How to install a gas pool heater

A gas heater is the fastest way to warm a pool or spa, but it's also the install with the least room for error. Undersized gas lines, missing clearances and bad flow cause most of the heater problems we hear about. This guide covers sizing, gas supply, placement, plumbing and bypass valves, and it's clear about which parts of the job belong to a licensed plumber.

Step 1: Size the heater

It takes about 8.34 BTUs to raise one gallon of water by 1°F. A 20,000-gallon pool therefore needs about 166,800 BTUs for every degree. Most standard gas heaters deliver roughly 82–84% of their input rating as heat.

Wind, evaporation and cool nights will slow those rates, and a solar cover recovers a lot of that loss. Spas heat much faster because the volume is small. A 400,000 BTU heater can often take an attached spa from the 70s to over 100°F in well under an hour. A bigger heater costs about the same per degree to run. It just gets there sooner.

Use our pool calculator to get your gallons.

Step 2: Choose your fuel and check your gas supply

Heaters are built for either natural gas or propane (LP). Order the one that matches your supply. Conversion kits exist, but a qualified technician has to install them.

Natural gas: check the meter

Natural gas carries about 1,000 BTUs per cubic foot, so a 400,000 BTU heater burns about 400 cubic feet per hour at full fire. Many standard residential meters are sized for household loads like the furnace, water heater, range and dryer. Adding a pool heater often pushes the total past the meter's capacity. If it does, your gas utility (Atmos Energy, CenterPoint Energy or Texas Gas Service, depending on where you live in Texas) will need to upgrade the meter. Call them early, because it can take a while.

Gas line sizing

The right pipe size depends on the heater's BTU load, the total pipe length from the meter (fittings count), the pipe material and the gas pressure. Your plumber sizes it from the fuel gas code tables. As a rough guide, a 400,000 BTU natural gas heater on a typical backyard run often needs 1-1/4" pipe or larger, and a 3/4" line is rarely enough.

An undersized line causes hard starts, sooting, short cycling and weak heating. We see it more than any other gas heater problem.

Gas pressure

Supply pressure is measured in inches of water column (in. w.c.). Natural gas supply is commonly around 7 in. w.c. and propane around 11 in. w.c., but your heater's manual lists the exact minimum and maximum. Pressure should be checked with a manometer while the heater is firing at full rate, not just at rest.

Propane

A gallon of propane holds about 91,500 BTUs, so a 400,000 BTU heater can burn over 4 gallons an hour at full fire. Small tanks can't vaporize gas fast enough to keep up, especially in cold weather. Your propane supplier should size the tank and regulators for the heater.

Step 3: Placement and clearances

Clearance requirements vary by model, and your heater's installation manual is the final word. These are the items almost every manual covers:

• Vent to openings: The fuel gas code commonly requires the vent to be at least 4 ft below or 4 ft horizontally from any door, operable window or gravity air inlet, or 1 ft above it. It must also be at least 3 ft above any forced-air inlet within 10 ft.
• Overhangs and roofs: Outdoor heaters vent straight up. Many can't go under a roof overhang, deck or awning at all, and others need a large clearance. A covered or enclosed location usually means an indoor-rated install with a vent kit and combustion air, which is a job for a pro.
• Pool edge: Many manuals require at least 5 ft between the heater and the inside wall of the pool or spa unless a permanent barrier separates them. Local codes may add to this.
• Service access: Leave room to remove the front panel and reach the controls and burner tray.
• Other equipment: Keep heat pump and A/C condenser exhaust from blowing across the heater.
• Water and landscaping: Aim sprinklers away from the heater, and keep shrubs and debris clear of the air openings.

The pad: Set the heater on a level, non-combustible pad, raised above any spot where water collects. In Texas, check periodically for ants, wasps and rodents. They nest in heaters and cause ignition faults.

Step 4: Plumbing

1. Position: Install the heater after the filter (and after solar, if you have it) and before any chlorinator or salt cell.
2. Check valve: Put a check valve between the heater outlet and the chlorinator so concentrated chlorine can't flow back into the heat exchanger when the pump stops.
3. Unions: Use them on both connections so the heater can be serviced or replaced without cutting pipe.
4. Pipe material: Follow the manual for the pipe and fittings allowed at the heater connections. Some models require heat sinks or specific connectors.
5. Height relative to the water: If the heater sits well above or below the pool's water level, the water pressure switch may need adjusting. The manual gives the thresholds.

Step 5: Flow and bypass valves

Every heater has a minimum and a maximum flow rate.

• Too little flow and the pressure switch won't let the heater fire, or the heater overheats.
• Too much flow wastes heat transfer and can damage the heat exchanger over time.

Many residential heaters include an internal automatic bypass that handles a typical flow range, often up to about 120 GPM. Check your model's spec. If your pump can push more than the heater allows, for example on a big pool or one with high-flow features, add an external manual bypass. That's a parallel pipe with a valve that sends part of the flow around the heater. Set it so the heater sees flow inside its rated range.

Variable-speed pumps: Low overnight speeds may not close the heater's pressure switch. Set a dedicated "heater" speed, or let your automation raise the pump speed whenever the heater calls for heat.

Step 6: Electrical and controls

• Most heaters accept 120V or 240V for the controls and ignition. Set the voltage as the manual shows.
• Connect the bonding wire (#8 solid copper) to the heater's bonding lug.
• If you have automation, wire the heater's remote or fireman's switch connection and turn on heater cooldown. That keeps the pump running a few minutes after the burner shuts off, which protects the heat exchanger.

When to call a plumber

A handy owner can set the pad, place the heater and handle the water plumbing. Leave these to a licensed professional:

• Any gas piping: running, sizing, connecting or extending the line
• Meter upgrades and regulator changes (coordinated with your gas utility or propane supplier)
• Pressure testing, purging and leak checking the gas line
• Measuring supply and manifold pressure with a manometer at start-up
• Indoor or enclosed installs, vent kits and combustion air
• Permits and inspections, which most cities require

In Texas, fuel gas piping is licensed plumbing work. Also check your warranty, since some manufacturers require installation by a qualified professional for full coverage.

Start-up and first firing

1. Confirm the pump is running and water is flowing through the heater.
2. Have the gas line leak-tested and purged, and the pressure verified.
3. Turn on the heater and set the target temperature.
4. A light odor on the first firing is normal. It's manufacturing oils burning off.
5. Brief condensation when the water is cold is also normal.

After that, keep your water balanced. Low pH and low alkalinity eat copper heat exchangers. If you have a salt pool, confirm your heater is rated for it.

Gas pool heater FAQs

What size gas heater do I need?

Most residential pools, especially those with an attached spa, are best served by a 400,000 BTU heater. Smaller pools, or pools heated only occasionally, can use 250,000–300,000 BTU.

What size gas line does a pool heater need?

It depends on the BTU rating, the run length and the gas type. For a 400,000 BTU natural gas heater, 1-1/4" or larger is common on typical runs. Have a licensed plumber size it from the code tables.

Can I install a gas pool heater myself?

You can do the pad, placement and water-side plumbing. Gas piping and the gas connection should be done by a licensed plumber, and most cities require a permit.

Natural gas or propane?

Where natural gas is available, it's usually cheaper to run. Propane works well when there's no gas line at the house, but the tank has to be sized for the heater.