Spa Guts Explained - What’s Inside Your Hot Tub and How It All Works

Spa Guts are the internal parts of a hot tub - the components in the spa that keep the water hot and the hydrotherapy massage working.

Your hot tub isn’t just a tub of water—it’s a complex system of components working together.

Circulates water through jets and filtration system.
Some spas may have a separate 24 hour circulation pump (runs continuously for filtration/heating) and jet pump(s) (provides massage power) while other spas use the low speed of a 2-speed pump to provide circulation and heating duties.
Importance: Ensures clean, warm, and properly circulated water. Allows water to circulate through a heater to warm up to the desired temperature.

Maintains desired water temperature
The spa heater is found in the main circulation line along with the circulation pump (or the main pump that runs on low speed when heating the spa). Typically heaters in hot tubs are rated at 1kw for 120v spas and either 4kw or 5.5kw for 230v spas. The heater is usually mounted below the "spa pack" or control box (flow through heater) but can also be mounted on the back of the box (slide heater). Most heaters have unions on either side of the heater to connect to plumbing hose or pipe. Heater line plumbing size depends on if the heater is a low flow type connected to a 24 hour dedicated circulation pump, or the larger inner diameter of the heater tube (heater manifold) which typically connects to either 1 1/2 inch or 2 inch plumbing.
Safety feature: The control system includes a high-limit sensor connected to the heater to prevent overheating. The hi limit sensor is either clipped onto the heater manifold or found inside the heater (like Balboa M7 type heaters that have both a temperature sensor and hi limit sensor inside the heater)

PVC or flexible tubing connecting pumps, heater, jets, and filter
Directs water flow efficiently to all parts of the spa. Various plumbing sizes are used in the spa to assure the proper flow rate to each jet, valve and component. Manifolds in the plumbing line split the water flow as needed
Proper plumbing layout prevents pressure loss and ensures strong jet performance
Plumbing sizes are NOMINAL which means everything connected to the plumbing refers to the size of the inner diameter of the connecting hose or pipe/plumbing system. For example a 2 inch PVC coupling refers to the 2 inch inner diameter flex hose or pipe that is connected to the coupling, NOT the measurement of the coupling or inside the coupling itself. If you think of nominal plumbing as including the word "for" then it is easier to understand - as in a 2 inch coupling FOR 2 inch flex hose or pipe.
For a pump union connection, the spec given may be 2 inch plumbing connection for 2 inch flex hose, but that would correspond to a 3 1/16" measurement across the actual pump threads. Nominal measurements help stay with the same plumbing size across multiple components and connect spa guts components and PVC parts together.

Provide hydrotherapy massage by forcing water and air into the tub. The water is typically supplied to the jet bodies by flex hose, usually 3/4" (inner diameter flex hose) and the air is supplied to the jet with 3/8" inner diameter tubing
Jet NozzleTypes: directional, rotational, dual roto, massage and combination jets.
Adjustable flow: Many can increase/decrease water/air mix for a personalized experience by turning the ring around the jet face of any adjustable jet.

Filters out dirt, debris, and contaminants
Usually includes a removable filter cartridge (sized by square foot of filter pleats)
Essential for water clarity and sanitation.

Optional component that helps reduce chemical use.
Injects ozone into water to kill bacteria and oxidize impurities during filter cycles/when the circulation pump is running. For ozonators connected to 2-speed pump lines, the ozonator typically shuts off if the pump goes on high speed.
Works alongside chlorine or bromine for cleaner water.

The spa pack/control box is the brain of the spa. The circuit board inside the box controls pumps, heater, lights, jets, and timers.
The topside control panel has an overlay graphics sticker with the correct button layout and allows users to set temperature, jet patterns, and filtration cycles.
Modern systems may include Wi-Fi connectivity and smartphone apps such as the Balboa BP Series. The older VS series is not Wi-Fi compatible. Setting a spa up for wifi requires adding a compatible wifi module. There are different wifi modules available for Balboa, Gecko, ACC and other spa control manufacturers.

Manages air intake for the jets via a valve, enhancing massage effect.
Adjustable knobs allow more or less air to mix with water.
Some spas have a dedicated air blower that forces air into the air manifolds and dedicated air injectors.

Regular cleaning of filters and changing them annually, inspecting plumbing for leaks.
Proper Water Chemistry: Keeping spa water balanced to prevent scale and corrosion. See our section on how to maintain a hot tub.

Understanding spa guts helps you familiarize yourself with the components "under the hood", troubleshoot problems, perform maintenance, and enjoy your hot tub to the fullest.
Knowing what’s inside makes sourcing parts easier, making repairs less intimidating and ownership more rewarding