Building codes require an outdoor air supply to help control indoor air quality with the outdoor air representing a significant portion of total HVAC building load. An ERV recycles energy from the normally exhausted building air to pre-condition incoming ventilation air. By recycling energy contained in the exhaust air, ERVs lower total HVAC energy usage. This is illustrated in Figure 1 below. This process is commonly referred to as load shifting. Shifting the outdoor air ventilation load to the ERV reduces the energy demand on traditional HVAC equipment, allowing the now smaller ERV-based HVAC equipment to operate more efficiently.
Passive Energy recovery relies on the temperature and humidity difference of the two opposing air streams to perform the energy exchange. The larger the difference, the more results the ERV will provide. This trait allows energy recovery components to reduce energy consumption on days that are the hottest, coldest, and most humid. As shown in Figure 2 below, reducing energy demand allows for a more energy efficient system year round for the majority of U.S. climate zones.
While Figure 1 illustrates a wheel and Figure 2 illustrates a plate exchanger, these Figures are intended to apply to all technologies. ERVs are sometimes referred to as air-to-air heat exchangers (AAHX) as in Figure 3 below, which illustrates a generic configuration.
The primary benefit in applying an ERV is the heating, cooling, and humidification ventilation load reductions throughout the year. Paybacks and load reductions can be calculated based on AHRI certified and verified rating points listed below. Performance data based on heating and cooling conditions include:
- Sensible effectiveness at 100 & 75 percent of the rated airflow
- Latent effectiveness at 100 & 75 percent of the rated airflow
- Total effectiveness at 100 & 75 percent of the rated airflow
Performance data measured at room ambient conditions include:
- Pressure drop of exchanger at 100 percent of the rated airflow
- EATR — Exhaust Air Transfer Ratio between the entering exhaust (Figure 3, Station 3) air and the leaving supply air (Figure 3, Station 2) at zero pressure differential
- OACF — Outdoor Air Correction Factor is the entering supply airflow (Figure 3, Station 1) divided by the measured (gross) leaving supply airflow (Figure 3, Station 2) at zero pressure differential
- Purge angle setting (rotary heat exchanger) or Tilt Angle (heat pipe heat exchanger)
In addition to the certified ratings listed, manufacturers have the option to certify EATR and OACF different pressure regimes to provide more detail on how the air-to-air heat exchanger performs under varying pressures. System designers must understand how the air moves between the two air tunnels under varying pressure regimes. Participants have to certify their technology under two of the following pressure differentials:
- -5.00, -3.00, -1.00, -0.50, 0.50, 1.00, 3.00, 5.00 in H20
[-1250, -750, -250, -120, 120, 250, 750, 1250 Pa]
Note: These pressure differentials are between the entering exhaust air (Figure 3, Station 3) and the leaving supply air (Figure 3, Station 2).
As certified effectiveness values typically drive overall operating savings and load reductions, it is important not to overlook EATR and OACF from an operational standpoint.
The sensible exchange is accomplished by the heat transfer matrix and the latent transfer is accomplished by the addition of a desiccant onto the transfer matrix media. Passive energy recovery relies on the difference in entering conditions of the opposing air streams to perform the exchange. An ERV’s “effectiveness” determines what the leaving condition will be. Effectiveness is further split into both sensible and latent values. A manual selection can be accomplished by using the following equations:
The ventilation load shifting accomplished by energy recovery reduces the capacity required by the chiller and boiler in a central system and that of the DX packaged unit. While it is optional to take advantage of the benefits in the central system, it is mandatory in the DX system to achieve proper control of humidity. If this is not done, the system becomes over-sized and could result in excessive moisture and occupant discomfort.
Exhaust Air Transfer Ratio (EATR)
EATR is defined as the tracer gas concentration difference between the Leaving Supply Airflow (Figure 3, Station 2) and the Entering Supply Airflow (Figure 3, Station 3) divided by the tracer gas concentration difference between the Entering Exhaust Airflow (Figure 3, Station 3) and the Entering Supply Airflow (Figure 3, Station 1) at the 100 percent rated airflows, expressed as a percentage.
If a designer is concerned about the contaminants exhausted from the building or using and ERV in a source controlled application, they need to pay particular attention to EATR. For guidance, review ASHRAE Standard 62, section 5 “Air Classification & Recirculation” on maximum EATR levels. It should be noted that if specified outside air volume is being measured at Station 2 (leaving supply airflow, Figure 3), actual outside air volume is not equal if the EATR is greater than zero.
Require 1000 cfm (1696 m3
/hr) of ventilation air for a given space
EATR is 5 percent for the ERV under your specified pressure regime. Measured airflow at Station 2 (Figure 3) is 1000 cfm (1699 m3
Actual outside air ventilation volume is:
1000 cfm (1699 m3
/hr)* (1-EATR) = 950 cfm (1614 m3
To compensate for EATR, one must increase the volume at Station 2 to 1053 cfm (1789 m3
Airflow at Station 2 = Required Airflow for ventilation / (1-EATR)
Airflow at Station 2 = 1000cfm (1699 m3
/hr) / (1-0.05)
Airflow at Station 2 = 1053 cfm (1789 m3
Outdoor Air Correction Factor (OACF)
OACF is defined as the entering Supply Airflow (Figure 3, Station 1) divided by the measured (gross) Leaving Supply Airflow (Figure 3, Station 2).
When specifying an ERV, it is highly recommended that the designer pay attention to the OACF, which is an indication of leakage across the heat exchanger. This leakage can occur within the heat exchanger and/or the seals between the supply and exhaust airstreams.
If the OACF is greater than one, the air is leaking from the supply air over to the exhaust air. Depending on fan configuration, it is likely one will end up requiring additional fan energy. If the OACF is less than one, the air is leaking from the exhaust air over to the supply air. The same parameters need to be taken into consideration when dealing with OACF values of less than one. Fans will not necessarily see the scheduled air volumes unless this is taken into consideration.
How should I apply OACF & EATR into my designs?
AHRI Certified manufacturers can provide details on how scheduled air volumes and overall performance are impacted by the certified EATR and OACF values. It is important to understand what the pressure differential is across the heat exchanger so that the correct OACF & EATR factors can be applied. This is why manufacturers offer multiple ratings for various pressure regimes. The AHRI Directory should be consulted for all certified ratings. When applying the technology, consult a manufacturer or representative to ensure that scheduled air volume and brake horsepower requirements are accurate.
Control and Prevent Frost
In cold climates, frost may occur in the exhaust air side of ERV-HRV. Means to avoid frost or removing frost are required to prevent damage to the heat exchanger and/or increase of pressure drop across the heat exchanger.
Some of the methods that can be used for all types of exchangers:
- Exhaust only: When frost is indicated on the component, the intake blower is de-energized for a period of time to allow exhaust air to warm component, melt frost, and drain or evaporate condensate. Alternatively, exhaust-only operation is determined based on outside air temperature to avoid frosting.
- Preheat: For outdoor air preheat, heat is provided in the outdoor air intake to heat the air to a temperature above the exchanger frost threshold before it enters the component. Alternatively, for return air preheat, heat is provided in the return air before it enters the exchanger to prevent frost from forming.
- Re-circulation: When return air or exhaust air re-circulation is used, return air or exhaust air is substituted, in whole or in part, for outside air passing through the exchanger for defrost purposes.
- Bypass: A portion of the outdoor air is bypassed around the exchanger until the frost threshold is avoided.