A furnace synonym is actually a name given to a specific brand or model of furnace that is used in many different homes and they have been in use for many years. They are used to describe the same model of furnace but in different ways. You can look up the furnace synonym of a model and they will give you all the information that you need to know about it. The model that you have is usually referred to as the main furnace but this is not the case with most models as there are some that have the ability to be run off of gas. This is not always the case as some models of the main furnace have an option to run on diesel, which has been known to run quite well. This is why you will often hear the words gas furnace used when referring to the main furnace in homes.
The main furnace is generally called a furnace because it is the main unit that is responsible for heating and venting the room that it is located in. A furnace synonym is used because of the fact that it is the same model but has different features that are found on other models. You will often hear the term furnace used to refer to the main unit as it is the one that does the heating and venting. You will also hear the furnace used in a way to describe the model that is used to replace the main unit that was used previously in that room. A new furnace may run very well and be very efficient but it will not be the same as the old model.