can't say for sure without a serial number, but any hotspring heaters sold after 2005 (approx) were PDR (power down reset) the reset buttons on older heaters were a poor design. i'm not a tech but your symptoms (lukewarm, solid red light) sound more like a failed heater relay board. good news is the part costs less than half what a heater costs. check out some youtube vids on hotspring heater board, see if it sounds like your issue, and whether you want to tackle it.