Types of Heating Systems
Forced air applies to all furnaces that have ductwork and a blower which "forces" air through the ducts and up into the home. Forced air furnaces can be gas/propane, oil, or electric. This is the only system that can have central air conditioning added on to it without the addition of ductwork.
Hydronic applies to any of the systems that have a boiler that heats water. This could mean either a steam or hot water system. The principle difference between the hot water and steam system is the temperature of the water that is forced through the pipes in the system. The most familiar "hydronic" systems are those that have the old fashioned radiators, but there are also systems that pump water through pipes in the floor or baseboard type radiators. Hydronic systems can be either gas or oil.
Gravity systems are often referred to as the old octopus type systems. You will have a large box type furnace in the basement with large round ducts coming off in all directions. The gravity systems work on the principle that hot air rises. There are no moving parts with this system. These systems, even though they do have ductwork, will not have central air because they do not have a blower motor. Gravity systems are always gas or oil.
The heat pump system is basically a reversible central air conditioning system. In the summer, the heat pump is a very high efficiency central air system. In the winter, the heat pump reverses itself and pulls heat out of the outside air and puts the heat energy into the customer's home. In cooler areas, a heat pump will always have some type of back-up furnace, either gas, oil or electric. The heat pump itself is always electric.
Baseboard electric heating is a lot like a space heater, but is attached to the walls and is generally controlled by either a room-to-room thermostat or a whole house thermostat. This type of heat does not come with ductwork and does not have central air. Baseboard heat does not need to be cleaned and checked professionally.