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Hot tub not heating: what it is, in order of likelihood
A cold hot tub in Connecticut is the call we get most from October to April. The good news is that the causes are few and they sort themselves by how the tub is behaving. Here is the ladder we climb on every visit.
1. The heater is not getting enough water
Heaters only run when the control sees flow. A filter that has not been cleaned in a couple of months, a valve someone closed, or an air lock after a refill will starve it, and the control either shows FLO, FLC, dr or dry, or quietly refuses to heat. This is the first thing we check and the cheapest fix. Half of these are solved on the diagnostic visit with a filter swap and a bleed of the pump.
Try before you call: pull the filter and run the tub without it for 20 minutes. If the water warms and the code clears, the filter was the problem. Replace it, do not just rinse it, if it is more than a year old.
2. The high-limit tripped
A safety sensor on the heater shuts it down when the water in the heater tube gets too hot, usually because of the same low-flow problem. On some controls you reset it by cutting power at the breaker for a minute. If it keeps tripping with a clean filter and good flow, the sensor or the heater itself is failing.
3. The heater element failed
Elements corrode, especially in tubs where the water chemistry has run acidic or the sanitizer has run high. A failed element either heats nothing or trips the GFCI the moment the heater kicks in. Replacement is a one-visit job on nearly every Balboa and Gecko heater: $350 to $650 for the element, $500 to $900 if the whole heater assembly has to go because the tube is pitted or the unions have fused.
4. A temperature sensor is lying
The control reads two sensors. If one drifts, the display shows a temperature that is not real, the tub heats to the wrong point, or the display throws SN1, SN2, SnA or SnB. Sensors are $200 to $400 installed and we carry the common ones.
5. The control pack or a relay
Rarest, and the one we rule out last. A heater relay on the board can weld open or closed. On older packs a board repair is sometimes possible; otherwise a replacement control pack runs $700 to $1,500 and we will price it against the age of the tub before recommending it.
Lukewarm but not cold
A tub that sits at 95 instead of 102 in January is often fine and simply losing heat faster than the heater can make it. A waterlogged cover is the usual culprit; a saturated cover weighs 60 pounds and insulates like a wet towel. A cover that has gone heavy is worth replacing before the heater.
If it is below freezing
Leave the power on and the cover closed. Most controls keep a freeze-protection cycle running even in an error state, and the pump heat alone can hold a covered tub above freezing for days. A tub with no power in a cold snap is the emergency: that is the one that cracks a manifold or a heater tube. Tell us in the form if the tub is dead and it is cold, and the visit moves up.