Sounds like you are considering purchasing this one?
If this spa has been stored outside, and if the spa was subject to freezing, then I'd make absoultely sure all of the plumbing was drained.
If it was properly winterized, then this probably happened, but who winterized it? A pro or the owner?
Just draining thru the drain and mopping up the bit on the bottom that the drain wont take isn't enough.
At that point there is still a lot of water inside the plumbing- enough to cause major problems if the water was to freeze.
Any way to fill the spa and get it running before you commit to purchasing it?
Brewman