The spa pack will not run the heater unless it is sure water is moving through it. A pressure switch or a flow switch has to close first; if it does not, the display shows FLO, LF, dr, OH or a flashing temperature and the heater stays off. The owner sees cold water and calls about the heater. Nine times in ten, the heater is fine.
Why flow fails in Seattle
Trees. Filters under firs and cedars in Wallingford, Laurelhurst, Ravenna, Finn Hill and Shoreline load with needles from September on. A loaded filter starves the pump, the switch never closes, the pack protects the heater. The same thing happens after a refill if air is trapped in the circ line, and when a circulation pump is getting lazy with age.
The five-minute check
Pull the filter and run the tub without it for a few minutes. If the code clears and the heater light comes on, you have found it: clean or replace the filter. If the tub was just refilled, loosen the union on the circ pump a quarter turn until water weeps out, then tighten — that bleeds the air. If neither helps and the pump sounds different from usual, stop there and call; a weak circ pump or a worn impeller is a parts job.
When it really is the heater
Flow is good, the heater relay clicks in, and the water still does not warm — or the GFCI trips the moment the heater is called. That is an element, usually one etched by soft water. We confirm it with a resistance-to-ground reading before anything is ordered, replace it, and add calcium to the fill so the new one lasts.
The cover
One more non-heater cause: a tub that heats but never reaches the set point in winter often has a saturated cover. If it takes two people to lift, it is losing more heat than the element can add. We weigh covers on every visit for exactly this reason.
Questions
What does FLO mean on my spa?
Can I run the tub without a filter to test it?
Related services
Next note: Repair or replace? An honest guide for older Seattle hot tubs