Hot Tub Won't Heat? Common Causes and Safe Checks

When a hot tub stops heating, the cause is often simple: a setting, a dirty filter, low water or an air lock that stops water flowing past the heater. Spa controls are designed to switch the heater off when flow is poor or the water gets too hot, so a “won’t heat” problem is often the safety system doing its job. Other causes, like a failed heater or an electrical fault, need a technician.

The short answer

Read the control panel message first and look it up in your owner’s manual. Then check the set temperature and heat mode, clean or check the filters, make sure the water level is correct and the cover is on. Hot tub maker Master Spas notes that if the heater has failed, most of the time it will trip the GFCI breaker. If the GFCI keeps tripping, or the problem is still there after the simple checks, call your dealer or a qualified technician.

Key takeaways

Error codes differ by control system and brand. The codes in this guide are examples from specific manufacturers, not universal meanings.
Dirty filters and air locks are common reasons for flow errors that stop heating.
An overheat message means stay out of the water.
Do not open control boxes or test electrical parts yourself.

Safety first

Hot tubs combine water and high-voltage electrics. Follow your manual for switching power off at the GFCI before touching filters or equipment.
The Balboa control documentation published by PÜR Spas warns not to attempt service of the control system, and says installation must be done by a licensed electrician.
If a display shows an overheat message, do not enter the water.

Read the display

Master Spas says to check the control panel for messages and look them up in the diagnostic section of the owner’s manual. Codes differ between brands and control systems. As an example, the Balboa VS300 documentation from PÜR Spas lists:

Code (Balboa VS300 example)	Meaning per that document	Owner action per that document
HL / LF	Temperature sensors differ significantly; this could indicate a flow problem (LF: persistent)	Check water level; make sure the pump is primed; call a technician if it persists
DR / DY	Possible inadequate water, poor flow or air in the heater	Check water level; make sure the pump is primed
OH / HH	Overheat; the spa has shut down	Do not enter the water; remove the cover and let it cool

Your spa may use different codes for the same problems. Always use your own manual.

Simple settings first

Master Spas’ troubleshooting steps start with the basics:

Set temperature: confirm it is above the current water temperature.
Heat mode: Master Spas says the tub should be in ready/standard mode. Economy or sleep modes on many spas only heat during filter cycles.
Filters: check for dirty filters and clean them if necessary. Hudson Bay Spas lists “the heater is running more frequently but the water is not heating evenly or normally” as a sign the filter needs cleaning or replacing. See hot tub filter cleaning.
Give it time: if the heater indicator is on, Master Spas suggests waiting approximately 1 hour with the insulating cover on and secured to see if the temperature changes. For normal heat-up times, see how long a hot tub takes to heat up.

Flow problems

Most spa heaters will not run without enough water flow. Hot Spring’s troubleshooting chart says a spa that won’t heat while jets and lights work may have an open pressure switch, a tripped circulation pump thermal cut-off, or an air lock created after filling.

Clogged filters: Hot Spring says to check for clogged filters, and that the switch should reset once water flows through the heater again.
Low water: fill to the level in your manual.
Air lock after refilling: Hot Spring’s fix for its spas is to place the hose down the main filter in the filter compartment and fill the spa completely to push the air lock out. Check your manual for your model’s method.
Circulation pump cut-off: Hot Spring says to check for clogged filters and air locks, and to disconnect power so the pump can cool.

Overheat and high-limit trips

A high-limit device shuts the heater or spa down if it senses too much heat. Hudson Bay Spas notes that a dirty filter can limit flow or trip the high limit thermostat, which turns off the spa. The Balboa VS300 document shows an OH overheat when the spa water reaches 110°F (43°C), and says not to enter the water and to remove the cover and let it cool.

For its spas, Hot Spring says to disconnect power for 30 seconds to reset the high limit and check for clogged filters. It says to call for service if the heater or thermostat trips frequently. Repeated trips are a sign of an underlying problem, not something to keep resetting.

Start with power

If nothing heats and the spa seems dead, check the GFCI. Master Spas notes that a failed heater usually trips the GFCI breaker. It also says GFCI tripping may be caused by a component on the spa or by an electrical problem, and that an electrician may be needed. Reset a tripped GFCI once. If it trips again, leave it off and call for service.

Check the actual temperature

Public Health – Seattle & King County recommends keeping an accurate thermometer in the water because the hot tub’s thermostat may be in error. It says hot tub water should not be warmer than 104°F. The Hudson Bay Spas manual cites the U.S. Consumer Product Safety Commission as saying spa water should never exceed 104ºF (40ºC). If the thermometer and the display disagree, mention it to your technician.

In cold weather, a spa may struggle to keep up if the cover is off or damaged. See hot tub water care in winter.

When to call a professional

The GFCI trips again after one reset.
Flow or overheat codes keep returning after cleaning filters, filling and clearing air locks.
The heater or high-limit trips frequently, which Hot Spring says needs spa service.
You see scorch marks, smell burning, or find water inside the equipment bay.
Anything needs the control box or heater opened.

FAQ

Why are the jets working but the water is cold?

Jets can run while the heater is blocked by a flow or pressure switch. Hot Spring lists an open pressure switch, a circulation pump thermal cut-off and an air lock as likely causes.

Is my pool heater guide relevant?

Only partly. Pool heaters are often gas or heat pump units with different checks. See pool heater not heating for those.

Sources

Research checked October 1, 2026. We only claim hands-on testing when we’ve actually tested the product ourselves. Manufacturer guidance applies to the named products; follow the manuals and labels for your own equipment.