Your hot tub is cold, heats too slowly, or flashes a code like FLO, dr, HL or OH and shuts down. Most hot tub heating problems come down to water flow through the heater, a setting you did not know about, or a temperature safety trip. This page explains how a spa heater works, which parts fail, what the common Balboa and Gecko codes mean, what you can safely reset, and when to call for service.
How does a hot tub heater work?
A hot tub heater is an electric element inside a metal tube. Water must flow through that tube whenever the element is on. The control system, called the spa pack, runs the pumps and heater and watches the sensors.
- The pump moves water. A circulation pump or the low speed of pump 1 pushes water through the heater. In Balboa’s Ready mode, pump 1 low may run at least 1 minute every 30 minutes just to check the temperature.
- Flow is checked. A pressure switch or flow sensor confirms the heater is full and water is moving. Balboa also compares two temperature sensors; if they drift apart while heating, it reads that as low flow.
- The thermostat calls for heat. When the water drops below your set temperature, the pack turns the element on.
- Limits stand guard. If water in the heater gets too hot, the pack shuts everything down: at 118 F on Balboa systems and 119 F on Gecko systems.
- Freeze protection. In freezing weather the pumps run on their own. Balboa keeps them on for at least 4 minutes after the freeze condition ends.
Which hot tub heating parts fail, and what are the signs?
| Part | What it does | Sign it is failing |
|---|---|---|
| Filter | Catches dirt before the pump | FLO, HFL or LF codes that clear when you pull the filter out |
| Pump or circulation pump | Moves water through the heater | Flow codes or humming, often after a refill |
| Pressure switch or flow sensor | Proves water is moving | FLO with a clean filter and a pump that is clearly running |
| Temperature sensors | Read water temperature in the heater | Sensor messages (Prr on Gecko, Sensor A or B on Balboa), or dry and flow codes with good flow |
| Heating element | Heats the water | Heater icon on, no error, but the water never warms |
| Control board | Switches the pumps and heater | Water keeps heating past the set point; Balboa’s “a pump appears to be stuck on” message |
| GFCI breaker | Cuts power if current leaks to ground | The whole tub is dead, or the breaker trips again after a reset |
What should I check first when my hot tub is not heating?
- Set temperature and range. On Balboa panels the low range can top out at 99 F and the high range at 104 F. If you are in low range, the tub will stop short.
- Ready or Rest mode. Balboa’s Rest mode only heats during filter cycles. Switch to Ready mode if the tub never seems to warm up.
- Priming mode. After every power-up, Balboa systems run a priming mode of up to 4 minutes, and the heater cannot run during it.
- Water level. Top up to the fill line. Low water is a top cause of flow and dry heater codes.
- Filter. Gecko suggests running the tub with the filter out for a test. If the code clears, clean or replace the filter.
- Valves and jets. Open all valves. Balboa notes that too many closed jets can restrict flow.
- Trapped air. After a refill, air can block the heater. Gecko’s fix: run the pump, slowly loosen a union nut on the heater until air escapes, then tighten it.
Safety first. Turn off power at the GFCI breaker before you open the equipment bay. Never bypass the GFCI, a pressure switch or a high limit. If the panel shows OH or HL (Gecko) or a pump stuck on (Balboa), the makers say do not enter the water. The CPSC says hot tub water should never go above 104 F, and pregnant women should stay at 100 F or below. Always check the water with a thermometer, and keep children supervised and the cover locked.
Hot tub heating numbers to know
These numbers come from Balboa Water Group and Gecko Alliance guides and the US Consumer Product Safety Commission (CPSC).
| Spec | Balboa | Gecko |
|---|---|---|
| Safe maximum water temperature (CPSC) | 104 F; high range set between 80 and 104 F | 104 F recommended maximum |
| Thermostat error (CPSC) | Can be off by as much as 4 F | Can be off by as much as 4 F |
| Spa water too hot | OHS at 110 F; resets by itself below 108 F | OH at 108 F |
| Heater too hot | OHH at 118 F in the heater; reset below 108 F | HL at 119 F in the heater |
| Minimum flow through the heater | See your model’s manual | 18 GPM |
| Low flow response | HFL: heater retries after about 1 minute. LF: heater off until you press a button | FLO: no flow while the main pump runs |
| Dry heater shutdown | dr: spa shuts down for 15 minutes | See your model’s manual |
Balboa vs Gecko: what do the codes mean, and what can you reset?
Many US hot tubs use a Balboa or Gecko spa pack. Some Balboa panels show short codes; others spell out a message with an M number.
- Balboa HFL and LF. Reduced flow through the heater. HFL retries on its own; LF disables the heater until you fix the flow and press a button.
- Balboa dr. The heater may be dry. The spa waits 15 minutes; a button press restarts heating.
- Balboa OHS and OHH. Water too hot in the tub, or in the heater.
- Gecko FLO. No water flow while the main pump runs.
- Gecko HL and OH. Heater at 119 F, or spa water at 108 F.
Watch out for HL: on Gecko it means high limit, but Balboa’s service manual groups HL with its heater flow low messages. Balboa’s current guides do not list dY or ICE; the closest are “Heater is Dry” (M027) and a freeze protection message, which Balboa calls operational, not an error. Check your spa’s own manual for older codes.
What you can safely reset: Balboa says messages marked with a star clear with any button press, and some need the power turned off and back on. Gecko’s guide resets by turning the spa off and back on. Fix the flow first, or the code returns. A pump stuck on or a sensor fault needs service, not a reset. Inflatable spas use different codes; see our inflatable hot tub problems page.
When should you call a pool or spa pro?
- Testing the element or board. Gecko’s guide checks for 120 or 240 volts at the heater terminals. That is live high-voltage work.
- Sensor faults. Balboa says to call for service if sensors stay out of sync or a sensor fails.
- A pump stuck on. Balboa says power down the spa and stay out of the water.
- A GFCI that trips again. See our pump, jets and GFCI page, then call an electrician or spa tech.
Spa techs bill by the visit or by the hour, so do the free checks first.
Common questions
How do I reset the heater on my hot tub?
Fix the cause first: clean the filter, top up the water, open the valves and bleed out air. Then press any button on a Balboa panel, or turn the spa off and back on at the breaker. Balboa’s OHH message only resets once the water is below 108 F.
Why is my hot tub running but not heating?
Flow is the most common reason. A dirty filter, low water or trapped air keeps the heater from starting. If there is no code, check the set temperature, the temperature range, and whether the tub is in Rest mode.
What are some common hot tub error codes?
Flow codes (FLO, HFL, LF), dry heater codes (dr), and overheat codes (OH, OHS, OHH, HL) cover most heating problems. Prr on Gecko means a temperature probe problem. Meanings differ by brand, so match the code to your control maker.
My hot tub isn’t heating, but there’s no error code. What could be the problem?
Settings are the usual cause. Rest mode heats only during filter cycles, low range can stop at 99 F, and priming mode blocks the heater for up to 4 minutes after power-up. If the heater icon is lit and the water still stays cold, the element or its power supply needs testing.
How do I know if my hot tub heating element is bad?
The sign is a heater icon that stays on with no code while the water does not warm. Gecko’s guide has a tech measure the voltage at the heater terminals; if it is correct, the heater gets replaced. That is live-wire work for a pro.
Why does my hot tub keep saying the heater is too hot?
Usually low flow: the water sits in the heater and gets too hot before the sensor sees the tub warm up. Check the filter, valves and water level. In hot weather, Gecko also suggests taking the cover off to cool the tub and shortening filter cycles.
Why won’t my hot tub get up to temperature?
Check the temperature range first, since a Balboa low range may stop at 99 F. Then check the cover, since heat escapes fast without a good one. A flow code that comes and goes can also cut heater run time.