Quick answer
A swim spa heater is almost never the gas unit this site covers for full-size pools — most swim spas hold only 500-1,500 gallons, well under the volume where gas makes sense, so the overwhelming majority ship with a built-in 240V electric-resistance heater or heat pump instead. A single-zone spa needs one heater and holds one temperature, typically 80-85°F for swimming. A dual-zone (dual-temp) spa splits the water into two independent sections with a divider wall — a cooler swim channel and a warmer soak end up to 100-104°F — and that design genuinely needs two separate heaters, not one bigger one, which is the detail most buyers coming from pool-heater shopping don’t expect.
Why swim spas skip the gas-vs-heat-pump debate this site covers for pools
Our gas vs. heat pump pool heater guide is written around pool-scale volumes — 10,000 gallons and up — where a gas heater’s $1,500-$3,500 installed cost and fast heat-up genuinely compete with a heat pump’s lower running cost. That math doesn’t carry over to a swim spa. At 500-1,500 gallons, a gas heater sized for that volume is a tiny fraction of the BTU output this site’s gas reviews cover, while installation still requires the same outdoor-only placement and professional gas-line hookup as a full pool unit — a lot of fixed cost for very little water to heat. That’s why swim spa manufacturers build in electric heating by default rather than offering gas as a standard option.
| Heater type | Typical swim spa use | Heat-up speed | Electrical/install need |
|---|---|---|---|
| Built-in electric resistance | Most common on single-zone spas | Slow — roughly 1-3°F per hour, often slower in cold weather | 240V, 30-50A circuit (4.0-11 kW), licensed electrician |
| Built-in heat pump | Increasingly common, better running cost | Slower still, best for holding a steady temperature rather than fast changes | 240V circuit, similar amperage to resistance but lower draw per BTU delivered |
| Gas (pool-style) | Rare on swim spas | Fast — 1-2°F per minute | Outdoor-only, fuel line, professional gas connection |
Sizing is the same math, smaller numbers
The underlying formula is identical to the one on our pool heater sizing chart: gallons × 8.34 × desired temperature rise, divided by target hours. The difference is scale. A common rule of thumb for electric heat pumps is about 4 BTU per gallon in mild summer conditions, rising to 5 in spring/fall and 6 in colder weather, according to sizing guidance cited by pool-heater retailers — so a 1,000-gallon swim spa lands somewhere in the 4,000-6,000 BTU range depending on season, nowhere close to the 100,000+ BTU gas and heat-pump units this site reviews for full pools. On the electric-resistance side, typical built-in units run 4.0-5.5 kW on a standard 30-amp/240V circuit for smaller spas, up to 11 kW on a 50-amp circuit for larger ones — and even the bigger element is genuinely slow, managing roughly 2-3°F per hour in a small, well-insulated, covered spa, dropping to as little as 1°F per hour in cold or windy conditions per installer forum reports.
Swim spa thermal cover
- Several spa-industry sources point to the cover as the single biggest factor in running cost — a gapped, worn, or missing cover lets heat escape faster than any heater can economically replace it, especially on a dual-zone spa paying to hold two different temperatures at once.
- Look for a cover rated for your specific spa's footprint rather than a generic tarp-style cover; swim spas are long and narrow, a different shape than most hot tub covers are built for.
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Dual-zone swim spas need two heaters, not one bigger one
This is the detail that catches buyers off guard most often. A dual-zone (also marketed as “dual-temp”) swim spa isn’t one body of water with a single heater working harder — it’s two genuinely separate bodies of water in one shell, divided by a solid wall, each with its own independent heating and filtration. Sundance Spas describes the swim side running roughly 80-85°F for exercise while the soak side holds up to 104°F for therapy, and Master Spas confirms its dual-zone combo units ship with two separate heaters specifically so each side can hold its own setpoint. The electrical consequence is real: running two heaters at once, rather than one, is why some manufacturers specifically call out needing 100-amp electrical service for dual-zone models — well above the 30-50 amp circuit a single-zone spa’s one heater typically needs, and worth checking against our pool heater electrical requirements guide framing before assuming your panel has the spare capacity.
The bottom line
If you’re shopping for a swim spa heater, you’re very likely shopping for a built-in 240V electric unit, not the gas or stand-alone heat pump models this site reviews for full-size pools — check the spa’s own spec sheet for the exact kW rating and circuit size rather than assuming pool-sizing rules apply directly. The one decision that actually matters as much as it would for a full pool is single-zone versus dual-zone: a single heater keeps things simple and cheaper to wire, while a dual-zone setup buys the ability to swim cool and soak hot in the same unit, at the cost of a second heater and meaningfully more electrical capacity.
Sources: Sundance Spas — Dual Temp Swim Spas, Master Spas Blog, PickHVAC — Pool Heat Pump vs Gas Heater Costs, Efficiency, Sizing, TroubleFreePool — Electric heater for hot tub/spa