Air to Air Energy Recovery

Capacity control can be accomplished either with control valves (two way or three way) on the supply air unit or variable flow pumps.  A variable flow pump works best if there is one supply air coil.

The Miami Psychrometric chart shows the design day cooling and heating recovery of the rotor.  The horizontal blue line shows a sensible rotor can deliver (14.7 Btu/h-cfm) and is based on the design drybulb day (best day for a sensible only device).  The more vertical blue line shows the performance (63.8 Btu/h-cfm) for an enthalpy rotor and is based  on the most humid day.  It is clear that in a location like Miami where dehumidification is key, an enthalpy recovery device is critical.  The red line shows the rotor on a design winter day.

The psychrometric chart shows the results for Montreal.  Montreal is not as hot or humid as Miami so the energy savings are smaller but still significant.  Montreal still benefits from enthalpy energy recovery which delivers almost 4 times the energy savings as just a sensible device.

In winter, the energy recovery is significant.  The bulk of the energy recovery is sensible heat so the enthalpy and sensible performances are almost the same (84.5 Btu/h-cfm vs. 70.7 Btu/h-cfm for just sensible).  Being able to recover humidity in the winter has some key advantages.  First, a building in this climate can be very dry in the winter (less than 10% RH).  An enthalpy recovery device can return the humidity to the building and raise the relative humidity.  Operating humidifiers requires a high energy budget (1000 Btu/lb of water).  The second advantage of enthalpy recovery in winter is the significant improvement in defrost.

The psychrometric chart shows Phoenix, a hot a dry climate.  It benefits with almost twice the sensible energy recovery of the other two locations (30.6 Btu/h-cfm).  Phoenix also benefits the least from enthalpy recovery.

[make same chart but add the color blocks for hours by psych condition]

While this analysis is good for recognizing the downsizing of supplemental heating and cooling equipment its weakness with the previous analysis is it does not consider the amount of time and thus the amount of energy savings that will occur.  Phoenix Energy Recovery with Hours shows Phoenix again but with hours the outdoor weather spends at different psychrometric conditions.  This makes it clear that while enthalpy may help a little, the total number of dry hours limits the savings.

Go back to Montreal, and consider what happens in winter.  The return air is an energy source that can be used to warm the outdoor air.  As the energy is transferred, the return air temperature drops.  At some point the return air temperature can reach freezing conditions (32°F (0°C)) If the return air is saturated  (at its dewpoint) then frost will start to form on the recovery device rendering it useless and possibly damaging it.

In the psychrometric chart, the red line S_sa shows the sensible heat gain in the supply air stream – it is a 65 °F (36°C) temperature rise.  Assuming the supply and exhaust airflow rates are equal, then the return air temperature will drop from 72 °F (22°C) to 6.8 °F (-14°C).  The green line S_ra shows the sensible heat loss in the return air stream.  In this example, the return air has 20% relative humidity (RH).  This means the return air will reach saturation as it reaches freezing temperatures and frost will occur.  Had the return air relative humidity been below 5% frosting would not have occurred.  It is very difficult to know when and if a frosting condition will happen because it is very sensitive to the building humidity level.

The red E_sa and green E_ra lines show the same process but with an enthalpy recovery device.  The key difference here is while the return air temperature will drops as the energy is transferred just like a sensible device, humidity is also being transferred which lowers the saturation point (the lines have slope on psychrometric chart).   Depending on the climate, this can increase the annual energy savings by 5 to 15% because you can extract more sensible heat from the return air stream without frosting.

Technically the rotor is capable of delivering the winter heat recovery but it cannot sustain it (it will frost). Most equipment selection programs will show the full energy recovery capability but provide a warning that frost may occur. In a properly designed ventilation unit, the recovery capacity will be modulated so the unit does not frost. This does not mean the recovery stops, it means no matter how much colder it gets outside, a fixed amount of energy will be extracted from the return air and used to heat the outdoor air. As the outdoor air temperature continues to drop, so will the supply air temperature but the heat transfer will stay constant.

Passive House Institute has rating programs for commercial and residential energy recovery ventilators.  This includes the energy recovery device efficiency, the fan energy use, leakage, sound etc. so it is a full unit test standard.  The Passive House equation includes a term for fan energy.    The same unit tested to Passive House, AHRI and ASHRAE Standard 90.1 will all yield different values because the equations are different.