Hot Tub Won't Heat? A Nashville Tech's Diagnostic Walkthrough

Of all the calls we run across Nashville and Middle Tennessee, "my hot tub isn't heating" is far and away the most common. The good news is that the failure modes are limited and well understood. The bad news is that figuring out which one your spa is doing usually means opening the cabinet and putting hands on the equipment. Here's how a tech actually thinks through it.

Start With the Symptom, Not the Part

Before opening anything up, a tech wants to know exactly what "won't heat" looks like for your spa. There are three common patterns:

• Cold from the start. The display shows a set temperature, the pumps run, but the water never gets above ambient. This usually points at the heater circuit — element, high-limit, or relay.
• Heats, then quits. Water gets warm, then the heater shuts off (often with an error code) and refuses to come back. Classic flow problem or high-limit trip.
• Won't even try. The pumps don't kick into heat mode at all, even though the panel is on. This is usually a control board, sensor, or wiring issue rather than the heater itself.

Once we know which pattern you're seeing, the diagnosis narrows fast.

Suspect #1: The High-Limit Switch

Every spa heater has a high-limit safety that cuts power to the element if water temperature gets above a safe threshold (usually around 110°F at the sensor). When this trips, the spa stops heating and often throws an OH (overheat) or HL code. Most can be manually reset; some need to be replaced.

What causes them to trip? Almost always poor flow. A clogged filter, an airlock in the pump, a closed valve, or a failing circulation pump all reduce flow past the sensor, and the heater "thinks" the water is hotter than it really is.

Suspect #2: A Flow Problem

Flow is the secret villain of hot tub repair. A heater that doesn't see enough water moving past it will either refuse to come on or kick off with a FLO, DR, or SnA error. Common flow killers:

• A filter that hasn't been cleaned in months and is now caked solid.
• An airlock in the pump after a drain & refill (water sits in the pump volute and won't prime).
• A failing circulation pump that spins but doesn't move water.
• A stuck or closed gate valve on the heater plumbing.

Suspect #3: A Bad Heater Element

If flow is fine and the high-limit isn't tripped, the next likely failure is the heater element itself. Elements are essentially heavy-duty stainless steel rods that sit inside a flow-through tube. They corrode, scale up, or short to ground. Once an element is open or shorted, replacement is the fix — they aren't field-repaired.

A tech checks elements with a multimeter, looking for a specific resistance value (usually 9–12 ohms) and verifying it isn't shorted to the steel sheath. Both tests take about a minute.

Suspect #4: Sensors and Control Boards

Spas use two main sensors at the heater: a temperature sensor and a flow/pressure sensor. When either reads wrong, the board makes safe decisions and shuts the heater off. Sensors are inexpensive parts and they fail with age, especially in Tennessee well water with high mineral content.

Less commonly, the control board itself fails. Boards are pricier than sensors, so a tech rules out cheaper failures before pointing at the board. The good news: most boards can be swapped in 30–40 minutes once the diagnosis is locked in.

What You Can Check Before Calling

If you want to do five minutes of homework before reaching out, these are the safe checks:

• Pull the filter and look at it under daylight. If it's brown, slimy, or you can't see the pleats, that's likely a flow issue starting.
• Check the topside panel for error codes and note exactly what flashes.
• Listen to the circulation pump (the small one). It should hum quietly. If it's silent or rattling, that's a clue.
• Check that the GFCI breaker hasn't tripped and that all gate valves at the equipment are open.

Anything more than that — especially anything involving wiring, the heater housing, or the control board — is best left to a tech. The voltages inside a spa cabinet can be unforgiving.

The Diagnostic Visit

When we come out, we follow the same flow every time: check the panel and pull error code history, verify flow with the cabinet open, check resistance on the heater element, check the high-limit and temperature sensors, and verify board outputs. Most of the time we know what's wrong inside 20 minutes and can quote the repair on the spot.