How to Winterize a Gas Pool Heater Without Trapping Water

Let's be honest about winterizing a gas pool heater: turning it off is the easy part. The real challenge is making sure water does not remain trapped inside the heat exchanger, headers, or connected plumbing when freezing temperatures arrive. Even a relatively small pocket of trapped water can expand as it freezes, potentially cracking expensive components. Understanding where water hides, how your particular heater drains, and which precautions matter most can help you avoid an unpleasant surprise when swimming season returns.

A gas pool heater needs more attention than simply switching the thermostat to OFF. The safest approach combines proper shutdown, complete drainage, attention to equipment layout, and a final check that the heater cannot accidentally restart while empty.

Why Trapped Water Is Such a Serious Problem

Gas pool heaters warm circulating pool water as it passes through a heat exchanger. Depending on the model, water travels through metal tubes, headers, manifolds, and internal passages before returning to the pool.

When temperatures fall below freezing, water remaining in these areas can turn to ice. Because ice occupies more space than liquid water, it can place substantial pressure on enclosed components.

Possible damage includes:

• Cracked heat exchanger tubes or headers.
• Split plastic manifolds and plumbing fittings.
• Damaged water pressure switches or their connections.
• Leaks that become apparent only when the system is restarted.
• Reduced heating performance or equipment failure requiring professional repair.

Freeze damage may not be visible from outside the heater cabinet. A heater can appear perfectly normal throughout winter, only to leak when water circulation resumes in spring.

Start With the Heater's Exact Winterizing Instructions

Before touching drain plugs or plumbing connections, locate the manufacturer's winterization procedure for your heater's exact model. This is essential because gas pool heaters do not all drain the same way.

Some models have a drain plug on the inlet/outlet header. Others include a second drain on the opposite side of the heat exchanger. Certain designs require draining through a plumbing union or following additional steps involving a pressure switch.

Do not assume that removing one visible plug empties the entire heater. A procedure that works for one model may leave water trapped in another.

Identify the heater's model number, find its specified drain locations, and determine whether any steps require a qualified pool or gas-appliance technician. Never disassemble gas connections, combustion components, or electrical controls as part of a homeowner winterization attempt.

How to Winterize a Gas Pool Heater Step by Step

The following sequence explains the major stages of winterization. It is a general guide, not a replacement for model-specific instructions or professional service.

1. Shut Down the Heater and Allow It to Cool

Begin by turning the heater off using its normal controls. Follow the manufacturer's shutdown sequence, including any required cooldown circulation period, before stopping the pool pump.

Disconnect electrical power to the heater at the appropriate breaker or disconnect, and shut off the gas supply using the designated manual shutoff valve in accordance with the manufacturer's instructions.

Allow the equipment to cool completely before working around water connections. Hot components and pressurized water can cause injuries.

2. Stop Water Flow and Identify the Plumbing Layout

Turn off the circulation pump and prevent automatic schedules or freeze-protection controls from restarting it during winterization.

Next, examine how the heater connects to the rest of the equipment pad. Determine whether the heater sits above or below the pool's water level.

This difference matters. A heater installed above pool water level may drain naturally through its designated openings. A heater below pool water level can receive continuing water flow from the pool unless the plumbing is properly isolated.

Where appropriate, close existing isolation valves according to the equipment manufacturer's instructions. If the system lacks suitable valves or the plumbing arrangement is unclear, have a pool professional handle the draining.

3. Open Every Required Heater Drain Point

Locate the drain plugs identified in your heater manual. Place a suitable container beneath the drainage area if needed, then open the specified drains using the recommended procedure.

Watch for water leaving the heater. The flow may begin strongly and gradually slow as the internal passages empty.

On certain heaters, a return-header drain is located behind an access panel opposite the main plumbing connections. Missing this second drain is a common reason water remains inside an otherwise winterized heater.

Do not force a stuck plug, damage its sealing surface, or remove unidentified fittings. If access requires disassembly beyond the manufacturer's homeowner instructions, contact a qualified technician.

4. Address Water Trapped in Manifolds and Connections

Draining the main heat exchanger does not necessarily empty every connected passage. Depending on heater design and installation, water can remain in a plastic manifold, horizontal pipe section, or fitting positioned below the natural drainage route.

Some heater manufacturers specify loosening or disconnecting a particular water union to drain these areas. Others provide dedicated drain openings or additional instructions for pressure-switch connections.

Follow only the method approved for your exact heater. Never assume that loosening an arbitrary fitting will safely release trapped water.

5. Use Air Purging Only When Specifically Approved

Some gas pool heater manufacturers include a controlled air-purging procedure to help evacuate residual water. However, using compressed air incorrectly can create dangerous pressure inside plumbing and equipment.

Never connect an ordinary high-pressure shop compressor to a heater and attempt to blow water out without approved equipment and instructions. Where air purging is permitted, the required pressure, airflow, connection point, and sequence are model-specific.

If you cannot verify those requirements, leave the procedure to a qualified pool professional.

6. Complete the Heater's Winter Storage Procedure

Once the required drains are complete, follow the manufacturer's instructions for storing or reinstalling drain plugs, protecting exposed openings, and preventing insects or debris from entering the cabinet.

Keep removed parts labeled and stored securely. Some models require plugs or components to remain removed during storage, while others specify reinstallation after draining.

Do not wrap the entire heater in airtight plastic or block ventilation and exhaust openings with an improvised cover. Use only a manufacturer-approved protective arrangement.

Where Water Commonly Gets Left Behind

The most troublesome winterization mistakes often involve parts that are not immediately visible.

The Return Header

On some gas heaters, the return header is on the opposite side from the inlet and outlet connections. Draining the accessible plumbing-side header may not empty this separate area.

Low Sections of Connected Plumbing

Horizontal pipes, elbows, and low points near the equipment pad can hold water even after the heater itself drains. These sections need to be winterized as part of the overall pool plumbing system.

Pressure-Switch Passages

Some heater designs have small water-filled passages associated with pressure switches. Manufacturers may specify additional drainage or removal procedures for these components. Because improper handling can damage safety controls, follow the exact instructions or use a qualified technician.

Shared Pool and Spa Plumbing

A pool with an attached spa may have more complicated valve positions, elevated water levels, check valves, and separate circulation paths. These arrangements can affect whether water drains completely or flows back into the heater.

Document valve positions before changing them, and make sure both pool and spa circulation routes are addressed.

Common Winterization Mistakes That Can Cause Freeze Damage

Another mistake is waiting until a hard freeze is already underway. Drainage is easier and safer when equipment is accessible, water is still liquid, and there is time to resolve an unexpected problem.

Do You Need to Winterize a Heater in a Mild Climate?

Not every pool owner closes their pool for winter. In warmer climates, pools and heaters may remain in service throughout the year.

However, even locations with generally mild winters can experience unexpected freezing temperatures. Outdoor equipment in exposed locations can be vulnerable, especially during extended cold spells or power outages.

Some installations use automatic freeze-protection circulation, but these systems depend on electrical power, correctly functioning controls, and adequate water flow. They should not be confused with a fully drained winterized heater.

If the pool will remain operational, follow the heater manufacturer's cold-weather requirements and consult a pool professional about the protection appropriate for local conditions.

How to Tell Whether the Heater Was Properly Drained

There is no universal visual test that proves every internal passage of a gas heater is completely empty.

A more reliable approach is to confirm that all manufacturer-specified drain points were opened, the required additional drainage procedures were completed, and no plumbing arrangement could have refilled the heater afterward.

If you discover an overlooked drain after freezing weather has already occurred, do not restart the equipment simply to see whether it works. Arrange an inspection first.

Likewise, water appearing around the heater during spring startup may indicate a loose fitting, damaged seal, or freeze-damaged component. The source needs to be identified rather than assumed.

What About Unexplained Pool Water Loss?

Heater drainage and pool water loss are different concerns, but they can overlap during seasonal equipment checks. If a pool begins losing water after equipment is returned to service, inspect visible plumbing and heater connections for leakage before drawing conclusions about the pool shell.

When the source is not obvious, the Mini Bucket Test can provide a simple first step for comparing normal evaporation with possible leak-related water loss. It cannot locate a leak or establish that the heater is responsible, but it may help determine whether further investigation is worthwhile.

When Should You Call a Pool Professional?

Professional winterization is especially worthwhile when the heater has hidden drain locations, complicated spa plumbing, inaccessible fittings, or an installation below pool water level.

You should also seek qualified assistance if a drain plug is seized, water continues flowing unexpectedly, the equipment has previously experienced freeze damage, or the manufacturer's procedure involves internal safety components.

Gas piping, burners, and combustion controls should be serviced only by properly qualified personnel. Never attempt to test for gas leaks with an open flame or bypass a safety control to make a heater operate.

Getting the Heater Ready for Spring

Winterization is not complete without a plan for recommissioning the heater. Before restarting, verify that all required plugs and water connections are correctly restored, valves are positioned properly, and the circulation system is ready.

The heater must not operate until its manufacturer-required water flow is established. Check for visible leaks while the circulation system runs, and follow the specified startup and safety inspection procedure.

If there is evidence of freeze damage, unusual operation, or a gas-related concern, keep the heater off and arrange professional service.

The Bottom Line: Drain Every Required Passage, Not Just the Obvious One

Properly winterizing a gas pool heater means removing water from all the areas the manufacturer identifies, not merely shutting off the gas and opening the easiest drain plug.

The details that matter most are the heater's specific construction, the locations of its drain points, the elevation of the equipment relative to the pool, and whether connected plumbing can retain or reintroduce water.

Use the exact manufacturer's procedure, avoid unapproved compressed-air methods, and get professional help whenever the system's drainage path is uncertain. A careful winter shutdown gives your heater a much better chance of returning to service without freeze-related damage when the next swimming season arrives.