How to Winterize a Pool Heater to Prevent Freeze Damage

Safe Shutdown Checklist for Power and Fuel Supplies

Every pool heater requires a complete energy shutoff before you touch a drain plug or loosen a pipe connection. Leaving electrical lines live or gas valves slightly open invites serious equipment damage and physical danger once the water drains away. If a heater tries to fire without water moving through its internal piping, the metal scorches and destroys the internal components within seconds.

For natural gas and propane units, locate the external manual gas valve on the rigid pipe leading into the heater cabinet. This valve usually looks like a brass lever or a square nut resting along the black iron pipe. Turn this handle a quarter turn so it sits perpendicular to the pipe, which physically blocks the flow of gas. Then, open the front heater access door and turn the internal gas knob to the off position.

Electric power must be disconnected at the electrical source. Walk to your main household breaker panel or the dedicated pool subpanel near the equipment pad. Flip the heater breaker switch completely to the off position. If your unit is plugged into an outdoor GFCI outlet (a protected ground-fault outlet with a weatherproof cover), unplug the cord and secure it off the wet concrete.

Electric heat pumps require the exact same electrical precaution. Turning off the digital thermostat on the heat pump display screen is never enough, because the system still holds active voltage across its internal capacitors and compressor terminals. Switching off the breaker prevents digital timers or unexpected warm afternoons from firing up an empty compressor.

Here is a quick shutdown checklist to verify before opening any water lines:

• Dedicated heater circuit breaker flipped completely to the off position.
• Outdoor gas supply shutoff valve turned perpendicular to the supply pipe.
• Internal gas control knob inside the heater cabinet switched to off.
• Pool automation system set to winter closing mode or service mode.
• Circulation pump power breaker turned off so water does not surge into the unit.

If you smell rotten eggs or notice a hissing sound near a gas heater, stop working immediately. Do not touch any electrical switches, do not attempt to force a stuck valve with a wrench, and do not use a phone near the pad. Step clear of the pool area and call your local gas utility provider or a certified technician.

Locating and Removing Drain Plugs to Dump Trapped Water

Water expands by roughly nine percent as it turns into solid ice. Inside your heater, water flows through narrow tubes known as the heating core. If even a small cup of water stays inside these metal or titanium tubes during a hard freeze, the expanding ice splits the metal walls wide open. Replacing a ruptured heating core costs almost as much as buying an entirely new heater.

Draining begins by finding the factory drain ports. Most gas heaters have two drain plugs located near the bottom of the header, which is the heavy plastic or cast-iron end piece where the plumbing pipes enter and exit the unit. One plug sits on the front header directly beneath the pipe connections, while the second plug sits on the rear header on the opposite side of the heater casing. Heat pumps generally have a single drain plug or threaded cap placed low on the water inlet manifold.

Empty the standing water by following these numbered steps:

1. Turn off your pool circulation pump and open the air relief valve on top of your pool filter tank to bleed off standing system pressure.
2. Loosen the threaded unions on the plumbing pipes right where they attach to the heater. Loosening these pipe unions breaks the internal suction seal, allowing trapped water to rush toward the bottom of the heater rather than staying suspended like liquid in a straw.
3. Unthread the primary drain plugs on both sides of the heater using an open-end wrench or socket wrench. Have a shallow pan ready if your heater sits inside an equipment shed, or let the water drain directly across the outdoor equipment pad.
4. Disconnect the water flow sensor (a small switch that monitors water movement to protect the heater from dry firing). This switch threads into the plumbing header and uses a thin siphon tube that easily holds water. Loosen the brass compression nut on this tube so trapped water trickles out before freezing can crack the delicate brass sensor.
5. Inspect the rubber O-rings on the drain plugs for flat spots, cracks, or dry rot. Note down any worn rings so you can pick up fresh seals before opening the pool next spring.

If you are closing your whole filtration setup at once, you can coordinate this step with your other winter chores. If you manage a saltwater pool, following our guide on how to winterize a saltwater pool will help you empty your electrolytic cell and protect your equipment alongside the heater.

Clearing Remaining Water with Low-Pressure Air

Pulling the drain plugs empties standing water, but gravity alone cannot reach every pocket. A heating core consists of multiple horizontal tubes joined by tight U-shaped return bends. Water pools in the bottom curve of these bends, trapped where gravity cannot draw it out through the low drain openings. Pushing that residual water out with air is the only reliable way to prevent freeze damage in areas that face hard freezes.

You should never use a high-pressure automotive air compressor for this task. Compressors designed for pneumatic air tools run at eighty to one hundred pounds of pressure per square inch. That extreme pressure can shatter internal plastic manifolds, blow out header gaskets, or bend internal water flow sensors. A simple wet-dry shop vacuum set to blow or a dedicated swimming pool line blower creates high air volume at a gentle two to four pounds of pressure per square inch, which cleans out water safely without stressing the heater.

Clear the internal tubes completely by following these steps:

1. Attach your blower hose to the water inlet pipe where water travels from the filter into the heater. Wrap a hand towel around the nozzle or use a rubber plug with an air fitting to form a snug seal against the pipe.
2. Turn on the blower and watch the open drain ports and the water outlet pipe. You will see a heavy gush of water blast out across the pad, quickly followed by a sputtering mist.
3. Run the blower for two to three minutes without stopping until the water drops stop flying and the ports release only dry air.
4. Switch the blower hose to the water outlet pipe and push air in reverse through the heating core for another two minutes. Reversing the air current forces out water caught behind internal baffles and internal headers that the forward airflow missed.
5. Leave all drain plugs out of the unit once the lines run clear. Never thread the plugs back in for the winter, because leaving the ports wide open allows normal winter condensation to drain freely and lets air move through the cabinet.

Take your time on this blowing step. Rushing the process and leaving behind a single cup of pooled water in a bottom bend can split a copper tube during an overnight freeze in January.

Setting Detour Pipes, Storing Hardware, and Choosing the Right Cover

Once you clear out the moisture, adjust your plumbing valves to isolate the heater. If your equipment pad features a detour pipe (a bypass valve loop that routes water directly from the filter back to the pool), turn the valve handles to the bypass position. This setup isolates the dry heater completely from the rest of the circulation system. That way, if rising winter water levels enter the pool plumbing, or if you run winter lines later, no water can push into the dry heating core.

Loose drain plugs and rubber gaskets are notoriously easy to lose over the long winter months. Never leave tiny brass or plastic plugs resting loose on top of the heater or scattered on an equipment pad ledge. Place all plugs, O-rings, and sensor caps inside a clear zip-top bag. Store this bag directly inside the empty pump strainer basket with the pump lid screwed on loosely, or tape the bag to the inside face of the heater access panel. Every part will sit right where you need it when spring arrives.

Choosing a winter cover requires balancing moisture control against pest protection. Many pool owners wrap their heater tightly in an ordinary blue plastic tarp secured with tight bungee cords. An airtight plastic wrap creates a severe greenhouse effect, trapping ground humidity and evaporating moisture inside the metal cabinet. That trapped moisture causes heavy rust on gas burner trays, corrodes delicate circuit boards, and causes premature decay of exterior paint. Also, a dark, sealed cabinet creates a cozy haven for mice, which chew through wiring insulation and pack the heating core with nesting leaves.

Breathable vinyl or canvas covers specifically designed for pool heaters offer far superior protection. These purpose-made covers feature mesh ventilation strips along the side panels that allow trapped moisture to vent into the air while shedding snow, sleet, and falling leaves from the top. If you do not have a breathable heater cover, place a thick piece of exterior plywood across the top of the heater cabinet and hold it in place with a single concrete block. The wood keeps falling debris and heavy ice from dropping into the exhaust flue while leaving the sides completely open so air circulates freely and rodents stay away.

While you wrap up your pool equipment pad, remember that securing your pool deck is just as vital before the snow flies. If you install a winter safety cover across the water, check your deck fittings carefully. Reviewing the right steps for removing pool cover anchors without deck damage helps you keep brass anchor bolts in top shape without cracking surrounding concrete or stripping screw slots.

FAQs

Where are the drain plugs located on my pool heater?

Most gas pool heaters have two drain plugs located near the base of the front and rear plumbing headers where the water pipes enter the cabinet. Heat pumps generally have a single drain plug or screw-off cap positioned low on the water inlet or outlet manifold. If you cannot spot the rear plug on a gas heater, check the opposite side panel or review the equipment diagram on the inside face of the front access door.

Do I need an air compressor to blow out the lines?

No, an industrial air compressor is not required and can easily damage internal components if the pressure is too high. A standard wet-dry shop vacuum set to its blower port or a specialized pool line blower produces plenty of airflow at a safe pressure of two to four pounds per square inch. High-pressure garage compressors can crack fragile plastic manifolds, tear rubber gaskets, or destroy internal water flow sensors.

Should I leave the drain plugs out all winter?

Yes, you should leave all drain plugs out of the heater until you open the pool in the spring. Leaving the ports open allows any condensation or blowing moisture that gets inside the cabinet to drain out harmlessly instead of collecting in the bottom tubes. Place the loose plugs and O-rings inside a sealed bag inside your pump basket so they stay safe and easy to find.