I have a SpaBuilders LX-35 with a flo thru heater. It will not heat past 70 degrees. I runs constantly so I lowered the set temp to 65 to avoid high elec. cost until I figure out what is wrong. I have tested the terminals on the heater and it is getting 244VAC. Does that mean my element is bad? I am guessing it has some kind of mineral build up to prevent it from heating to its full potential. My major question is this: can I replace the element without draining the tub? There is 1 gate valve on 1 side but the other side has no valve and goes to 1 of the pumps.

Yes, pretty much that does indicate a defective element. You can verify this with the power turned off and taking an ohms reading across the heater terminals.

Look on the pump suction (other side of the pump) for the other cut off valve. There should be one somewhere unless it was built friday at 5.