Foam It
Foam It

Forced-Air Distribution System During Heating Season (PDF)

Forced-Air Distribution System During Cooking Season (PDF)
dafdafda HVAC

The HVAC (Heating, Ventilation & Air-Conditioning) Systems

OK.  We've talked about the envelope.  Now let's turn our attention to the HVAC system.  This system is made up of the following components.

  • Furnace
  • Distribution System
  • Air-Conditioner
    • Condenser
    • 'A' Coil

Each of these systems serves a specific purpose.  We'll discuss the furnace first.



The furnace is the thing that creates heat.  Through a thermal reaction it releases stored Btu's of heat in a fuel (gas, propane, electricity, wood, the earth, etc) into our dwellings to create a comfortable environment.  Furnaces have evolved over time just as the building envelope, but for our immediate purposes we'll simply talk about the most modern and common types of furnaces.

The most common fuel used in furnaces in today is natural gas.  Sometimes natural gas is simply referred to as gas.  Gas is primarily measured in units called therms. A therm is a commercial unit of heat energy.  Gas can also be measured in cubic feet, Btu's kBtu's (thousands of Btu's) or MMBtu's (millions of Btu's).  1 therm = 100,000 Btu's = 1 Kbtu = .10 MMBtu's = (approx) 96.7 cu.ft. of natural gas.

Gas furnaces have an efficiency rating that describes how well they convert the stored Btu's of heat in the fuel into actual Btu's of heat.  Furnaces are sized according to the number of Btu's heat they can produce/hour of operation. 

If you look at your furnace's ID tag it might say the brand name and then it might say something like 90kBtu output, 90%EFF.  First 90kBtu output means that your furnace can supply a maximum of 90,000 Btu's/hour of heat output.  Second 90%EFF means that of the total number of Btu's of heat energy stored in the fuel it utilizes (in this case gas), it can convert 90% of those stored Btu's into actual heat output.  So for every therm (100,000 Btu's) of gas your furnace brings in off the gas line, it can create 90,000 Btu's of heat output.  (Remember that 1 therm = 100,000Btu's of heat.  90% of that amount = 90,000Btu's or 90kBtu.)

Efficiency on furnaces is also measured in AFUE, HSPF & COP.  These are less common and are utilized mostly by manufacturers and pointy headed lab nerds, but you still should know what they are. 

  • AFUE = Annual Fuel Utilization Efficiency (utilized on furnaces & boilers)
  • HSPF = Heating Seasonal Performance Factor (utilized on air-source heat pumps)
  • COP = Co-efficient Of Performance (utilized on ground-source heat pumps)

Furnaces are not to be confused with distribution systems.  They literally just convert stored Btu's in a particular fuel into actual Btu's of heat.  How you move those Btu's throughout your home is dependent on the distribution system.


Distribution System

The distribution system in a home is the system that carries the heat from the furnace to the rooms.  There are two primary types of distribution systems such as:

  • Forced-air
  • Hydronic

HVACForced-air is the most common.  A forced-air system employs a series of enclosed ducts and a large blower to move air through the furnace and out to the rooms and back to the furnace.  It cycles the air frequently in order to move Btu's efficiently.

Your furnace probably utilizes this system.  If you look at your furnace you'll probably notice a large metal trunk going into the bottom (return air drop).  There is probably a place for a filter (which you probably haven't changed for several monthsJ) and a return air grill.  If you turn your furnace fan on, you'll notice that this grill will begin to suck air.  This trunk is called the return or return air drop


Above the furnace you will notice another trunk and perhaps several different ducts branching off of it (typically trunks are square, ducts are round).  If you feel these after your furnace has been running for a while in the winter, they should feel warm.  These ducts are called the supply.  These ducts are connected to registers in your floor located throughout your home and supply heat from the furnace to those registers.  There are lots of these kinds of registers.  The return is connected to only a few grills throughout your home.  They are typically square grills located on the return air drop or on a wall either at the top near the ceiling or at the bottom near the floor.  They suck the stale air out of the rooms and 'return' it to the furnace so it can be heated and ‘supplied' back to the rooms through the floor registers.  The air is moved by a large blower situated near the bottom of the furnace.  It can work independently to circulate air regardless of whether the furnace is heating or the A/C is cooling. 


Furnaces operate primarily through convective heat movement, and remember that heat movement through convection requires a medium that can absorb heat, move to a new location, and then release heat.  In forced-air systems this medium is air.  In hydronic systems the medium is water.

HVACHydronic distribution systems utilize water to move the Btu's of heat. Typically a boiler is utilized to heat cold water. The most common location for these systems is in the floors of a building. These systems utilize water lines embedded in the floor system (often times embedded in concrete, sometimes just attached under the sub-floor) and hot water from a boiler or furnace is cycled through these lines and back to the boiler or furnace. The water is cycled by a pump.

Both distribution systems operate the same way. Take heat from a source, transfer it to a medium, move the medium to a new location and the heat is transferred to (experienced by) the objects and/or people in that new location. The process is repeated all winter long in millions of homes around the world.

Air Conditioner (A/C)

HVACAir conditioners 'condition' by removing heat and humidity from the air. The same blower and air distribution system that supplies the warm air in winter also supplies cool air the summer. Instead of the burners doing the work, now the ĎA' coil does the work. Below is a diagram of how an A/C works.

An A/C operates because physics dictates that a gas refrigerant (like Freon) will hold a greater amount of heat in a gaseous state than it will in a liquid state. Under pressure, one can change the state of a refrigerant from gas to liquid. If the pressure is released it will return to a gas. In simple terms, the refrigerant in gaseous state is pumped through the coil located in the house where it absorbs many Btu's of heat. Then the refrigerant is pumped outside, compressed it into a liquid, and pumped through another heat exchanger coil. As a liquid it releases those Btu's and a large fan blows air through the coil removing the Btu's of heat from the liquid in the heat exchanger coil. Then refrigerant is pumped back into the house, the HVACpressure is released as it flows into the evaporator coil and the liquid is allowed to return to a gaseous state so it can absorb more Btu's of heat and the cycle is repeated over and over as a means of moving Btu's of heat from inside to outside.

This process is accomplished with by utilizing a compressor (which sits outside) and an evaporator coil that is incorporated into the forced-air system and is situated directly above the supply plenum. The evaporator coil is very cold and therefore absorbs Btu's of heat from the air that blows across it. It also removes humidity because as the air goes over the coil its temperature drops and it looses its ability to carry water vapor. The vapor condenses on the coil and then runs down to a condensate line which flows into a floor drain.