What's this device (circled in blue) that's attached to my furnace? It recently got replaced and I forgot what the HVAC technician called it, or what its function is.
I do rember that they said to put some vinegar in the U-bend (circled in red) once or twice a year. I forgot to ask why this is necessary, but I'd guess that it tends to collect moisture, and the vinegar will prevent mold?
And a bunch of other microbes too. A single species may dominate the population, but it’s highly likely that there are several other species hanging around as well. Adding some vinegar will make the conditions more challenging for most of them, so many will just die while others may struggle and grow very slowly.
In ELI5 speak: Gross bugs growing in there. Add vinegar to make it a bit nicer.