Basic types of spas & hot tubs

Most of the market for “hot tubs” today is more accurately for spas: technically, the term “hot tub” refers to the older, wooden tubs with bench seating, but the terms are fairly interchangeable these days. Because wooden hot tubs have increased maintenance requirements and higher purchasing and operating costs, they are less common today; however, some buyers strongly prefer the aesthetics of wooden hot tubs.

The most common type of spa purchased today is a molded acrylic shell with built-in seating and water jets. Beneath the shell is often fiberglass or wooden framing, and a composite or synthetic cabinet. One or more motors pipe water to the jets for massage and relaxation, and to filters and other cleaning systems to maintain the water quality.

Buyers who don’t want to permanently mount their hot tub in one spot can choose portable spas: smaller, preassembled spas that sit aboveground. These usually accommodate only one or two people, but still include motors and jets similar to the full-size permanent models.

Sizes
Small portable spas and hot tubs seat one or two people and hold approximately 100 gallons of water. The largest models seat 10 or more and hold approximately 1,000 gallons. Depths usually range from 3' to 7', and most are 3' to 9' in diameter. Hot tubs and spas are available in round, oval, square, or rectangular shapes.

A 7' tub is a fairly common size and typically seats about four to six people. The number of people who can share one hot tub isn’t a fixed number, as it depends on how friendly your hot tubbers are with each other, among other factors. In general, though, you’ll often find that filling a hot tub to its rated capacity results in a lack of foot space.

Heaters
There are two primary options for heating the water in your hot tub: electricity or gas. Electric heaters are more common and somewhat less expensive to install; natural gas heaters can reduce your operating costs over the long run.

Some buyers look for wood-fired, solar, or propane heaters for their spas and hot tubs. If you have a strong preference for one of these alternative heating methods, be prepared to spend extra—potentially a lot extra. You may even have to buy a kit or aftermarket add-on to convert the tub yourself.

Generally, you’ll need 220v service to power the pumps in a hot tub, regardless of what you use to heat the water, so you’ll probably need a professional electrician to do the wiring for you.

Insulation and cabinet
The spa shell is the innermost layer that holds the water. The outside—the cabinet—is most often made of synthetic material made to look like wood. The primary reason for this is the significant maintenance required to keep a real wood cabinet in shape. Of course, real wooden hot tubs usually feature wooden cabinets as well, which requires additional maintenance.

Between the shell and the cabinet is a layer of insulation designed to contain the heat of the water. The best choice for energy efficiency is full-foam insulation: a sprayable foam that fills the interior spaces, including those around the pipes and motors.

Full-foam insulation can make it somewhat more difficult to repair internal leaks. However, since it stabilizes internal pipes and prevents vibration from working them loose, it reduces the likelihood that those leaks will occur. It also helps reduce motor noise.