My wife and I bought a house a year ago, and there was a spa in the backyard, but didnt know if it worked. Its an old clunker. No idea what brand it is. There is no LCD readout or anything. It has a little membrane cup thing for your finger with a hole and an LED inside that switches the jets around. We had a guy come out and get it running. He got it working, but the performance leaves alot to be desired. There is a valve that switches pressure from one set of jets to another. That thing is totally seized up, and WD-40 and a wrench didnt help much. Only jets that work are the 2 main ones ontop, in opposing corners. Also, the little bubble jets work. (little pinholes all over) The temperature adjustment pot was bad, so they replaced it. After trying to heat the spa up the other day, after 3 hours, the water hadnt heated up at all. I pulled the switch out, and this guy had clipped one of the 3 leads off the pot and turned it into an on/off switch. Anyway, the spa had worked before. Just 3 days prior, I had drained it and filled it, and in 4 hours it had heated up pretty good. With the heater switch turned on (actually "off" with the way this hack wired it up), my electric meter spins like a top, so I assume the heater is pulling a good amount of current. I was thinking there was an air bubble after filling it, but those top jets flow really hard, and the spa had heated up the day before. The plumbing around the pump confuses me. I know how centrifugal pumps work, so I dont understand why there is a manual valve before and after the compressor. (one after the pump is actually after the heater) I cant see what it would accomplish. Anyway, with the heater turned off, the fuse that goes to the pump will trip and the pump will shut off after maybe 30 seconds. With the heater on, it wont. I dont get that at all. Shouldnt it trip when its drawing too much current?
I hear about the circulating pump in some posts here, but there is only 1 pump that I know of on this one. When its on, the pump is loud and those 2 opposing jets in the corners flow hard. Those 2 jets would seem to be the circulating jets given their location, and that is what my loud pump is working. Are they supposed to flow that hard?
Also, we cant find the right filter for this thing. The people in spa stores cant figure it out. We got one that isnt threaded right, so it just sits there in the sump. I guess it does some filtering but not alot. I figured if some pipe was clogged, those jets wouldnt be flowing that hard.
Anyway, I wanted a spa my whole life and finally got one. But my total lack of spa knowledge isnt making things easy when things go wrong. I want to use it this weekend, but if the water isnt heating up, whats the point?

That manual valve you mention is an older spa's way of getting by with a smaller pump, and switching the jets from one bank to the other. A modern day fix to this would be to upgrade the pump to one big enought to run them all, or run dual pumps, one for each bank. But to keep things simple, we'll work on what you got. The valve should be able to be disassembled, cleaned, and lubricated with petroleum jelly. That should cure that problem.

The valves you mention on both sides of the pump are gate valves, and their purpose is to cut the water so you can remove the spa pack without draining the tub. Worthless things usually end up leaking out of the shafts, but make filter changing easy. I assume you have a filter that is located near the pump, under the cabinet, yes? If so, see if there are any markings on the housing. Most common filter type that I have ran across is the Hayward C225 filter housing, and the filters are easy to come by.

The individual jets may be independantly adjustable. If so, they will have grips around the outside edge, and it will turn. This will turn them off and on separately. You MAY or MAY NOT have this.

The pump should be a two speed model, low and high. On low is when it should heat. If you aren't getting a low speed, it is possible the low speed capacitor is defective (in the motor) and would explain why the motor tripps off after about 30 seconds. Does the motor hum and not pump just before the breaker trips? That would be the symptom.

The pot should only have 2 wires going to it. It's just simply a variable resistor. It just sounds as though the wire was clipped on the wrong side, which you can remedy by moving the one outside wire to the other terminal.

If you have any pictures you could provide regarding the questionable parts, that would be helpful.

Here is a general wiring diagram for a tub similar to yours, and it might work as a guide to help you decipher what is going on: