Spa Heater Circuits

Typical Hot Tub Heater Circuits

We begin with the 120 volt 1.5kW circuit. Generally, there are 2 main controls (switches) in the “loop” that operate the heater. These include the thermostat (temperature knob) and high limit (also a temperature sensing switch, designed to trip at a certain “high” temperature is usually 120 degrees) In hot tubs built in the late 80’s on a pressure switch or flow switch, may be present as well. 240 volt hot tubs require a heavy duty relay or contactor to handle the voltage required by the heater and maybe to activate the low speed pump. For more information on the many different types of switches and relays click here.


The black wire indicates the path of the voltage. It is your hot wire in to the heater junction box. The voltage path is: 1. To the thermostat 2. Out of the thermostat and into the high limit 3. Out of the high limit and to the element. 4. The red wire exiting the box is used to activate the low speed pump in some systems. The neutral (white) goes directly to the element and ties with one of the indicator light wires. Obviously the green goes to ground. It’s important to note that this path is the same no matter what color the wires are. For some reason some thermostats will have a factory red and a factory black. The electricity still follows the same path. This type of thermostat is just an open or closed switch (also known as a single pole switch).

Power passes through it if it’s closed and doesn’t if it’s open. The wire going to the high limit is still following the same electrical path if it’s black or red.  Electricity doesn’t care what color the wire is.


Note that the red wire to the low pump still resides on the downstream side of the high limit. This allows the pump to be activated first so that the pressure switch will close allowing the voltage to go through to the element.

The pressure switch is there as a safeguard to assure water flow before allowing the heater to fire. I call it a conditional switch. The condition of water flow must be present before the switch will close.


Many times just opening your heater junction box (with the power off of course) will be quite illuminating. If water comes pouring out you have a problem! Element replacement may be necessary! If your controls are corroded or wires are burnt or melted the fix could be as simple as replacing a burnt wire.

This can happen when there are low voltage situations (not a dedicated line) or loose screws, wire nuts not tight, or loose connectors. When replacing a wire be absolutely sure it is of the right size to carry the current you are sending through it.

120 volt heaters at 1.5 kW draw 12 amps and wires should be #12 minimum and copper! Lamp cord is not acceptable! 240 volt elements at 6 kW draws 24 amps and should
have a minimum wire size of #10.

240 volt heaters utilize a contactor that may or may not be present in the heater junction box. It is often located in the controller box. Convertible 120/240 volt controls also use this method.

The contactor is there to send power to the heater element after power makes the loop through the controls.

This allows for the use of smaller wire sizes through the control loop since they will only be operating the “coil” of the contactor.

This is referred to as “pilot duty” as the smaller wires through the controls only activate the coil of the contactor. The heavy current will flow through the contactor points and utilize larger wires on the path to the element when the coil of the contactor is energized.

If upon opening your heater you find two small wires through the thermostat and two more small wires through the high limit, with two heavier wires to the element this will be a “pilot duty” situation and a contactor will be involved somewhere.

The control loop is the same: T-stat, high limit, pressure switch (or flow switch) and then to the contactor coil. When the T-stat is engaged, the low pump activated and all other switches are closed you will hear the heavy click of the contactor being pulled closed as the heater activates.

There are often extra wires present on both the coil and the high current sides of the contactor.

Neutrals are often bunched together on the coil connector.

Tracing the hots is the key
to finding out what you want to know.

Follow that wire back to it’s source. Find out what the source is.

It’s called seat of the pants trouble shooting. In the process you’ll find out how the pump comes into the heater equation.

It will be there I promise you! You may have another relay there for the sole purpose of turning on the low pump.
To learn more about switches and relays click here.


If you’ve figured out your problem, congratulations! It’s not so hard once you know how things are supposed to work! If you don’t find what you need in the Spa Babe’s catalog
please call us…..we have access to anything and everything, even the parts “no one can get!”
Call Us 813 235 4574
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If you are still stumped we’re a phone call away! Or if you are enjoying your quest, the answer may await in the next category! Click on!

Check List Spa Heater Wiring and logic Identify your spa heater Symptoms and Cures

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