Wiring diagram for ff32atc control box

Water heater can be added to any incoming water line, either on hot or cold.
http://waterheatertimer.org/How-to-install-under-counter-water-heater.html
Under-counter water heater located near tub will supply hot water just like ordinary household water heater. But it will not reheat water that circulates inside the spa tub.
Otherwise, a spa heater is made with electric element that can work with 120V or 240V. Water from tub circulates through tank that holds element. Element must be fully immersed in water.
To retrofit spa tub with this heater would require cutting water line near pump, running both ends of water line into tank with element.
Then of course you would need to retrofit adjustable thermostat to control when element is on. And integrate element operation into air control switch at tub that can override thermostat so element is not heating when tub is not in use.
Unless there is a pre-packaged kit to retrofit heater, I think adding heater would be difficult.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Check your heater/pump/lights/accessories. These should all have electrical ratings. Your supply wire should be large enough to carry the combined amperage of everything running together. I personally would separate each system in the breaker box, 1 for pump, 1 for heater, 1 for accessories.

1) Safety question first
Why hasn't the GFCI tripped.
If your spa is not protected by ground fault., then you are at serious personal risk.
http://waterheatertimer.org/How-to-wire-GFCI.html#arc-fault

2) Replace the element.
http://waterheatertimer.org/How-to-test-water-heater-element.html

3) If element is cracked, it can continue heating to ground wire, causing runaway heating.
This applies to spas, and water heaters of all type.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

OH stands for Over Heat situation.
If the motor is not working at all, you are getting the OH code because of that. The OH happens when there is not enough circulation in the hot tub which causes the water near the heater to overheat the sensor located in that area. It sounds like either the pump has failed, the wiring has shorted to the motor, or the control board is not telling the pump to come on.
Its probably a bad motor since the heater is obviously activating, which it wouldnt if the pumps motor was getting no power. Replace motor and all should be well again.

Most hot tubs have a freeze protection that will lock you out and run the pumps full blast with no heater if the temerature is under 50 degrees F.The circulating pumps create heat until the water temp is above 50 degrees, and then the heater will come on after that.
Hot tubs do this to protect themselves and it is normal operation.

You will need to check on the control box under the hot tub. Most tubs can be wired ether way. Others no. If it can be wired ether way 220v runs cheaper and will heat the water a lot faster. But you will have to check on the face or door of the control box. I had a 110v outlet by mine then found it only runs on 220v. It cost me to run the 220v but I'm glad I did, it heats from 75 to 100 in less than an hour. Less run time and less power when it runs.

Check for a bad heater controller board or burnt wire connection going to the heater. You can check for voltage across the white and black leads going to the heater from the heater controller board (upper left corner of the black control box

I think you have a small leak somewhere that is causing the GFI to kick off. ... and it is probably quicker than 2 seconds.

Look carefully inside the control box - that is where I found the leak in mine.

After you get it fixed (and you may have to buy a new control) pat yourself on the back for having saved yourself at least $250.00.

Good luck with your project.

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Make sure that the inline heater has power. It should not take very long to heat since your putting hot water into it anyway. The heater is to keep water warm.

Sounds as if the 'mode' for the tub is set to sense the temperature of the water. see if there is a setting on the control box to a 'timed' setting for filtering and heating instead of controlled by the temperature setting. the problem is the the control box is away from the water and when it calls for heat it is because the water is cooler there than in the hot tub. so when it calls for heat and the water starts to circulate it gets the hot water from the body of the hot tub and then shut down because it senses the 'hot water'. make sense?