Hot Tub Heaters Explained: Types, Sizing, kW, Flow Switches and Troubleshooting

Last updated: September 2026

Whether you’re a new hot tub owner or a seasoned pro, heat is going to be a priority. I’ll rephrase that: it’s definitely going to be a priority. A hot tub with no heat is, after all, just a tub!

How well your tub heats depends on the heater, of course, but also on its condition, the insulation, the cover, the weather and where the tub sits. This guide covers how hot tub heaters work, how to size one, why they need flow, how to keep the heat in, and what to check when the water’s cold.

How a hot tub heater works

On most modern hot tubs, the heater is part of the spa pack (the control box). Water is pumped through a stainless steel heater tube with an electric element inside it. The element heats the water as it passes through, and the water carries that heat back to the tub.

Alongside the element there are sensors:

• A temperature sensor tells the spa pack how hot the water is.
• A high limit sensor cuts the heater if the water gets dangerously hot.
• Flow detection: a pressure switch, a flow switch, or (on Balboa’s M7 systems) a pair of sensors that compare temperatures. This makes sure water is moving before the element fires. More on this below.

Yes, you can replace or upgrade the heater. The one that came with your tub is a bit like a car’s stock stereo. It works, but it’s not the best you can have. Elements, heater tubes and whole spa packs can all be swapped, and you can add a separate heater such as a heat pump alongside the spa pack.

Types of hot tub heater

Here’s the overview. I’ve gone deeper on which one to pick in What is the best method to heat my DIY hot tub? and on my recommended options in DIY best hot tub heater, so I won’t repeat all that here.

Heater type	Good for	Watch out for
Spa pack electric element	Standard on almost every tub; cheap to buy and replace	Most expensive to run per unit of heat; slow on big tubs
Air source heat pump	Much lower running costs; what I use on my own tub	Higher upfront cost; output drops in very cold weather
Gas (natural gas or propane)	Fastest heat-up; big tubs and occasional use	Gas supply, flue and installer requirements
Heat exchanger (from a boiler or solar)	Uses heat you already make	Needs a separate circuit and careful controls
Wood-fired	Off-grid tubs	Hands-on; hard to hold a steady temperature

Even if you add a heat pump or gas heater, you still need a spa pack to run the pumps, filtration and controls. Many people keep the spa pack element as a backup or boost. For the specifics, see air source heat pumps for hot tubs and hot tub gas heaters.

kW, amps and your supply

The easiest way to think about heater size is a room heater: a 1kW fire doesn’t put out as much heat as a 3kW one. The higher the kW, the faster the heater warms your water.

But the kW you can have is limited by your electrical supply:

• UK 13A plug-in tubs are limited to about 2kW of heat, and the heater can’t run at the same time as the jet pump. On a cold day with the jets on, the water slowly cools.
• UK hard-wired tubs (typically on a 32A supply) can run a bigger heater, often up to 3kW, and can usually heat with the jets on. A 3kW heater draws about 13A at 230V on its own.
• USA 120V tubs are usually limited to around 1kW. On 240V, 4kW and 5.5kW heaters are common. The same Balboa M7 heater rated 5.5kW at 240V gives only 1.375kW at 120V, so check that your heater matches your supply voltage.

A bigger heater needs a bigger supply, so talk to your electrician before upgrading. There’s more in A guide to hot tub electrics.

How long will it take to heat? (sizing)

Rough heat-up times, such as 4–8 hours from 24°C, are fair for a typical tub, but they depend entirely on the size of the tub and the heater. So here’s the maths instead.

It takes about 1.16kWh to raise 1,000 litres by 1°C. So:

°C rise per hour ≈ heater kW × 860 ÷ litres
(or °F per hour ≈ heater kW × 3,412 ÷ (US gallons × 8.34))

Water volume	2kW	3kW	5.5kW
1,000 litres (264 US gal)	1.7°C / 3.1°F per hr	2.6°C / 4.6°F per hr	4.7°C / 8.5°F per hr
1,500 litres (396 US gal)	1.1°C / 2.1°F	1.7°C / 3.1°F	3.2°C / 5.7°F
2,000 litres (528 US gal)	0.9°C / 1.5°F	1.3°C / 2.3°F	2.4°C / 4.3°F
2,800 litres (740 US gal), my tub	0.6°C / 1.1°F	0.9°C / 1.7°F	1.7°C / 3.0°F

These are best-case figures with the cover on. Wind, cold nights, a tired cover and poor insulation all slow it down. So a 1,500 litre tub starting from 10°C (50°F) mains water needs around 28°C of rise: roughly 16 hours on a 3kW element, and longer in winter. That’s why big DIY tubs on a standard spa pack heater feel slow, and why so many of my customers add a heat pump. For pools and bigger tubs, try my heater size calculator.

What does it cost to run?

A heater uses its kW rating for every hour it’s firing. So a 3kW element running for an hour uses 3kWh. At the UK Ofgem price cap for October to December 2026 of 26.32p per kWh, that’s about 79p an hour. In the USA, multiply by your own tariff from your bill. A heat pump delivers several kW of heat for each kW of electricity, which is how I cut my own costs. That story is in How I saved 66% on my hot tub electricity cost.

Flow switches, pressure switches and why heaters need flow

An element sitting in still water, or worse, in air, will overheat in seconds. That can ruin the element, crack the heater tube or melt the fittings. So every spa pack checks there’s flow before it lets the heater fire. There are three common ways of doing it:

• Pressure switch: screwed into the heater tube, it senses the pressure from the pump. It’s common on many packs. It can be knocked out of adjustment or fail, and a dirty filter can drop pressure enough to trip it.
• Flow switch: a paddle or sensor that physically detects moving water.
• Dual sensors (Balboa M7): no switch at all. Two sensors at either end of the heater compare temperatures. A difference of 3°F or less is normal, while 6°F or more suggests a flow problem. That gives an HFL message, and repeated faults lock the heater out with LF until you reset it.

Whatever the system, the lesson is the same: most heating faults are really flow faults. Before you blame the heater, check the water level, the filter, the valves, and for an air lock. My FLO error guide goes through it step by step.

Keeping the heat in

Whatever heater you have, the cheapest heat is the heat you don’t lose.

The cover

Most heat is lost from the water’s surface, so keep the cover on while heating and between soaks. An old cover that’s waterlogged and heavy has lost most of its insulation value, and it can harbour all kinds of funky grime too. When it gets like that, replace it, as it’ll pay for itself. See the ultimate guide to hot tub covers.

A floating thermal blanket

If you’re still struggling, add a floating thermal blanket on the water under the cover. They’re not heated and don’t add a second heat source. They’re simply an extra layer of insulation that cuts evaporation and heat loss, and they don’t use any electricity.

Shelter from the wind

Wind increases evaporation and cooling, making your heater work harder both to reach temperature and to hold it. Trees, shrubs, fences and privacy screens around the tub all help, and they make it more private too. Who’d have thought a few well-placed plants could help the thermodynamics of your tub?

Insulation

For a shop-bought tub, you can add insulation inside the cabinet panels (keeping it clear of the pumps and spa pack so they can breathe). For a DIY in-ground tub, insulation under the slab and behind the walls makes a real difference.

Should you leave your hot tub running?

I see this question every week in the hot tub Facebook groups I follow. The answer depends on how and when you use it.

If you live somewhere cold and use your tub a few days a week, running it at a lower temperature when you’re not using it and turning it up when you are saves time and money. That’s exactly what I do. I never turn mine off unless I’m going away. During the week I set it to 33°C (91°F), then turn it up to 39°C (102°F) from Friday to Sunday. Most spa packs have modes that do some of this for you. See Balboa heat modes explained.

If your climate is milder and you use the tub only occasionally, it can make sense to shut it down between uses. With sub-zero winters and an outdoor tub, though, don’t let it get cold enough to freeze, or it’ll take a long time to heat up again. Personally, winter is my favourite time for the tub. It extends the use of the garden. If you do shut down for winter, follow the steps in Winterizing your hot tub.

What temperature is safe?

The US Consumer Product Safety Commission advises keeping spa water at 104°F (40°C) or below. I find 38–39°C (100–102°F) most comfortable, and that’s what I set my own tub to.

Hot tub heater troubleshooting

There’s nothing worse than getting into a cold hot tub. Before you touch anything electrical, turn off the power at the isolator. If you’re not confident, get a qualified electrician or spa technician out.

Symptom	Likely causes	What to check
Not heating at all	Low flow, failed element, tripped high limit, heater relay, mode or settings	Error codes, filter, water level, air lock, set temperature and mode
Heats, but very slowly	Heater too small for the tub, heat loss, 13A tub running jets	Do the sizing maths above; check the cover and insulation
Heats, then won’t stay hot	Clogged filter, poor circulation, waterlogged cover, sensor	Clean the filter; check the cover; compare with a floating thermometer
Temperature swings up and down	Failing temperature sensor, or on older tubs a mechanical thermostat	Compare the display with a floating thermometer; replace the sensor
Trips the RCD/GFCI when heating	Element leaking to earth (often a split element)	See why your hot tub trips the breaker
FLO, HFL, LF or overheat codes	Almost always low flow or low water	Filter, valves, water level, air lock, pump

Pro tip: keep a floating thermometer in the tub and compare it with the reading on your topside control. If they disagree by more than a degree or two, suspect the sensor.

Low flow? Replacing the pump is rarely the first answer. A dirty filter, a closed valve, low water or an air lock are far more common, and all are free to fix. Only once you’ve ruled those out would I look at the pump itself.

For a full walk-through, see Help! My hot tub is not heating.

Replacing the heating element

An element can’t be repaired. When it fails, you replace it. Luckily, elements and heater tubes are not too expensive and are straightforward to change.

1. Turn off the power at the isolator and check it’s dead.
2. Close the valves either side of the heater, or drain the tub if you don’t have any.
3. Remove the heater tube or element. There are usually external nuts or bolts, and on some models the heater sits inside the spa pack.
4. Disconnect the wires, noting exactly where each one goes.
5. Fit the new element or tube with new seals, reconnect and tighten everything.
6. Open the valves, bleed out any air and check for leaks before you restore power, so the element never fires dry.

I’ve done this on video in How to replace your spa pack heating element and Easy spa pack heater tube swap.

Looking after your heater

• Keep the water balanced. Poorly balanced water corrodes elements and builds up scale on them.
• Keep air out of the system. Bleed the air after every refill or repair, because air around the element causes premature failure.
• Clean the filter regularly. A clogged filter starves the heater of flow.
• If in doubt, get a pro out. Water and electricity don’t mix.

Need parts or planning a build?

For replacement heaters, elements, sensors, spa packs and heat pumps, head to parts4tubs.co.uk in the UK or parts4tubs.com in the USA.

If you’re building your own tub, design the heating in from the start: the right spa pack, room for a heat pump and good insulation. My DIY hot tub plans include the plumbing layout and parts list to do exactly that.

FAQs

What size heater does a hot tub need?

Most spa pack heaters are 2–3kW in the UK and 4–5.5kW on 240V in the USA. Use the table above: bigger tubs, colder climates and faster heat-up all need more kW, or a heat pump alongside.

How long does 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 most of a day.

Why won’t my hot tub heat when the jets are on?

On a UK 13A plug-in tub, the heater is designed to switch off while the jet pump runs, to stay within the supply. On a hard-wired tub, check for an error code.

Is it cheaper to leave a hot tub on?

If you use it several times a week, usually yes. Keep it at a lower temperature between soaks rather than reheating from cold each time.

What does a FLO or HFL error mean?

The spa pack thinks there isn’t enough water moving through the heater. Check the filter, water level, valves and for an air lock first.

Can I add a heat pump to my existing hot tub?

Usually, yes. It plumbs in alongside the spa pack heater. See my heat pump guide.

Got a heating question? Drop it in the comments below. I read them all.

Cheers,
Andi

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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