comprehensive code.
NFPA 54 (National Fuel Gas Code): synonymous with IFGC essentially.
NFPA 58: liquid propane specific.
ANSI Z21: gas appliance standards.
Local AHJ + permits: required for new gas installations OR major modifications.
Common gas system mistakes
Wrong orifice for gas type: appliance burns wrong; soot + CO.
Skipping bonding on CSST: lightning + fire risk.
Cross-threaded fittings: leaks.
Wrong PTFE tape (white on gas): not rated; long-term leak.
Inadequate combustion air: incomplete combustion.
Improper venting: back-drafting + CO.
Working live (gas on): ignite + explode.
Forgetting pressure test: invisible leaks.
NEVER work on gas lines while gas is flowing. Shut off at meter (NG) or tank valve (LP) before opening any joint. Cap any open ends immediately. Even brief opening releases enough gas to create explosion risk. The handful of fatal residential gas explosions each year share one cause: tech working hot. Shutoff is 30 seconds; ignoring it = death + lawsuit + license revocation. Always shutoff. Always.
References
- IFGC International Fuel Gas Code (current edition)
- NFPA 54 + NFPA 58
- ANSI Z21 (gas appliance standards)
- AGA (American Gas Association)
- Manuall internal: Gas Line Pressure Test, Gas Line Installation Reference