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Hot Tub vs Pool Heater

Different jobs

A portable hot tub might hold 300 to 500 gallons and keep it near 100 to 104°F under an insulated lid. A pool holds 10,000 to 30,000 gallons or more at roughly 78 to 84°F, often without a lid. The spa needs a high temperature but small volume; the pool needs a modest temperature but huge volume and surface. That is why equipment and running costs differ so much.

How hot tubs are heated

Self-contained hot tubs use built-in electric resistance heaters, typically a few kilowatts, run on 240 V or sometimes 120 V. The insulated shell and lid keep losses low, so the heater runs only part of the time. Some owners add a small heat pump to reduce running cost. In-ground spas attached to pools are usually heated by the pool's gas heater or heat pump, with valves sending water through the spa during spa mode.

How pools are heated

Pools use gas or propane heaters, heat pumps or solar. Sizing comes from surface area and temperature gap, with volume deciding heat-up time. Our pool heater size calculator covers this. Because pools rarely have insulated lids, surface loss dominates.

Heating a spa with a pool heater

A pool-and-spa combination shares pump, filter and heater. When the spa is in use, valves divert water to the spa only, and the heater lifts that small volume quickly. Gas heaters excel here because they bring a few hundred gallons to temperature in well under an hour or so. Heat pumps can do it but slowly, especially in cool air. Controls, valve actuators and spillover design are best left to a pool professional.

Energy maths

Energy to heat water equals gallons × 8.34 × degrees of rise. A 400-gallon spa rising 40°F needs about 133,000 BTU; a 20,000-gallon pool rising 10°F needs about 1.67 million BTU. The spa takes less total energy but much more per gallon. Holding temperature is where lids matter: a covered spa may lose little overnight, while an open pool loses heat continuously.

Running costs

A hot tub's monthly electricity depends on climate, lid quality and use; insulated tubs in mild weather can be modest, while older tubs in cold climates use much more. A pool heater's cost depends on hours and fuel. Use the operating cost calculator for the pool; for an electric spa heater, multiply its kW rating by hours run by your price per kWh.

Safety

Spas run hotter than pools, so temperature limits matter. Many health authorities advise a maximum around 104°F and limits on soak time; pregnant people, young children and people with heart conditions should check with a doctor. Electrical work on either system belongs to a licensed electrician with GFCI protection and bonding. Gas heaters need a licensed HVAC or pool professional. Follow chemical labels; spa doses differ from pool doses because volumes differ.

Which should you prioritize?

If you want warm water most evenings in a cool climate, a covered hot tub delivers more comfort per dollar than heating a whole pool. If you want daytime swimming, heat the pool and use a cover. A combination pool and spa with a gas heater gives quick spa heat and occasional pool boosts.

Worked example

Figures below are illustrative examples (2026-10-29), not quotes or guarantees.

Assumptions: 400-gallon spa from 60°F to 102°F with a 5.5 kW electric heater (about 18,800 BTU/h) and low losses. Energy ≈ 400 × 8.34 × 42 ≈ 140,000 BTU, so roughly 7.5 hours. The same spa on a pool's 250,000 BTU/h gas heater at 84%: about 140,000 ÷ 210,000 ≈ 40 minutes, plus plumbing losses.

Lids, covers and habits

A spa lid in good condition is the biggest saver for a hot tub, just as a cover is for a pool. Waterlogged or cracked lids lose much of their value and can multiply running cost. Keep the lid closed and strapped when not in use, lower the set point a few degrees during long absences, and check for gaps around the edges. For pools, the same idea applies with a solar or thermal cover; see solar cover guide.

Choosing a setup for a new build

When planning a new pool with an attached spa, discuss with your builder how often you expect to use each and in which months. Daily spa use in winter favours a fast gas heater or a dedicated spa heater; summer-only pool use with occasional spa evenings may suit a heat pump. Ask for the heater size, plumbing layout, valve control and expected heat-up times in writing.

Electrical and gas needs side by side

Portable hot tubs commonly need a dedicated 240 V circuit with GFCI protection and a disconnect within sight of the tub, installed by a licensed electrician; some plug-and-play models use 120 V and heat more slowly. A pool heater may need a separate 240 V circuit for a heat pump or a gas line for a gas heater, plus bonding to the pool's equipotential grid. If you plan both, ask the electrician to check whether your panel has capacity for both loads, since adding a tub and a heat pump together can push some homes toward a service upgrade. Plan both loads together so you only pay for one panel review.

FAQ

Can one heat pump serve a pool and spa?

Yes, but spa heat-up is slow; many owners prefer gas for that.

Is a hot tub cheaper than heating a pool?

Usually per use, because the volume is small and the lid holds heat.

What temperature is safe for a spa?

Many guidelines cap around 104°F; follow the manufacturer and medical advice.

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