Hot tub not heating unless jets are on.

[Jmarlow]

I have a Vita Spa, it is probably 25 years old.  It has worked fine over the last several years, until lately.  It had a small leak without my knowledge and the water level went low.  Now the unit will not heat unless the jets are turned on.  Does this sound like an issue anyone knows about?  Please let me know anything I need to check to figure this issue out.  I am new to working hot tubs, so any help is much appreciated.  Thank you!!

[LEADMEN]

not sure if its corrected but sounds like a pressure switch or just dirt in or around the p switch try cleaning it or adjust it its a spring adjusted micro hope this helps

[Kev B]

 The flow of water on low speed is too weak to engage heater and only high speed will do it. Take out filter and run it without it and see if it kicks on. If it does...replace filter.
 Other possibilities
  1- partially plugged pump impeller (You wouldn't believe some of the things I have found in impellers)
  2- BAd pressure switch....replace, do not adjust.