Hot tub not heating: what to check before you call
A hot tub that runs but will not heat is usually a flow problem, not a heater problem. Low water, a dirty filter, a closed valve or an airlock starves the heater and the safety switch shuts it off. Check water level and the filter first, then the high limit and the sensors.
Why does a hot tub run but not get hot?
Because a spa heater only fires when water is moving through it at a sufficient rate, and there is a safety device whose entire job is to prevent it from firing when the water is not. That is not a fault, that is the design. Heating elements sitting in still water fail spectacularly, so the tub refuses.
So the question is almost never why is the heater broken. It is what is stopping the water. Low water level, a filter loaded with body oils, a closed or partly closed valve, an airlock in the plumbing after a refill, or a failing circulation pump all produce the same symptom: the tub appears to be running and the water stays lukewarm.
The second family of causes is sensing. A high limit switch that has tripped, a temperature sensor that has drifted, or a control board that has lost its mind will all report the wrong thing and refuse to heat with perfectly good flow.
What can you safely check yourself?
Water level first. It is the single most common cause and it costs nothing. The level should be above the highest jet and comfortably above the skimmer intake. A tub that has evaporated an inch over two weeks can drop below the point where the pump gets a solid column of water.
The filter second. Pull it out and look at it. If it is gray, if the pleats are packed, or if it has not been cleaned this quarter, that is very likely your answer. Here is the free diagnostic: run the tub for a minute with the filter completely removed. If it starts heating, you found the problem and you owe the filter a proper soak or a replacement.
Then the cover, which people never think of. A worn cover that has absorbed water and lost its insulation lets a tub lose heat faster than the heater can add it, particularly on a cold night. If the tub reaches temperature during the day and is cool by morning, look at the cover before you look at the heater.
Then the panel. Note the exact error code and write it down. It is worth real money on the phone, because a technician who knows the code before arriving can bring the right part.
Is it the heater, the flow, or the sensor?
Feel the plumbing. With the tub running, the pipe going into the heater assembly and the pipe coming out should differ if the heater is actually firing. If both are the same temperature as the water, the heater is not working. If neither has any flow you can feel, it is a circulation problem.
Watch the jets and the surface. Weak jets, a surface that barely moves, or air spitting from a return all point to flow. Strong jets with cold water point away from flow and toward the heater or the control side.
Listen at start up. A circulation pump that hums but does not move water, or one that has become noticeably louder, is failing. A tub that trips its breaker when the heater engages is an electrical fault and it stops being a diagnostic exercise at that point.
A high limit that has tripped can often be reset, and if the reset holds and the tub heats normally, fine. If it trips again, do not keep resetting it. That switch is doing its job and something is genuinely wrong upstream.
Why does an attached spa behave differently from a portable tub?
An attached spa shares its pump and filter with the pool, so its heating depends on the valve position and on the pool equipment being healthy. A spa that will not heat is often a pool problem: an actuator that failed to rotate, a valve stuck between positions, or a filter clogged on the pool side starving the spa.
Automation is the other difference. Attached spas usually heat through a controller with a spa mode that repositions valves, changes pump speed and enables the heater in a specific sequence. If any step in that sequence fails, the spa never gets its flow, and the panel may show no error at all because from the controller point of view everything went as planned.
The physics also differ. An attached spa is a small volume of water bolted to a large cold one, so a leaking or partly open valve that lets pool water bleed into the spa will keep the spa from ever reaching temperature. If your spa heats to a point and then stalls short of the setting, look for a leak between the two bodies of water.
What does a heater diagnosis involve?
A trip plus diagnosis runs $175 to $275+. What that buys is a technician verifying flow, testing the heater element and its safety devices, checking voltage under load, reading the control board and confirming the sensors, and then telling you in writing what failed.
That process matters because heater symptoms overlap heavily. A tripped high limit, a scaled element, a bad flow switch and a failing pump can all present as a tub that will not heat, and three of the four are cheap while one is not. Buying a heater on a guess is an expensive way to discover you had a flow problem.
When the repair is approved, the diagnosis goes toward the work rather than sitting on top of it.
How to tell which one you have
- It holds 98 degrees and never reaches the setpoint
- The heater is working and losing the race. That is usually a tired cover, an exposed windy location, or an element scaled enough to lose output. It is a slow decline rather than a sudden failure.
- The display disagrees with a thermometer in the water
- The sensor is reading wrong. A sensor packed with scale reads slow and low, so the tub either overheats or never satisfies. Compare the panel against a thermometer before buying any parts.
- It heats on jets and stops once they switch off
- The circulation pump is not delivering enough flow on its own. Many tubs heat on the low speed circulation loop, and a weak or airlocked circulation pump shows up in exactly this pattern.
- A rotten egg or hot plastic smell from the equipment bay
- Stop and cut the power. That is either stagnant water in a biofilm or something electrical getting hot. Neither is a wait and see item on 240 volt equipment sitting right next to water.
- The level drops an inch a week with the cover on
- A covered tub loses very little to evaporation, so steady loss points at a leaking union, a jet body or a pump seal. Low water then starves the heater, and nobody looks for the real cause.
Spend on a new cover or on heater work
| New cover | Heater repair | |
|---|---|---|
| Symptom it fixes | Reaches temperature then cools overnight | Never reaches temperature at all |
| How to tell | The cover is heavy, sagging or waterlogged | Inlet and outlet pipes are the same temperature |
| Cost | A known price for a stock size | Diagnosis first at $175 to $275+ |
| Side benefits | Lower running cost all year, less debris | None beyond the repair itself |
| If you guess wrong | You still gained efficiency | You paid to diagnose a cover problem |
Short version. Press on the cover before you call about the heater. A waterlogged cover is the most common reason a tub loses heat overnight, and it is the cheaper of the two to be wrong about.
What moves the price
A range is only honest if you can tell which end you are on. These are the things that decide it.
| Factor | What it does to the number |
|---|---|
| Which part of the chain failed | A filter or a cover is a known purchase. A flow switch or a sensor is a modest part. An element or a control board is larger, and a failed circulation pump sits at the top of the range. |
| Access to the equipment bay | A tub built into a deck or surrounded by tile is far harder to work on than a freestanding one with removable panels. Sometimes the tub has to be lifted, and that is labor before any repair begins. |
| Scale on the heating element | Hard fill water at 150 to 250 parts per million of calcium coats an element over years. A scaled element is an insulated element, so it heats slowly and eventually trips the safety switch. |
| Portable tub or attached spa | A portable tub has its own heater, pump and controls. An attached spa heats through the pool equipment, so the fault may be a valve actuator, a pool side filter or the automation, which widens the search. |
How long it takes
Check water level, filter and cover
Fifteen minutes
Run one minute with the filter removed
One minute
Write down the exact error code and model
Two minutes
Diagnosis of flow, element, sensors and voltage
One visit
What people get wrong here
Refilling with a hose and going straight to full temperature
Cold fill water and a big jump make the heater run flat out, and hard local fill water leaves more calcium on the element each time. Fill it, then let the temperature climb at its own pace.
Washing the filter cartridge with dish soap
Detergent stays in the fabric and comes back as foam and cloudy water for weeks, which also masks how the tub is really flowing. Use a proper filter cleaner, rinse well, and let it dry before refitting.
Running the same water for many months
Dissolved solids build until the water stops responding to chemicals, and biofilm in the plumbing eats sanitizer. A tub that will not hold sanitizer is often telling you the water itself is worn out.
Leaving a full tub switched off through a cold snap
A small volume with no circulation and no heat freezes faster than a pool. If it will not be running through a freeze, it needs to be properly drained rather than turned off and left standing.
Through a North Texas year
- The first cold night in NovemberWhen failures surface. A tub coasts through mild weather because the heater barely works, then a 30 degree night exposes a tired cover or an element that has been scaling all year.
- December to FebruaryFreeze risk outranks comfort. Know how to drain the tub if the power goes out, because a small volume with no circulation freezes quickly on a 20 degree North Texas night.
- March to MayPollen loads spa cartridges fast, and a blocked filter is the most common flow fault of the year. Check the cartridge every couple of weeks through this window rather than monthly.
- June to SeptemberHeavy use season. Body oils and sunscreen blind a cartridge within weeks, and a busy tub burns sanitizer quickly, so both flow and chemistry need more attention than in the cold months.
Our water. The North Texas hot tub failure has a season, and it is the first genuinely cold night after a mild stretch. Tubs coast through a sixty degree December looking perfectly healthy because the heater barely has to work, and then a twenty degree night arrives and reveals a tired cover, a marginal circulation pump or an element that has been scaling all year. That scale is the local part. Hard fill water builds calcium on a heating element faster than softer water does, and a scaled element is an insulated element, so a tub that took forty minutes to reach temperature in its first winter can take three hours in its fifth. If the tub is outdoors and the power goes out on a freezing night, it has no circulation and it will freeze faster than a pool, so drain it if the outage is going to last.
When to stop and call
Call once water level, the filter and the cover are ruled out, or immediately if the breaker trips, if you smell burning, if there is any scorching at the equipment, or if a reset high limit trips a second time. Spa equipment is 240 volts sitting next to water, and that is a hard stop.
Why is my hot tub not heating up, and where the homeowner checks stop
Start with the filter and the water level, because that is where most of these end. A dirty cartridge starves the circulation pump, and low water lets air into the line. Both of those answer why is my hot tub not heating up without any tools at all. Clean the filter, top the water, and give it an hour. If the temperature climbs, you just saved yourself a service call.
Here is where you are out of your depth. Once the easy checks pass and you are still asking why is my hot tub not heating up, the next suspects are the flow switch, the high limit, and the heater element, and all three sit next to line voltage in a wet box. Do not open that panel. A spa that trips its breaker while you poke at it is telling you to stop.
Common questions
How long should a hot tub take to heat up?
Most residential tubs raise water temperature a few degrees per hour, so a full heat up from cold is measured in hours, not minutes. What matters is whether it is slower than it used to be, because a gradual slowdown is usually scale on the element or a tired cover.
Should I leave my hot tub on all the time?
Generally yes. Holding temperature costs less than repeatedly heating from cold, and in freezing weather a tub left off with water in it is at risk. A tub that will not be used for months should be properly drained rather than switched off and left full.
The tub heats then cools off overnight. What is that?
Almost always the cover, sometimes an air leak or a failing thermostat. Lift the cover and press on it. If it is heavy, sagging or waterlogged, it has lost its insulating value and the heater is fighting the night air.
Related guides
- Hard Water and Scale in North Texas PoolsCalcium only goes one direction in this region unless you take water out. Here is how scale starts, what it damages first, and when a partial drain is genuinely cheaper than fighting it.
- Winterization and Freeze Protection in TexasDown here the plan is not closing a pool. It is keeping water moving through the two or three nights a year that can crack a pad, and knowing what to do when the power goes out.
- Repair or Replace a Pool PumpA pump is a motor bolted to a wet end, and the two fail differently. Knowing which half failed tells you whether you are buying a part or a pump.
Start this week
Send the city and a pad photo. You get the plan before the first stop.