Why is my hot tub not heating?

Start with water flow, the filter and the display

A hot tub that will not heat is often a flow problem rather than a dead heater. One manufacturer's manual says the spa's heating system will only function with the proper amount of water flow through the system.^[1]

So look at what you can see from the outside: whether the cover is on, whether the water is moving, and what the display says. Write down any message word for word. It narrows the search before a technician picks up a tool, and it is the first thing a service desk will ask for.

The filter

A dirty filter is a common cause. The manual says a clogged dirty filter makes the heater and pump work harder than they need to, possibly causing them to fail.^[1] Even if a filter looks clean, scale and other particles can hide deep in its fibers and restrict flow, and Cal Spas recommends cleaning the filter at least once a month or every two weeks depending on use.^[1] A cartridge that has not had a deep clean in months is a likely reason for a tub that is warm but never reaches the set temperature.

Filter cleaning is routine owner care, and your own manual covers it. If a clean cartridge changes nothing, the cause is further in and needs a technician.

Air in the pump, and a tub that is just slow

If the pump hums but the water barely moves, air may be trapped. When a pump has an airlock, continuing to run it can damage the pump.^[1] Do not keep running a pump that is not moving water; a technician can clear the airlock.

Also rule out the ordinary causes of a slow tub. After a first start-up the heater can take 10 to 15 minutes to engage.^[1] Its heater status icon turns red when the heater is on and white when it is off, and it flashes during heater start-up, which is normal.^[1] On a cold day, with a tub that was allowed to cool a long way, heating back up simply takes hours. Cal Spas says that letting the water fall more than 10°F below the temperature you want to use, then reheating it, makes the heater run longer than holding the tub about 5°F below.^[1] The cover works like a lid on a thermos and lowers the amount of time the heater has to run.^[1] A cover that is waterlogged or does not seal well loses heat the heater may not keep up with.

When it is the heater, a sensor or the controls

If the flow is good, the filter is clean and the tub has had time, what is left is the heater element and housing, the sensors that tell the control pack when to heat, the pressure or flow switch, and the control pack itself. These all sit behind the cabinet, connected to the tub's dedicated circuit, and diagnosing them means measuring live parts.

Trips tell a technician a lot. A breaker that trips again as soon as it is reset probably means a direct short, and you should not try to reset it again.^[2] On the GFCI plug or breaker, if the GFCI will not reset, unplug the spa, call the dealer for service and do not use the spa.^[1] Cal Spas says not to open the control box, because of the shock hazard.^[2]

The service visit is covered in what a hot tub repair technician checks. What it costs is in how much hot tub repair costs in Virginia, and if the tub is old, whether an old hot tub is worth repairing.

What to have ready for the technician

• The brand, model and serial number.
• Any message on the display, and when it first appeared.
• What the water temperature is against the set temperature, and how long it has been that way.
• When the filter was last deep cleaned or replaced.
• Whether the breaker or GFCI has tripped.

When to call a licensed pro

Call when the filter is clean, the pump is moving water and the tub still will not heat, or any time the breaker or GFCI trips. In Virginia no individual may work, or offer to work, as an electrical, plumbing or HVAC tradesman without a tradesman licence, unless working under the supervision of a tradesman licensed in that specialty.^[4][3] DPOR also says that if the work is valued at $1,000 or more, a valid Virginia contractor's licence is required for the licence category in which the contractor is to work.^[5] Check any business you hire in the DPOR licence lookup.

Do not open the control box, keep resetting a tripping breaker or keep using a tub whose GFCI will not reset.

For more, see pool equipment repair answers or browse pool and spa businesses.

To get the tub looked at, describe the problem once and local businesses respond.