hot tubs

How Acrylic Hot Tubs are Made

Today’s energy-efficient spas incorporate different materials, technology and accessories

When Roy Jacuzzi incorporated therapy jets into the side of a hot tub in 1968, he was not only creating a relaxing environment for water therapy, but also pioneering a progressive and emerging industry.

Today’s energy-efficient spas incorporate different materials, technology and accessories to entice a growing and sophisticated clientele. This friendly competition among manufacturers has improved product quality and offers myriad choices for consumers to select their ideal spa.

While many companies use the same construction procedures, each manufacturer incorporates unique components, equipment and jet technology to differentiate its spas.

Molding and Vacuum Forming

Flat, thermoplastic acrylic sheets are drawn into molds by vacuum to form the contours of spa seats, lounges, pillows and filter wells. Time and temperature are tightly controlled during this critical process, which also shapes the top edge and outlines where plumbing fixtures will be located. The thermoplastic materials form a continuous outline of the spa that is waterproof, fade resistant and impervious to separation.

Shell Strengthening

When it is cool enough to move, the acrylic spa shell is gently removed from the mold and sent to a station where a coating is applied to the underside. Since the acrylic has little structural rigidity, the coating gives the shell enough strength to contain hundreds of pounds of water and bathers. Some manufacturers spray fiberglass fibers and resin; others roll on polyester resins. In either case, the shell achieves rigidity and strength during this phase.

Trimming and Drilling

Holes are drilled in the spa shell to accommodate each fitting and its associated plumbing. As the coatings under the acrylic are hard and tough, carbide and diamond tools are often used both for speed and cleanliness of cut. Each hole must be precisely cut and flush to ensure a watertight fitting.

Just as important is trimming the edge of the spa so it will mount flush on the spa frame. This stage ensures an even weight distribution around the perimeter of the spa shell.


Spas have a lot of plumbing compared to a swimming pool. Each jet has to be individually plumbed, sometimes in a loop configuration to ensure even pressures. And, unlike pools, air lines must be plumbed in addition to the water lines.

Flexible PVC pipe is widely used due to its inert properties with spa water. To make the process more automated, less expensive and less likely to leak, many manufacturers are using pre-assembled plumbing harnesses. They are simpler to install and fit well within the confines of the cabinet.

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The frame creates a cavity where plumbing, pumps, insulation, heaters and electronic controllers are located. The most common material used for hot tub frames is premium-grade, treated lumber to support substantial weight. Their design uses support similar to roof trusses, and the lower members are elevated to allow air passage and avoid rot.

Some manufacturers use galvanized steel for frames because it is up to 20 times stronger than wood and warp resistance, however it is more expensive and thus will increase the cost of the spa overall. Still other manufacturers use patented technology for their framing, such as forming frame molds from acrylonitrile butadiene styrene (ABS), which is a common thermoplastic polymer that is both strong and water resistant.

Interior Components

Pumps are plumbed and wired into place in the lower area of the frame. Commonly, other water maintenance devices like ozonators, ionizers, purification cells and blowers are installed at this time as well.

The components of today’s spas are a far cry from those used in the wooden hot tubs of 15 to 20 years ago. Solid-state electronics are used in most spas, controlling everything from water temperature and filtration times to aromatherapy and massage cycles. Many are equipped with filtration and smart features to make the spa nearly maintenance-free. They can also be set to take advantage of off-peak energy rates, ensuring your spa will operate in the most economical way possible. Some are even equipped with time-out features that will automatically turn off a pump or lights should you accidentally leave it on after using the hot tub.


Next, insulation is installed to retain water temperature and reduce noise from pumps and blowers. Some manufacturers use a full-foam technique where the interior cavity is filled with spray-in insulation, while others use blankets of insulation around the spa shell.

Water Testing

Spas receive routine testing and inspection throughout the manufacturing process, but filling the spa from a tank, running the pump and so forth while the hot tub is still in the factory ensure its optimal functionality. Fittings that pass through the shell are visually inspected for integrity. Pressure testing of all plumbing lines ensures even the tiniest leak is detected. Many manufacturers even test the electrical current draw of the pump motors to be certain they run efficiently.

Exterior Cabinets

Since spas are covered more than they are in use, cabinets are the single most noticeable component on the spa. Most spa cabinets are constructed from polymers, which combine the appearance of wood with strength, rigidity and ease of maintenance. These will not crack, delaminate or be subject to other problems wood might incur during the life of the spa. Companies offer polymer cabinets that are molded to look like rock, furniture or other finishes you might see in a backyard setting.

Final Inspection

The product that leaves the plant represents the reputation of the spa manufacturer, so final inspections remain a point of pride for them. Some manufacturers use spot-checking throughout the process, where inspectors can test a pump or look for material flaws. Others rely on redundant testing before the spa is wrapped for shipping.

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