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 stream.

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 latent energy.

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Last modified: 11/18/15