Gas Heaters for Hot Tubs: Costs, Sizing and Safety (UK and USA)

Last updated: September 2026

Gas heaters are one of the oldest ways to heat a hot tub, and they’re still the fastest. They’re also the option that catches people out, because the tub is the easy bit and the gas supply, flue and safety rules are where the cost and the headaches live.

This guide covers how gas heaters work, how to size one, what natural gas and propane cost compared with electricity and a heat pump, and the installation rules you can’t skip. It’s written for both sides of the Atlantic, because gas is common in US backyards and much rarer in UK gardens.

How gas hot tub heaters work

A gas heater is a boiler for your tub. The circulation pump pushes water through the heater, gas burns in a sealed combustion chamber, and the heat passes into the water through a heat exchanger (usually copper or cupro-nickel tubes). The heated water goes back to the tub.

1. Flow first. A flow or pressure switch confirms water is moving before the burner is allowed to fire. No flow, no flame.
2. Ignition. Modern heaters use electronic ignition, and older ones use a standing pilot light. A flame sensor checks the flame has lit.
3. Heat exchange. The hot gases pass over the exchanger, and the exhaust leaves through the flue.
4. Temperature control. A thermostat or the spa pack switches the burner on and off to hold your set temperature. A high-limit switch shuts everything down if the water gets too hot.

Newer heaters add digital controls, adjustable output, low-NOx burners and self-diagnostics. They’re useful, but none of them changes the basic job. Watch the pump too: if it’s too small or the filter is clogged, the heater will trip on flow before it ever gets hot. My guide to hot tub heaters covers the electric side of things.

Natural gas or propane (LPG)?

Both burn cleanly and give similar performance. The difference is how the fuel gets to you, and what it costs.

	Natural gas (mains)	Propane / LPG
Supply	Fixed pipe from the mains	Cylinders or a bulk tank you have refilled
Fuel cost per kWh	Usually the cheapest	Usually noticeably higher
Running out	Never	Yes, so you need to keep an eye on it
Best for	Homes with a gas connection near the tub	Rural properties, or where a gas run isn’t practical
Energy in the fuel	About 29.3 kWh per therm (100,000 BTU)	About 26.8 kWh per US gallon (91,500 BTU)

A heater is built and certified for one fuel. You can convert between natural gas and propane, but only with the manufacturer’s conversion kit, fitted by a qualified engineer. Don’t try to change the jets yourself.

Sizing a gas heater

The maths is simple. Every gallon of water needs 8.33 BTU to rise by 1°F. In metric, every litre needs 4.186 kJ to rise by 1°C, which works out at about 1.16 Wh.

Energy needed = water volume x temperature rise (x 8.33 in gallons and °F).

Take a 1,500 litre (400 US gallon) tub and heat it by 20°C (36°F), say from 18°C to 38°C. That’s about 35 kWh, or 120,000 BTU, of heat. A heater sold as 100,000 BTU/h (about 29 kW) input at roughly 80% efficiency puts about 80,000 BTU/h (23 kW) into the water, so the job takes around an hour and a half. Divide the energy needed by the heater’s output to get your heat-up time.

Tub volume	Typical gas heater output	Notes
Up to about 1,000 litres (260 gal)	20 kW / 70,000 BTU/h	Small tub, insulated and covered
1,000 to 2,000 litres (260 to 530 gal)	23 to 30 kW / 80,000 to 100,000 BTU/h	Most family tubs
Over 2,000 litres (530 gal), or poorly insulated	30 kW and up / 100,000+ BTU/h	Big tubs, wind and cold air need more

These are rules of thumb, not a substitute for the maths above. The pool heaters commonly sold in the USA (200,000 to 400,000 BTU/h) are usually far bigger than a hot tub needs. Oversizing wastes money, and it can make the burner cycle on and off. What matters is your water volume, how quickly you want it hot and how cold and windy the site is. Heat loss is a big factor, so see hot tub energy efficiency and heater size for your pool or hot tub for more.

What it costs to run

Here’s what one heat-up of that same 1,500 litre tub (35 kWh of heat) costs, using the Ofgem price cap for October to December 2026 (26.32p/kWh electricity and 7.97p/kWh gas) and the US EIA average of 18.31 cents/kWh for electricity. I’ve assumed 80% efficiency for the gas heater and a COP of 3 to 4 for the heat pump.

Heater	UK cost per heat-up	USA cost per heat-up (electricity)
Gas heater (80% efficient)	About £3.50	Depends on your gas or propane price
Electric element (100%)	About £9.20	About $6.40
Heat pump (COP 4)	About £2.30	About $1.60
Heat pump (COP 3)	About £3.10	About $2.15

The lesson is that gas beats a direct electric element easily, but a good heat pump is in the same ballpark as gas per kWh of heat, and costs nothing for fuel storage or flues. Heat pump efficiency drops in the cold, so the real gap depends on your winters. To work out gas or propane for your own area, find the price per therm, gallon or kWh on your bill, multiply by the energy you need and divide by 0.8.

The bigger costs with gas are the installation: a gas supply run to the tub, a flue, a qualified engineer and, in many places, a permit. Get proper quotes before you fall in love with the idea. If you already have a gas boiler, look at heating a hot tub from your home boiler, which is often much simpler than a separate gas heater.

UK and US rules you can’t skip

In the UK

• Gas work must be done by a Gas Safe registered engineer. It’s the law, and it’s your insurance too.
• A gas heater needs a flue that takes the exhaust gases outside. Outdoor-rated heaters have their own flue arrangements. Anything in a control room needs a proper flue and ventilation routed to the outside.
• Fit a carbon monoxide alarm wherever people could be affected. Never enclose a gas heater in a small sealed space.
• Propane heaters need cylinders or a tank sited away from ignition sources, with the right regulator and hoses.
• Gas heaters for hot tubs are much rarer in the UK than in the USA, so check the model is certified for UK use before you buy.

In the USA

• Use a licensed gas fitter, and ask your local building department whether you need a permit and an inspection.
• Follow the manufacturer’s clearances from walls, decking and overhangs, and keep flammable materials away.
• The heater’s electrical supply and the pump still need GFCI protection. For the electrical side, see hot tub wiring explained.

Gas or a heat pump?

I’ve tested both routes, and I fitted a heat pump to my own tub. Gas wins on raw speed, especially for big temperature jumps and in hard frost. But a heat pump has no flue, no gas supply, no fuel deliveries and no combustion, and it holds the temperature cheaply. I ran mine outdoors at -4°C, and it kept working (read I ran my air source at -4°C). My own Comfortline is listed as 20kW (formerly 21kW), and it took my tub from 33°C to 39°C in about 1 hour 20 minutes on a mild day.

	Gas	Air source heat pump	Electric element
Heat-up speed	Very fast	Moderate (slower in the cold)	Slow
Running cost	Low to moderate	Lowest	Highest
Installation	Gas supply, flue, engineer	Pipework and a power supply	Simplest
Cold weather	Excellent	Check the unit’s rating	Unaffected
Best for	Occasional use, big tubs, quick heat-ups	Frequent use, lowest bills	Small tubs, tight budgets

For the full picture, read the guide to air source heat pumps, the best method to heat my DIY hot tub and how to heat a hot tub for free. If you’re off-grid, wood-fired and solar options are worth a look.

Maintenance and troubleshooting

Follow your manual, and have a qualified engineer service the heater every year. Between services:

• Keep the filter clean, because low flow is the number one reason a heater won’t fire. See how to clean a hot tub filter cartridge.
• Keep the area around the heater and flue clear of leaves, webs and nests.
• Keep your water balanced (pH 7.2 to 7.6, alkalinity 80 to 120 ppm). Low pH corrodes the exchanger, as covered in the chemical balancing guide.
• Test the carbon monoxide alarm.

Problem	Common causes
Won’t fire or trips out on flow	Dirty filter, closed valve, air lock, weak pump. See air locks
Slow to heat	Undersized heater, low gas pressure, dirty exchanger, missing or poor cover
Temperature swings	Faulty sensor or thermostat
Pilot won’t stay lit	Dirty pilot, weak thermocouple, gas supply problem. Leave this one to an engineer

If you can smell gas, switch off the appliance, open windows, don’t touch any electrical switches and call the gas emergency service (0800 111 999 in the UK). Never work on gas parts yourself.

Planning your own build?

Deciding on heating early saves a lot of pain, because the heater, pump size and pipework all affect each other. My hot tub plans include the plumbing layout, and I can draw up a full CAD and 3D plumbing design around your choice of heater. If you’re after heat pumps, take a look at my store at parts4tubs.co.uk (UK) or parts4tubs.com (USA).

FAQs

Are gas heaters faster than electric?

Yes. A gas heater delivering around 23 kW can raise a 1,500 litre tub by 20°C in roughly an hour and a half. A 3kW element would take about 12 hours.

Do I need a permit or engineer to install a gas heater?

In the UK you need a Gas Safe registered engineer for any gas work. In the USA you need a licensed gas fitter, and most areas need a permit and inspection. Never install one yourself.

Can I put a gas heater in an underground control room?

Only if it has a proper flue to the outside, fresh air ventilation and a carbon monoxide alarm, and the heater is designed for that use. Check with the manufacturer and your engineer. A sealed room isn’t safe.

Is gas cheaper than a heat pump?

Gas is cheaper than a direct electric element, but a good heat pump often costs about the same or less per kWh of heat. Gas comes into its own when you need speed or live somewhere very cold.

How long does a gas hot tub heater last?

With yearly servicing and balanced water, 8 to 12 years is a fair expectation. Poor water chemistry shortens it.

Can I convert my hot tub from propane to natural gas?

Sometimes. The manufacturer has to offer a conversion kit for your model, and a qualified engineer must fit and test it.

Got a question about your own tub? 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