Is my DIY hybrid water heater setup going to starve for air?

The biggest issue you're going to face with a hybrid in a tight closet is short-cycling the air. If that intake doesn't get enough fresh, warm volume, the unit just sits there chilling its own exhaust air and the efficiency drops to basically nothing. That cardboard shroud is a decent temporary fix, but you really need to maximize the free area of your door vents. Most of those standard louvered doors don't actually provide the 750-1000 cubic feet per minute these units often want. I'd swap the door for one with a much larger high-low vent configuration or just leave it off entirely while you're testing the recovery times.

Running two anodes isn't going to cause a catastrophic chemical reaction, but it is redundant. Since you couldn't pull the factory rod, that powered anode is still going to do its job by modulating the voltage to stop corrosion. The passive magnesium rod will eventually just sit there and slowly sacrificialize itself alongside the electronic one. It's not "double protection" in a linear sense, but it won't hurt the tank. Just make sure the powered anode is getting a good ground to the tank shell or it won't do anything at all. Regarding the circulation pump on a smart switch, that's actually a solid way to handle it. Those small wet-rotor pumps are designed for high cycles, so turning it on and off based on occupancy isn't going to kill the motor any faster than a standard timer would. The main thing is to make sure your **check valve** is working properly so you don't get a crossover between your cold and hot lines when the pump is off. If you start getting lukewarm water at your cold taps, that's usually the culprit.

You should definitely fix that inlet line clearance sooner than later. If you can't pull the filter, the evaporator coil is going to get gummed up with dust within a year, and cleaning those fins is a nightmare compared to just rinsing a plastic mesh filter. Since you're already in the studs, maybe look into adding a dedicated **condensate neutralizer** if you're running that drain into a cast iron stack or near a concrete floor, as that discharge can be surprisingly acidic over time.

Nice setup, but have you ruled out noise levels with that closet being stripped to the studs? Those hybrid fans can get pretty loud, and without the drywall mass, it might vibrate the whole frame. Also, regarding the smart switch for the pump—are you sure it's actualy saving money? Every time the pump stops, the water in the lines cools down, and the heat pump has to work harder to recover that loss. Sometimes the constant reheating of a dead line ends up being a wash compared to just letting it run. Have you monitored the actual power draw during those recovery cycles?

I'm just starting to learn about these hybrid units, so bear with me, but what exactly is a condensate neutralizer? I didn't realize the water coming off a heater could be acidic enugh to mess up a floor. Also, since you mentioned the check valve and lukewarm water issues, is that somthing a total novice can install, or does it require more of those special plumbing tools? I'm worried about my own setup now becuase I definatly don't want my cold water coming out warm.

Since you have already stripped the drywall, you should verify that your powered anode is not experiencing signal interference from the residual sacrificial rod's potential; while genrally benign, an old magnesium rod can sometimes skew the reference electrode's voltage readings on higher-end electronic units, leading to premature depletion or false error codes. Also, regarding that smart switch, ensure your pump isn't experiencing 'dry fire' scenarios if the smart logic triggers while the lines are drained for maintenance, as even a few seconds without lubrication can score the ceramic bearings and cause a permanent, high-pitched whine that will resonate trough those bare studs.

If you’re worried about that airflow in a tight closet, grab the Rheem UV12993 duct adapter kit. It’s way better than a cardboard shroud and lets you pull warm air from the attic space you already opened up. Since you've got the studs exposed, I’d also toss a Grundfos Comfort PM Auto into that loop instead of a basic smart switch. It learns your usage patterns and is way more efficient than just cycling on the bathroom lights. Also, don't sweat the dual anodes. I’ve run a Corro-Protec alongside a factory magnesium rod in my AO Smith Voltex for three years with zero issues; the electronic one just takes over the heavy lifting.