What size gas line for a pool heater at 100 feet?
A typical 250,000 BTU pool heater on a 100 foot run needs 1-1/4 inch Schedule 40 pipe for natural gas. That is the smallest size in NFPA 54 Table 402.4(2) whose capacity at 100 feet still meets the demand.
The math, step by step
A residential gas pool or spa heater. This is usually the single largest load on a house. Always confirm the real rating on the appliance data plate before sizing anything.
- Load: 250,000 BTU per hour.
- Demand: natural gas carries 1,000 BTU per cubic foot, so 250,000 divided by 1,000 gives 250 CFH.
- Length column: a 100 foot run rounds up to the 100 foot column.
- Result: 1-1/4 inch pipe carries 400 CFH at 100 feet, which covers the 250 CFH demand with 150 CFH to spare.
How the nearby sizes compare at 100 feet
| Pipe size | Capacity at 100 ft | Against 250 CFH |
|---|---|---|
| 1/2 inch | 50 CFH | not enough |
| 3/4 inch | 104 CFH | not enough |
| 1 inch | 195 CFH | not enough |
| 1-1/4 inch | 400 CFH | enough |
| 1-1/2 inch | 600 CFH | enough |
| 2 inch | 1,160 CFH | enough |
If a second appliance shares the run
This is where most undersized lines come from. Put a 100,000 BTU furnace on the same pipe and the connected load becomes 350,000 BTU, or 350 CFH — which needs 1-1/4 inch at 100 feet, the same size, so the original run still works. Size the pipe for everything that will ever hang off it, not just what you are installing today.
Propane on the same run
On propane the same pool heater draws only 99 CFH because LP carries 2,516 BTU per cubic foot, and PlumbCalc returns 3/4 inch for this run. That figure is deliberately conservative — it is sized against the natural gas table, so it will be equal to or larger than a dedicated LP table. Confirm LP work against an LP table and local code.
Before you cut any pipe
These figures come straight from NFPA 54 Table 402.4(2) for Schedule 40 metallic pipe, natural gas at 0.60 specific gravity, inlet pressure under 2 psi and an allowable pressure drop of 0.5 inches water column. That is the ordinary residential case, and it is the same table the PlumbCalc gas line sizing calculator uses.
Two things change the answer on a real job. First, the code sizes the total connected load on a run, not one appliance in isolation — if a second appliance shares the pipe, add its BTU rating before you size. Second, a 2 psi system, a CSST or copper material, or a longer allowable pressure drop all use different tables entirely. Size to the longest run, size to the full load, and confirm against your local code and the appliance data plate before installing.
The governing standard is the National Fuel Gas Code, published as NFPA 54 and adopted in most of the United States — see the National Fuel Gas Code for background on the code itself.
Related sizing references
- Full gas pipe sizing chart, every size and length
- Gas line sizing calculator
- How many BTU 1-1/4 inch pipe carries
- Natural gas pipe sizing explained
- Gas line size for a pool heater at 20 feet
- Gas line size for a pool heater at 40 feet
Frequently asked questions
What size gas line does a pool heater need at 100 feet?
A typical 250,000 BTU pool heater on a 100 foot run needs 1-1/4 inch Schedule 40 pipe for natural gas. The demand is 250 CFH and that size carries 400 CFH at the 100 foot column of NFPA 54 Table 402.4(2).
Can I use a smaller pipe if the run is shorter?
Often yes, because capacity rises as the run shortens. Measure the actual longest run from the meter to this appliance, including vertical rises, then round up to the next table column. Do not measure the straight-line distance across the room.
Do fittings and elbows count toward the length?
NFPA 54 Table 402.4(2) already builds in an allowance for a normal number of fittings, so you generally use the measured pipe length. A run with an unusually high number of elbows, tees or a long vertical drop deserves either the next size up or a proper equivalent-length calculation.
What if the appliance rating is different from 250,000 BTU?
Use the plate rating, not the typical figure. Divide the real BTU input by 1,000 for natural gas to get CFH, then read down the 100 foot column until you find a size that meets it. The PlumbCalc gas line sizing calculator does exactly that.
Does this cover a 2 psi system?
No. This is the ordinary under-2-psi residential case with a 0.5 inch water column drop. A 2 psi system with appliance regulators uses a different table and typically allows smaller pipe over long runs. Size it against the table that matches the system you are installing.