A pool heater that won't fire, won't heat, or keeps shutting off is one of the more complex service calls in the field. Gas heaters and heat pumps fail in completely different ways, and diagnosing one while thinking about the other wastes time. This guide covers both types systematically.
Gas appliance service that involves combustion components, gas lines, or heat exchangers should only be performed by licensed technicians in most jurisdictions. Know your state's licensing requirements before doing hands-on gas heater work beyond basic checks.
Modern gas heaters (Hayward H-Series, Pentair MasterTemp, Jandy LXi) display error codes on the control panel. Common codes:
| Code | Model | Meaning | Common Fix |
|---|---|---|---|
| AO | Hayward H-Series | Air/Oxygen — combustion air problem | Check flue, bird nests, vent clearance |
| BD | Hayward H-Series | Bypass Detect — low flow | Check filter pressure, pump speed, bypass valve |
| E05 | Pentair MasterTemp | High limit switch tripped | Check flow, scale on heat exchanger |
| E06 | Pentair MasterTemp | Stack flue sensor fault | Sensor replacement or combustion issue |
| E01 | Jandy LXi | Ignition failure | Check gas pressure, igniter, flame sensor |
| SFS | Various | Service Filter Soon — high pressure drop | Clean or backwash filter |
Most heater lockouts are flow-related. The heater's pressure switch requires a minimum flow rate to allow ignition. Common causes of insufficient flow:
For variable speed pump pools: the pump must run at a minimum speed when the heater is calling for heat. Many automation systems handle this automatically, but check that the heater-on speed is programmed correctly (typically 2,500–3,000 RPM for most heaters).
If gas supply is confirmed but the heater won't ignite:
A pool heat pump is essentially a reverse air conditioner — it extracts heat from ambient air and transfers it to pool water via refrigerant. This makes them extremely efficient (COP 5–7: 5–7 BTUs of heat per BTU of electricity) but dependent on ambient temperature. Below 50°F ambient, efficiency drops sharply; below 45°F, most units lock out.
| Symptom | Likely Cause | Fix |
|---|---|---|
| Unit runs but pool doesn't heat | Ambient temp too low; refrigerant low | Check ambient temp; call HVAC tech for refrigerant |
| Icing on evaporator coil | Low ambient temp; restricted airflow; low refrigerant | Clear debris around unit; check temp; check refrigerant |
| Unit won't start | Low flow lockout; low ambient temp lockout; electrical fault | Check flow and pump speed; check ambient; check breaker |
| FL error code | Flow switch not sensing flow | Increase pump speed; check flow switch |
| High-pitched screeching | Compressor bearing failure | Compressor replacement — major repair |
A heat pump must have unrestricted airflow around the evaporator coil and fan. Common installation errors that reduce performance:
Log heater model, error codes, and service history per account in SplashLens. Repeated E05 flow errors on a Pentair MasterTemp signal a scaling heat exchanger — catching this early avoids a $500–1,500 heat exchanger replacement.
SplashLens stores heater model, recurring error codes, and all service notes per account — so repeat heater issues get resolved permanently, not re-diagnosed from scratch every visit.
Open SplashLens Free →The most common causes are insufficient water flow (dirty filter, low pump speed, closed bypass valve), a failing pressure switch, an overheating heat exchanger from scale buildup, or a faulty high-limit switch.
Most heat pumps require ambient air temperatures above 50°F to operate efficiently. Below 45°F they may lock out entirely. For cold climates, a gas heater is more reliable for shoulder-season heating.
A gas heater raises pool temperature 1–2°F per hour. A heat pump raises it 1–2°F per hour in good conditions. Heating a 20,000-gallon pool from 60°F to 80°F typically takes 1–2 days with an appropriately sized heater.
AO (Air Oxygen) usually indicates a combustion air issue — blocked vent, bird nest in flue, or inadequate combustion air supply. BD (Bypass Detect) indicates a flow problem — the heater senses insufficient flow and shuts down safely.