Start Here — The Flow Check (Fixes 90% of Problems)
A clogged filter creates low water flow. Every pool heater has a pressure switch that shuts the heater off when flow is inadequate — this is a safety feature, not a malfunction. Before calling a technician, work through the basic flow check:
Backwash or Clean Your Filter
Sand and DE filters: backwash until the sight glass runs clear. Cartridge filters: remove and rinse the cartridge. A dirty filter is the #1 cause of heater shutoff.
Check the Pump Basket for Debris
Leaves, debris, and dead bugs in the pump basket restrict flow significantly. Clear it out completely before testing the heater.
Verify Pump Is Running at Full Speed
Variable-speed pumps set too low won't generate enough flow to satisfy the heater's pressure switch. Run the pump at full speed when operating the heater.
Retry the Heater
After cleaning filter and basket, wait 2–3 minutes for flow to stabilize, then attempt to start the heater again. Most shutoff issues resolve at this step.
Gas Heater Problems
Ignition Failure / Won't Light
Check that the gas supply valve is fully open. On older models with a standing pilot, confirm the pilot is lit. On electronic ignition models, listen for the igniter clicking — no click indicates an igniter or control board issue. Verify the gas meter hasn't been shut off by your utility.
Pressure Switch Error
Almost always flow-related. The pressure switch opens when water flow drops below the minimum threshold. Clean your filter first — this resolves the error the vast majority of the time. If the error persists after cleaning, the switch itself may have failed or the impeller may be worn.
Heat Exchanger Scale
Calcium and mineral buildup inside the heat exchanger tubes reduces heat transfer efficiency and can eventually cause the exchanger to crack. Symptoms: heater runs but water barely warms, plus potentially visible white scale deposits. Descaling service is required — this is not a DIY job on a gas heater.
Common Gas Heater Error Codes
- Hayward "LO" — Low flow. Clean filter, check pump.
- Pentair "HLS" — High limit switch tripped. Usually overheating from low flow or a failed thermostat.
- Raypak "E05" — Pressure switch error. Flow issue — clean filter first.
- Jandy "Flow SW" — Flow switch error. Same root cause as above.
Heat Pump Problems
Not Heating in Cold Weather
Heat pumps extract heat from ambient air. Below approximately 50°F, there isn't enough heat energy in the air to make the unit efficient — output drops sharply and operating costs spike. This is normal operation, not a malfunction. If you need to heat your pool in early spring or fall, a gas heater is the better tool for that temperature range.
Flow Switch Error
Same root cause as gas heaters: dirty filter restricting flow. The heat pump's flow switch shuts the unit off to protect the titanium heat exchanger from running dry. Clean your filter and verify full pump speed before troubleshooting further.
Low Refrigerant
The only visible symptom is the unit running normally (fan spinning, compressor on) but producing no heat. Low refrigerant requires a certified HVAC technician — refrigerant handling is regulated and not a homeowner repair. Do not attempt to add refrigerant yourself.
Titanium Heat Exchanger Corrosion (Saltwater Pools)
Saltwater pools with improperly balanced chemistry — particularly high salt levels or acidic water — can corrode the titanium heat exchanger over time. Look for visible pitting, greenish staining, or actual leaks around the heat exchanger housing. Professional replacement is required.
Solar Heater Problems
Solar Sensor Not Triggering (Pump Not Diverting to Panels)
Solar controllers divert water to the panels only when the panels are meaningfully warmer than the pool — typically an 8–10°F differential. If your pool is already near panel temperature (cloudy day, late afternoon), the system won't run. This is normal. Check the differential setting in your solar controller; if it's set too high, the system will never activate.
Panels Clogged or Air-Locked
Solar panels can accumulate scale, algae, or debris inside the tubes, especially after winter. Flush them from the bottom with a garden hose. Air locks in the panels prevent water from flowing through — purge by opening the air release valve at the top of the panels if equipped.
Diverter Valve Stuck or Not Actuating
The three-way diverter valve routes water either to the panels or bypasses them. If it's stuck, water never reaches the panels. Manually rotate the valve to confirm it moves freely. Failed actuators (motorized versions) can be replaced without replacing the full valve body.
Insufficient Panel Coverage
Solar heating requires roughly 50–75% of the pool's surface area in panel square footage to achieve meaningful temperature rise. An undersized system will run constantly but provide little warming. This is a design issue, not a malfunction — additional panels are the only fix.
When to Call a Professional
Situations that require a certified pool or HVAC technician:
- Error codes that persist after cleaning the filter and verifying flow
- Heat exchanger replacement (gas, heat pump, or solar)
- Refrigerant handling on heat pumps (legally required)
- Any gas line work, gas valve replacement, or burner assembly
- Control board or ignition module failure
- Leaks from the heat exchanger or plumbing connections
For everything else: try the flow check first. If the heater still won't run after confirming clean filter, full pump speed, and adequate flow — then call a tech with the specific error code in hand.
Frequently Asked Questions
Why does my pool heater keep shutting off?
Almost always low water flow from a dirty filter. Clean or backwash the filter, then retry. If the heater shuts off again after cleaning, check the pump basket, verify the pump is at full speed, and look for any closed valves in the circulation line.
How long should a pool heater last?
Gas heaters: 7–12 years. Heat pumps: 10–15 years. Solar: 15–25 years with proper maintenance. Lifespan is heavily influenced by water chemistry — keep pH and calcium balanced to avoid heat exchanger damage.
Can I run my pool heater with a dirty filter?
The heater will shut itself off via the pressure switch. You can't run it effectively — clean the filter first. Repeatedly trying to force the heater on with low flow can damage the pressure switch and potentially overheat the heat exchanger.
Why won't my heat pump work in cold weather?
Heat pumps are inefficient below 50°F ambient air temperature. This is by design — there simply isn't enough heat energy in cold air for the refrigerant cycle to work effectively. Use a gas heater if you need to heat your pool in early spring, fall, or winter conditions.
How do I know if my heat exchanger is bad?
Gas heater: look for water in the firebox or combustion chamber, rust stains around the heater base, or water dripping from the bottom of the unit. Heat pump: running but not heating despite correct flow and confirmed refrigerant charge. Both situations require professional inspection — do not operate a heater with a suspected cracked heat exchanger.
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