Manufacturers use asterisks in legends and dashed boxes on HVAC schematics to mark optional components like humidifiers and UV lights. New technicians should check the legend first to confirm a missing part is optional, not broken, before diagnosing a problem.
Hey, are you new to the trades? Here's something you may not know about optional components on a schematic. One diagram has to cover a dozen versions of the same unit— with a humidifier, without one, with a UV light, without one. So manufacturers built in a way to mark what's optional. Watch for a small asterisk next to a component's label in the legend. An asterisk means maybe. It tells you that part may be factory installed, or it might be something that gets added later, or never added at all. You'll also see dashed boxes drawn around a whole section of circuitry— that's the manufacturer flagging an accessory circuit, or a sales option, not a mistake in the drawing. This matters the first time you open a furnace and the schematic shows a humidifier or an IAQ accessory that simply isn't there. New techs panic and think a wire's missing or the board's wrong. Missing doesn't always mean broken. Check the legend for that asterisk or that dashed box first. If it's marked optional, the unit's running exactly as designed. That accessory was just never installed, and that's not your problem to fix today. Now that you know, you can help someone else know.
