Heater cores are small radiators placed somewhere within the dash. They pass hot engine coolant through them in the same fashion the car's main radiator does, with the blower motor then forcing air through the heater core and into the cabin. This allows the hot coolant from the engine to heat the air and thus the cabin, providing a convenient source of heat. In some instances, the heater core can even be used to aid in cooling an overheating engine by cranking the heat all the way up and turning the fan on maximum. While this is not a long term or efficient method of cooling the engine, it can give a driver the added cooling power needed to make it that extra mile. Don't forget to roll down the windows!
Heater cores can fail in a few ways. Corrosion of the metal can cause leaks, as can a faulty connection to the lines which direct coolant to the core from the main cooling circuit. When this happens, often time windows will become foggy and the slightly sweet smell of coolant will be noticable in the car. Window fog from coolant is not easily wiped off, as it will leave residue and streaks behind. They may also become clogged from debris which can often build up in engine cooling systems, causing them to stop functioning.
Replacing a heater core is usually a tedious and time consuming job. It is one of those repairs that will almost always cost a lot more in labor than parts. Make sure it is done right the first time, as it's not a job you will want to have done again any time soon!
We stock and sell a wide variety of quality heater cores from these manufacturers: