Reverse Engineering An Oil Burner Comms Board, With A Few Lucky Breaks

Here’s a question for you: How do you reverse engineer a circuit when you don’t even have it in hand? It’s an interesting problem, and it adds a level of difficulty to the already iffy proposition that reverse engineering generally presents. And yet, not only did [themole] find a way to replicate a comms board for his oil burner, he extended and enhanced the circuit for integration into his home automation network.

By way of backstory, [themole] has a wonky Buderus oil burner, which occasionally goes into safety mode and shuts down. With one too many cold showers as a result, he looked for ways to communicate with the burner controller. Luckily, Buderus sells just the thing — a serial port module that plugs into a spare slot in the controller. Unluckily, the board costs a bundle, and that’s even if you can find it. So armed with nothing but photos of the front and back of the board, the finding of which was a true stroke of luck, he set about figuring out the circuit.

With only a dozen components or so and a couple of connectors, the OEM board gave up its secrets pretty easily; it’s really just a level shifter to make the boiler talk RS-232. But that’s a little passé these days, and [the78mole] was more interested in a WiFi connection. So his version of the card includes an ESP32 module, which handles wireless duties as well as the logic needed to talk to the burner using the Buderus proprietary protocol. The module plugs right into the burner controller and connects it to ESPHome, so no more cold showers for [themole].

We thought this one was pretty cool, especially the way [themole] used the online photos of the board to not only trace the circuit but to get accurate — mostly — measurements of the board using an online measuring tool. That’s a tip we’ll keep in our back pocket.

Thanks to [Jieffe] for the tip.

3 thoughts on “Reverse Engineering An Oil Burner Comms Board, With A Few Lucky Breaks

  1. I had a very similar problem with the same buderus controller a while back. I also had thought about some form of remote system to check on the thing for errors. But in the end i ended up just repairing the damn thing. Luckily buderus provided full circuit diagrams with the unit. Took me just a a short time to figure out what was happening. The whole thing uses miniature relais to switch 240v. Do this for 10+ years and like tens of times a day and they fail. In my case it was even two faults. The “firing controller” inside the burner itself as well as the relais inside the logamatic control. They most likely died together. The first part was exchanged by a service company and i had about 50% less problems. The failng relais inside the logamatic were not diagnosed by them, but by me after the quoted me the price for a replacement. I ended up finding a guy on the web who specializes in replacing the relais. I bought a refurbished board, exchanged it myself and send him my broken board to be repaired for his next customer. No more problems ever since and no more need for remote solutions.

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