Flow / Low-Flow Error

When a pool heater or heat pump refuses to fire because it can't confirm water is moving through it, the cause is almost always upstream of the equipment itself, not inside it.

What's usually going on

Gas heaters and heat pumps don't measure flow directly — they infer it from a pressure or flow switch that only closes once water moving through the unit creates enough pressure to trip it. That switch won't close, and the equipment won't fire, unless several things line up: the pump has to actually be running, and on variable-speed setups it has to be set to a high enough speed to generate that pressure; the filter has to be clean enough not to choke flow before it reaches the heater; every valve on the path has to be open rather than throttled; and the water level has to be high enough, with no air pocket trapped in the plumbing, to keep flow steady. Only after all of those check out does a genuinely stuck or failed switch become the likely explanation. Because the switch is a safety device — it exists so a heater never fires with no water moving through its heat exchanger — none of these brands let you bypass it, and none of these codes should be reset repeatedly without addressing the actual flow restriction underneath.

Codes that show this symptom

• Hayward LO — Limit String Open (includes the water pressure switch, among other limit switches in the same string)
• Pentair ERR PS — Water pressure switch open
• Raypak PRS — Low water pressure
• Raypak E3 — Flow fault on Crosswind heat pump pool heaters

Full code lists: Hayward, Pentair, Raypak.

Checks that apply regardless of brand

1. Confirm the pump is actually running and, on a variable-speed system, that it's set to a high enough speed for the heater — this is the single most common cause across every brand listed here.
2. Clean or backwash the filter and clear out the pump's strainer basket; a dirty filter is the second most common cause.
3. Make sure every valve feeding the equipment is fully open, and bleed any trapped air at the filter's air relief valve.
4. On a heat pump specifically (Raypak E3), check the filter separately, since the heat pump has no pump of its own and depends entirely on the filter pump already running.
5. Once flow looks normal, clear the fault and let the equipment retry — a genuinely stuck or failed switch is possible, but it's the least common cause of the four.