A hot water furnace is a system that heats air through the use of heated water running through pipes and into a radiator. The heat emanating from the radiator warms the air throughout the rest of the room. Thus, the home can be heated just through the use of water. This is one of the most common types of furnaces, especially for older and larger buildings.
The use of the term hot water furnace may be a bit of a mischaracterization. The term furnace usually applies to the heating of the air, which is then circulated throughout a building. However, a boiler heats water. Boilers, especially large ones, require constant attention and may even have someone watching them full time in some operations.
There are advantages to a hot water furnace when compared to other types of furnaces. For example, it is usually more efficient to move hot water than it is to move hot air. This is because water retains its temperature longer than air does. Also, it is easier to direct the water where you need it, more so than air. For example, if a room is not in use, a valve can shut off the water to that room.
As far as types of heaters go, a hot water furnace can also have some disadvantages. It is usually easier to detect a leak or other problem area with this type of furnace. While this may, on the surface, seem like an advantage, it is not. The reason it is easier to detect those problems is because the problems are often very messy and destructive. Also, if there is some sort of power failure and the furnace fails to operate, there is a chance of the water freezing. This could cause a massive problem inside a building if a water pipe were to burst.
There may be some safety disadvantages to a hot water furnace also. The radiators can get quite hot. Accidentally touching them could cause burns and many of the radiators are exposed and can be touched by children and adults. To help prevent this, some find objects to put in front of their radiators. However, if placed too close to the radiator, the object could become damaged if it is sensitive to heat.
While the main use of a hot water furnace is in larger buildings, there are some models that are made for home use. These models are typically not used very often, as most prefer the convenience that comes with an air furnace.