Spa or No Spa: What an Attached Spa Really Costs You

Every pool builder will offer you a spa. None of them will show you a measured number, and that is not an oversight — for an attached spa, no measured number exists.

That is the honest place to start. Not with the pros and cons, which every page has, but with the reason the pros and cons are all adjectives.

The thing nobody tells you: the cheap one is measured and the expensive one is not

A $600 inflatable spa is a regulated appliance. It is tested to a national standard, ANSI/APSP/ICC-14, under defined conditions — 102°F water, 60°F air, the manufacturer’s own cover on, for at least 72 hours. The result is normalised and printed on a removable label stuck to the shell, showing standby watts, annual kWh, annual running cost, and which cover was certified with it. California requires the label, and Oregon, Washington, Connecticut, Arizona and Florida have adopted the standard.

The $30,000 spa your builder is quoting has none of that. The Department of Energy’s own coverage determination says so directly, excluding from the rule “units that are site-assembled, such as portable electric spas that are permanently installed in the ground or attached to a pool.”

No test. No label. No database. No standard.

You can check that asymmetry yourself in about two minutes, and it costs nothing. California’s appliance certification database is public and needs no login. Go to cacertappliances.energy.ca.gov, open Advanced Search, pick Pool Products and then Portable Electric Spas. One step matters and the page does not tell you: under “Select Fields to Display”, tick “Portable Electric Spa Normalized Standby Watts” and “Portable Electric Spa Fill Volume” — they are off by default, so the table you get otherwise has no watts in it. Then set the Brand filter (it is a dropdown of every certified brand, not a text box) and hit Search.

Here is what comes back. On 7 September 2026 there were 2,187 certified portable spa models in the file, and the Hot Spring Sovereign’s row read:

Read that row again, because it is doing four things no builder quote does. It names the exact cover the spa was tested with. It gives a measured standby draw, not a claim. It puts that draw next to the legal ceiling for a tub that size, so you can see the Sovereign comes in 19% under its own cap. And Non Federally-Regulated is the Energy Commission stating the DOE exemption from the other side.

Now do it for the $600 inflatable. Intex has fifteen certified model numbers in there, each with a tested cover and a filed standby figure — 231 watts on the 210-gallon tubs, 271 on the 290-gallon ones. Then try to do the same for the spa your builder is quoting. There is nothing to look up, because nothing was ever required to be filed.

One honest limit while you are in there: the database publishes standby watts and volumes, not annual kWh or annual running cost. Those two live on the removable label stuck to the shell.

So when a builder tells you the attached spa is cheap to run because it shares the pool equipment, understand what kind of statement that is. It is not a measurement, and it is not falsifiable. The least efficient tier of spa is the only one nobody measures.

That is the spine of this whole page, and everything below is us doing the arithmetic that the regulation does not require anyone to do.

Your attached spa is cold

This is the single biggest misconception, and it changes what you are buying.

A hot tub is hot. You lift the cover and get in. An attached spa is not a hot tub — it is a cold vessel you heat on request. It shares water and equipment with the pool, so unless it is valved off it sits at pool temperature: mid-80s in a Florida summer, far colder for the rest of the year and anywhere else.

Getting 500 to 700 gallons from there to 102°F takes roughly 45 to 75 minutes on a properly sized gas heater. The clearest statement of what this means in practice comes from the Trouble Free Pool owner community, which has no product to sell:

“Pool with Spas are designed to be in the pool mode (with spillover) most of the time. They are not designed to keep the spa hot or even warm. Basically, when you want to use the spa, you turn it on and wait (forever) for the spa to heat up.”

And the part that appears on no builder’s page: while the spa is heating, your pool is not being filtered. In spa mode the valves put both suction and return on the spa. You can only heat one body of water at a time, and you can only filter one at a time, and for that hour it is not the pool.

Ask your builder one question and make them answer it in words: is the spa valved so it can be isolated and held at temperature, or is it a shared-water design that will always sit at pool temperature? Those are different products. Both get called an attached spa.

What it takes out of your swim

Here is a number we worked out ourselves, because we could not find it published anywhere.

A 7-foot round spa is about 38.5 square feet. Cut into the corner of a rectangular pool, that is a modest share of the surface. But it is not cut out of the surface evenly — it takes a bite out of one end, and the end is the dimension you swim.

Across every common pool size, the swim length you lose is roughly two to three times the surface area you lose. In a 12x24, a 7-foot spa turns a 24-foot swim into a 17-foot swim.

The reason this catches people is the drawing. A builder shows you a plan view, where the spa is a small circle in a large rectangle, and you evaluate it on area, because area is what a plan view shows. Then you get in and swim the long way.

Put the other way: a 12x24 pool with a 7-foot spa is 288 square feet of which 250 is actually pool. You bought a 12x24 and you swim in about a 12x21.

By spa size, share of water surface consumed:

The rule that falls out: below about 14x28 an attached spa is a swim-lane decision, not a luxury decision — decide it on that basis and nothing else. Above about 16x32 it costs you little. And there are two designs that cost you no pool surface at all: a spa raised beside the pool rather than cut into it, and a separate portable tub, which is the option nobody frames this way.

Our figures assume a round spa cut into a corner, with the diameter coming off the long axis. Surface area only — spa depth varies and does not affect the swim lane.

What it costs to heat, with the assumptions everyone else leaves out

Rival pages assert that the built-in is cheaper to run. Almost none of them states a usage assumption or a rate assumption, and the claim is meaningless without both.

Here is the arithmetic. A portable spa costs you standby and no per-soak. California’s legal cap for a 300-gallon standard spa is 208 watts, which is 5 kWh a day, about $25 a month at $0.17 per kWh. It is hot whenever you want it, with no wait.

An attached spa costs you per-soak and no standby, because it is switched off. Using 8.34 pounds per gallon and a gas heater at 82% efficiency:

Divide the portable’s monthly standby by the attached spa’s per-soak and you get the crossover — the number of soaks at which the attached spa stops being the cheaper machine. It is highly rate-dependent, which is exactly why a single number would be dishonest:

The honest answer is a range: somewhere between about four and twelve cold-weather soaks a month, depending on what you pay for gas and electricity. On propane, the attached spa is essentially never the cheaper machine. Below your crossover the built-in genuinely does win — but only because it is switched off, which is another way of saying you are not using it.

These are heat-up figures. They exclude standby while you soak, evaporation with the cover off, and pump electricity, on both sides of the comparison.

Why gas, and not a heat pump

Everyone says to specify gas for a spa. The reason usually given is heat-up speed, and that is true — 500 gallons from 60°F to 102°F is about 32 minutes on gas against 78 to 119 minutes or more on a heat pump.

But there is a better reason that nobody publishes. Every pool heat pump’s capacity rating is measured at 80°F entering water. We checked ten manufacturers — Pentair, Hayward, AquaCal, Raypak, Jandy, Heat Siphon, Gulfstream, Nirvana, Thermeau and Madimack — along with AHRI’s own certification directory, and it is uniform. A spa runs at 102°F, which is 22 degrees above every published rating point in the industry, and not one of them publishes the capacity you actually get up there. The rating standard’s only trip anywhere near spa temperature is a survival test at 101°F: the unit has to not fail, and nobody has to say how much heat it makes.

Three of those ten will not reach spa temperature at all. Heat Siphon caps its thermostat at 98°F, Nirvana and Thermeau at 95°F. And Jandy ships a spa mode whose factory default setpoint is 102°F while still publishing capacity only at 80°F water.

On top of that, pool heat pumps lose capacity as the air cools — from 80°F down to 50°F air, expect to retain roughly 58 to 75% of capacity, with the coefficient of performance dropping from about 5.5 to 4.0. That 4.0 is not a coincidence: it is the code minimum, so makers are reporting to the floor. Inverter models fall further, one popular unit to 42%. And one major manufacturer’s manual notes defrost cycles beginning near 50°F that “will vary from 15 minutes to several hours depending on ambient air conditions.”

Be sceptical of heat-up claims generally. One widely copied dealer page claims a 1,500-gallon spa goes from 65°F to 102°F in 15 minutes on a 400,000 BTU heater. The real answer is about 84 minutes. The claim is wrong by more than five times, and it has been copied across the trade with the conditions stripped off.

The spillover is the most expensive switch on the pad

A spillover spa is beautiful and it is the reason many people buy one. Understand what running it does.

With the spillover open, the pool and spa are one body of water. You are no longer heating 600 gallons, you are heating everything:

The multiplier is simply the combined volume divided by the spa volume. And it does not buy you a warm pool as compensation: dumping one heated spa-full into a 20,000-gallon pool raises it by less than one degree. The heat is conserved and devalued.

There is a common builder defect worth checking for: an offset return valve that sends a fixed share of flow over the spillway permanently. It looks lovely and it means you can never heat the spa separately. It also drives pH up continuously by aeration, for the same reason every water feature does — see our guide on what water features do to your water. The practitioner setting is 15 to 20 minutes, twice a day, not continuous.

The cover matters more here than on the pool

At spa temperature the heat leaves through the surface. Running the Florida Solar Energy Center’s model for a 7-foot spa at 102°F in 50°F air with a light breeze puts roughly 69% of the loss in evaporation, 20% in convection and 11% in radiation, with conduction through the shell well under 1%. That last figure has a practical consequence — insulating the shell of an in-ground spa is close to pointless, because that is not where the heat is going.

One caution about that split, including our version of it. It is modelled, not measured. If you go looking you will find 68/17/15 quoted all over the web as though it were a spa measurement; it is FSEC’s worked example for a pool at 80°F, which is a different problem, and the radiation share in particular does not survive the move to spa temperature. What holds at any temperature is the shape. Evaporation is around two thirds to seventy percent of the loss, and evaporation is the part a cover stops.

Uncovered, that spa is shedding somewhere between 5 and 8 kW, or 17,000 to 29,000 BTU/hr. The width of that range is not sloppiness — it is which empirical correction you apply to the same formula, and we would rather show you the band than pick the flattering end of it.

Covered, the picture changes so completely that the only honest way to describe it is with the measurement that exists — and it exists only for portable spas, because those are the ones the law tests. California’s certification database puts the sales-weighted average standby draw of a portable spa at 218 watts, and 180 watts for spas meeting the current standard. That is the whole heat loss of a hot tub sitting outdoors in winter, covered. Against a five-kilowatt uncovered figure, the cover is doing well over ninety percent of the work.

The best controlled test of why is Cal Poly’s, which ran twelve identical pools against an uncovered control with the leakage backed out, and measured foam, bubble and solid covers all suppressing evaporation by about 95%. A spa cover is not an accessory.

Efficiency: what is real and what is a word

Three findings here are worth more than any brochure.

“Fully insulated” means nothing. In the California market study, more than 99% of spas claimed it. The distinction that matters is closed-cell foam versus fibreglass, and fibreglass, in the study’s words, “if it gets wet, essentially loses all insulating properties.”

An efficient spa does not cost more. A regression across 107 spas found that efficiency had no statistically significant effect on price. Brand, volume and jet count drove price. Efficiency did not. You are not choosing between a cheap spa and an efficient one.

Inflatables are allowed to be roughly twice as leaky, by law. California caps standby power by volume, with a separate and much looser formula for inflatables. Work the two formulas and you get this:

A 250-gallon inflatable is legally permitted more standby power than a 500-gallon hard-shell. A 500 hard-shell gets the same allowance as a 248-gallon inflatable. The regulator has effectively conceded that inflatables are about twice as leaky per gallon and written the concession into the tolerance.

One thing to watch for while shopping: there is no ENERGY STAR category for spas. It does not exist. Any listing claiming the badge is misusing it.

Do the jets do anything?

Every spa is sold on a jet count, and the jet count is close to meaningless. Here is the number that would tell you something, and which no manufacturer prints.

A jet’s flow demand is set by its orifice. From the watershaping trade literature, corroborated by a jet manufacturer’s own tables: a quarter-inch jet wants 7 to 8 gallons per minute, five-sixteenths wants 10 to 12, three-eighths wants 15 to 18, seven-sixteenths wants 22 to 25.

Now do the multiplication. Ten five-sixteenths jets want 100 GPM, which is the entire output of a 2 HP spa pump.

So what is a 90-jet spa? It is 90 orifices, of which perhaps 20 to 30 are live at any moment, with diverter valves moving water between zones. That is not a scandal — zoning is sensible engineering. But it means the advertised count is a number the machine can never deliver simultaneously. Comparing two spas on jet count compares two numbers that describe neither machine.

A related trap: the horsepower on the label is electrical input, not output. A pump sold as 5 HP draws about 5 HP of electricity and delivers perhaps 3.5 to 4 HP at the shaft. And the flow figures in retail listings — “210 GPM”, or a maximum flow beside a maximum head — are the two endpoints of a pump curve. You never get both at once.

What to ask instead: how many jets run at once, on which pump, and at what flow per jet. If nobody can answer, the spa has not been specified, it has been decorated.

And what the research says about jets specifically

There is a genuine literature on warm-water immersion, and it is not nothing. Immersion produces real measurable effects — raised heart rate, lowered mean arterial pressure, substantially increased limb blood flow, lower cortisol.

The question is whether the jets add anything to that, and one 2024 randomised crossover trial set out to isolate exactly that, comparing air jets, water jets and no massage in the same water. Its finding, verbatim: “There was no additional benefit of water-based massage, while air-based massage blunted some positive vascular responses.” Participants did not even rate the jets more enjoyable.

Two honest caveats. It is a small study, twenty people, conducted in an inflatable spa rather than a high-end acrylic one. And the broader balneotherapy literature that shows benefits mostly uses mineral spring water on multi-week residential programmes, which does not transfer to tap water and chlorine in your backyard; its own authors flag the lack of blinding, and the one trial measuring a hard outcome — return to work at one year — found no significant effect.

But the direction is clear, and one detail makes it harder to wave away: the jet study was funded by a spa manufacturer. It is a finding against the sponsor’s interest.

The reasonable reading: buy the warm water, which is well evidenced. Treat the jets as a preference, not a therapy.

The portable option, and the bubbles that cool your water

For a lot of people the honest answer is a separate tub. It is hot on demand, it takes no pool surface, its energy use is measured and labelled, and it leaves with you.

The inflatable tier has one feature you should understand before you buy. Those “120 bubble jets” are air bubbles from a blower, not water jets from a pump. On one common model the blower is 800 watts, the heater is 2,200 watts, and the entire water circulation is 460 gallons per hour — 7.7 gallons per minute, less than half the flow of a single three-eighths-inch therapy jet.

Blowing ambient air through hot water is also an excellent way to cool it, and you do not have to take our word for that. The manufacturer’s own manual lists “the air bubble function is activated when heating the spa water” among the causes of slow heating, alongside freezing weather and wind. And for the error code that means the water is too hot, the manual’s prescribed solution is to press the bubble button. The headline feature is, in the maker’s own troubleshooting table, the chiller. The trial above measured it: about a 1.1°C greater drop over thirty minutes than sitting still.

The 120-volt limit is arithmetic, not a design choice. A 15-amp circuit at 120 volts, under the electrical code’s 80% continuous-load rule, gives you 1,440 watts. A 1,500-watt heater is already over that on its own. This is why a plug-in spa cannot heat and run the blower at once — and it is not purely a voltage issue: one major manufacturer notes that a 240-volt spa on a 40-amp breaker has the same problem. The real variable is available continuous watts.

The clearest illustration is a natural experiment: the same model of inflatable ships with a 1,300-watt heater in the United States and a 2,200-watt heater in the United Kingdom, heating at 2 to 3°F per hour against 3 to 4.5°F. Same tub, different grid.

On running cost, metered owner reports cluster around 4 kWh a day at 60 to 70°F ambient and 9 kWh a day at 45°F. The enormous $20-to-$200-a-month range you will see quoted is mostly your electricity rate and your climate, not the tub.

Aesthetics: what the design press says, and where it disagrees with your builder

There is a real disagreement here, and only one side has a commercial interest in the outcome.

Raised versus flush is the main fork. Builders default to raised, and a raised spa earns more. The watershaping design press is markedly cooler on it — one journal argues spas look best “tucked inside a corner of their associated pools at the waterline,” where they are the least visual noise.

The reconciliation is that the real dispute is not height, it is water volume. The same critic admires raised spas whose spillways move small, asymmetric quantities of water. What he objects to is the gusher: “spillovers that are oversized for their surroundings — the volume of falling water is too great for the pool and/or spa, and the sound it generates is overwhelming.”

That is corroborated from the engineering side. A sheer-descent manufacturer’s own technical sheet puts the requirement at 10 to 12 gallons per minute per foot of weir, and adds two specifications you can hold a builder to: do not install the waterfall more than 3.5 feet above the water or the sheet breaks up, and if the waterfall flow exceeds 40% of filtration flow, specify a separate pump. A related rule from the trade press: one pump should not be asked to do pool filtration, spa filtration, therapy jets and a spillover.

Elevation’s real job is what the bather sees. As one Genesis 3 designer puts it, “what you see from inside the spa is absolutely crucial” — his cautionary example is an architect who set an ocean-view spa at grade and recessed the water below it, “completely depriving the bathers of the ocean view.”

The height band converges on 12 to 18 inches. Eighteen inches is the height at which the wall works as a seat for someone talking to you from the deck. Twenty-four inches is a difficult step for some people. And there is a documented failure at the other extreme: an owner mid-construction discovered their builder’s default was four-foot walls — “I sat inside the spa and realized I can not see over the walls” — and the fix was compromised because the jets were already set.

The regret owners write down, and builders do not mention

This one is worth the whole section. From an owner, after the fact:

“We also have a raised spa on the house side of the pool and the one regret I have is that I cannot see on the other side of it. So with any kids you never know what is happening behind that wall if sitting on deck or in the house looking out… A little thing I never even thought about.”

A raised spa between the house and the pool creates a blind spot in the pool. One builder refuses that layout outright for exactly this reason; another defended it on the grounds that moving it “would mess up his design.” An owner in the same discussion noticed the convention and worked out why: “most people have their spas off to the side or back corner of their pools, and this may be why.”

There is a genuine trade on the other side, and it is social rather than aesthetic: a spa on the far side of the pool protects the view but takes the soakers out of the conversation happening on the patio.

Three tests, all cheap, all before you sign:

1. Pick the location standing at your main indoor sightline — usually the living room or the kitchen door. The spa should terminate that view without blocking how people move through the yard.
2. Judge privacy sitting down, at water level. Neighbouring windows and second floors appear from that height in a way they do not when you are standing.
3. Put flags in the yard. Owners who have been through it are consistent that renderings mislead and flags do not.

Continuity, and what dates

The best-sourced rule about tile is a construction rule rather than a colour rule. Dimension the spa to the tile module. As one designer puts it, “because a spa is small and the entirety of the perimeter can be taken in at a glance, any flaws in construction are extremely noticeable” — so set the dimensions so tiles fit without cuts or odd trim.

Matching coping and waterline tile across pool and spa is the prevailing convention rather than a law, and a contrasting spa is a legitimate choice when the contrast is doing sculptural work. The buyer’s test: ask your builder to say, in words, what the contrast is for. If there is no answer, match.

On the interior finish, the argument for tile or polished aggregate in a spa is tactile rather than visual — you are in contact with it, and it “feels great to the touch.”

Two material warnings. Textured ledger stone and porous marble under a spillway are a mistake, because the ledges make calcium scale nearly impossible to clean; a smooth, glazed, non-porous surface takes cleaning products. And an uncoped open-face spillway is a real slip hazard: “open face spillways are dangerous to walk on when wet.”

What dates a spa is easy to name, because the industry has been through the cycle once already. Artificial rock is the clearest verdict, and the objection is structural as well as visual: faux rocks of the 1980s and 90s “were made of unprotected concrete that eventually cracks. Water migrates into the rocks and they decay from the inside out, like a disease that spreads” — eventually reaching the bond beam. The same designer calls that generation “aesthetically horrendous.”

The instructive part is the timeline. In 1989 a newspaper declared kidney shapes “as out of fashion as Twiggy-shaped models”; in 1990 the same paper was selling the replacement — free-form lagoons with waterfalls and “exotic rockscapes” — in the language of timelessness. That is precisely the look now being torn out. The cure became the next disease inside a single renovation cycle.

The durable line, from a builder with 800-plus projects, is between “trying to fully mimic nature” and “acknowledging that what we’re building is architectural and man-made, but informed by natural textures, colors, and forms.” The stone that dates is the stone pretending to be nature.

On lighting: colour-changing LED is now simply the standard fixture, so the hardware is not a fad. What dates is using it — standing colour and cycling shows. The durable specification is warm white as the everyday setting, on its own circuit from the pool, with colour as an occasional scene. One rule worth following: do not aim a fixture at the water surface, because the reflection puts glare straight into the eyeline of everyone sitting in it. And note that a small vessel is easy to over-light — a single conventional niche light in a spa is the hot spot.

Safety, and one legal asymmetry nobody covers

A standalone hot tub satisfies the child-barrier requirement with its own lockable cover — the federal definition of a barrier explicitly includes, “with respect to a hot tub, a lockable cover.”

An attached spa cannot do that. It sits inside the pool enclosure, has no cover, and inherits the pool’s fencing regime entirely. If barrier compliance is a live question on your property, that is a real difference between the two purchases and we have not seen it discussed anywhere else.

It is also worth remembering that Virginia Graeme Baker died in a spa, not a pool — small volume, high pump flow, small drain area, bodies close to the drain. The law that followed binds the drain cover on residential spas. A commonly repeated claim that federal law requires a safety-vacuum-release system on a residential spa is wrong: that requirement applies to public pools and spas, and residential rules come via state law and the building code.

One maintenance item that appears in no buyer’s guide: spa drain covers have a service life of five to seven years and must be replaced when it expires or when cracked.

So: spa or no spa?

Buy the attached spa if your pool is 16x32 or larger so the swim-lane cost is small, you have natural gas rather than propane, you will use it fewer than roughly eight times a month in cold weather, you want the spillover as a visual and acoustic feature, and you accept a 45-to-75-minute wait each time.

Buy a separate tub if you want to actually soak on a Tuesday night without planning it, your pool is under 14x28, you are on propane, or you want an appliance whose energy use is measured and labelled rather than asserted.

Buy neither if you are honest about the last question, which is the one the industry does not ask. The best survey we could find — 2,000 owners, commissioned by an insurer with no stake in the answer — found that 55% of hot tub owners now rarely or never use theirs, with running costs the most-cited reason among those who stopped. A separate survey put hot tubs seventh among most-regretted purchases, with 36% going unused. Against that, a manufacturer’s own survey of 700 of its customers reports 16.7% using theirs four or more times a week. Both numbers are weak — the first is British, from 2022, and a lockdown cohort; the second is vendor-run and self-selected — and we would rather show you both with their conflicts than pick the flattering one.

The decision is not which spa. It is whether you are one of the 45%.