Finally figured out why my hot water was always lukewarm

I ran into this exact situation when I moved into my 1950s ranch house. We had a 40-gallon Rheem that would barely fill a bathtub before going lukewarm. I spent months thinking the thermostat was shot or the elements were scaled up. When I finally pulled the insulation back to replace the unit, I realized the cold supply was feeding into the top hot port. It’s wild how much heat you lose when that dip tube isn't doing its job to keep the cold water at the bottom.

My setup was a bit different because I didn't want to cross the lines right at the tank with flex hoses; it looked like a mess. I ended up cutting into the copper about two feet up and soldering a crossover section. It took a few extra 90-degree elbows and some flux, but getting that flow corrected made an immediate difference. Now we get a full 20 minutes of high-heat water for the dishwasher and showers simultaneously. Just make sure you double-check the check valve if you have an expansion tank, because those can get finicky when the flow direction changes.

I had a similar headache with a 50-gallon AO Smith ProLine unit a few years back, but my situation was a bit diferent. My lines were color-coded correctly with the blue and red plastic rings, but the previous homeowner had used flexible corrugated connectors that were kinked so badly they restricted the flow. I thougt for sure it was a dip tube issue becasue the hot water whould just die out after a few minutes, exactly like you described. I’m a bit skeptical about your plumber saying the whole house would run hot if he swapped them properly at the heater. On my old setup, once we crossed the copper leads to match the tank's inlets, it isolated the fix right there. I ended up installing a thermostatic mixing valve anyway because the temp spikes were getting unpredictable. It’s funny how these "simple" tank swaps turn into a full investigation of the copper runs in the crawlspace.

That plumber’s logic about the toilets is gold. If the whole house is backwards, then crossing 'em at the tank is literally the only way to fix it without ripping out drywall. I’ve seen some DIYers try to just swap the dip tube to the other side, but that’s a total pain and usually leaks.

The flex line 'X' look isn't pretty, but it's way better than lukewarm showers. Just make sure those hoses aren't kinked too hard. If they're stressed, they'll pinhole sooner than you'd think. You're gonna love that first winter with a tank that actually works.

Quick update—the new setup is working perfectly. I’m glad I didn't try to swap the dip tube myself like some folks suggested; crossing the lines at the tank was definitly the smarter play. You were right about the plumber’s logic, too—it saved me from ripping out any drywall or dealing with hot toilets. It’s a relief to finally have a normal shower again!