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  Heating, Ventilation and Air-Conditioning (HVAC) systems are designed specifically for each home, based on the number of rooms, the size of the house and the climate where the house is located. These systems may be single source HVAC systems or multiple source HVAC systems, meaning that there can be one or more HVAC systems per home.
  
  

Single HVAC System

• A single HVAC system is comprised of one set of HVAC equipment--a heat source and a cooling source. The heat source must be either fuel burning (natural gas, fuel oil, or propane) and the cooling source is optional, although in southern climates, a viable cooling source is required. Because most HVAC systems are based on moving air through the house to regulate temperature, forced air systems are very popular for heating and cooling needs. These systems incorporate a heating coil inside the air handler and an evaporator coil above the furnace in the ductwork supply plenum for cooling. The compressor unit of the air conditioning system sits outside.
  
  Other systems use water to heat a home through a boiler system. These systems distribute the heated water through plumbing lines that either radiate from baseboards or radiate through the entire floor system. Some boilers are equipped with geothermal pumps that pump water through underground pipes where it gathers heat and is used for heating purposes.
  
  

Multiple HVAC Systems

• Multiple HVAC systems are simply systems that have more than one heating and/or more than one cooling component for the home. These systems separate the house into heating/cooling zones that require their own thermostatic controls and usually their own dedicated piece of equipment. In forced air systems, one furnace and air conditioning unit will supply one floor. Another set of units will supply a separate floor.
  
  With water heating systems, one boiler can usually supply two different zones, but the zones are much smaller. For large zone heating, two boilers are required. Many of these systems also tie one of the boilers and the hot water heater together to make it easier for the water heater to heat up the water. Homes that have these systems along with air conditioning have to have a separate HVAC forced air-cooling system to supply the house with cool air. These systems are double the cost of a normal multiple HVAC system but allow the most versatility in terms of zoning area and energy efficiency.
  
  

HVAC System Selection

• Besides the size of a home, there are other reasons to choose a multiple HVAC system. If you are looking for total control of multiple rooms in a home or have rooms that need specific temperature controls, such as wine cellars, a multiple HVAC system will be needed. For heating, these types of situations are easier to control with multi-zone boiler systems because forced air can only be zoned in one area per unit without extremely expensive commercial equipment.
  
  If you have a standard sized home of 3,000 square feet or under, you should not need to have a multiple HVAC system because most conventional single systems will be able to handle the heat and cooling load.
  
  Single HVAC systems are half the cost of Multiple HVAC systems for the simple reason that the extra equipment and distribution systems are more intensive.