GEA's Global HVAC Technology Blog Blog

GEA's Global HVAC Technology Blog

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Residential Energy Recovery & Humidity

Posted February 26, 2014 9:09 AM by larhere

With improvements in new residential building construction to make homes more efficient envelope leakage rates have been reduced to levels as low as 0.2 air changes per hour (ACH). This is well below minimum ventilation ratings required by ASHRAE 62.2 for acceptable indoor air quality. Consequently to satisfy these standards we are seeing a trend with HVAC installing contractors including supplemental mechanical ventilation equipment in their bids for new residential construction. To minimize heating or cooling losses while bringing in fresh outside air the most common type of ventilator specified for residential applications are fixed plate cross flow heat exchangers. Two versions of this type of equipment are heat recovery ventilators (HRV) and energy recovery ventilators (ERV). The differences between them are:

  • The HRV transfers only sensible heat between a supply air stream and the exhaust stream with a heat transfer effectiveness of approximately 70-80% depending on the device construction and air conditions.
  • The ERV is able to transfer both latent and sensible heat between the two streams by substituting a permeable material vs metal for the heat exchanger core to allow for moisture transfer. Moisture transfer effectiveness is generally in the 50-70% range depending on core size and partial pressures of the two air streams.

ERV's are very effective in warm humid climates where it can reduce the latent load on an air conditioner to maintain a comfortable humidity level. Too high humidity (typically >50%) can result in respiratory problems as well as potential mold, mildew and structure issues (buckling of hardwood floors for example). In colder climates an ERV is effective in slightly increasing the humidity of the occupied air space to help lessen the requirement of a humidifier in a very tight structure. However it has been the experience of this writer that many contractors appear to overstate the effectiveness of an ERV as a total replacement for a humidifier in cold climates. Unless the home has a very high latent load (e.g. excessive laundry usage, frequent bathing, high occupancy-space ratio, etc) the humidity can be unacceptably dry (<20%) which can result in respiratory problems, dry cracking skin, nose bleeds and also structure issues (e.g. shrinking hardwood floors, cracks in caulking, etc.) Extremely cold climates can also create condensation in the ERV which can significantly reduce its effectiveness. For best conditions to minimize comfort and structure issues discussed above relative humidity (RH) levels should ideally range between 35 & 45%.

Most manufacturers of these devices will typically recommend ERV's for warm humid climates with longer cooling seasons and recommend HRV's for colder climates with long heating seasons as well as dry arid climates. If an ERV is specified for a Zone 1 (upper third of US) residence the contractor or owner should not rule out the addition of a humidifier to maintain an acceptable humidity level in the home in winter.

Editor's Note: CR4 would like to thank Gene McNurlen, GEA Associate, for contributing this blog entry.

Gene McNurlen is a professional mechanical engineer and project manager with over 40 years' experience in various engineering industries including HVAC, process and plastic molding. Skilled in the development and application of commercial and industrial air handlers, heating and cooling coils, boilers, burners, incinerators and plastic products.

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