Hot Tub Not Heating? What to Check, in Order (and How to Fix It)

Last updated: September 2026

There’s nothing more frustrating than finishing your DIY hot tub build, or getting a brand new tub delivered, filling it, and finding the water is still cold hours later. Before you panic or pay for a call-out, work through the checks below. In my experience, most “not heating” problems turn out to be a setting, a flow problem or simply not waiting long enough. Only a few are a failed part.

I’ve put the checks in the order I’d do them: free and easy first, then the bits that need a meter or an electrician.

Safety first

A hot tub combines water and mains electricity, so before you take a panel off or touch anything inside the spa pack, turn the power off at the isolator and make sure it can’t be switched back on. Looking at settings, the filter and the water level is safe for anyone. Testing elements, relays and fuses is for people who are competent with electrics. If that’s not you, stop at that point and get a qualified electrician or spa technician in.

1. Check the set temperature and heat mode

It sounds obvious, but in the rush to get in, people often forget to turn the set temperature up on the topside control (the keypad you use for the jets and lights). If the water is already at or above the set point, the heater has no reason to switch on.

Turn it up to 38–40°C (100–104°F). You should hear a relay click in the spa pack a short time later, and on many topsides a heat icon or light comes on. Don’t go above 40°C (104°F), which is the maximum the US Consumer Product Safety Commission recommends.

Then check the heat mode. On a Balboa pack in Rest mode, the tub only heats during filter cycles, so if the filter cycles are short or overnight, it can look as though the heater isn’t working. Switch to Ready mode while you test. I’ve explained the modes in Balboa heat modes explained. Gecko and other brands have similar economy settings, so check your manual.

2. Give it time: how fast should it heat?

Heating takes hours, not minutes. Here’s the rule of thumb I use across the site: °C per hour ≈ heater kW × 860 ÷ litres of water (or °F per hour ≈ kW × 3,412 ÷ (US gallons × 8.34)). That’s the best case with the cover on and no heat loss.

Heater	1,000 litres (264 US gal)	1,500 litres (396 US gal)
1kW (typical US 120V)	~0.9°C / 1.6°F per hour	~0.6°C / 1.0°F per hour
2kW (UK 13A plug-in)	~1.7°C / 3.1°F per hour	~1.1°C / 2.1°F per hour
3kW (UK hard-wired)	~2.6°C / 4.6°F per hour	~1.7°C / 3.1°F per hour
5.5kW (US 240V)	~4.7°C / 8.5°F per hour	~3.2°C / 5.7°F per hour

So a 1,500 litre tub filled with 10°C (50°F) mains water needs about 28°C of rise. On a 3kW heater that’s around 16 hours, and longer on a cold, windy night. On a 120V heater it can take a couple of days. Put the cover on while it heats and check the water with a floating thermometer after a few hours, rather than expecting it to be hot by teatime. If you want to work out your own tub, use my heater size calculator.

3. Is it a 13A or 120V tub with the jets on?

UK 13A plug-in tubs are limited to about a 2kW heater, and the heater can’t run at the same time as the jet pump. It isn’t a fault. If you sit in the tub with the jets on in winter, the water will slowly cool, and it only starts heating again once the jets go back to low speed or off.

Many US 120V tubs work the same way. They’re also limited to about 1kW of heat. The Balboa M7 heater rated 5.5kW at 240V gives only 1.375kW at 120V, which is a quarter of the output, so a tub running a 240V heater on a 120V supply will feel as though it isn’t heating at all.

4. Is the spa pack set up for your supply?

Some spa packs, mainly US ones, can run on either 120V or 240V. The wiring and the pack’s configuration (DIP switches or setup menu) have to match the supply and the heater. If they don’t, you can get odd behaviour: the pump runs and everything looks fine, but the heater runs at a fraction of its output, only heats when the jets are off, or doesn’t heat at all.

If you’ve just installed a new or replacement pack and it won’t heat, check the configuration against the manual. For Balboa packs, see How to configure a Balboa spa pack. Anything to do with the supply wiring itself is a job for your electrician. There’s more background in A guide to hot tub electrics.

5. Look for flow errors

This is the big one. Every spa pack checks that water is moving through the heater before it lets the element fire, because an element in still water, or in air, will overheat in seconds. Depending on the pack, that’s done by a pressure switch, a flow switch or, on Balboa M7 heaters, a pair of temperature sensors.

Lights on but nobody home? Look at the jets. Can you see water coming out? Is there an error on the topside? Common codes include:

• FLO: the pressure or flow switch isn’t seeing enough flow. See my FLO error guide.
• HFL and LF (Balboa M7): the two heater sensors differ by 6°F or more on a 240V system, which suggests low flow. If HFL happens five times in a day, the pack shows LF and stops heating until it’s reset.
• dr and dry (Balboa M7): the heater may be dry, usually because there’s air in it. Three dr faults in a row without a good heating cycle shuts the heater down.
• PSOL / PSOH on ACC controllers: see PSOL and PSOH errors.

Whatever the code, work through the free fixes first:

1. Water level. It should be up to the level in your manual, usually around the middle of the skimmer. Too low and the pump sucks air.
2. The filter. Take it out and run the tub without it for a few minutes. If the heater now fires, the cartridge is clogged. See How to clean a hot tub filter cartridge.
3. Valves. Make sure every gate valve and slice valve in the equipment bay is fully open. On a DIY build, check the bypass valves too.
4. Air locks. These are very common after a refill. If the circulation pump hums but nothing is moving, or you can hear it running but see no flow in the tub, you most likely have an air lock. Slackening the union on the top of the pump for a moment to let the air out usually sorts it. The full method is in Hot tub air lock: how to clear it, and How to fill a hot tub the correct way shows how to avoid it next time.
5. The pump. Only once you’ve ruled out the above would I look at the pump itself, such as a seized motor or a failed capacitor.

6. External heaters and heat pumps have their own flow switches

If you heat with an air source heat pump, a gas or propane heater or a heat exchanger, that unit has its own flow switch, and it won’t heat without enough flow either. Most heat pumps also wait a few minutes for steady flow before the compressor starts. On the Comfortline, it’s about three minutes.

• Air in the heater. This is the usual culprit after a refill. Loosen the union on the highest connection of the heater slightly and let the air hiss out until water appears, then tighten it again by hand.
• Too little flow. A bypass valve that’s too far open, a small circulation pump or a long pipe run can all leave the heat pump below its minimum flow. The Comfortline 7kW, for example, wants 2–4m³ an hour (roughly 9–18 US gallons a minute).
• Two thermostats. If the heat pump is controlled by flow, it heats to its own set point, not the topside’s. If it’s set lower than the topside, the tub will never get there. Keep the two the same. I explain why in How to control a heat pump with any hot tub spa pack.
• Cold weather. A heat pump’s output falls as the air gets colder, so a small unit may barely keep up in winter.

For heat pumps, work through Help! My air source heat pump is not heating my hot tub as well.

7. The electrical checks (power off first)

If there’s no error code, the flow is good and the tub still won’t warm up, it’s time to look at the heater circuit. Isolate the power before you open the pack.

Has the RCD or GFCI tripped?

If the tub trips its RCD (UK) or GFCI (USA) whenever it tries to heat, the element is often leaking to earth, usually because it has split. Don’t keep resetting it. See Hot tub tripping the breaker?

Has the high limit tripped?

The high limit cuts the heater if the water or heater gets too hot, often after a flow problem. Many packs show an OH, HL or similar code and need a reset once the cause is fixed. If it keeps tripping, suspect flow or a failing sensor.

Check the fuses

On some packs, the heater circuit has its own fuse, and there’s usually a wiring diagram and fuse list printed inside the lid. Even if electrics isn’t your thing, you should be able to read the list. It takes quite a lot to blow a heater fuse, though, so if you replace it and it blows again, something else is wrong. On a new spa pack, a blown heater fuse points to a bigger problem.

Test the element

A competent person can test the element with a multimeter, power off and wires disconnected. As a rough guide, the resistance should be close to voltage² ÷ watts: about 17–18Ω for a 3kW element at 230V, or about 10–11Ω for a 5.5kW element at 240V. An open circuit (infinite resistance) means it’s failed. Any continuity between either terminal and the heater’s metal body means it’s leaking to earth. An element can’t be repaired, but they’re not too expensive and they’re straightforward to swap. I’ve done it on video in How to replace a hot tub heating element.

Relay, sensors and the board

If the element is fine and the pack calls for heat (the icon’s on) but the element never gets power, the heater relay or the circuit board may have failed. A faulty temperature sensor can also make the pack think the water is already hot. Compare the topside reading with a floating thermometer: if they differ by more than a degree or two, suspect the sensor. Whether to fix the board or swap the whole pack is covered in Replace spa pack or just the circuit board?

Heats, but slowly or won’t hold temperature?

If the heater works but the tub is always playing catch-up, the problem is usually heat loss, not the heater. Most heat escapes from the water’s surface, so a waterlogged, heavy or badly fitting cover makes a huge difference. Wind, an uninsulated cabinet or DIY shell, and a heater that’s simply too small for the volume of water all add up. My hot tub heating and energy efficiency guide covers how to keep the heat in, and Hot tub heaters explained covers sizing.

Quick reference

Symptom	Most likely cause	What to do
No heat icon, no errors	Set temperature reached, or Rest/Economy mode	Raise the set point; switch to Ready mode
FLO, HFL, LF, dr codes	Low flow or air in the heater	Water level, filter, valves, air lock
Heats only with jets off	13A or 120V load limits	Normal; consider a bigger supply or a heat pump on its own supply
Heats very slowly	Small heater, big tub, heat loss, wrong voltage	Do the maths; check the cover; check the pack configuration
Trips RCD/GFCI when heating	Element leaking to earth	Test and replace the element
Heat icon on, water stays cold	Failed element, fuse or relay	Power off; test the element, fuse and relay
Heat pump runs, tub stays cool	Bypass too open, set point too low, cold weather	Close the bypass a little; match the thermostats

If all else fails

If you’ve been through all of this and the tub still won’t heat, contact the retailer or manufacturer of your shop-bought tub, especially if it’s still under warranty. Opening up the electrics yourself can void it. If you’ve built your own tub, drop your symptoms and any error codes in the comments below and I’ll help where I can.

For replacement heaters, elements, sensors, spa packs and heat pumps, have a look at parts4tubs.co.uk in the UK or parts4tubs.com in the USA.

If you’re still planning your build, design the heating in from the start, with the right spa pack, room for a heat pump and good insulation. My DIY hot tub plans and The Ultimate Hot Tub Control Room Guide cover the equipment side in detail.

FAQs

Why is my hot tub not heating but the jets work?

Usually because the heater isn’t getting enough flow (a dirty filter, low water, a closed valve or an air lock), because the set temperature has been reached, or because the pack is in an economy mode. On 13A and many 120V tubs, the heater also switches off while the jets run on high.

How long should a hot tub take to heat up?

Roughly heater kW × 860 ÷ litres gives you °C per hour. A 1,500 litre tub on a 3kW heater gains about 1.7°C an hour, so heating from cold mains water can take 16 hours or more.

Can a dirty filter stop a hot tub heating?

Yes. A clogged filter cuts the flow through the heater, which can trigger a FLO or HFL error and stop the element firing. Try running the tub briefly without the cartridge to test it.

How do I know if my hot tub heating element has gone?

With the power off and the wires disconnected, test it with a multimeter. An open circuit, or any continuity to the heater body, means it needs replacing. Tripping the RCD or GFCI when the heater comes on is another common sign.

Why does my hot tub stop heating when I use the jets?

On a UK 13A tub, and on many 120V US tubs, the supply can’t run the jet pump and the heater together, so the pack switches the heater off while the jets are on high. It’s normal, not a fault.

Still stuck? Drop your symptoms and any error codes in the comments below. I read them all.

Cheers,
Andi

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Thanks - Andi

 

Hi, Andi here. I own Buildahottub.com and also write all of the articles and info pages on the site. Some years back now, I built my own hot tub but struggled to find the information I needed. So, once my tub was complete, I started this website to help others in their own pursuit of hot tub and plunge pools DIY building information.

Fast forward to 2025, I've helped over 1400+ DIY customers just like you all over the world build hot tubs and pools. Have a good look around the site, there are lots of resources here. Please do get in touch if I can help you. - Cheers, Andi