Pool/Spa Combo - Heater Installation

Hello!

I've just moved in to a house with a beautiful Pool/Spa heater combo, the only issue is of course, it has no heater. Which it seems obvious that at one point it obviously did (or else that would be one useless spa), but there isn't one there anymore. I'll, be asking a few questions and posting a few pictures to correspond. Excuse my ignorance with terminology, this isn't exactly my forte.

To start off, here is an overview of the setup:
http://i.imgur.com/LdvrrxQ.jpg

Pretty much, there are two lines into the system it seems to be, and I just want some help verifying what you guys think is the line that should go IN to the heater, and which one goes back in to the system.

heres a picture of the line with the hose valve:
http://i.imgur.com/KTTJ0Nw.jpg

Here's a picture of the other exposed line, isn't this typically used for draining?
http://i.imgur.com/N3K0nRh.jpg

Here's both:
http://i.imgur.com/vi41yLZ.jpg

Here's the overview without the text:
http://i.imgur.com/JMtfpn7.jpg

And the gas line:
http://i.imgur.com/cdcwYGa.jpg

So to sum everything up. I just want to know which goes in the heater, and which is used as the outtake back into the system. Thanks in advance for your help!

Pool/Spa Combo - Heater Installation - HELP PLEASE!