Hot Tub Won't Heat? The Diagnostic Order That Saves You a Heater

“No heat” is the most misdiagnosed hot tub complaint, because the heater is the last thing that usually failed. Spas cut heating on purpose for a dozen protective reasons; the repair skill is reading which one.

Check in this order

1. The display. FLO/FLC codes mean flow, not heat: dirty filter (the cause in half of all cases — clean or pull it and retest), low water, closed valves, an airlock after a refill, or a tired circulation pump. The heater is fine; it’s being protected.
2. The obvious embarrassments: mode set to economy/sleep, a schedule that only heats at night, a tripped GFCI breaker half-thrown (reset fully off, then on).
3. The high-limit reset — many packs have a red button on the heater housing; a one-time trip after a filter clog is normal, repeated trips are a flow problem again.
4. The pressure/flow switch — the little switch that certifies water is moving. They stick and fail cheap ($15–$40) and mimic heater death perfectly.
5. Only now, the element. Kill power, disconnect leads, meter it: a healthy element reads roughly 9–12 ohms; open (infinite) is dead, and any continuity to ground means it’s leaking current and likely tripping the GFCI too. Elements run $30–$80; a full heater assembly $150–$400.

The corrosion subplot

Elements rarely die of age — they die of water chemistry. Low pH etches the sheath pinhole-thin until it grounds out. If you’re on your second element in two years, the repair is a test strip habit, not another part.

Repair vs. replace vs. call

Filter cleans, resets, flow switches, and element swaps are honest DIY on a depowered spa (photograph the wiring first, and treat the 240V pack with the respect it will not extend to you). Board-level faults, relays that chatter, or anything you can’t confidently de-energize is technician territory — a $150 service call is cheaper than being the conductive path.