ERV’s and HRV’s: What’s the Difference?

In the quest for more energy efficient and comfortable working and living spaces, dPoint Technologies has developed innovative solutions to conventional problems, playing a key role in redefining the HVAC industry (such as the recent project with Tridel). As building envelopes tighten, adequate ventilation continues to grow as one of the most pressing issues. Since the 1980’s, the phenomena of sick building syndrome has led to greater recognition of the need for proper airflow, both in quantity and quality. However, increased ventilation in buildings has a greater energy demand, and mitigating that cost is the purpose of ERV’s and HRV’s.

Heat Recovery Ventilators

When fresh air is brought into a building, it carries with it the potentially undesirable properties of the ambient outdoor air, including temperature and humidity. Conversely, the exhaust air from the building is already conditioned to the requirements of the building users. Heat recovery ventilators (HRV’s) take advantage of the temperature gradient between the airflow to transfer sensible energy. Sensible energy can be thought of as the energy that is a result of changing a substances temperature. An HRV runs the outgoing air past the incoming air separated by an impermeable divider, using counter-current flow to strip the heat from the building exhaust and deposit it in the fresh outdoor air traveling into the building. This exchange cuts energy consumption by reducing the need to preheat the outdoor air before it enters the building. While an advantage over no energy recovery, HRV’s still fail to capture critical elements of the energy balance. Enter the energy recovery ventilator or ERV.

Energy Recovery Ventilators

Energy recovery ventilators (ERV’s) possess the same capabilities as the HRV, but take greater advantage of the full spectrum of energy contained within the airflow. Latent heat is the energy required or produced when a substance changes state (or phase) while remaining at an even temperature. Consider a pot of room temperature water on a stove-top. When the stove turns on, the water absorbs energy from the element and begins to increase in temperature. This is a sensible heat transfer as mentioned above. However, as the water approaches 100 ° C, it begins to undergo a phase change, from liquid to gas. From this point, it is latent heat that is being exhibited, as the water stays at about the same temperature until it is all boiled off. The corollary to this in HVAC systems is the humidifying and dehumidifying process. Forcing water out of the air in the humid summer and into the air in the dry winter comprises a large part of the energy consumption in a typical building. ERV’s are able to capture not only the energy savings of the heat exchange, but also of the latent energy of the water (humidity) in the air. When considering humidity in a building, this property of ERV’s reduces the cost of moisture control while simultaneously reducing the temperature control aspect of the energy bill.

A common misconception about ERV systems is that they are only economical in climates with extreme indoor/outdoor humidity gradients, such as in tropical climates. A summer 2007 study by the National Research Council of Canada conducted tests on identical houses in Ottawa, Ontario. One was equipped with a conventional HRV unit and the other with an ERV to evaluate the significance of savings attainable with full energy recovery. What they found indicated that savings on dehumidifying can be as high as 20% and cooling costs are reduced by as much as 12%. This study showed that ERV’s are adaptable to a far broader range of climates than previously considered economical.

To read the study: http://www.nrc-cnrc.gc.ca/obj/irc/doc/pubs/nrcc47733/nrcc47733.pdf

What makes a Good ERV?

A number of systems are able to efficiently carry out the functions an ERV is intended to provide, but there are certain types that carry advantages. Fixed plate exchangers have become the more forward thinking option as the technological development of the materials used in the cores has progressed. While previously used materials such as cellulose based products have had issues with mold and lifespan reduction, polymer membranes like dPoint’s are quickly becoming recognized as the superior choice in this application. Mold resistant, washable, and able to withstand extremes temperatures, polymer membranes can be specifically tailored in their pore sizes and electrical potential to provide the best air quality and the highest energy savings available. By optimizing the physical structure of the core, static pressures can be reduced to a minimal or non-existent level. Static pressure involves the fan capacity required to force the air through a core to overcome the resistance posed by the passage of the air through the material.

This fairly technical explanation can be easily summed with two basic concepts that anyone can get excited about: better quality of life in your apartment or office, and cheaper energy bills. dPoint is truly on the cutting edge of this technology with critical patents that set them apart from the competitors in the field. This has much to do with the reason why Tridel, Ontario’s largest condominium developer, recently chose dPoint’s membrane for their ERV systems. Read more: http://greenangelenergy.ca/news/dpoint-greens-building-developer-tridel/