State-of-the-Art Spa Details

photo of a custom spa

While portable spas have advanced their technology over the last 20 years, most field-constructed spas have made little progress. I am often retained as an expert to review construction defect issues and am amazed at how many shotcrete spas are currently being built with improper plumbing, equipment and chemistry systems. It’s also disappointing that oftentimes, no thought or planning was invested into the comfort of the vessel — which should be a primary goal. After all, the intent of the spa is primarily therapeutic in nature.

The pool and spa industry is very innovative when considering the product and manufacturing space, but less innovative when we look at builders. There are certainly creative builders that dream up wild pool and spa designs, but distracted by their busy sales and project management roles, they tend to build things the same way again and again without thinking about how to make each project better than the previous one. It’s a shame because there are many opportunities to take classes, read articles and get enlightened.

Getting Comfortable

I took my first Genesis 3 class nine years ago. Back then, they were promoting a detail where the coping on a spa was not cantilevered over the waterline tile because it is uncomfortable to have a projection into your back or neck. It’s a simple concept and the group has detailed over 20 different spa coping configurations that don’t cantilever into your neck so we know it’s easy, but I continue to see new spas built with that uncomfortable, traditional detail.

Commercial codes often require a cantilevered grip for safety, but we have versions that comply with the health department requirements without the projection. You wouldn’t buy a family room couch with stone projections through the pillows so let’s not build our spas that way.

Another uncomfortable yet common feature is the projecting spa jet fitting. Are you kidding me? Bulging eyeball fittings can be flush-mount eyeballs. Larger spinning therapy jets can be recessed, too — the portable spa industry has been doing this for years.

One of my favorite details that we developed several years ago was a solution for those annoying anti-vortex suction outlet covers that project up from the floor: the channel drain.

When I saw my first channel drain, I decided I would place the split pairs at the walls right at floor level, which would clean as well as any other floor-located outlet (after all, we have to push the debris to the outlets as they can’t actually suck the debris to themselves). This quickly led to our signature toe kick detail that improves safety and aesthetics, simplifies the structural design of the floor and adds a practical benefit that functions just like the toe kick in your kitchen.

With our toe kick detail firmly established as our “standard” configuration for outlets, we began improving aesthetics by hiding other things in the shadows such as the un-jetted return lines from the filter, LED bullet lights and even LED strip lights that cast a glow across the floor, providing enough illumination for the whole spa. No other light source is necessary or visible.

Using these design methods, our spas are starting to look unusually and beautifully free of plastic fittings, bulging stainless steel rings and glass lenses. The floor is larger and curiously appears to extend well beyond the walls of the footwell, even though the toe kick is only 6 inches deep. Have you ever seen suction outlet covers that were not geometrically aligned on the floor? This is really noticeable in an all-tile spa — even plain plaster finishes can’t hide misaligned fittings — but these issues simply disappear with the toe kick. Now the floor is an empty canvass, waiting for an artist’s mosaic of tile, free from interruption in both aesthetics and the texture of the covers.

Some of our clients that have built the toe kick have said they won’t build them any other way now. The improvement is so popular it helps them win more contracts and, of course, their new clients anticipate the same high level of attention that they witnessed when they toured some of the builder’s existing work.

The homeowner that is paying for a custom spa expects to be comfortable, and you, as a designer and builder, should get comfortable with trying new things. If you don’t have the confidence to build the toe kick, then just try using channel drains on the walls of the footwell without recessing them — at least the floor will be clear.


Beyond eliminating the projections, there are other things we can do to make constructed spas more comfortable. Portable spas have become more luxurious with sloping seats, built-in pillows, lounge chair configurations and smooth contours that seem to wrap around your body. Why not incorporate some of the contours that the portable industry enjoys into a concrete vessel? (While such features are nice, personally, I would prefer one side of these elaborate spas to be simplified so that I don’t always have to be seated in a pocket. I also don’t think they are deep enough and most have extremely small footwells, but these space concerns are not a problem with field constructed spas that don’t need to fit through a door or gate in one piece.)

Some builders have tried to emulate portable spa ergonomics and succeeded — others, not so much. I saw one failed attempt at a Las Vegas resort — I won’t say which one, I’ll just say that they are open for all four seasons. This otherwise high-end resort includes a spa where the builder sloped the back of the seats. Great idea, except that when you slope the back you also need to tilt the seat bottom so that the two planes intersect in roughly a 90 to 100 degree angle. By leaving the seat bottom level, your body slips forward when you lean back, which requires frequent repositioning and removal of the wedgie caused by the swimsuit sticking to the seat while you slip. I needed an entirely different type of therapy after trying to enjoy that spa.

Increased depth is one big advantage that constructed spas have over their portable cousins. We started building 42-inch deep spas ten years ago. We’ve built deeper ones for tall clients, particularly professional athletes who have educated us on the issues of long bodies in shallow spas. We had a client specifically request a 36-inch deep spa to then pay us $6,000 to remove the floor and increase it to 42 inches. We haven’t proposed a shallower version since.

Standing wells have also been incorporated into some of our work. Typically, we do this as a 3-foot diameter pocket with two columns of jets and stainless steel handles so that the client isn’t blown off the wall. It’s quick and thorough hydrotherapy for the impatient — the well depth is usually set with the client’s shoulders right at water level and custom nozzles can extend above water level directed right at their neck and shoulders.

diagram of a custom spa suction outlet at toe kick detail

diagram of a custom spa channel outlet configuration

Detail for a completely unblockable split pair channel drain (bottom) and toe kick suction outlet (top), both of which eliminate ugly protrusions at the floor of the spa.

Behind the Scenes

Vessel details aside, there are plenty of things we can do to improve the mechanical systems. We should always start with safety: suction outlets should always be at least split pairs (if not more) and never single suction outlets, even if the outlets are “unblockable.” The reality is that the confined volume of spas brings people in close proximity to multiple outlets, and there should be no risk of entrapment and no need for SVRS devices as a backup. Size the lines appropriately, ensure adequate depth in the sumps and always follow the manufacturer’s installation instructions, which may include specific requirements or limitations that were mandated by the nationally recognized testing laboratory when the product was listed.

Fortunately, the capacity of the channel drains that we specify exceeds the combined flow rate of the filtration and typical eight-jet configuration that we use, meaning that for most projects we only need two channel drains (see Detail P405). Adding more jets simply requires additional split pairs of channel drains.

Skimmers are typically not used in residential applications where the spa spills into the pool, but I’ve seen independent spas without skimmers and the water surface is coated with oils, floating debris and biofilm that is both unsightly and, more importantly, unhealthy. Spas should have a skimmer even if constructed with a pool in a combined system. When a plastic skimmer is used, don’t use an equalizer located above the bench. The equalizer could easily become a single suction outlet in this configuration and, combined with the poor location above the bench, could be an entrapment problem in multiple ways (e.g., body, limb, hair, mechanical).

Moving past the safety issues, all of the equipment, including pipes, fittings, and valves, need to be properly sized and compatible with each other. A common 8-foot spa only needs about 30 gpm to maintain a 30-minute maximum turnover, but some 400,000 Btu/hr heaters require 40 gpm to operate properly, so components not only require compatibility with each other but compatibility with the plumbing and spa details.

When plumbing the spa jets, our most common configuration is eight jets at 15 gpm each on a 2-hp pump. A detailed TDH analysis accounting for parallel looped systems shows that the TDH should be between 50 and 55 feet of head. There are common 2-hp pumps in the industry that match 120 gpm @ 55 feet very well without even needing a variable frequency drive. Our configuration requires no blowers and the jets are strong.

Now let’s say our client asks for two more jets. A detailed analysis will show that the TDH really doesn’t change because the additional jets are parallel (or “looped,” as a hydraulic engineer would describe them) with the original eight jets. Now we look at a family of pump curves and we find that we need to jump to a 3-hp pump for the two additional jets. This is because the performance curves drop off and the pumps produce less flow and head. Comparing the two systems, we use about 0.25 hp per jet with the 8-jet system, but we use 0.30 hp per jet with the 10-jet system — obviously less efficient.

So when we are asked to provide more jets, we often don’t just jump to a bigger pump — it is often more advantageous to add more dedicated jet pump systems. This also provides redundancy and keeps the pipe sizes reasonable, like multiple 4-inch suctions and 3-inch returns instead of 6-inch lines. Every project is different and some constraints may outweigh others and force us into less desirable configurations but we should always start with the safest, most efficient and long-lasting system that we can conceive.

Killing the Petri Dish

Spas, with their high bather load sweating it out in a small volume of hot water, are akin to Petri dishes for culturing bacteria. Short turnovers of 30 minutes are required, and if we expect the spa to be overloaded with kids at a resort, we’ll even shoot for 20 minutes. More importantly, we like to hit the water hard with a robust dose of ozone and a contact/degas tank (a small UV ozone generator won’t do it). This does not eliminate the need for a chlorine and pH monitoring and control system, but it significantly improves the health of the water.

There is only one system better than ozone: ozone and UV sterilization. This advanced-oxidation combination has proven to be even more lethal to the pathogens in the water than either system alone. The good news is that they are both very compatible with each other, although the initial cost has made advanced-oxidation unreachable for all but the commercial and higher-end residential markets. As the industry gives more attention to water quality I believe that designers, builders and our clients will place more value on advanced-oxidation and we will see this segment grow.

Part Two

I have outlined some simple considerations when designing a better field-built, spa but there is a lot more to this discussion. We can get more specific with the math. We can review detailed schematic diagrams and accurately calculate TDH. We can talk about control systems and how certain details once thought problematic are easily solved with certain plumbing configurations or control system setups. We can do this together in Orlando when I present ENGINEERING 201: State of the Art Spa Details — you can see all of this in action and ask questions in an interactive environment.

Comments or thoughts on this article? Please e-mail [email protected].

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