A pool heater's gas line size depends on its BTU rating and the pipe length from the meter. A 400,000 BTU natural gas heater burns about 400 cubic feet an hour: usually 1-1/4 inch pipe on short runs, 1-1/2 or 2 inch on long ones. The meter must carry the whole load too.
How do you size a gas line for a pool heater?
You size it from the heater's input rating, the total pipe length, and the code or manufacturer table for your gas pressure. The fuel gas code Phoenix uses says gas piping is sized from tables in the code, from a listed piping maker's tables, or by engineering methods, and the flow to plan for is the sum of every appliance on the line running at full capacity at the same time.
Here is the process a plumber follows:
- 1Read the heater's rating plate. It shows the input in BTU per hour and the gas type. The Pentair and Hayward heaters covered here range from 150,000 to 500,000 BTU. A propane heater is a different model from a natural gas one; see natural gas vs propane at home.
- 2Convert to gas flow. Divide BTU per hour by about 1,000 to get cubic feet per hour for natural gas.
- 3Measure the real pipe length from the meter to the heater, following the route the pipe will take, not a straight line across the yard. Pentair's manual says to add 3 feet of straight pipe for every elbow.
- 4Add every other appliance on the same run. If the new line tees off a line that also feeds the house, that shared section has to carry both loads.
- 5Pick the pipe size from the table for that flow and length, then check the meter and the supply pressure.
The heater itself sets a pressure window. Pentair's MasterTemp manual calls for a natural gas supply of no less than 4 inches and no more than 14 inches of water column, with at least 4 inches still there while the heater is running. The manual says pressure outside that range may cause improper burner operation, and Hayward's manual says to check for undersized piping when pressure at the heater is too low.
What pipe size do 250K and 400K heaters need?
A 250,000 BTU heater usually needs 1 inch pipe for a short run and 1-1/4 inch for a longer one, while a 400,000 BTU heater needs 1-1/4 inch on short runs and 1-1/2 or 2 inch as the run gets longer. Here are two real tables side by side, so you can see how the numbers work.
Pentair MasterTemp, natural gas, maximum equivalent pipe length:
| Heater | 3/4 inch | 1 inch | 1-1/4 inch | 1-1/2 inch | 2 inch |
|---|---|---|---|---|---|
| 175K to 200K | 30 ft | 125 ft | 450 ft | not listed | not listed |
| 250K | 20 ft | 70 ft | 250 ft | 600 ft | not listed |
| 400K | not listed | 20 ft | 100 ft | 200 ft | 400 ft |
"Not listed" means the manual gives no length for that pipe size and heater.
Fuel gas code Table 402.4(1), Schedule 40 metallic pipe, natural gas under 2 psi, 0.3 inch water column drop, capacity in cubic feet per hour:
| Pipe length | 1 inch | 1-1/4 inch | 1-1/2 inch | 2 inch |
|---|---|---|---|---|
| 50 ft | 215 | 442 | 662 | 1,280 |
| 100 ft | 148 | 304 | 455 | 877 |
| 150 ft | 119 | 244 | 366 | 704 |
Read the code table this way. A 400K heater needs about 400 cubic feet per hour. At 50 feet, 1-1/4 inch pipe carries 442, so it works. At 100 feet it carries only 304, so you step up to 1-1/2 inch, which carries 455. At 150 feet even 1-1/2 inch falls short at 366, so the run goes to 2 inch.
Why do the two tables differ a little? They are built on different assumptions. The code table assumes a 0.3 inch pressure drop and gas with a specific gravity of 0.60. Pentair's table assumes 0.65 and is built around its own heater's pressure window. The plumber sizes the job to the code the city enforces and to the heater maker's instructions, since the code requires pool heaters to be installed to those instructions. When in doubt, sizing up once beats digging the trench twice.
Two traps to watch for:
- Flexible connectors. Pentair says flex connectors are not recommended unless sized for the heater's rating, and Hayward says not to use one unless it is approved for outdoor use and marked with a BTU capacity at least equal to the heater's input. A thin connector at the end can choke a well-sized line.
- Reducing at the wrong spot. The supply line is usually bigger than the heater's gas valve inlet. Pentair says to make that reduction as close to the heater as possible.
Can my gas meter handle a pool heater?
Maybe not, and it is the step people forget. Hayward's manual says the gas meter "must have the capacity to supply enough gas to the pool heater and any other gas appliances," and gives the example that a 225 meter supplies about 225,000 BTU per hour. A standard house meter sized for a furnace, water heater, range, and dryer may not have room for another 400,000 BTU on top.
Add up the ratings on every gas appliance in the house, plus the pool heater. If that total is more than the meter can deliver, the fix is a bigger meter. The meter is Southwest Gas's equipment, so that request goes to the utility. Hayward says to consult the local gas utility if there is any doubt about the meter size, and its gas pressure section lists a low-capacity meter as a cause of inadequate gas pressure at the heater.
A plumber can check the existing load and the pressure at the heater while it runs. If pressure sags when the heater and the house are both firing, the pipe, the meter, or both are too small.
Do you need a permit to run gas to a pool heater in Phoenix?
Yes, a new gas line to a pool heater needs a Phoenix permit, because it is new gas piping and Phoenix requires a city permit, a city inspection, and a city gas clearance for any new gas piping installation. Replacing an old heater with a bigger one counts too: the city's table lists an appliance upgrade that increases BTU load and needs pipe sizing as permit work.
The line usually runs underground across the yard, and the code sets rules for that:
- Burial depth. Underground gas piping has to be at least 12 inches below grade.
- Plastic pipe is allowed outdoors underground only, and it needs a yellow insulated copper tracer wire, at least 18 AWG, laid alongside so the line can be found later.
- Steel pipe underground needs real corrosion protection, like a factory coating. The code says zinc galvanizing alone does not count.
- Pressure test before the trench is backfilled, since the code requires testing before piping is concealed.
- Shutoff, sediment trap, and union. Pentair's manual calls for a manual shutoff valve, a sediment trap, and a union outside the heater cabinet.
Pool heaters themselves must be listed to their safety standard and installed to the manufacturer's instructions, which is how the code handles them. Once the gas is on, Pentair also says pool heaters must be used with carbon monoxide detectors installed near the heater. Our page on whether a gas water heater can cause carbon monoxide covers CO warning signs that apply to any gas appliance.
Who owns and maintains the line after it is installed?
You, the property owner, own and maintain the line from the meter to the heater. Southwest Gas owns the pipe up to and including the meter. The line from the meter to the pool heater is customer-owned buried piping, and the utility's own diagram lists lines to pool heaters as the customer's responsibility. Southwest Gas also warns that customer buried piping can't be located by calling 811. Keep the as-built drawing or a photo of the trench before it is backfilled, so the next person who digs knows where it runs.
If you are planning a heater, get the line sized before you buy, especially for a 400K unit or a run past 100 feet. HQ Plumbing & Air sizes, installs, and pressure tests gas lines to pool heaters, grills, and fire pits across the Phoenix area, with free estimates and the price up front before any work starts. For a new line to the backyard, see also our guide on running a natural gas line to a grill, since the two often share one trench.
Size the line to the heater, check the meter, and test before you bury. That's the job done right.