From NachiWiki

There are four common types of heating units:

• A furnace provides heat through a forced air distribution system.
• A boiler provides heat through a hydronic distribution system. (Hydronic systems are also referred to as hot water systems.)
• A space heater supplies heat directly to the room where it is located.
• A heat pump extracts heat from the air, ground or water outside the house and usually delivers it through a forced air distribution system.

Most heating systems need air for combustion. Furnaces, boilers and space heaters that burn fuels need a supply of air to be able to burn properly, and a vent to the outdoors so that combustion gases can escape from the house. Electric heaters do not need to be vented. Combustion is a two-step process: air in, and gases out.

Air in

In the past, there was usually plenty of air leaking into a house to keep the furnace, boiler or stove burning well. Modern homes, however, are better sealed and use controlled ventilation, rather than uncontrolled leakage, to provide greater comfort and energy efficiency. Vents that supply air for heating units should never be blocked. It is important to ensure that there is an adequate supply of combustion air available, even when other air exhausting equipment is in use.

Gases out

Venting used to be done through a chimney. Today, however, many models of natural gas, oil and propane equipment can be vented by pipe directly through the wall, which greatly simplifies installation. Remember that combustion gases cannot escape from your home unless you provide air to replace them. That’s why venting problems can often be traced to air supply problems.

Controls

The indoor temperature is automatically controlled by a thermostat. Two important considerations are location and type. Central systems are normally controlled by a single thermostat. To achieve proper temperature control, the thermostat must be located in an area where it will sense the “average” indoor temperature. Locations exposed to localized temperature extremes (outside walls, drafts, sunlight, hot ducts or pipes, etc.) should be avoided.

Different types of thermostats are available. Basic types maintain a fixed indoor temperature. However, you can reduce your heating costs by installing a set-back thermostat which can be programmed to automatically lower the temperature when no one is home or everyone is in bed, and then warm up the house before you get home or wake up. Savings will vary, but a set-back of 3ºC for eight hours daily could reduce your heating costs by about 5%.

Where space heaters are used, each unit will likely be individually controlled by its own thermostat – which is usually the basic type. This allows you to keep unused areas at a lower temperature than those areas you do use.