Hot Tub Not Heating but Jets Work? Start Here

The first winter we had our hot tub, I lifted the cover on a cold January evening, ready for a soak, and found lukewarm water with the jets roaring away like nothing was wrong. I was sure the heater had died and spent the next hour pricing replacement parts on my phone.

It turned out to be a filter so clogged that the spa wasn’t moving enough water to let the heater turn on. Ten minutes with a garden hose fixed it. That’s why I always tell people the same thing: when the jets work but the water won’t heat, start with flow and settings, not the heater. Here’s the order I check things in.

Why the jets can run while the heater stays off

Your spa’s jet pump and its heater are separate parts. The jets can blast away while the heater sits idle, because the control system only lets the heater turn on when several conditions are met at once. If any one of them fails, the heater stays off to protect itself and you.

• Enough water flow. A pressure or flow switch, or flow sensors, confirm water is moving past the heater. Without flow, a heater element can overheat in seconds, so the system won’t fire it.
• Safe temperature readings. A high-limit sensor shuts the heater down if water near it gets too hot.
• A call for heat. The setpoint has to be above the current water temperature, and the spa has to be in a mode that allows heating at that moment.
• Power to the heater circuit. The heater has its own relay and draws a lot of current, so it can fail separately from the pump.

Most “dead heater” calls turn out to be one of the first three. That’s good news, because those are usually free or cheap to fix.

The fix, in order

1. Read the panel. Look at the topside display before you touch anything else. Codes like FLO, FL1, dr, dry, OH, HL or a flashing temperature are your best clue. Write down exactly what it shows, then look the code up in your owner’s manual. Codes vary between brands, and the manual tells you what yours means.
2. Check the setpoint and mode. Make sure the set temperature is actually above the water temperature. Many spas have economy, sleep, away or rest modes that only heat during filter cycles or keep the water at a lower range. If someone in the house changed the mode, the spa may be doing exactly what it was told.
3. Pull and rinse the filter. With the power off at the breaker, remove the filter cartridge and hose it off thoroughly, working between the pleats. Then run the spa briefly without the filter to see if heating starts. If it does, a dirty or worn-out filter was restricting flow. Rinse it every week or two with regular use, deep-clean it with a filter cleaner periodically and replace it when the pleats are flattened or torn.
4. Check the water level and suction. Low water lets the skimmer draw air, which drops flow. Top it up to the level marked in your manual. Clear leaves or debris from the skimmer and suction covers in the footwell.
5. Open every valve and diverter. Slice valves and gate valves under the cabinet are sometimes closed during service and never reopened. Make sure they’re fully open. Turn jet diverters to their middle position and open any closed jets so water has somewhere to go.
6. Bleed out an air lock. This is especially common right after draining and refilling. Air trapped in the pump or heater housing blocks flow even though the jets seem to run. Many spas have a bleed screw or a union you can loosen slightly to let air escape. Follow your manual’s priming procedure.
7. Reset a tripped high-limit. If the panel shows an overheat or high-limit code, let the water cool, fix any flow problem you found, then reset the system the way your manual describes. On some spas that’s a reset button on the control pack; on others, turning power off and on at the breaker. If it trips again right away, stop and call a technician.
8. Look at the cover. A waterlogged, cracked or poorly sealing cover can let heat escape almost as fast as the heater adds it, especially on cold, windy nights. If the water warms slowly but never reaches the setpoint, the cover may be the real problem.

If the heater itself is the problem

Once flow, settings and the cover all check out, the problem may be in the heating system itself: a failed element, a bad heater relay, a faulty temperature sensor or a pressure switch that’s out of adjustment. Signs include no error code at all, the setpoint well above water temperature, plenty of flow, and still no heat.

Diagnosing these parts means testing live electrical components with a meter. If you’re not trained for that, this is the point to call a spa technician. Tell them what you’ve already ruled out; it can save time and money on the service call.

When to call a spa pro

• The GFCI or breaker trips when the heater tries to turn on.
• A high-limit or overheat code comes back after you’ve reset it and restored flow.
• You see burn marks, smell burning or hear buzzing from the equipment area.
• Flow is good, settings are right and the heater still won’t come on.
• The spa is under warranty. Opening the control pack yourself may void it.