AquaCal pool heat pumps are among the most energy-efficient pool heating options available, but the FLO error code stops them cold — literally. FLO means the unit's flow switch did not detect adequate water flow through the titanium heat exchanger. The unit shuts down immediately to protect the compressor and refrigerant circuit from operating without water cooling. Here is how to diagnose and fix every cause of FLO.
Unlike gas heaters that heat a relatively small heat exchanger with combustion, pool heat pumps transfer heat from refrigerant to pool water via a large titanium tube-in-tube or plate exchanger. If water flow through this exchanger is inadequate, the refrigerant side temperature builds rapidly, the high-pressure safety trips, and the compressor shuts down. The flow switch is the first line of protection before the refrigerant-side safety devices.
AquaCal units require a minimum flow rate of 20–25 GPM for smaller units (50K BTU) up to 45+ GPM for large commercial units. Always verify your heat pump's minimum flow requirement against your actual measured flow rate.
This is the most common cause of FLO in the field — and the easiest fix. A filter that needs backwashing increases system head pressure, which reduces flow rate. On variable-speed pump installations, the pump may try to compensate by increasing RPM, but pressure-side clogs will ultimately restrict flow below the heat pump's minimum. Clean the filter first, always.
Many heat pump installations use a variable-speed pump that runs at low speed for circulation and higher speed only during scheduled pump cycles. If the heat pump is programmed to run during low-speed periods, flow may be insufficient. Check the VSP program schedule — ensure that any schedule that enables the heat pump also runs the pump at high enough speed (typically 2,500–3,000 RPM minimum) to achieve the required flow.
The flow switch itself — typically a paddle-type or thermal-dispersion style — can fail. The paddle can break or stick in the closed position, while the thermal sensor type can drift out of calibration. Test by measuring continuity across the switch with the pump running at full speed. If the switch reads open with confirmed good flow (measure with a flow meter), replace the switch. AquaCal flow switches are typically $20–45.
Air trapped in the heat exchanger or plumbing creates a partial blockage that prevents the flow switch paddle from lifting fully. This is common after startup from winterization or after any plumbing work. Run the pump at high speed with all valves open to purge air. Watch the heat pump's water connections for gurgling or irregular flow sounds.
If the bypass valve that allows water to bypass the heat pump is too open, flow through the unit drops below the minimum. Verify bypass position — most heat pumps should have the bypass completely closed or nearly closed during operation.
AquaCal units are often installed with isolation valves for winter removal. A valve that's not fully reopened after winter is a surprisingly common FLO cause in spring startup season. Verify all valves in the heat pump's plumbing loop are fully open.
| AquaCal Model | Min Flow (GPM) | Flow Switch Part |
|---|---|---|
| SuperQuiet 55 / SQ55 | 20 | Contact AquaCal for model-specific part |
| TropiCal T115 | 25 | Contact AquaCal for model-specific part |
| HeatWave SuperQuiet 120 | 30 | Contact AquaCal for model-specific part |
Warning: Never bypass the AquaCal flow switch. Operating a heat pump without adequate water flow causes high refrigerant pressure, which trips the refrigerant-side safety and can damage the compressor — a $400–800 repair.
Look up AquaCal error codes and flow requirements by model in PoolLens — available offline at the equipment pad.
PoolLens covers heat pump FLO errors and fault codes across all major brands. Free, offline, works without cell service.
Open PoolLens Free →FLO on an AquaCal heat pump means the flow switch is not detecting adequate water flow through the heat exchanger. The unit shuts down to prevent operating without water cooling the refrigerant circuit.
The flow switch is located on the water inlet or outlet pipe inside the AquaCal unit, typically a paddle-type switch installed in the 1.5 or 2-inch plumbing run through the unit.
Yes. If the pump speed is set too low (below the heat pump's minimum flow requirement, typically 20–30 GPM), the flow switch won't close. Always confirm the pump's low-speed program meets the heat pump's flow requirement.
With the pump running, locate the flow switch wires and measure continuity across the switch terminals. A properly functioning paddle-type flow switch should show continuity (closed) when water is flowing. An open reading with flow present means a failed switch.
The FLO code is standard across AquaCal's SuperQuiet, TropiCal, and HeatWave series. The flow switch part number and location vary by unit size, but the diagnostic process is the same.