Hot Tub Not Heating? Here’s The Complete Guide to Diagnosing and Fixing Heating Problems
Safety Notice: This article is provided for informational purposes only. RV systems may include electrical, propane, plumbing, and mechanical components that can pose serious safety risks. Always follow manufacturer guidelines and proper safety procedures. When in doubt, seek assistance from a qualified technician
Few things are more frustrating than stepping outside, lifting the hot tub cover, and discovering that your spa is ice-cold instead of warm and inviting. A hot tub not heating is one of the most common issues spa owners encounter, but the good news is that most heating failures can be diagnosed and often fixed without calling a technician.
To get a more general idea as to why your hot tub isn’t heating visit here: Hot Tub Isn’t Heating, causes, tests and what to check
Safety Notice: This article is for general informational purposes only. Pools, spas, and hot tubs involve electricity, heat, water, and chemicals that can be dangerous if handled improperly. Always follow manufacturer instructions and safety guidelines. If you are unsure or uncomfortable performing any repair, consult a qualified pool or spa professional
Understanding How Hot Tub Heating Works
Before you can fix a heating issue, it helps to understand how heating is supposed to work.
A modern hot tub relies on a simple but interconnected system:
1. The Water Pump
Water must flow continuously through the heater. Without proper flow, the heater will shut down to prevent overheating.
2. The Heater Element
Inside the heater assembly is a metal element—similar to a kettle—that warms the water as it passes. If the element fails, the spa will not heat.
3. Pressure Switch or Flow Switch
This acts as a safety device. It ensures that the heater only activates when water is flowing properly.
4. The Control Board
This is the “brain” of the hot tub. It receives temperature readings and activates the heater accordingly.
5. The Temperature Sensor
Also called a thermistor, this device tells the control board what the water temperature is.
When any of these components malfunction, the hot tub can stop heating.
Most Common Reasons Your Hot Tub Is Not Heating
Let’s break down the primary causes homeowners encounter—ranked from simplest to most complex.
1. Dirty or Clogged Filter
This is by far the most frequent cause. I could not tell you how many service calls ive been to where the problem was just a dirty filter. When filters are clogged with debris, oils, or minerals, water flow decreases. Low water flow triggers safety devices and prevents the heater from turning on.
Symptoms
* Water heats slowly or not at all
* Error codes like FLO, FL1, or FL2
* Jets feel weaker than normal
Fix
* Remove the filter
* Run the hot tub without the filter
* If heating returns, replace the filter immediately
2. Low Water Level
If the water level drops below the skimmer or filter intake, the pump can suck in air—which stops proper circulation.
Symptoms
* Gurgling sounds
* Bubbling from the skimmer
* Heater shuts down intermittently
Fix
* Add water until it reaches the midpoint of the skimmer
* Restart the spa
3. Tripped Breaker
Heater elements require significant power. If your spa experiences a electrical short or a surge, the breaker may trip.
Symptoms
* Now power to the spa
Fix:
* Check the GFCI outlet
* Check the main breaker panel
* Reset both and wait for the heater to restart
If the breaker trips again, an electrical short is causing the breaker to trip, the most common cause is usually the heating element. Calling a professional should be considered.
4. Airlock After Refilling
An airlock happens when air gets trapped inside the pump or heater tubes, preventing water from circulating.
Symptoms
* Pump runs but water doesn’t move
* Heater stays off
* High-limit error codes
Fix
* Open the jets fully
* Loosen a union on the pump until air escapes
* Restart the tub
5. Faulty High-Limit Switch
This safety device protects your tub from overheating. But if it’s faulty, it can mistakenly shut the heater off.
Symptoms
* “HL,” “OH,” “HOT,” or similar codes
* Water is lukewarm or cold
* Heater turns on briefly then shuts off
Fix
* Press the high-limit by turning the breaker off for about 30 seconds. Then turn it back on.
* If tripping continues, investigate low flow or replace the sensor
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6. Pump issues
The heater needs water flow in order for it to work. Some spas use a dedicated circulation pump for the heater and others use a jet pump. If either of these don’t work, the spa won’t heat.
Symptoms
* No water flow
* FLO or FLC errors
* Heater never clicks on
Fix
* Check for clogged filters
* Check for a blown fuse on the spa’s circuit board
* Have a technician test the pump windings
7. Broken or Burned-Out Heater Element
Heater elements fail over time, especially if water chemistry is unbalanced.
Symptoms
* Water barely warms
* Breaker trips when heater engages
* No error codes, but no heat
Fix
* Test heater element with a multimeter
* Replace the entire heater assembly if necessary
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8. Faulty Temperature Sensor
If the sensor reads the wrong temperature, the control board might think the water is already hot.
Symptoms
* Incorrect temperature reading on the display
* Tub only heats partway
* PRR, PRH, PR or SN1/SN2 error codes
Fix
* Replace the thermistor (inexpensive and easy)
9. Control Board Failure
Although less common, a malfunctioning control board can prevent the heater relay from turning on.
Symptoms
* Heater light doesn’t activate
* No sound coming from the relay
* No error codes, but still no heat
Fix
* Repair or replace the control board
Detailed Step-By-Step Troubleshooting Guide
Use the following process to diagnose your heating issue systematically—like a professional spa technician.
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Step 1: Check the Water Flow First
Low water flow is the #1 cause of heating failure.
* Remove the filters
* Turn jets on
* Check the water level
* Look for airlocks
If the tub starts heating after removing the filters, you’ve found the issue.
Step 2: Verify the Settings
Before assuming a hardware problem, check your control panel settings.
Make sure:
* The temperature is set correctly
* Economy or Sleep mode is not enabled, (ec, eco or sl message)
* Filtration cycles are scheduled properly
Economy mode heats only during filter cycles; many owners confuse this with a heating problem.
Step 3: Listen for the Heater Relay
Inside the spa pack is a small relay switch that clicks when the heater activates.
You should hear a clear “click” when the heater engages. If you don’t hear the click, the control board may be failing.
Step 4: Look for Error Codes
Common codes include:
* **FLO / FL1 / FL2**: Flow problem
* **OH / HOT / HL**: Overheat
* **SN1 / SN2**: Sensor issues
* **DR / DRY**: No water in heater
These codes help pinpoint the issue quickly.
Step 5: Test Power to the Heater
This step is only for owners comfortable with electrical work. You’re working with electricity up to 240 volts and 50 amps. Serious injury or even death could occur. Proceed at your own risk.
Check:
* Voltage across heater terminals
* Continuity of the heater element
If no power is reaching the heater, the control board is likely the problem. If there is power, then the heating element is the problem.
Step 6: Check the Pump
Verify that the pump that pushes water through the heater is operational, if not, check fuses
Step 7: Consider the Ambient Temperature
In winter, very cold air may cause:
* Slower heating
* Heat loss through the shell
* Freeze protection mode
Ensure the cover is insulated and tight.
Why Your Hot Tub Is Not Heating in Winter
Cold weather introduces new complications:
1. Heat Loss Exceeds Heat Output
If your hot tub cover is old or waterlogged, the heater can’t keep up.
2. Poor insulation allows heat to escape
Many budget spas have weak insulation.
3. Freeze Protection Mode
The spa may prioritize running pumps over heating.
4. A failing heater element worsens in cold weather
Low-performing heaters struggle in freezing temperatures due to the increase in demand
Fixing Heating Issues: How to Safely Repair a Non-Heating Hot Tub
Here are the most effective DIY fixes:
Clean or Replace Filters
Frequency:
* Clean weekly
* Replace every 12–18 months
Dirty filters cause 80% of flow-related heating problems.
Reset the Heater
Some older heaters (20 plus years) have a small red reset button on the heater tube.
Press it once the spa has cooled.
Fix an Airlock
The quickest method:
* Set all jets to open
* Turn off the pump
* Loosen pump union until air hisses out
Here are some repairs you should consider calling a professional for:
Replace a Heater Element
A new element costs $30–$120.
If the element tests bad:
* Shut power off
* Disconnect the heater leads
* Replace with identical wattage
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Replace the Temperature Sensor
Sensors screw into the heater tube or plug into the control pack.
Signs it’s the problem:
* Incorrect temp readings
* OH or SN codes
Replace the Pump
If low speed is dead:
* The pump may need new bearings
* The motor capacitor may be failing
* The entire pump may need replacing
Replace or Repair the Control Board
A technician typically handles this because:
* The heater relay can burn out
* Circuit traces may be damaged
* Solder repairs require precision
Preventing Future Heating Problems
Preventive maintenance can eliminate most heating failures.
1. Maintain Proper Water Chemistry
Imbalanced water corrodes heaters.
Target levels:
* pH: 7.2–7.8
* Alkalinity: 80–120 ppm
* Calcium: 150–250 ppm
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#2. Clean or Replace Filters Regularly
Filters older than 2 years are ineffective.
3. Keep Water Level Above the Skimmer
Low water = low flow = no heat.
4. Check Your Cover Condition
A heavy, waterlogged cover loses massive heat.
Replace if:
* It smells musty
* Vinyl is torn
* It sinks into the water
5. Insulate Plumbing and Cabinet
Adding foam insulation can significantly improve heating efficiency in winter.
6. Schedule Annual Maintenance
A technician can spot issues before they cause heater damage.
When to Call a Professional
DIY is great—but some issues require a trained spa technician.
Call a pro if:
* Breaker trips every time heater turns on
* You smell burning plastic
* Water leaks from heater or pump area
* Control board shows erratic behavior
Electrical issues should be handled by professionals to avoid damage or injury.
A hot tub that won’t heat is annoying, but the cause is usually simple: clogged filters, low water flow, a tripped safety switch, or an aging heater. With the troubleshooting steps above, most homeowners can identify the issue quickly and often fix it themselves.
However, if electrical problems or component failures are involved, don’t hesitate to bring in a professional—especially during winter, when freeze damage can occur quickly.
Written by Dan Harvickson, pool & spa repair technician