Pool Heater Not Heating — Gas Error Codes, Heat Pump Faults & Heat Exchanger Service

Common Symptoms

• Gas heater displays error code IF, HH, AFS, or PS and will not heat
• Hayward H-series shows LO, HL, or SFS on the display
• Heat pump runs continuously but pool temperature rises less than 1°F per hour
• Heat pump displays E1, E2, E3, or HP error code
• Heater fires briefly but shuts off after 2–5 minutes without reaching setpoint
• Pool water temperature is 10°F or more below the set temperature
• Heater makes repeated ignition attempts (3 tries) then locks out

Most Likely Causes

1. 1

   Pentair IF Error — Ignition Failure (Gas Pressure or Electrode Gap)

   IF (Ignition Failure) on Pentair MasterTemp 400 (460737) is triggered after 3 failed ignition attempts. The three most common root causes: (1) Gas pressure below 3.5" WC for natural gas (11" WC for propane) — verify with a manometer at the gas valve inlet. (2) Hot surface igniter gap outside the 1/8" specification (measure with a feeler gauge — a gap over 3/16" won't arc reliably). (3) Dirty or oxidized flame sensor rod (clean with fine steel wool, restore bright metal). IF is distinct from AFS (combustion blower) and HH (high limit) — each requires a different fix.

2. 2

   Pentair HH Error — High Limit Trip (Scale or Low Flow)

   HH (High Header temperature) on MasterTemp fires when the heat exchanger outlet water temperature exceeds 135°F — almost always caused by restricted water flow or heavy scale inside the heat exchanger tubes. Reduced flow from a dirty filter, partially closed valve, or slow pump allows water to overheat locally before exiting. Scale from calcium carbonate buildup in hard-water areas reduces heat transfer and restricts flow simultaneously. Both conditions require different fixes: flow issues need valve/filter service; scale requires muriatic acid descaling.

3. 3

   Pentair AFS Error — Air Flow Sensor (Combustion Blower Fault)

   AFS indicates the combustion air blower is not achieving the required airflow for safe combustion. The most common cause is a partially blocked combustion air intake (leaves, dirt dobber nests in the vent screen). Second most common: a failed combustion blower motor (listen for it — it should be audible within 3 seconds of calling for heat). Third: the pressure switch sensing tube has become kinked or disconnected. Clear the intake screen first before condemning the blower.

4. 4

   Hayward LO Error — Low Water Pressure (Flow Switch Fault)

   LO on Hayward H400FDN indicates the pressure or flow switch on the heater inlet is not seeing adequate water flow. Minimum required flow is 25–40 GPM depending on the heater BTU rating. Most LO codes after a pump startup are caused by air in the heater that hasn't purged yet — wait 3 minutes after starting the pump before calling for heat. Persistent LO: check filter pressure (dirty filter restricts flow), check pump speed on variable-speed systems (minimum 2400 RPM for most Hayward heaters), and verify the bypass valve is not diverting too much flow around the heater.

5. 5

   Heat Pump E1 Code — Low Refrigerant or Low Ambient Temperature

   E1 on Hayward HeatPro HP50HA, Pentair 460933, and most heat pump brands indicates either low refrigerant charge or ambient air temperature below 50°F (10°C). Heat pumps extract heat from ambient air — when air temperature drops below 50°F, the heat pump cannot operate efficiently and enters protection mode. This is normal behavior, not a fault. If the ambient temperature is above 55°F and E1 persists, the refrigerant circuit has a low charge — refrigerant addition requires an EPA 608-certified technician.

6. 6

   Heat Pump E3 Code — High Pressure (Dirty Coil)

   E3 (high discharge pressure) is almost always a dirty evaporator coil on the heat pump. The coil extracts heat from ambient air; when it is clogged with pollen, grass clippings, or cottonwood fluff, the unit cannot exchange heat effectively and the compressor discharge pressure rises until the high-pressure safety trips. Clean the coil monthly during heavy pollen season with a low-pressure water rinse (garden hose from inside out, never a pressure washer).

7. 7

   Heat Exchanger Scale Buildup — Reduced Heating Efficiency

   In hard-water areas (calcium hardness above 400 PPM), calcium carbonate deposits inside the heat exchanger tubes over 3–7 years. Scale is a thermal insulator — a 1/16" scale layer can reduce heating efficiency by 30–50%. Symptoms: heater runs at full power but pool temperature rises very slowly; HH codes appear despite normal flow. Diagnosis: inspect the heat exchanger end caps for white mineral deposits. Fix: muriatic acid flush procedure (see Step 7).

Quick DIY Checks

1. 1Step 1 — Record the full error code sequence: Before touching anything, document the exact error code shown on the display. For Pentair MasterTemp: IF = ignition failure, HH = high header limit, AFS = air flow sensor, PS = pressure switch. For Hayward H-Series: LO = low water pressure, HL = high limit, SFS = service gas valve. Each code requires a completely different test procedure — do not skip straight to parts replacement without confirming the root cause. On MasterTemp units, hold the Mode button for 3 seconds to display the service history log, which shows the last fault code and how many times it has tripped.
2. 2Step 2 — Gas pressure verification (IF, SFS codes): Connect a manometer to the gas valve inlet test port (1/8" NPT fitting on the valve body, usually labeled 'P1'). With the heater calling for heat and the gas valve receiving 24V (you can verify with a multimeter), record the gas pressure. Minimum required: 3.5" WC for natural gas, 11" WC for propane. Below minimum: call the gas utility (natural gas) or check propane tank level and regulator (propane — minimum 20% tank fill). If pressure is adequate, proceed to igniter gap and flame sensor checks.
3. 3Step 3 — Igniter and flame sensor test (IF code, DSI and hot surface): Pentair MasterTemp uses a hot surface igniter (HSI) — silicon carbide or silicon nitride. Test: disconnect the igniter connector and measure resistance. A good SiC igniter reads 40–90Ω; a cracked or degraded igniter reads OL (open) or <20Ω. Gap the electrode at exactly 1/8" (3mm) using a feeler gauge. For the flame sensor rod: use fine steel wool or light sandpaper to clean the ceramic-insulated sensing rod until it shows bright metal. A contaminated flame sensor produces <1 µA DC signal — the control board interprets this as 'no flame' and shuts the gas valve after 4 seconds of flame proving. Hayward H-Series uses a spark igniter — inspect the ceramic insulator for cracks and clean the spark gap.

1. 4Step 4 — Flow verification (LO, HH, HL codes): Confirm minimum water flow is reaching the heater. With the pump running, measure filter pressure — it should be within 5 PSI of the clean baseline. Check that the heater bypass valve (if present) is set to allow maximum flow through the heater. For variable-speed pumps: the heater requires a minimum pump speed — typically 2400 RPM for Hayward H400FDN and Pentair MasterTemp 400. Verify the automation system is running the pump at adequate speed when calling for heat. LO and HH codes triggered by flow restrictions clear immediately once flow is restored — no manual reset required on most Hayward and Pentair units.
2. 5Step 5 — High limit switch test (HH, HL codes): The high limit switches on pool heaters are manual-reset or automatic-reset bimetal thermostats. On Pentair MasterTemp: locate the high limit switches (typically two — one for each header) on the inlet and outlet headers. With the heater cooled completely, test continuity across each switch — a good switch shows continuity at ambient temperature. If a switch reads OL when cold, it has failed in the open position and must be replaced. CRITICAL: Never bypass or jump a high limit switch to get the heater running. The high limit switch is the last line of defense against heat exchanger burnout — bypassing it can destroy the heat exchanger (a $300–$800 part) or cause a fire.
3. 6Step 6 — Heat pump coil cleaning (E3 code): Turn off the heat pump at the breaker. Using a garden hose with a spray nozzle (no pressure washer — coil fins are soft aluminum and bend easily), rinse the evaporator coil from the inside out: insert the hose nozzle between the fan guard and the coil and spray outward through the fins. For heavy pollen, use a fin cleaning spray (available at HVAC supply houses) applied from outside, soaked 10 minutes, then rinsed inside-out. After cleaning, restore power and start the heat pump. E3 should clear within 5–10 minutes once the compressor discharge pressure normalizes. Clean the coil monthly during peak pollen season.
4. 7Step 7 — Heat exchanger descaling procedure (muriatic acid flush): Turn off the heater and pump. Close the inlet and outlet valves at the heater. Disconnect the inlet and outlet plumbing unions at the heater (have towels ready). Connect a small circulation pump (a pool submersible pump works) with hoses to the heater inlet and outlet connections. Fill a 5-gallon bucket with a 1:10 solution of muriatic acid and water (always add acid to water, never water to acid). Circulate the solution through the heat exchanger for 30–60 minutes. CAUTION: do this in a well-ventilated area away from the pool — the acid fumes are irritating and the reaction generates CO2. Neutralize thoroughly: drain the acid solution, then circulate a baking soda solution (1 cup per gallon) through the exchanger for 15 minutes until no fizzing occurs when tested. Flush with plain water for 5 minutes before reconnecting to the pool system.
5. 8Step 8 — AFS combustion blower check: Remove the heater front panel to access the combustion compartment. With the heater calling for heat, verify the combustion blower (a small centrifugal fan on the burner tray) starts spinning within 3 seconds. If the blower doesn't start, test 120V at the blower motor terminals — no voltage means the board or relay is not energizing the blower. If voltage is present but the blower doesn't spin, the motor has failed. Also inspect the combustion air intake screen (usually on the side of the heater cabinet) for spider webs, wasp nests, or debris — a blocked intake reduces airflow enough to trigger AFS without any component failure.
6. 9Step 9 — Heat pump E1 ambient temperature and refrigerant check: If ambient air temperature is above 55°F (13°C) and E1 persists, the refrigerant circuit has a low charge. Signs of low refrigerant: ice forming on the evaporator coil, compressor running but pool temperature doesn't rise, high suction superheat on gauges. Refrigerant work requires EPA 608 certification — this is not a DIY repair. When calling a refrigerant technician, specify: (1) the refrigerant type (most residential pool heat pumps use R-410A), and (2) whether you have a saltwater pool — salt pool owners MUST have a titanium heat exchanger (not cupro-nickel), as copper alloys corrode rapidly in chlorinated saltwater. Confirm this was specified at purchase.

Repair vs Replace

Recommended Tools & Parts

• Pentair 460737 MasterTemp 400 Control Board

  OEM main control board for Pentair MasterTemp 400 BTU heater. Controls ignition sequence, error code display, high limit inputs, and gas valve output. Replace after confirming igniter, gas pressure, and sensors test correctly.

  $180–$280

  Buy on Amazon →

• Hayward H400FDN Igniter — IDXLIGN1930

  OEM hot surface igniter for Hayward H400FDN and H-Series FD heaters. Silicon nitride construction. Gap at 1/8" before installing. Also covers H150FDN, H200FDN, H250FDN, H300FDN.

  $25–$55

  Buy on Amazon →

• Pentair 472359 High Limit Switch

  Replacement high limit thermostat switch for Pentair MasterTemp and Max-E-Therm heaters. Trips at 135°F to protect heat exchanger. Replace when switch reads OL (open) at ambient temperature. Never bypass.

  $18–$35

  Buy on Amazon →

• Raypak P-R406A Gas Valve Assembly

  Combination gas valve for Raypak P-R406A and R-406 series pool heaters. Controls main burner gas flow on natural gas models. Test for 24V signal at valve terminals before replacement.

  $120–$200

  Buy on Amazon →

• Hayward HeatPro HP50HA Flow Switch

  Flow switch for Hayward HeatPro HP50HA heat pump. Triggers E2 code when water flow falls below minimum. Test by confirming adequate pump flow before replacing switch. Also covers HP21404T and HP21004T.

  $30–$55

  Buy on Amazon →

• Nu-Calgon Coil Cleaner (Heat Pump Evaporator)

  No-rinse foaming coil cleaner for heat pump evaporator coils. Dissolves pollen, cottonwood, and organic buildup from aluminum fin coils. Monthly application during heavy pollen prevents E3 high-pressure codes.

  $12–$20

  Buy on Amazon →

• AquaCal TropiCal T135 Defrost Control Board

  OEM defrost control board for AquaCal TropiCal T135 heat pump. Controls the HP defrost cycle sequence in ambient temperatures below 55°F. Replace when defrost cycle runs continuously or will not terminate.

  $80–$140

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

Related Repairs