If the air coming from the vents in your home has a fishy smell, the source may not actually be your HVAC system. However, it is a potentially serious and even dangerous problem that needs to be addressed immediately.
The most common cause for an unexplained fish odor in the home is not mold or dead animals in the crawlspace, as many people might think. Instead, it is overheated plastic in electrical components. The plastic used in these components contains a chemical that emits a fish or urine-like smell when it gets extremely hot or burns. The ultimate source of that fish smell in your home could be one of many things—an outlet, a switch, an electrical breaker, wiring, even an overloaded extension cord.
The reason that the smell seems to be coming from the HVAC system is usually that the odor is being drawn in through the intake vent, then dispersed through the home, although it is possible that the electrical issue is occurring inside the heater or air conditioner itself.
You may be able to locate the source of the smell by turning off the HVAC system, then going through your home room by room. If there’s no odor at all when the HVAC is turned off, the problem may be in the HVAC. On the other hand, if some part of the house still smells “fishy” even when the HVAC isn’t running, then that may be where your electrical problem is happening. You can further try turning off lights and unplugging appliances to narrow down the source of the odor.
Whether the odor is coming from your HVAC system or from another source in your home, overheating electrical components clearly pose a serious fire hazard. So if you smell a fishy odor, don’t simply hope that it will go away. Contact a licensed electrician or HVAC technician to fix the problem before it results in something much worse than a big stink.