I am an electronics technician and I briefly had a chance to look at my brother's hot tob. I am not sure the brand but it looks pretty simple. Here is the problem. This is a 220VAC 50 amp GFCI, when both pumps are running the hot tub seems fine. Once you shut off one pump, it doesn't matter which one, the GFI trips. So, we opened it up and noticed that the Heater out relay closes when one pump is operating. I unplugged the heater and ran one pump, there was no breaker trip. Now, that means it is probably the heater right? Why is it when we run both pumps that heater out relay never closes. I tested across the heater terminals and got 0VAC. There is some sort of switch to the right of the heater terminals that interupts electric current to the heater terminals. I assume that is some sort of flow switch or high temp shut off. There is another sensor that plugs into the right of the heater terminals. That plug directly into the circuit board and is labled flow switch. So that must control whether or not the relay activates or not. Also there is a metal sensor that appears to be a temperature sensor. It was just lying in there. It says high temp sensor on the board where it is plugged in. I assume that would go in some well but I wouldn't think it would affect this relay. There is something that I don't know. Please help. I can troubleshoot more stuff but unless I know exactly how it supposed to work it is kind of hit and miss. If the temperature is set below the actual temp, the heater should be turning on if both pumps are running, right?

Yikes.
Man you got any clue as to any kind of brand or model on this controller?

Any names, numbers... anything?

Perhaps you could post a photo of this thing here?

Thanks.

It is a sweat water spa. Is it possible that the heater isn't supposed to run with both pumps. I tested the resistance and got 10.5 ohms. I also tested each heater terminal to ground and got 6Kohms. I guess that would be a short which is causing the heater to trip the GFCI. If the heater isn't supposed to run with both pumps that I think the problem is solved. If it is, then there is something else wrong. Let me know what you think.

That thing should normally be set up for full amperage (50amp) availability so heating with both pumps is nbd. And, if you've got 6k to ground from any leg of the element, try disconnecting both leads from the element and then do the check again... from the element to ground. You should have infinity. If not then you're element is compromised, and in need of replacement. http://spapartsnet.com/Electric-Heaters... 6_1_1.html