How do I size a gas line for a 100,000 BTU furnace and 40,000 BTU water heater?
Quick answer
For a combined load of 140,000 BTU/hr at standard residential conditions (natural gas at 0.5 inch water column pressure drop, schedule 40 steel pipe), the typical answer is 3/4 inch black iron pipe for runs up to about 50 feet from the meter. Beyond 50 feet, jump to 1 inch.
But "typical" isn't code. Code requires you to size from the manufacturer's table or the IFGC/NFPA 54 tables based on:
- Total connected load in BTU/hr (sum every appliance fed by the line)
- Length of the longest run from the meter to the most remote appliance
- Specific gravity of the gas (0.60 for natural, 1.50 for propane)
- Allowable pressure drop (typically 0.5 in. w.c. for natural gas)
- Pipe material (schedule 40 steel, CSST, copper)
Worked example using NFPA 54 Table 6.2(a)
Load: 140,000 BTU/hr ÷ 1,000 BTU/cu ft = 140 cubic feet per hour
Length to furthest appliance (say furnace): 60 feet
Look up the 60-ft row in the 0.5 in. w.c. natural gas table:
- 1/2 inch pipe: 73 cfh — too small
- 3/4 inch pipe: 152 cfh — works
- 1 inch pipe: 285 cfh — works (overkill)
Answer: 3/4 inch from meter to the tee, then 1/2 inch to the water heater (40k = 40 cfh) and 1/2 inch to the furnace (100k = 100 cfh) since each branch is shorter.
Common mistakes
- Forgetting the longest-run rule. The whole table is sized by the longest run, not each branch's length.
- Using altitude-uncorrected BTU. Above 2,000 ft, deratings apply — check the manufacturer's spec.
- Mixing pipe materials without re-sizing. CSST has different friction; copper has different tables.
- Skipping the drip leg / sediment trap. Required at every appliance per IFGC 408.
What this means on the job
For most residential installs with under 50 ft of run and 140k total load: 3/4 inch trunk, 1/2 inch branches. For longer runs or higher loads, pull the table. Don't guess.
Want more like this?
Master Mechanical Buddy walks you through NFPA 54 sizing tables, CSST bonding, drip legs, and every other gas-piping topic that shows up on the Master Mechanical exam.