Adding Load to an Existing Low-Pressure Gas System
A 1955 house has a 100,000 Btu/h natural-draft furnace and a 40,000 Btu/h water heater fed from a 7 in. w.c. meter through a single 1/2 in. black iron line about 25 ft long. The owner is adding a 65,000 Btu/h gas range in the kitchen and a 75,000 Btu/h vented garage heater; the flue location makes the run from the meter to the garage heater the longest path at 70 ft. Gas heating value is 1,030 Btu/ft³. The building department requires a permit and a pressure test before the meter is turned back on. The homeowner's handyman has already bought 1/2 in. black iron pipe for the new drops.
1. Determine the connected load in CFH and identify the first capacity check that has nothing to do with pipe size.
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Connected load: 100,000 + 40,000 + 65,000 + 75,000 = 280,000 Btu/h.
Convert to flow: 280,000 ÷ 1,030 Btu/ft³ ≈ 272 CFH.
First check - the meter and service, not the pipe. A typical residential diaphragm meter is rated around 250 CFH at a 0.5 in. w.c. pressure drop. At 272 CFH connected load this service is at or beyond the meter's capacity, and an undersized meter shows up as low manifold pressure at the appliances whenever several are firing - a symptom no amount of downstream pipe sizing can fix. Contact the gas utility to verify the meter rating, the service regulator, and the service line size before sizing a single joint of pipe.
Second check - delivered pressure. With a 7 in. w.c. meter outlet and 3.5 in. w.c. manifold target at natural gas appliances, the standard low-pressure allowance is 0.5 in. w.c. of drop from the meter to the farthest appliance (verify against the adopted fuel gas code table). If the utility cannot guarantee that, the design converts to a 2 psi service with a listed line regulator at each appliance - a legitimate solution, but it changes the pipe sizing basis entirely and must be labeled as such.
2. Size the piping using the longest-length method and comment on the handyman's 1/2 in. pipe.
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Method: longest-length, natural gas, 0.5 in. w.c. drop (verify the exact capacity table in the locally adopted fuel gas code - do not size from memory or from a supplier's chart that uses different pressure-drop assumptions).
Key rule: in the longest-length method, every segment is sized at the total length from the meter to the farthest outlet (70 ft here) and the total Btu/h connected downstream of that segment. You do not shorten the length for segments closer to the meter.
| Segment | Load | Length basis | Approximate requirement |
|---|---|---|---|
| Meter to first tee (trunk) | 280,000 Btu/h | 70 ft | 1-1/4 in. black iron; 1 in. is far short |
| Branch to range | 65,000 Btu/h | 70 ft | 3/4 in. |
| Branch to garage heater | 75,000 Btu/h | 70 ft | 3/4 in. |
| Branch to water heater | 40,000 Btu/h | 70 ft | 3/4 in. to be safe; 1/2 in. is marginal at this length |
| Branch to furnace | 100,000 Btu/h | 70 ft | 3/4 in. minimum |
On the handyman's 1/2 in. pipe: 1/2 in. black iron at a 70 ft equivalent length carries roughly 45,000–55,000 Btu/h at 0.5 in. w.c. drop. It cannot serve the 75,000 Btu/h garage heater, the 65,000 Btu/h range, or a trunk at 280,000 Btu/h. Using it will produce low manifold pressure, delayed ignition, and nuisance lockouts at the farthest appliance - and it will not pass inspection. Return the pipe.
If the trunk run is actually shorter than the garage branch, still use 70 ft for every segment in this method. Also add the equivalent length of every fitting and valve, or use the code's allowance for fittings.
3. List the installation and test items the inspector will be checking.
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- Permit and pressure test: cap the system with the meter removed and test per the adopted code; for low-pressure systems that is commonly 1.5 times working pressure but not less than 3 psig held for at least 10 minutes (verify the edition). Do not use appliance gas valves or connectors as test boundaries.
- At each appliance: a listed shutoff valve ahead of the union, a sediment trap/drip leg on the downstream side of the shutoff and ahead of the appliance controls, and a connector no longer than 6 ft where a flexible connector is used, protected from damage.
- Garage heater: ignition sources and burners elevated at least 18 in. above the floor in a garage, with protection from vehicle impact and a thermostat located so the appliance is not cycled by door openings.
- CSST, if substituted for black iron: only listed product, routed with strike protection where required, and directly bonded per the manufacturer's instructions and the electrical code's metal-piping bonding requirement - a bonding failure in CSST is a lightning/arc hazard, not a paperwork issue.
- Start-up and combustion: purge the lines, leak-test every joint with an approved bubble solution or detector, verify manifold pressure at each appliance against its nameplate (typically 3.5 in. w.c. for natural gas), then combustion-test for CO and CO2 and adjust the air shutters.
- Documentation: leave the sizing calculation, the pressure-test record, and the combustion readings with the permit file.