If you detect the smell of antifreeze around your heating system, there's only one instance in which it could actually be antifreeze and that's if you have a boiler and hydronic heaters, such as radiators or baseboard heaters. It's common to add antifreeze to the water circulating through the system to prevent freeze-up in winter, and if you detect that smell, that probably means there's a leak. If so, look on the floor for a puddle and the leak should be in the pipes somewhere above that. Call an HVAC repair technician to fix the leak as soon as possible.
If you detect the smell of antifreeze from any other type of heating system, it probably isn't antifreeze you're smelling but something else. The most likely culprit is refrigerant from an air conditioning system. If you have a heat pump or an outdoor air conditioning unit, there are evaporator coils inside the air handler — where heat is also produced — and they could be leaking.
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How to Recognize a Refrigerant Leak
If your HVAC system has a heat pump or a split-system air conditioner, you'll see evaporator coils inside the air handler when you open the door. The coils are usually arranged in a triangle, and they are mostly covered by a set of metal fins, but the bends are exposed on the ends. If refrigerant is leaking, you'll often see discoloration and even rust on the ends of the fins where the coils emerge. If you don't see discoloration on the front of the coil unit, get a mirror with a long handle and a flashlight to check the back because that's where leaks occur most often.
If refrigerant is leaking, you'll probably notice an uptick in your energy bill because the air conditioner has to work harder and longer to maintain a comfortable temperature in the house. You might even notice that the house isn't as cool as it should be. If you suspect a leak, call an HVAC tech because this isn't something you can repair yourself.
You Could Be Smelling Gas
A furnace doesn't circulate water, so it doesn't need antifreeze, and it doesn't cool, so it doesn't need refrigerant. That means if you're actually smelling antifreeze, the odor can't be coming from the furnace, and if there's no air conditioner, you can't be smelling refrigerant. Look around the furnace room for other possible sources of the odor, such as open solvent containers or spills. If you can't find anything that could be producing the odor, take another whiff to see if you can identify what it might actually be. It could be gas.
Natural gas and propane have much stronger and more pungent odors than antifreeze, but there may be a small enough concentration of gas in the air to attenuate the odor and give the impression of antifreeze. A gas leak is a serious matter because it could mean the furnace heat exchanger is cracked and is releasing carbon monoxide throughout the house. If you suspect a gas leak, turn off the furnace and leave it off until the HVAC technician arrives. Secure the door to the furnace room to keep people out because gas could be lingering in the air.
Odors From an Oil Furnace
Home heating oil has an odor similar to kerosene and can also mimic kerosene in low enough concentrations. An oil leak in an oil furnace can often be corrected by tightening a fitting in the oil line with a pipe wrench, but if you can't find a leak, the problem may be an oil burner malfunction, which needs professional service. If you just had the furnace installed, an oil smell is normal during the first few days of operation, but if the smell persists, call for service.