Hot tub not heating: flow, filter and heater checks

Common symptoms

• The water is cold or lukewarm, and the display shows no error at all.
• The display flashes FLO, HFL, LF, dr or a “Heater Flow is Reduced” message.
• The tub heats a little, then stops well short of the set temperature.
• The heater icon is on, but the water never warms up.
• After a refill, the pumps gurgle or hum and the tub will not heat.
• The whole tub is dead, including the panel and the lights.

Quick checks before anything else

• The set temperature is above the current water temperature.
• The tub is in the high temperature range, not the low range.
• A Balboa tub is in Ready mode, not Rest mode.
• The water is up to the fill line.
• The filter is clean, or the tub runs without it for a test.
• All valves are open and most jets are open.
• The breaker and the GFCI (ground fault circuit interrupter, the safety breaker for the tub) are on.

Why is my hot tub not heating?

• A setting Low range, Rest mode or a low set point stops heating with no error code.
• Low water flow A dirty filter, low water or closed valves make the control turn the heater off and show a flow code.
• Trapped air After a refill, an air pocket keeps water out of the heater, often with a dry heater code.
• Priming mode or a start delay Right after power-up, the heater is blocked for a few minutes on purpose.
• A failed heater or sensor The heater icon is on with no code, but the water stays cold.
• No power A tripped GFCI or breaker shuts off the whole tub.

Step-by-step: get a hot tub heating again

When your hot tub is not heating, we start with settings, because they cost nothing and cause a lot of “broken heater” calls. Then we move to water flow, which causes most error codes. Our hot tub heating problems page explains how the heater, pumps and sensors work together.

1. Read the display. Note any code before you reset anything. Flow and dry heater codes (FLO, HFL, LF, dr) send you to steps 5 to 8. Overheat codes (OH, OHH, OHS, HL) mean the water or heater is too hot, not too cold; stay out of the water. Our hot tub error codes guide covers each one.
2. Check the set temperature and range. Balboa systems have two ranges with separate set points. The high range might be set between 80 and 104 F, and the low range between 50 and 99 F; your spa maker may narrow these. If the low range arrow is showing, the tub will stop short of what you expect. Switch to the high range and raise the set point.
3. Switch from Rest to Ready mode. On Balboa, Rest mode only heats during programmed filter cycles. In Ready mode on tubs without a circulation pump, pump 1 runs on low at least 1 minute every 30 minutes to check the temperature, then heats if needed. On a Gecko panel, a lit heater indicator means it is heating, and a flashing one means heat was asked for but the heater has not started yet.
4. Wait out priming mode after power-up. Every time a Balboa system powers up, it runs a priming mode of about 4 minutes, and the heater is not allowed to run. The display may also show no temperature until the pump has run for 1 minute. If you just reset the breaker, give it a few minutes before deciding anything is wrong.
5. Top up the water. Balboa’s flow checks start with low water level. Fill to the line marked on your tub so the skimmer and suction fittings stay under water.
6. Test without the filter. Gecko’s fix for a FLO code is to run the system with the filters taken out. If the code clears or the tub starts heating, clean or replace the filter. Put it back once you are done testing, since the filter protects the pump and heater from debris.
7. Open the valves and jets. Balboa lists closed valves, suction restrictions and too many closed jets as flow problems. Open every valve on the equipment side and turn open most of the jets in the seats.
8. Bleed trapped air. Air in the plumbing is common after a refill, and the pump may sound rough. Gecko’s fix: start the pump, slowly loosen one of the union nuts on the heater until air escapes, then tighten it again. Balboa’s priming mode lets you run each pump on high to push air out. If a pump has not primed after 2 minutes, Balboa says to turn it off and try again, and not to repeat this more than 5 times.
9. Check the real water temperature. Use an accurate thermometer. The CPSC (Consumer Product Safety Commission) says hot tub thermostats can be off by as much as 4 F. If the tub is warm but the display says cold, or the other way around, a sensor may be reading wrong, and that needs service.
10. Keep the cover on while it heats. An open or badly fitting cover lets heat escape while the heater works, which you notice most in cold weather. Close and latch it while the tub warms up.
11. Check power if the tub is dead. If the panel is blank, check the breaker and the GFCI. Reset a tripped GFCI once. If it trips again, see our hot tub GFCI keeps tripping guide before you try anything else.
12. Stop at the heater element. If the heater icon is on, there is no code, and the water still will not warm, the element or its power is the next suspect. Gecko’s guide has a technician measure 120 or 240 volts at the heater terminals with the power on. That is live high-voltage work, so call a pro.

Balboa vs Gecko: how each one handles heating faults

Most US hot tubs use a spa pack, the control box that runs the pumps and heater, from Balboa or Gecko. Both stop the heater for the same reasons, but the codes and resets differ.

Inflatable spas use their own heater and codes, so see our inflatable hot tub not heating guide for Intex, Bestway and SaluSpa models.

When to call a pool or spa pro

• The heater icon is on, there is no code, and the water stays cold after a few hours.
• A flow code returns with a clean filter, full water level, open valves and no trapped air.
• Balboa shows Sensor Balance is Poor, a sensor failure, or a pump stuck on.
• The GFCI trips again after one reset.
• The pump hums but will not run; our hot tub pump not working guide covers what you can check first.

Common questions