If your hot tub pump is running but the water stays cold, the problem is usually flow, sensors, or settings — not always a dead heater. Work through these steps in order before paying for a service call.
1. Quick safety checks
- Any burning smell from the equipment area?
- Breaker feels hot or trips when heat kicks in?
- Visible scorch marks or melted plastic?
2. Flow basics – what the heater needs
Hot tub heaters will not turn on unless there is proper water flow. A flow or pressure switch monitors movement of water through the plumbing. If flow is weak, the heater will stay off to protect itself.
Most systems need:
- Clean filter with no heavy restriction
- Water level above the skimmer or intake
- All slice valves fully open
- No large air pockets in the plumbing
3. Filter & water level checks
A dirty or collapsed filter is one of the most common reasons for “pump runs but no heat.”
- Turn power off at the breaker.
- Remove the filter cartridge(s).
- Rinse thoroughly from top to bottom with a garden hose.
- Check water level – it should be above the skimmer opening.
- With filter still out, turn power back on and run the pump.
4. Air lock and closed valves
After draining or moving a hot tub, air can get trapped in the lines, or a valve can be left half-closed. Both can cause flow switches to keep the heater off.
- Visually check all slice valves near the pumps – they should be fully up / open.
- Listen for gurgling or “whooshing” sounds when the pump runs – that can be air.
- If your pump has a bleed screw, crack it open slightly with power off to vent air.
5. Pressure / flow switch checks
Many systems use a pressure or flow switch to confirm there is enough water movement through the heater. If it misreads flow, the heater will stay off even though the pump is running.
- Check for any visible kinks or crushed sections in plumbing.
- Look in the manual for pressure or flow switch location and settings.
- Never bypass a safety switch unless guided by a qualified technician.
6. When it’s likely the heater
Only after you’ve ruled out flow problems should you suspect the heater itself. Signs include:
- Good, strong jet action but no temperature increase over hours
- Breaker trips only when heat is called for
- Visible corrosion or damage around heater terminals
At this point, it’s usually more cost-effective (and safer) to have a technician test with a meter rather than guessing and replacing parts blindly.
7. When to stop and call a technician
- Breaker trips repeatedly when the heater tries to run
- You smell burning or see melted plastic / wiring
- No circulation even with clean filter and full water level
- Repeated error codes that come back after resets
8. Hot tub not heating – FAQ
Why is my hot tub running but not heating?
Most of the time it’s a flow or safety issue – dirty filter, low water, air lock,
closed valve, or a flow/pressure switch problem.
Can I keep using the hot tub if the water is just warm?
It’s usually safe only if there are no burning smells, no tripping breakers, and
no error codes. If anything feels off, shut it down and check the basics.
Should I replace the heater right away?
No. Always check filters, water level, valves and air locks first. Many expensive heater
replacements were just flow problems in disguise.