Heat Exchangers: Interesting Applications Part I: HRV’s and ERV’s

Call me a nerd, but I actually find heat exchangers pretty cool.  My first exposure to them was in high school when my parents’ bathroom fan quit and I was tasked with purchasing and installing a new one.  When I went to a big-box home store to survey my replacement options, most of the fans were in the $50 to $150 range.  I was shocked to see one with a $500 price tag.  The unit was a “Panasonic FV-04VE1 WhisperComfortTM Spot ERV.”  The unit’s box was printed with the following image:

ERV-1(picture from http://www.panasonic.com)

It’s pretty clear from the picture what the ERV (energy recovery ventilator) does, at least regarding sensible heat.  In winter, warm air from inside a conditioned space heats fresh cold air drawn in from outside.  The reverse process occurs in summer.  This is most often accomplished by separating the fluids by a thin membrane with a very large surface area.  HRV’s (heat recovery ventilators) are units that only exchange sensible heat.  ERV’s like the one pictured above actually exchange latent heat as well,  recovering some of the energy lost due to a phase change of water.  This is commonly done via a moisture-permeable membrane separating the fluids.  Here’s a crude diagram of what’s going on:

Scanpsychrometric_chart

Both ERV’s and HRV’s are a great way to decrease indoor air pollutants in today’s tight, low-infiltration homes, while not significantly increasing space-conditioning energy use, as would traditional forced HVAC ventilation or natural ventilation would.  However ERV’s are pretty much required (when compared with HRV’s) in extreme climate conditions.  When HRV’s are used in situations with a large temperature gradient between air streams & at least one of those air streams is even slightly humidified, condensation is sure to occur in the unit, which can result in mold and actually make bad indoor air worse.  ERV’s should also not be used without considering moisture issues; some conditions may overcome a unit’s ability to exchange moisture and condensation can still occur.  A great example of ERV misuse would be the story I told before; that Panasonic ERV should have never been by the bathroom fans, since using it in that application, as a device intended to evacuate shower steam, would likely be more moisture than it was designed to cope with.

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