Another 3-wire/4-wire question (sorry)

The situation: I'm rebuilding my brewery from scratch (had to sell it before I moved) but I'm working with limited options as far as heat sources go; I live in an apartment on the 5th floor with no balcony, so propane is out, and my electric stove doesn't stand a chance. I've decided to go electric. I don't know tons about electrical stuff but I tried to do my homework as much as possible.

The only 240V line in my apartment is for my AC unit. It's connected by a NEMA 6-15 plug (240V/15A, 3-wire, hot-hot-ground, no neutral). I'm pretty much limited at 15A because I checked the wires going to the outlet and they're 12-gauge and the breaker in the box is 20A. Should be fine for a 3500W element, which is what I'm planning on using.

The issue is that I ordered a 60A spa panel because it was a great deal on Amazon and I got free shipping. At the time I didn't realize there was a difference between 3-wire and 4-wire panels.

MY QUESTION: Given that the wires coming from my wall are hot-hot-ground, can I hook up the GFCI in the spa panel leaving the neutral spot empty, using only the GFCI hot connections and the ground to the box? Do I need to just buy a different panel? Is my house going to burst into flames?

Btw, I shut off the breaker and pulled the 6-15 outlet out of the wall to check if there was a 4th wire (saw that tip on some of the other posts). There was an wire in there. In a last ditch effort I suppose I could use it, but then I would need a workaround to use my air conditioner (the plug is permanently attached to the AC unit). It would be much simpler to do this all using the NEMA 6-15 connectors (plus it's the right connector for the wire/breaker). Keep in mind this is an apartment, so I'd like to keep things as non-permanent as possible.

Thanks in advance for the help, everyone.

I will let the experts chime in here (I most certainly am NOT), but I believe you are fine running the 3 wires you have into your spa panel, and going 4 wires from your spa panel to your control panel.

Your spa panel will just split your ground wire into a Ground AND Neutral.

Yes, you can do that. Search the forums and you should find a picture PJ has posted showing the wiring. As I understand it though, you really should not use the neutral out of the spa panel for any 120V circuits. If you need 120V for a pump, take that from another circuit.

Thanks guys, I appreciate the responses. Any 120V gadgets I might run (I don't have any right now) will be on the normal 120V circuits in the wall, not run through the box. My heater element and temp controller are both 240V. My brew setup is gonna be in my oddly large bathroom, so I already have GFCI protection on all those outlets.

If the GFCI will not function without the neutral connection, you might be able to connect the ground to the GFCI's neutral to keep it happy. It's probably not completely kosher though. BUT if doing that you should absolutely not use the neutral out of the GFCI for line to neutral loads. That would force the ground wire in the outlet to carry the imbalance, and the ground wire should not be used as a neutral like that. So right any 120V loads I would just plug into another outlet. If you need 120V only for control power, you could just get a 240V/120V xfmr in the VA you need for your panel.

Do not connect the ground from your kettle, panel, pump... ect ect to the neutral of a spa panel or gfci breaker. It will basically make your gfci protection useless. The gfci would think that any current passing thru the ground (neutral) is normal and would not trip.