If your hot tub isn’t heating, it’s rarely a mystery — even if it feels like one at first. In most cases, the heater is being prevented from turning on because the spa is detecting a condition that isn’t safe, such as low water flow, a sensor problem, or an electrical issue.
The key to fixing a hot tub that won’t heat is not guessing or replacing parts blindly. It’s identifying what condition is stopping the heater and addressing that first.
This guide walks you through the most common reasons a hot tub isn’t heating, how to diagnose the issue step by step, and what to check before replacing expensive components.
Start Here: Quick Diagnostic Checklist
Before troubleshooting, answer these questions:
- Is the water temperature staying the same or dropping?
- Does the heater turn on briefly, or not at all?
- Do you see an error code (FLO, FL, OH, HL, etc.)?
- Are the pumps running normally?
- Did the problem start after a refill, power outage, or maintenance?
These details help narrow the problem quickly.
What “Not Heating” Usually Means
When a hot tub doesn’t heat, one of three things is happening:
- The heater is not being allowed to turn on
- The heater turns on but shuts off quickly
- The heater is working, but heat isn’t transferring to the water
Each scenario has different causes — and different fixes.
Most Common Reasons a Hot Tub Isn’t Heating
1. Dirty or Restricted Filter (Very Common)
Low water flow is the #1 reason hot tubs stop heating.
When the filter is clogged:
- flow drops
- the heater overheats internally
- safety sensors shut it down
What to check
- Remove the filter and run the spa briefly
- If heating resumes, clean or replace the filter
Never run the spa long-term without a filter.
2. Circulation Pump Not Running
Most modern hot tubs rely on a circulation pump to move water through the heater.
If the circ pump:
- isn’t running
- is air-locked
- is weak or failing
…the heater won’t engage.
What to check
- Look for water movement at the return
- Listen for a quiet hum or vibration
- Bleed air from the pump after refilling
3. Flow or Pressure Switch Issue
Flow and pressure switches confirm water movement before heating.
If the switch:
- sticks
- drifts out of adjustment
- is clogged with debris
…the heater will stay off.
Common signs
- FLO or FL error
- heater won’t start despite pumps running
- intermittent heating
4. Heater Turns On Then Off
This usually means the heater is overheating due to restricted flow or a sensor issue.
Possible causes include:
- dirty filter
- air in plumbing
- failing temperature sensor
- scale buildup in the heater tube
This is a very common misdiagnosis scenario.
5. Temperature or High-Limit Sensor Failure
Sensors tell the spa when to heat — and when to stop.
If a sensor drifts out of range:
- the spa may think water is already hot
- heating may stop prematurely
- error codes may appear intermittently
Sensor resistance should match manufacturer specs at room temperature.
6. Electrical Supply Problems
Heaters require full voltage under load.
Possible issues:
- tripped breaker or GFCI
- loose wiring
- failing relay or contactor
- voltage drop during heating
A heater may appear “dead” even though the spa powers on normally.
7. Heating Element Failure
Heating elements do fail — just not as often as people think.
Signs include:
- breaker trips when heating starts
- heater turns on briefly, then shuts down
- failed resistance or ground-fault tests
Always test before replacing.
8. Control Board or Relay Failure
If everything else checks out, the control board may not be sending power to the heater.
Clues include:
- no voltage at heater terminals
- clicking relays with no heat
- burn marks or corrosion on the board
This is typically a last-step diagnosis.
Simple Tests to Narrow It Down
Filter Bypass Test
- Remove the filter
- Run the spa briefly
- If heating resumes, the filter was restricting flow
Pump and Flow Test
- Confirm circulation pump operation
- Check for air bubbles or surging flow
- Verify all valves are open
Voltage Test
- Measure voltage at the heater when it should be heating
- Significant drop under load indicates an electrical issue
Common Misdiagnoses That Waste Money
- replacing the heater when the filter is dirty
- blaming the control board without testing voltage
- ignoring air locks after refilling
- replacing sensors without checking flow
Most heating issues start with water movement, not electronics.
When to Stop DIY and Call a Professional
Call a qualified spa technician or electrician if:
- voltage testing is uncomfortable
- breakers trip repeatedly
- internal board damage is suspected
- insulation resistance testing is required
Hot tubs combine water and electricity — safety matters.
How to Prevent Heating Problems
- clean or replace filters regularly
- maintain proper water chemistry
- address flow issues early
- avoid running the spa with low water
- inspect equipment after refills or outages
Related Articles
- Hot Tub Heater Turns On Then Off
- Hot Tub FLO or FL Error Code
- Hot Tub Not Heating Past 80 degrees
- How to Tell If a Hot Tub Heating Element Is Bad
- hot Tub High Limit
- Hot Tub Pressure Switch
Written By Dan Harvickson, Pool and Spa Technician