Spas use relays to turn the heating element on and off. A spa's relay contains two separate electrical circuits. When the spa's thermostat detects cold water it allows electricity to energize the relay control-circuit's electromagnet. The relay's load circuit has constant voltage applied to its input terminal. When the control circuit energizes, its electromagnet closes an internal switch in the load circuit. This energizes the output terminal and allows the voltage to flow to the spa's heating element.
Check the spa's identification tag for the input voltage. The identification tag, located in the electrical service area, states the spa's model number, serial number, input voltage (VAC) and the system's amperage.
Turn a voltmeter to VAC. Test the input voltage at the terminal block. Place one voltmeter lead on each terminal. The wires from the circuit breaker lead to the terminal block. Systems with a 240 VAC input rating should read between 210 and 240 VAC. Systems with a 120 VAC rating should read between 110 and 120 VAC.
Turn the thermostat off.
Find the heater relay. The wires from the heating element lead to the relay. Some relays have an exposed coil in its control circuit; some relays use self-contained coils, usually in a black box. In either case the relay has a chart identifying the pins and the control and load voltage rating.
Identify the control circuit pins. Place one voltmeter lead on the green ground screw near the terminal block. Test each control circuit pin with the other voltmeter lead. The meter should read zero.
Identify the load circuit's input pins. These pins usually lead directly to the terminal block. Place a voltmeter lead on each input pin. The voltmeter reading should equal the reading taken at the terminal block.
Identify the load circuit's output pins. Test the voltage across these pins. The voltmeter should read zero. If not, then replace the relay.
Turn the thermostat to the high-heat position. This should turn the spa on and send voltage to the heat relay's control circuit.
Place a voltmeter lead on the ground screw. Test the voltage at each control-circuit pin. Both pins should have the same voltage reading and this reading should equal the relay's stated control-voltage rating. If one pin has voltage but the other does not, then replace the relay.
Place a voltmeter lead on each load-circuit output pin. The voltage reading should equal the voltage reading at the input pins. If not, then replace the relay.