Hot tub not heating? Check these first

A heater only runs when water is flowing through it. A pressure or flow switch checks that, and a high-limit switch shuts everything down if the water gets too hot; both trip when flow drops. That is why Hot Spring's first answer to “does not heat, jets and lights work” is a clogged filter or an air lock, not the heater. Settings come next: Bullfrog's X Series leaves the factory in an Energy Saver mode that holds the set point 20°F lower during daytime hours, which can look like a heater fault.

1. Read the display. A code names the fault: FLO means no water flow, dr means the heater may be dry; for the rest, see hot tub error codes.
2. Check the set temperature and any economy, sleep or weekender mode.
3. Turn the tub off, then pull and clean the filters.
4. Top up the water to the level your manual shows.
5. After a refill, purge the air lock as your manual describes; Jacuzzi's uses a garden hose at the filter fitting.
6. Turn the power off for at least 30 seconds and back on, which Hot Spring says resets its heater high limit.
7. Keep the cover on while it heats.

Usually trapped air. Filling leaves pockets in the pump and heater lines that block flow, so the flow switch keeps the heater off. Jacuzzi's manual describes priming the pump after a refill and letting air out by removing the filter cartridge. Hot Spring's troubleshooting points to air locks in the plumbing too. See weak hot tub jets for the jets side of the same problem.

It depends on the heater's size, the gallons and how cold the water starts. See hot tub heaters for heater sizes by brand and what they mean for heat-up time, and hot tub temperature for safe settings.