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How A Hot Tub Works (Parts And Plumbing Explained)

Ever wondered how a hot tub works?

You know the water circulates, but there’s a whole process that happens in between to help keep everything running smoothly — and more importantly, keep the water clean and clear.

This article will walk you through a complete hot tub system, including all the parts and plumbing from start to finish.

The Control System (Spa Pack)

The control system (also called the spa pack), is the brain of your hot tub.

It controls everything the hot tub does, from turning on the jets, to how much heat is injected in the water, and even turning on any lights and music.

Over time, or with heavy use, this system will wear out. On average, you can get about 7-8 years of life from it, but it’s not unheard of to squeeze 10 years.

This doesn’t mean your hot tub has to be replaced though. All you’ll have to do is buy a new control system and you’re ready to go.

Topside Or Spa-side Control

There are two types of control systems to run hot tubs: topside or spa-side.

  • Topside control systems are found on hot tubs that are standalone shells. This type of tub is the most popular, and the control panel is located on the top edge of the tub. This provides you with controls at your fingertips while you’re in the tub, making it convenient when you want to make changes to the environment.
  • Spa-side control systems are located away from the tub with the pump, filter, and heater. These systems are usually used in custom hot tubs which are built from the ground up and integrated into an existing pool and landscaped backyard. Many spa-side systems can be used with a remote to trigger the different functions of the tub. 

While wireless remotes do exist, hard-wiring is more common. This requires running a line from the system to a point on the spa’s deck where the remote can be accessed easily while in the tub.

This line should be installed when building the tub, as most high-end designs are surrounded by concrete.

The Operational Parts (Plumbing)

The plumbing of a hot tub is a closed-loop system that’s designed to endlessly cycle your water for as long as it’s running.

This whole process prevents stagnation, distributes chemicals, allows the filter and heater to do their job, and provides a steady stream of water for the jets to do their “hot tub thing”.

Here’s how it works:

  • The suction side is what sucks the water out of the tub and into the filtration system, and it includes the pump (on low speed, for sucking), skimmers, and suction lines.
  • The filtration system is what filters and eventually heats the water being sucked in, and it includes the pump (to keep things moving), filter, heater, and sometimes an ozonator.
  • The return side is what pushes the clean, heated water back out to your hot tub, and it includes the pump (on high-speed, for pushing), return lines, manifolds, PVC tubes, and jets or hydrojets.

Let’s go over the plumbing system in more detail, following the flow of water leaving the hot tub (the suction side).

Suction Line

The suction line is the water’s point of entry into the hot tub circulation system.

Water is sucked into this line from two points in the tub: the skimmer, and the floor drain. These 2 lines then converge into one, and the water is sent to the next piece in the equipment chain (the pump).

Pump

You can think of the spa’s pump as the heart of your filtration system.

It’s responsible for pulling in water, moving it through the filtration system, and returning it back to the tub. Much like a human heart, without a properly functioning pump, everything else fails.

One side of the pump creates suction, which is what pulls water in from the hot tub. The other side of the pump moves in the opposite direction, creating pressure. 

You can think of it like a pendulum. If one side is sucking in the water, by default, the other side is going to be pushing the water along.

Filter

After the pump, the water is pushed through the pipes and sent through a dedicated filter.

The most common filter for hot tubs is a cartridge filter. This is a cylindrical, pleated filter made from polyester or paper. As water passes through the filter, pollutants that have been killed by chlorine or bromine stick to it and are removed from the water.

A sand filter or DE (diatomaceous earth) filter can also be used for this process, although they’re less common. Sand filters use spiky silica sand that contaminants stick to, while DE filters use grids and DE powder for their filtration process.

Note: Some tubs design their cartridge filter into the skimmer, where the filtration process begins. This is done to help remove large debris, so it can’t enter the spa system and damage any equipment.

Heater

If using a dedicated filter, the heater will come after it. If your filter is located in the tub’s skimmer, the heater will come after the pump.

Heaters come in a few different styles: electric, gas, and solar.

Electric heaters have elements inside the unit that run the length of a stainless steel pipe. This raises the temperature of the water as it passes through. 

Gas heaters are usually used with inground hot tubs and require a gas line. Running this line can be extremely costly, but the upside is that these heaters are extremely fast and effective when it comes to heating the water.

There’s also solar heaters that can be used with your hot tub for a more eco-friendly method. These harness sunlight to heat the pipes that the water travels through. While it’s an option, this heater’s reliance on sunlight makes it less than ideal if you want to use your tub at night.

Of course, hot tubs are meant to be hot, but too high a temperature can cause injury. For this reason they’re equipped with a failsafe to avoid scalding.

According to the US Consumer Product Safety Commission, the best temperature to keep the tub is between 100°F and 102°F. Most tubs will cut off at 104°F.

Ozonator (Optional)

Some spa owners are installing ozonators to help keep the water clean. If you decide to use one, it will be located after the heater in the equipment chain.

Ozonators work by generating ozone gas and injecting into the water. This process allows ozone to remove pollutants in the tub, while reducing the level of sanitizing chemicals that are needed.

There’s two types of ozonators you can pick from: ultraviolet light (UV) or corona discharge (CD). CD is the more widely used ozonator due to its longer lifespan, high efficiency, and smaller size.

Return Line

The hot tub’s return line is the PVC pipe that sends the water from the heater (or ozonator) back to the tub via the manifolds.

It’s like the suction line, but in reverse.

Manifolds

At the end of the return line sit the manifolds. These are used to split up the returning water, diverting it to each of the return jets.

Manifolds come in a variety of sizes to accommodate tubs with few, or many jets. 

PVC Tubes

Attached to the manifolds are flexible tubes. These tubes run from the manifold to the jets, delivering the filtered water to the tub.

They’re made of flexible PVC to easily wrap around the peaks and valleys of the tub’s shell. This also saves space and makes for cleaner lines.

Jets Or Hydro Jets

The calling card of any hot tub are the jets. These are the entry points where the water from the filtration system returns to the tub. 

The jets can be adjusted for water pressure, shooting the water in slow or fast. 

This makes them double as massagers, adding an additional layer of relaxation for your muscles.

Hot tubs jets have 3 components to them: water in, air in, and an air mixture outlet.

The hot tub’s pump moves a pressurized stream of water through the filtration system’s pipes, out the jet, and into the tub. Air is also mixed into this stream via a venturi, which is a small hole in the jet that water is directed through. This is referred to as the “Venturi effect”.

This occurs because when water takes up space in the pipe, the speed of it increases while the pressure decreases. This causes a reduction in the incoming water feed, forcing it through the small hole. 

The air outside the jet is of higher pressure than the water inside the pipe, so air is forced into the stream and mixes with the water. This makes a pressurized mixture of air and water, which is then shot out of the jets, producing the bubbles.

Note: Air blowers are sometimes used in hot tubs to produce bubbles. They’re usually installed on a tub that has a fairly long return line. The longer this line is (as a result of the pump being far away from the tub), the fewer bubbles you’ll get. If the equipment is fairly close to the spa itself, a blower will be overkill in most cases.

That’s How A Hot Tub Works!

So there you have it. Now you know how a hot tub works.

It’s not overly complicated, but the knowledge will come in handy if the tub requires troubleshooting, as you’ll know how to identify each step along the chain if any problems arise.

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