When a hot tub stops heating, the culprit is usually something far more mundane than a broken heater — in the vast majority of cases it comes down to insufficient water flow, typically caused by a clogged filter. The control system simply cuts power to the heating element before it can be damaged. This guide walks you through eight causes ordered from the most common to the rarest, and tells you exactly where the owner’s job ends and the technician’s begins.
Is it really not heating? First tell two very different problems apart
Before you take anything apart, work out which of these two situations you are dealing with. The diagnosis and the fix are different for each.
Not heating at all. The water temperature has not moved a single degree in several hours, or it is even falling. The panel may or may not be showing an error. Here you are almost always looking at a specific technical fault or a safety shutdown.
Not heating fast enough. The temperature is rising, just more slowly than you expected. In a large share of cases this is not a fault at all — merely unrealistic expectations, cold weather or an open cover. How many hours heating really takes is covered in the article How to heat a hot tub and pool faster.
How many °C per hour is normal?
A typical domestic hot tub holding 800–1,500 litres has an electric heater rated at 2–3 kW. With the cover closed, that works out at roughly 1–2 °C per hour. Heating from cold mains water (12–15 °C) up to an operating 37 °C therefore normally takes 12 to 24 hours — and in winter easily half as long again.
So if you measure a rise of around 1.5 °C/h, your hot tub is performing exactly as it should. A rise below 0.5 °C/h on a covered tub in mild weather, on the other hand, already signals a problem.
A five-minute check before you reach for a screwdriver
- Write down the temperature on the display and read it again after 60 minutes with the cover closed — that gives you an objective number rather than an impression.
- Check that the display is showing a set temperature higher than the current one (many panels show the target value when touched, and owners mistake it for the actual reading).
- Feel the jets: is the water coming out at a normal rate? A weak or intermittent flow is the strongest clue to the cause.
- Look at whether the panel is showing an error code or a heater indicator light.
8 reasons why a hot tub is not heating — from the most common to the rarest
The table below is a quick signpost: find the row that matches your symptoms and skip straight to the relevant section.
| # | Cause | Typical symptom | Who can handle it |
|---|---|---|---|
| 1 | Clogged filter | Weak flow from the jets, heating shuts itself off after a while | Owner, 15 minutes |
| 2 | Low water level or air in the system | Bubbling, gurgling, pump “running dry” | Owner, 20 minutes |
| 3 | Heating switched off / economy or sleep mode | Everything works, it simply does not heat outside the scheduled window | Owner, 2 minutes |
| 4 | Pump not running or running on low speed | Humming without starting, no circulation | Owner diagnoses, technician repairs |
| 5 | Scale on the heating element | Heats at first, then shuts down; repeated overheating | Technician (or prevention by the owner) |
| 6 | Faulty temperature sensor / flow switch | Nonsensical temperature on the display, false flow errors | Technician |
| 7 | Burnt-out heating element | Flow is fine, the temperature does not move at all | Technician / electrician |
| 8 | Poor insulation and frost | Heats, but fails to reach the target temperature in winter | Owner (cover, insulation) |
1. Clogged filter: by far the most common cause
The filter cartridge in a hot tub traps body oils, cosmetics and mineral deposits, and under heavy use it can become severely clogged within a few weeks. Once too little water passes through it, flow through the heating circuit drops — and the control unit cuts the heater as a safety measure to stop the element from overheating. In other words, the tub is “not heating” entirely on purpose.
It is easy to spot: the flow from the jets is weaker than usual, heating kicks in and shuts off again after a few minutes, or a flow error appears. The fix is to remove the cartridge, rinse it with a jet of water between the pleats and let it soak in a cleaning solution once a month. The complete procedure and intervals are covered in the article Cleaning a hot tub filter.
A five-minute test: take the filter out completely and let the tub run without it. If the flow improves immediately and the heating comes on, you have found your culprit. (Never run the tub without a filter long term, though, and do not bathe in it in that state.)
2. Low water level and air in the system
The second most common cause is related to the first: if the level drops below the skimmer, the pump starts drawing in air. An air pocket in the pipework blocks circulation just as effectively as a clogged filter, and the heater protection kicks in again.
The typical audible symptom is gurgling, intermittent “spitting” from the jets, or a high-pitched whine from the pump without any water actually flowing. It most often happens after a water change or after cleaning the filter.
How to put it right: top the water up to at least 3–5 cm above the skimmer mouth, switch the tub off and loosen the bleed screw on the pump housing (on most models a black plastic cap) until the air escapes and water starts to come out. Then tighten the screw and start the system. If the level keeps dropping, this is not evaporation but a leak — the difference is explained in the article Water loss vs. evaporation.
3. Heating switched off, or economy and sleep mode active
A trivial yet surprisingly common cause. Balboa, Gecko and other control units offer modes such as Economy, Sleep or Rest, in which the water is only heated during programmed filtration cycles. Outside those windows the tub may cool by several degrees, and the owner reads it as a fault.
Equally often, someone in the household has lowered the target temperature, or the panel has been left locked after maintenance. So always check these first: the set temperature, the active operating mode (ideally Standard) and the panel lock.
Hot tubs controlled by a mobile app add one more trap — a scheduled overnight setback to take advantage of a cheaper tariff. How to use it sensibly is described in the article Hot tubs and pools on spot-priced electricity.
4. Pump not running or running on low speed
No circulation means no heating — on the vast majority of hot tubs the heating element sits directly in the pipework behind the pump and must not switch on without water moving through it. So if the circulation pump does not start at all, the tub will not heat even when everything else is in perfect order.
Watch out for two-speed pumps as well: low speed drives circulation and heating, high speed drives the massage jets. If the pump gets “stuck” in the wrong mode, the heater may never receive the signal to switch on.
The characteristic symptom of a failing pump is humming without starting — the motor tries to spin up but cannot (typically a faulty start capacitor or a seized rotor). Diagnosis, and what you may still tackle yourself, is covered in the article Pump hums but will not start.
5. Scale on the heating element
In hard water, a layer of limescale gradually builds up on the heating element. It acts as thermal insulation: the element heats its own coating instead of the water, overheats and is repeatedly cut off by the thermal cut-out. The symptom is distinctive — the tub heats for a while, then the heating switches off, and once it has cooled it comes back on.
The scale layer also shortens the element’s life and increases electricity consumption, because it lengthens the heating time. The remedy is professional descaling (or replacing the element), and the prevention is correctly managed water hardness. How many °dH is still acceptable for a hot tub and how to treat the water is covered in the article Water hardness and scale.
6. Faulty temperature sensor or flow switch
The control unit decides whether to heat on the basis of two inputs: the temperature sensor and the flow detector (a mechanical flow switch or a pressure switch). If either of them fails, the unit will not switch the heater on as a precaution — even if everything else is physically working.
You can recognise a faulty temperature sensor by nonsensical values on the display (the water is lukewarm, the panel reports 40 °C) or by a difference of more than 2 °C between the display and a real thermometer. A stuck or corroded flow switch, meanwhile, generates flow errors even with a perfectly clean filter.
At this point you are into measurement and parts replacement, which is a job for a technician. If your panel is showing a specific code, start with the overview Balboa error codes; the most common flow message is dealt with in a separate guide, The FLO error and how to fix it.
7. Burnt-out heating element
The least common option, and the most expensive. An element typically burns out when it has been running for a long time with insufficient flow or under a layer of scale — so it is usually the consequence of one of the earlier causes rather than bad luck.
Indirect clues the owner can assess: the filter is clean, the level is fine, the pump is running and the jets deliver full flow, the panel shows no error — and yet the temperature does not rise by even a degree over several hours. Sometimes the heater trips the circuit breaker or the RCD the moment it switches on.
Checking the element’s resistance with a multimeter is no longer a job for the owner. It means opening the electrical section of the hot tub, where dangerous voltage levels are present even after the panel has been switched off. Replacement elements and other service components can be found in the category construction and spare parts, but leave the fitting to a professional.
8. Poor insulation and frost: the hot tub does not heat “enough”
The eighth cause is not a fault but physics. During a freezing week, an outdoor hot tub loses heat faster than a 2–3 kW heater can supply it — especially on cheaper models with thin cabinet insulation or with a worn, waterlogged thermal cover.
The cheapest and most effective measure is precisely that cover: a waterlogged or poorly seated thermal cover can increase heat losses by tens of percent. When replacement pays off is covered in the article Hot tub thermal cover. The whole winter operating regime, including freeze protection, is described in Hot tubs in winter.
What the owner can handle and where the technician’s work begins
The dividing line is simple: everything done with the breaker switched off and without opening the electrical enclosure belongs to the owner. Anything behind the cover of the electrical installation belongs to a technician or a suitably qualified electrician.
| Task | Owner | Technician / electrician |
|---|---|---|
| Cleaning or replacing the filter cartridge | yes | — |
| Topping up water, bleeding the pump | yes | — |
| Checking the mode, temperature and panel lock | yes | — |
| Restarting the tub by switching the breaker off for 30 seconds | yes | — |
| Descaling or replacing the heating element | — | yes |
| Measuring element resistance, replacing sensors and switches | — | yes |
| Work inside the electrical enclosure, replacing a contactor or board | — | yes |
Never open the electrical section of a hot tub while it is live, and do not work on it after merely switching off the panel either — the panel is not an isolator. What a correct electrical connection should look like, including the RCD, is described in the article Hot tubs and the electrical connection. If the tub is under warranty, any unqualified intervention in the equipment will usually void it.
Prevention: what maintenance can head off
Seven of the eight causes described share a common denominator — they can be caught before they show up as a “broken heater”.
- Weekly: rinse the filter cartridge and check the water level.
- Monthly: soak the filter in a cleaning solution, measure water hardness and check the condition of the thermal cover (waterlogging, seating).
- Quarterly: change the water, flush the pipework and check the tightness of the unions at the pump.
- Annually: have the RCD and the condition of the heating circuit checked during a service visit.
A complete annual schedule month by month can be found in the article Hot tub maintenance calendar. In the long run, the cheapest maintenance is the kind you never have to make up for with a repair — the service life of individual components and typical service intervals are covered in Hot tub lifespan and servicing.
Frequently asked questions when a hot tub is not heating
Does restarting the tub at the breaker help?
Often yes — and it is a legitimate first step. Switch the breaker off for about 30 seconds and back on again; the control unit runs its own self-test and clears false flow or temperature errors. If the same error returns within a few hours, the restart is solving nothing and you need to look for the real cause.
How long does it take to tell whether the tub really is not heating?
Measurement settles it: close the thermal cover, note the temperature and read it again two hours later. A rise below 0.5 °C per hour on a covered tub in mild weather is a clear signal of a problem, whereas 1–2 °C per hour is a perfectly normal figure.
Can bad water chemistry be the reason a hot tub is not heating?
Not directly, but indirectly very often. Unbalanced water with high hardness accelerates scale build-up on the heating element, and cloudiness or biofilm clogs the filter cartridge faster. Both end in the heating being shut down — so looking after the water means looking after the heater too.
How much does it cost to replace a hot tub heating element?
The price depends on the model and part availability; on ordinary domestic hot tubs the element itself runs to the low hundreds of euros, plus the technician’s labour. You will only get an exact figure after diagnosis — and it pays to have the sensors and the flow switch checked during the same visit.
Is it safe to use a hot tub that is not heating?
Bathing in lukewarm water is not dangerous, but do not put the problem off. Water below 30 °C loses disinfection effectiveness faster in a regime it was not set up for, and above all: if the cause is insufficient flow, both the pump and the heater are running under abnormal conditions. In winter there is the added risk of the pipework freezing.
Conclusion: work from the top down and save on service calls
The most valuable thing about this diagnosis is the order. Before you pick up the phone to a service company, work through the first three points — filter, water level and panel settings. Together they take under half an hour and resolve most cases where a hot tub has “stopped heating”. Only when the flow is demonstrably fine and the temperature still refuses to move are you looking at a failed component.
Filter cartridges, thermal covers and spare equipment parts can be found in the categories construction and spare parts and equipment, and chemicals for softening and balancing water in the water treatment section. More practical guides on operation and faults can be found in our Advice centre.
