Please post current pics of the way your valves are now set. I went through your plumbing with you in another thread. You have complex plumbing and one valve misset can create your problem.
Your heater only works with the black pump. You need to have the valves set to pull water only from the spa and return only to the spa using the black pump to get good heat to it.
Lets go back and review the position of each of your valves as we discussed here Spa Losing Water when Pump is off overnight
Album — Postimage.org here is how the valves are set
What gets me is that at one time - the way the valves are set now - it actually worked. It stopped working after successfully heating the spa