heat exchanger

a device that allows for an exchange of heat between two flows without actually mixing the two flows

An example of a simple heat exchanger would be running a pipe of hot water through a pipe with cold water, in order to heat the cold water.

In HVAC systems, heat exchangers are called HRV (Heat Recovery Ventilation). They extract a certain volume of stale air from a building, and replace it with an equal volume of fresh air from outside.

More complex systems incorporate a moisture exchange system for arid or very humid climates. These systems are called ERVs.

|from   |
|return |
|vents  |              ____________________________
|  |  | |_____________|__________     _____________|_____________
|  |  +----> stale      8        \   /        8  #       <----  outside
|  |     _______________8_   \    \ /     /  _8__#_______________intake
|  |    |             | ^ \   \    X     /  / ^  ^ |
|  v    |             | f  \   _| /X\  |_  /  f  f |
|       |             | a   \    /XXX\    /   a  i |
|air    |             | n    \  /heatX\  /    n  l |
|return |             |       \/exchngX\/        t | 
|duct   |             |       /\XcoreXX/\        e |
|       |             |      /  \XXXXX/  \       r |  
|  |    |             |     /    \XXX/    \        |
|  |    |             |    /      \X/      \       |
|  |    |_____________|___/        X        \______|_____________         
|  |  +-----   clean              / \                     -----> outside
|  v  |  _______________ ________/   \___________________________exhaust
|     v |             |____________________________|
|  to   |
|furnace|
|or A/C |

The Heat Exchange Core

                  /   
                 / /  
                / /o/   
               / /o/ /
    __________/ /o/ /o/              
    oooooooooo|/o/ /o/              
    ----------|o/ /o/        
              |/ /o/
    ----------| /o/
    oooooooooo|/o/
    ----------|o/
    __________|/

Thousands of tubules are stacked together, layered in alternating directions. When the air is forced thru this honeycomb arrangement, the temperature of outgoing and incoming air will equalize. This means that the stale air sent outside will be cooled by the fresh air coming in, itself warmed coming in. The process works both ways, so in the summer, the process is reversed.

Source: http://energy-publications.nrcan.gc.ca/pub/home/Operating_and_Maintaining_your_HRV.cfm