All electrical resistance heaters work the same way: current is passed through a special element, which creates heat.  When the current flow is interrupted or diverted, no heat is created.

Spa Heater Element Construction

Diagram shows construction of an electric heater element. Note the electrical connection terminal [1]. Inside is the heating coil [13], a filament encased in insulating powder [11].  It is surrounded by a sheath of Incoloy [12], a heat and corrosion resistant alloy which isolates the electrical coil and insulating filler from contact with water. Reference this diagram when performing diagnostics below.

Common Symptoms & Causes of Heater Failure:

• No heat

A heating element is similar to a light bulb in that its filament [13] is a heating coil which in time can break or burn out.  Assuming that the heater's connection terminals [1] are in good condition, it is properly energized by your hot tub's control system when it should be, and that the thermostat and high limit switches are functioning properly, no heat can indicate a burned-out or broken heating coil, which results in an open circuit.

Note: Catastrophic failure of the heater element can be caused by a dry fire.  This occurs when the heater is operated with little or no water present, or a greatly reduced water flow, which can cause the heating coil [13] and/or outer sheath [12] to actually melt.  The element will often have visible signs of damage when a dry fire occurs.  Although properly functioning high limit switches, pressure/flow switches, and thermostats are designed to help prevent this condition, like all devices, these too can fail in hot tubs.

• Reduced heat

Low heat is more often caused by a reduced water flow rather than an electrical problem with the heater itself.  Check your filter to make sure it is not clogged, and that there are no other obstructions restricting spa water flow.  Excessive scale buildup on the heater element, as a result of poor water balance, can reduce heating efficiency.  See Equipment Troubleshooting.

Preventing Scale Buildup Left is an example of a spa heater element suffering from calcium scale buildup. This buildup not only reduces heater efficiency, it eventually results in element failure. Regular use of Spa System Flush can help eliminate scale.

• GFCI tripping

A heater causing the GFCI to trip, even intermittently, more often than not indicates a short circuit caused by water intrusion into the heater element's outer sheath [12].  Water can seep inside at various other entry points such as the epoxy seal [5] or at the braze [9], but the most common cause will be a pin hole in the sheath caused by corrosion.  When the electrical current finds a path from the heating coil [13] to the water, the short will cause the GFCI to trip.  If you do see a hole in the sheath, there is no need to proceed with testing-- the element is toast!

Testing the Heater Element

The best and safest way to evaluate heater elements is to first take them out of circuit by disconnecting BOTH power leads from the heater terminals [1], and then take measurements with an ohm meter.

Important: Careful Removal

Be certain that all power is disconnected to the spa before making inspections, and before removal and testing of the heater.

Photo credit: Kaisa Williams

Acceptable Resistance Range To test hot tub heating elements for integrity, use an ohm meter on its lowest setting.  With the meter's test leads, measure the resistance between the two terminals as shown.  The acceptable resistance range for heaters used in most hot tubs is between 9 - 12 ohms (a few may be as high as 25 ohms, depending upon kilowatt rating). A reading which is too low indicates a bad unit: short circuit.  A very high (or infinite) reading indicates a bad unit: open circuit or limited conductivity.

Testing for Shorts

Now test for a short to ground.  On the meter's highest ohm range setting, measure between one element terminal [1] and the sheath [12] of the element.  You should get an infinite reading on the ohm meter, indicating no continuity to ground.  Any ohms reading indicates a short, and bad element.

Installing Replacement Spa Heater Element When connecting the electrical wires to a heater element, always use two wrenches to avoid terminal damage. Hold your lower wrench stationary while firmly tightening the top nut. This will prevent twisting of the terminal pin, or fracturing its epoxy potting.

Testing Associated Heater Components

If the heater element checks out OK, the problem may be with the high limit switch or the thermostat.  These can be tested by disconnecting both wires to each and checking for continuity.  Keep in mind that high limits and thermostats are merely on/off switches which are triggered by heat sensors.  The photos below show the ohm meter test points for each component.

If your hot tub's heater element, thermostat, and high limit switch all check out good, refer to our
Hot Tub Equipment Troubleshooting Guide.

Note: Flow and pressure switches are not pictured, but perform the same protective function by interrupting power to the heater when sufficient water is not flowing.  Either of these can wear out, and flow switches can be prone to malfunction due to debris interference.

An Ounce of Prevention

You can protect your spa heater, and prolong its life by observing a few simple rules:

• IMPORTANT: Be careful when installing new heater elements.  Never bend, push, or twist the electrical terminal.  This could cause a fracture of the epoxy seal [5] and lead to water intrusion.  Always use two wrenches to tighten or loosen the terminal nut.  Hold one wrench on the terminal hex [4] to keep it from turning, and one on the terminal nut [2] for tightening or loosening.
  
• Avoid air locks.  After servicing equipment or when filling a dry spa,  make sure all air is purged from the plumbing.  Dead air in the heater assembly tube can cause rapid overheating of the element, called a dry fire.  After refilling the spa, but before connecting the power, loosen a union enough to let trapped air escape (you'll hear it). Hand tighten when all the air has been purged. Make sure your plumbing system is free of leaks.
  
• Keep your hot tub's water balanced.  This will prevent corrosion of heater parts by water which is too acidic (pH to low) and prevent heater scale buildup caused by pH too high.  A few cents worth of balancing chemicals can save an expensive repair, and the trouble of replacing.
  
• Maintain sanitizer in hot tubs.  A slimy brown film on the inside wall of a spa may  be evidence of a metal dissolving bacteria that is slowly gnawing away at the heater sheath.  This will not be a problem if spa shock is periodically applied and sanitizer maintained.
  
• If your spa's control system allows for setting heating cycles by time of day, rather than thermostatic demand of heat cycles, this will prolong heater life.  Switching a heater on and off many times per day puts more strain on the element.  If your spa does not have a timer, don't lose any sleep over it.
  
• A defective control, such as a chattering contactor or thermostat, can pulse power rapidly on and off and cause premature failure. If a replacement heater burns out quickly, this is something to look for.
  

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