Hot tub not heating: causes and what to check

A hot tub that is not heating usually has a water flow problem, not a dead heater. Check the filter, the water level, and the display for an error code first. Then check the mode setting. Only after those are ruled out is the heater element, a sensor, or the circuit board likely. Most cases are fixed with a clean filter.

First, is it actually not heating?

Put a thermometer in the water, close the cover, and check again in two hours.

• 240V tub: expect a gain of about 3 to 6°F per hour.
• 120V plug-in tub: about 1 to 2°F per hour, and none while the jets run on high.

If the temperature is rising at those rates, the tub is working. A big tub, a cold fill, or a 120V supply just takes time. See 120V vs 240V hot tubs.

Causes, most likely first

Cause	Clues	Who fixes it
Dirty or worn filter	Flow codes, weak jets, filter over a year old	You
Low water or air lock	Just refilled, pump noisy, dry-heater code	You
Mode or settings	Economy, sleep, or rest mode. Set temperature lowered.	You
Tripped high limit	Overheat code earlier, heater will not restart	You (reset), then find the cause
Circulation pump failed	Flow code with a clean filter, no trickle from the circ return	Technician or confident DIYer
Pressure or flow switch	Flow code with strong flow	Technician
Heater element	GFCI trips when heat calls, or no heat with no codes	Technician
Sensor fault	Sensor code, temperature reading that does not match a thermometer	Technician
Board relay or fuse	Heat light on, no heat, heater tests good	Technician
Cover or weather	Heats but cannot keep up in cold. Heavy, waterlogged cover.	You

What to check, in order

1. Read the display. Any code tells you where to look. Flow codes such as FLO or HTR FLOW FAIL point to filter and flow. Use the AskTub code decoder for anything you do not recognize.
2. Check the water level. It should be above the highest jets. Top up if needed.
3. Clean the filter. Turn off power at the breaker, pull the filter, and rinse it. Then restore power and run the tub for 20 to 30 minutes with the filter out. If the heater kicks in, the filter is the cause. Deep clean or replace it. See how to clean a hot tub filter.
4. Check the mode and set temperature. Make sure the tub is in standard or ready mode, not economy, sleep, or rest. On Balboa VS panels, Ecn and SLP are modes that limit heating. Confirm the set temperature is where you want it.
5. Bleed any air lock. If you recently drained, open all jets and pulse the pump between low and high for 10 to 15 seconds at a time. See startup after draining.
6. Reset the power. Turn the breaker off for 30 seconds and back on. This resets a tripped high limit on many systems and clears stuck states.
7. Check that valves are open. With power off, look in the equipment bay for slice (gate) valves on each side of the pump and heater. The handles should be fully up or out.
8. Watch the heat indicator. Most panels show a heat light or icon. If it is on and the temperature does not rise after two hours with good flow, the heater circuit has a fault. Time to call a technician.

Understanding the heater's safeties

The control only allows the heater to run when all of these agree:

• Flow is proven, by a pressure switch, a flow switch, or a pair of sensors in the heater.
• The water is below the set temperature.
• The high limit has not tripped.
• Sensors read sensibly.

That is why a tub with a perfectly good heater so often will not heat. One of the permissions is missing, and it is usually flow.

When it is the heater element

Elements fail in two ways. They burn open, so no current flows and there is no heat and no code. Or the outer sheath corrodes through and current leaks into the water, which trips the GFCI. Low pH, scale, and running dry are what kill them. Testing the element means measuring at its terminals with the power off, which sits right at the control box, so leave it to a technician. More in heater element and heater replacement basics.

Heats, but not enough

• Cover. A waterlogged or badly fitting cover leaks most of the heat. If it is heavy to lift, it is done.
• Air controls. Open air valves and blowers inject cold air. Close them when you are not using the tub.
• Filter cycles in economy modes. The tub may only heat during set hours.
• 120V supply. A small heater cannot keep up with an open cover in winter.
• Low voltage. A long or undersized supply run weakens the heater. An electrician can measure it.

In freezing weather

A tub that cannot heat can freeze. Keep the cover closed and latched. A full tub holds heat for a day or more. Put a small space heater or work light in the equipment bay, clear of anything flammable and on a GFCI outlet, until the repair is done. Do not drain the tub in a hurry unless you can fully winterize it.

When to call a pro

Call a technician if the filter is clean, the water level is right, the mode is correct, and the tub still shows a flow, sensor, or overheat code, or shows no code and no heat. Also call if the GFCI trips. See hot tub tripping breaker.