What is a hot tub (spa system)?
Updated August 2026
A hot tub, or spa, is an insulated shell holding roughly 300 to 500 gallons of water that a heater element keeps at 100 to 104 degrees while jet pumps drive it through nozzles for massage. Under the cabinet it comes down to four parts working together: a small circulation pump, a heater assembly, a filter, and a control pack that decides what runs and when.
Where it is
The working parts sit behind one removable access panel, almost always the side below the topside control display. Pull it and you will find the heater as a stainless tube about 15 inches long with two wires on it, the control pack in a plastic box beside it, one or two pumps with unions you can loosen by hand, and the filter cartridge in a canister or in the skimmer well inside the tub. Photograph the model and serial sticker inside that panel before ordering any part.
What it does
Small volume plus constant heat is what separates a spa from a pool. Sanitizer burns off fast, pH drifts upward as the jets aerate the water, and dissolved solids build until nothing holds, which is why the water gets dumped every few months rather than nursed along for a season. Failures follow a pattern: the heater element scales over or dry-fires when flow is restricted, the flow or pressure switch sticks, and the control board gives up somewhere past year eight. A FLO, FL1, or FLC code on the display is not a heater failure. It is the tub reporting that water is not moving, and a clogged filter causes that more often than anything else.
Maintenance
Rinse the filter cartridge monthly, deep-soak it in filter cleaner every three months, and replace it yearly for $25 to $70. Test sanitizer and pH two or three times a week, then drain, wipe, and refill the whole tub every 3 to 4 months. Keep the cover latched and dry, because a cover that has soaked up water insulates poorly and can add $20 to $40 a month to the power bill. Cadence for all of it sits in the hot tub maintenance checklist and how often to clean the filter. Handled that way, a quality spa runs 10 to 20 years.
What it costs to replace
A replacement hot tub runs $4,000–15,000 for a new spa; $250–600 for a heater element and $300–800 for a control board.
OnOtto's photo scan identifies parts like this in your home and builds the maintenance schedule for you. See how the scan works.
Frequently asked questions
How much does it cost to run a hot tub?
Budget $30 to $80 a month in electricity for a modern, well-insulated 400-gallon spa held at 102 degrees, plus $10 to $25 a month in chemicals and filters. Cold winters, thin older insulation, and a waterlogged cover can push that past $100. Three things move the number more than anything else: cover condition, the temperature you hold between soaks, and whether the tub sits exposed to wind. A new cover at $400 to $900 often pays for itself inside two winters.
How often should I drain and refill a hot tub?
Every 3 to 4 months for a household that soaks a few times a week, and closer to every 2 months under heavy use. Spa techs use a quick formula: divide the gallons by three, then divide by the number of daily bathers, and the result is your days between changes. A 400-gallon tub used by two people works out to about 66 days. Refilling through a hose prefilter removes metals and hardness, which saves a full round of balancing.
Why does my hot tub keep shutting off or showing a flow error?
Nine times out of ten the filter is dirty or the water level is low, so start there. Shut the tub down, pull the cartridge, and run it briefly with no cartridge installed. If the error clears, you found it. Next check for a slice valve left closed after a service call, air trapped in the pump (crack the pump union to bleed it), and a jammed flow switch. Errors that persist with clean, moving water point to the switch or the control board.