Why Is My Hot Tub Not Heating?
Most hot tubs that won't heat have a water-flow problem (a dirty filter, low water, or an air lock), a setting problem, or a tripped breaker, so check those first. For reference, a 4 kW heater warms 400 gallons by about 4.1 °F per hour, or 20 °F in about 4.9 hours, before any heat loss.
On this page (9 sections)
Start with the calculator: it tells you how fast your tub should heat with its heater and gallons. A tub that is a little slower than this is normal in cold or windy weather. A tub that heats far slower, or not at all, has a cause you can usually find with the checklist below. Anything inside the equipment bay that involves wiring is a job for a licensed electrician or a spa technician.
How long should a hot tub take to heat?
Heat-up time depends on three numbers: gallons, how many degrees you want to add, and heater power in kilowatts (kW). Hours = gallons × 8.34 × °F ÷ (kW × 3,412). In the example, 400 gallons, a 20 °F rise, and a 4 kW heater give about 4.9 hours and 19.6 kWh of energy. The 4 kW heater is only an example: check the label on your heater or your owner's manual for its power.
The math assumes no heat loss. In real life an open cover, cold air, and wind all slow it down, so treat the result as the fastest your tub can do. Makers quote different real-world rates:
| Source | Rate |
|---|---|
| Jacuzzi (most hot tubs) | 3–6 °F per hour |
| Arctic Spas | About 1–2 °F per hour |
| Hydropool | About 5–10 °F per hour |
| Intex PureSpa (inflatable) | 2–3 °F per hour |
| Bestway Lay-Z-Spa (inflatable) | 3–4 °F per hour |
If your tub is well below its maker's rate with the cover on, something is limiting the heater. Check the list below.
120-volt (plug-in) tubs heat more slowly and often can't heat while the jets run on high. Bullfrog says the heater and pumps of its plug-and-play tubs can't run at the same time, and Jacuzzi says most 120 V systems can't run the heater and jets at full speed together. Long soaks with the jets on can cool the water.
Check these first, in order
- Read the display. Note the set temperature, the actual temperature, and any error code. Write the code down before you reset anything.
- Check the mode. Many tubs have an economy, rest, or sleep mode that heats only during filter cycles. Switch to standard or ready mode if you want heat now.
- Check the setpoint. Make sure it is above the water temperature. A tub set to 98 °F will not heat 99 °F water.
- Put the cover on. An open or waterlogged cover lets heat escape almost as fast as the heater adds it.
- Check the filter. A clogged cartridge cuts flow, and many heaters shut off when flow is low. Remove the filter, run the tub briefly without it if your manual allows, and see whether heating returns. Clean or replace it: hot tub filter cleaning.
- Check the water level. Water below the skimmer or filter intake lets the pump draw air. Top it up.
- Look for an air lock (air trapped in the plumbing) after a refill: jets that sputter or don't run. Follow your manual's steps to purge air.
- Check the breaker and GFCI. If the GFCI (the test/reset breaker that protects the tub) has tripped, reset it once. If it trips again, stop and call a licensed electrician or your dealer.
Hot Spring's troubleshooting guide names the same causes: an open pressure switch (usually from a clogged filter), a tripped pump thermal cut-off, or an air lock after filling. It says to call for service if the heater trips frequently. If the tub still doesn't heat after these steps, the problem is likely a part: the heater element, a flow or pressure switch, a temperature sensor, the high-limit switch, or the control board. Those repairs involve high-voltage wiring and should be done by a qualified technician.
Error codes: what the panel is telling you
Codes differ by control maker and panel, so check your manual for your exact codes. As an example, here is what Balboa's Spa Touch 3 panel reports about heating:
| Message | What happens | What to check |
|---|---|---|
| Low flow | Heater stops, then retries after about 1 minute | Filter, water level, closed valves, air lock |
| The heater may be dry | Spa shuts down for 15 minutes | Same as low flow; make sure the pump primes |
| The heater is dry | Spa stays off until reset by hand | Water level and flow; call service if it returns |
| Water too hot | Shown at 110 °F | Filter cycles in hot weather; setpoint; call service if it keeps rising |
| High-limit fault | Heater sensor read 118 °F | Call a technician |
Older panels show short codes instead, such as FLO, dr, or OH. Jacuzzi describes OH as often caused by a tripped high-limit switch from water that is too hot, and says to call your technician if it persists.
Hot tub not staying hot
If the tub reaches the setpoint but loses heat overnight, look for where the heat goes:
- Cover. A cover that is waterlogged, cracked, or doesn't seal at the edges is the most common cause. A heavy cover is usually a waterlogged one.
- Economy settings. In economy or sleep mode, the tub may let the water cool between filter cycles. That is by design.
- Cold, windy weather. Heat loss rises with the difference between water and air temperature. A windbreak and a good cover help.
- Air jets or blowers left on. Air jets cool the water fast. Turn them off when you're done.
- Leaks. If you keep having to add water, a leak may be the cause, and the cold refill water cools the tub.
Hot tub overheating
A tub that runs hotter than its setpoint, especially in summer, is often heated by its own pump: long filter cycles in hot weather add heat even with the heater off. Shorten the filter cycles, open the cover vents if your cover has them, or lower the setpoint. Most control systems also shut the heater off and warn you if the water gets too hot (see the table above).
The CPSC recommends that hot tub water never go above 104 °F. If the display shows a temperature well above the setpoint, or an overheat code keeps coming back, stop using the tub, turn it off at the breaker, and call a technician: a failed sensor or relay can keep the heater on. What temperature to set is on hot tub water temperature.
Inflatable hot tubs heat slowly by design
Intex rates its PureSpa heaters at 2–3 °F per hour and Bestway its Lay-Z-Spa at 3–4 °F per hour, so heating a freshly filled tub to soaking temperature can take a day or more. Both makers list cold air as a reason for slower heating. Intex names air below 50 °F, wind, running the bubbles while heating, and leaving the cover off. Bestway says the rate changes below 59 °F and tells you to keep the cover on whenever the heater is on. Intex says its tubs are for use in air no colder than 39 °F, and claims its insulated cover improves energy efficiency by as much as 50%. See inflatable hot tub care for chemicals and winter use.
When to stop and call a pro
- The GFCI or breaker trips again after one reset.
- You smell burning, see scorch marks, or hear buzzing from the equipment bay.
- Water is leaking near the heater or the control box.
- The display shows an error code your manual says needs service.
- The water is above the setpoint and keeps rising.
Don't open the control box or the heater housing yourself. The CPSC advises having an electrician qualified in pool and spa repairs inspect electrical problems, and Hot Spring warns that an improperly wired tub can cause electrocution or an electrical fire.
Safety
Turn off power at the breaker before you remove panels, change a filter in a tub without a filter shutoff, or work near the equipment. Never bypass a GFCI, a flow switch, or a high-limit switch. Electrical repairs belong to a licensed electrician or a spa technician. Keep the water at or below 104 °F, and don't soak if the tub is showing an error.
FAQ
How long does it take a hot tub to heat up?
It depends on gallons, heater power, and how many degrees you add. A 4 kW heater warms 400 gallons by 20 °F in about 4.9 hours before heat loss. Enter your own numbers above.
Why is my hot tub warm but not hot?
Check the setpoint and mode first: economy or sleep modes heat only during filter cycles. Then check the filter, the water level, and the cover.
Can a dirty filter stop a hot tub from heating?
Yes. A clogged filter cuts water flow, and most heaters turn off to protect themselves when flow is too low. Clean or replace the cartridge.
Why does my hot tub lose heat overnight?
Usually the cover: it is waterlogged, damaged, or not sealing. Economy settings, cold wind, and air jets left on also cool the water.
Sources
- Jacuzzi, Hot in a hurry (3–6 °F per hour): jacuzzi.com
- Arctic Spas, How long does it take to heat a hot tub: arcticspas.com
- Hydropool, How long does a hot tub take to heat: hydropoolhottubs.com
- Bullfrog Spas, Plug and play hot tubs (120 V): bullfrogspas.com
- Jacuzzi, Honest review of plug-and-play hot tubs: jacuzzi.com
- Hot Spring, Complete troubleshooting guide: hotspring.com
- Hot Spring, How to fix a tripped GFCI breaker (licensed electrician): hotspring.com
- Balboa Water Group, Spa Touch 3 messages: balboawater.com
- Jacuzzi, Understanding hot tub error codes: jacuzzi.com
- Intex PureSpa owner's manuals SSP-10 and SB-HWF10 (heat rate, slow heating, 39 °F; secondary host): manualslib.com
- Bestway Lay-Z-Spa owner's manual (heat rate, cold weather, cover; secondary host): manualslib.com
- Intex, Energy-efficient spa cover: intexcorp.com
- U.S. Consumer Product Safety Commission, Don't swim with shocks (qualified electrician, GFCI): cpsc.gov
- U.S. Consumer Product Safety Commission, hot tub temperatures (104 °F): cpsc.gov
- U.S. EIA, British thermal units (Btu) and energy unit conversions (3,412 Btu per kWh): eia.gov
- USGS, Water density (about 8.34 lb per gallon): usgs.gov