Why is warm water dumping into my basement drain while showering?

That is almost certainly a **trap primer**, and it's there by design. Building codes often require them on basement floor drains so the water in the P-trap doesn't evaporate over time and let sewer gas back up into the house. A realy common way builders set these up is by tapping a small feed line directly off the shower riser pipe upstairs. Because it's tied in after the shower valve, it only flows when water is diverted up to the shower head. That also explains why the water felt warm—it's just pulling the mixed water from the shower line. It's supposed to send a tiny shot or a small trickle into that drain every time someone uses the shower to keep the trap wet. If it’s gushing out like a fully open tap the whole time the shower is running, the valve on the primer might have failed open, but a small, steady trickle isn't out of the ordinary for older setups. As for your electric bill, this isn't the culprit. Even if it runs a slow stream for a ten-minute shower, the actual volume of hot water lost is just a few gallons at most. That's a drop in the bucket compared to what it takes to noticeably swing your power bill. You're much more likely looking at an auxiliary heat strip sticking on with your heat pump, a burned-out lower element on the water heater forcing the upper element to work overtime, or just seasonal weather shifts. To double-check the line, run the bathroom sink or fill the tub using just the lower spout. The water in the basement shouldn't flow untill the shower diverter is engaged.

Take a close look at how that flexible line terminates at the PVC pipe; if the end is just shoved down into the opening without an indirect connection via an approved air gap—specifically a vertical separation of twice the supply line's effective diameter, minimum one inch—you have an illicit cross-connection between your potable line and the drainage system. Also, if that yellowish tubing is tied to a simple continuous-flow orifice tee upstairs rather than a dynamic pressure-drop primer, the internal metering port could easily be enlarged from ten years of velocity erosion, which woud explain why you're suddenly hearing a much heavier, noticeable volume of water passing through it.

Keep an eye on that yellowish tube, too. If it's old vinyl or poly, they get crazy brittle with warm water running trough 'em. Had one crack behind my drywall a while back and it made a huge mess before I even realized what happened.

If it's dumping way too much water and driving you nuts, look into dropping a mechanical trap seal like a Green Drain right into the floor drain grate. Lets you cap that primer line off entirely if your local code allows it.

Had the exact same setup in my last place. Mine went from a quick squirt to a steady stream becasue hard water crust jammed up the little metering pin.

If you pull the shower handle and trim plate off upstairs, you can usally see where that small line tees off the vertical riser pipe. I ended up putting a cheap 1/4" inline needle valve on mine right inside the access hole and dialed it back so it only lets a few ounces trough per shower. Took about twenty minutes and stopped me stressing over all that warm water going down the drain.

Wait, whoa, I had no clue houses tapped water straight off a shower riser to keep a floor drain wet, that is wild!! Mind blown haha. Now it has me totally wondering though—if you swap out the showerhead upstairs for one of those high-pressure handheld wands or an ultra low-flow head, does the extra backpressure change how much water gets forced down into that tube? Or does whatever showerhead you screw on not even affect that little feed line?

Update: Thanks everyone, a trap primer makes total sense. I took off the shower trim plate upstairs and found that little tee on the riser. The wierd thing is, it runs a steady, constant stream the entire time the water is on, not just a quick burst. If that metering pin is stuck from hard water, is it usally cleanable with some vinegar, or am I looking at replacing the whole piece?