The type of system needs to be distinguished from other types of heating systems, evaluated and reported . You want to know if it is a furnace, which produces warm air or a boiler, which can produce hot water or steam, as well as a verity of other systems.  The means of heat distribution needs to be evaluated.  Furnaces use ducts and boilers typically use radiators or baseboards, however, there are some exceptions.  A hydronic forced warm air system is a combination that uses a boiler to produce hot water, but this hot water does not circulate through the house, it circulates through a coil in an air handler. Air that passes over the coil is heated and distributed through the house through ducts.  The most common systems run on gas or oil.  There are also systems that run on electricity.  There are heat pumps that produce both heating and cooling through ducts, electric furnaces and electric boilers are also found.  Radiant heat from electric wires in ceilings is less common but not unusual.  Radiant heat from pipes in floors heated from a boiler is also not common but not unusual.  Some homes  have electric baseboard heating.

Whatever type of heating system is present, it must be functioning properly and safely.  It must have a means of distributing the heat to all parts of the dwelling space. All combustion gasses if applicable must be safely vented to the exterior of the home.  Any significant deficiencies should be reported and a qualified heat service company should come for further evaluation and repair.
 

Gas Fired Furnace
 

Oil Fired Boiler
 

The photos above and to the left are one heating system.  It is a hydronic forced warm air system.  Water is heated in the boiler above and is pumped through a coil in the air handler to the left. Air passes over the hot coil and is heated as it is circulated through the ducts to the rooms of the house.
 

Once in a while you will still find one of these old gravity fed furnaces.  At times this type of unit is found in relative good working condition; however, they are an obsolete system that will need close monitoring if it is to be used. It will not heat very efficiently compared to new units.  In the final analysis, replacing it before too long even if it is working my save money in the long run.
 

AIR CONDITIONING:

Home inspectors are required to inspect central air conditioning systems and through wall units. They are not required to inspect window units.  Weather can restrict the evaluation of an air conditioning system.  In the colder months, running the system can damage it and, therefore, it cannot be evaluated.  Consistent outdoor temperatures above 60 to 65 degrees for at least 48 hours prior to inspection is required to safely run the AC system. 

The most common central air conditioning system is a "split system".  It is basically divided into two parts.  The condenser unit is what most people are familiar with.  It is the unit you find on the exterior of the home (see photo to right).  The other section is the air handler and evaporator unit.  In many cases the air handler is the same blower fan and ducts used for the heating system when a furnace is present.  In these cases, the evaporator is installed in the duct directly above the furnace.

Most condenser units last about 8 - 15 years before a major break down or replacement is needed.  We have often seen them older, but cannot predict how long a system will last. Annual maintenance by a heating and air conditioning service company is the best way to get the longest and most efficient use of your system.

A through-wall air conditioner can be a split system or a single unit (non-split). It can be made to cool one larger area of a building (below left) or be a central air conditioning unit (below right).  They are not window air conditioners that fit into a sleeve in a wall.  They are permanent, non-portable systems.

Cools large area

Central Air Conditioner