Types of ERV Systems
Generally there are four media-component choices for an ERV:
rotary heat exchanger (wheel); plate heat exchanger (fixed core); heat-pipe
heat exchanger (refrigerant); and run around coils (water). Each of these
choices is a proven technology that has been around for over 20 years.
A wheel is a plastic or metal device that rotates between the exhaust and
outdoor air streams. It picks up heat from one air stream and transfers it
to the other. Metal wheels only transfer heat (sensible energy), while
some plastic wheels, impregnated with a desiccant, absorb and
release moisture (latent energy) as well. Wheels are the most popular ERV
because of their relatively low initial cost, reasonable pressure drop, ease
of maintenance, and smaller physical footprint.
Fixed-core plates generally are larger and more expensive than wheels, but
have no moving parts and can be used in certain applications (such as
hospitals) where a wheel may not be permitted by code. Instead of a wheel
moving between air streams to transfer energy, the air streams pass by each
other through a series of channels, heating up or cooling down the material
between the channels and transferring energy. This is much like putting a
can of warm soda in a bucket of ice. The ice cools the metal can and the can
cools the soda without the ice ever actually touching the soda itself.
Fixed-core plates can be metal, plastic, or even paper. Metal cores can only
transfer sensible energy, not latent energy. Plastic cores can come in
sensible-only or sensible-plus-latent versions. Paper cores always are
sensible plus latent.
Heat pipes are somewhat limited because, as with HRV's, they cannot recover
latent energy. They are sensible-only energy-recovery devices. Heat pipes
are copper tubes with refrigerant inside of them. These tubes run between
the two air streams (exhaust and outside air). One air stream heats the
refrigerant in the tube, causing it to evaporate. That refrigerant vapour
then moves down the pipe to the other air stream. When that other air stream
cools the pipe, the refrigerant condenses, warming the cooler air stream in
the process. The newly cooled refrigerant then flows back to the warmer air
Run around coils share some similarities with heat pipes, but often are
preferred when the exhaust and outdoor airflows are separated by large
distances. This type of system requires the installation of a water coil in
the exhaust air stream and a second one in the incoming outdoor/ventilation
air stream. These two coils are piped together, filled with a water/glycol
mixture, and then the mixture is mechanically pumped between the two coils.
Heat is picked up in one air stream and released in the other. Like heat
pipes, these systems only are capable of transferring sensible energy, never