I've got my electronic button pusher all wired up now. Along the way, I found a couple of confusing things in the Navien NPE-A2/S2 installation manual. In the end, it turned out to be completely accurate, but it was still confusing. When I first wired things up and tried to configure it via the front panel, it refused and asked me if I had connected the HotButton and had set the DIP switch to enable it. "Well, heck, yeah," I thought. I assumed all along that the HotButton itself was a simple NO momentary switch, but I began to wonder if it had some kind of secret internals, like some particular resistor value so the heater would recognize it. That would be overly complex, but it's at least possible. I wrote to the company to ask about that. I haven't heard back yet (it was late on a Friday that I wrote to them, so I didn't expect an immediate answer). In the meantime, I figured it out.

Here's part of the installation manual:

In the first place, the image of the board is rotated 180 degrees from its actual position inside the heater. I wasn't really fooled by that trick since I have plenty of experience being fooled by it in electronics vendor documentation. So, even though it's highlighted showing the up direction of the DIP switches being "on", I rotated my brain and knew that down meant "on". (The other fun thing about that image is that they show a pair of wires coming in to unrelated terminals.) What actually fooled me was something I had to read about 50 times before noticing. You need to have the DIP switch set to "off" to enable the feature. It's right there in black and white. I had carelessly misread "Default: Enabled (OFF)" to mean "by default enabling the feature is turned off". I had flipped the switch to "on" and thereby disabled the HotButton feature.

I have to at least partially blame myself for that one. But just so I don't leave Navien unblamed too much, why in the world is the menu configuration for the HotButton (and recirculation in general) behind the password-protected "Installer menu"? Luckily, my installer kept the default published password of "1234". If you just read the user manual instead of the installation manual, you wouldn't know any of this. At least Navien does publish all the manuals as PDFs that anybody can download.