3.7.6.6 Energy Recovery

Exhaust to Outside Heat Recovery Effectiveness

Applicability

Any system with outside air heat recovery

Definition

The effectiveness of an air-to-air heat exchanger between the building exhaust and entering outside air streams. Effectiveness is defined as follows:

(Equation 3.7.6-14)

$$HREFF = \frac{EEA_{db}-ELA_{db}}{EEA_{db}-OSA_{db}}$$

where

HREFF	The air-to-air heat exchanger effectiveness
EEAdb	The exhaust air dry-bulb temperature entering the heat exchanger
ELAdb	The exhaust air dry-bulb temperature leaving the heat exchanger
OSAdb	The outside air dry-bulb temperature

Units

Ratio

Input Restrictions

As designed

Baseline Rules

Required for fan systems with a design supply air flow rate of 5,000 cfm or greater if the minimum outside air quantity is 70% of the design air flow rate. If required, the energy recovery system should have at least 50% effectiveness. Energy recovery is not required for the following situations:

Systems serving spaces that are not cooled and that are heated to less than 60°F.
Systems exhausting toxic, flammable, paint, or corrosive fumes or dust. This exception shall only be used if heat recovery is not used in the proposed design.
Commercial kitchen hoods used for collecting and removing grease vapors and smoke classified as Type 1 my NFPA 96. This exception shall only be used if heat recovery is not used in the proposed design.
Heating systems in climate zones 1 through 3
Cooling systems in climate zones 3c, 4c, 5b, 5c, 6b, 7 and 8
Where the largest source of air exhausted at a single location at the building exterior is less than 75% of the design outdoor air flow rate. This exception shall only be used if heat recovery is not used in the proposed design.
Systems requiring dehumidification that employ energy recovery in series with the cooling coil. This exception shall only be used if heat recovery is not used in the proposed design.

Condenser Heat Recovery Effectiveness
Applicability	Systems that use recover heat from a condenser
Definition	The percentage of heat rejection at design conditions from a DX or heat pump unit in cooling mode that is available for space or water heating.
Units	Percent (%)
Input Restrictions	As designed. The software must indicate that supporting documentation is required on the output forms if heat recovery is specified.
Baseline Rules	Not applicable for most conditions. Condenser heat recovery is required for service water heating in 24-hour-per-day facilities, if the total installed heat rejection of the water cooled system exceeds 6,000,000 Btu/h and the design service water heating load exceeds 1,000,000 Btu/h. In this case the heat recovery system is required to have the capacity to provide the smaller of 60% of peak heat rejection load at design conditions or preheat the peak service hot water to 85F. Exceptions to this requirement do not apply. If a condenser heat recovery system meeting the requirements described in Section 6.5.6.2 cannot be modeled, the requirement for including such a system in the actual building shall be met as a prescriptive requirement in accordance with Section 6.5.6.2, and no heat-recovery system shall be included in the proposed or baseline building designs.

Heat Recovery Use
Applicability	Systems that use heat recovery
Definition	The end use of the heat recovered from a DX or heat pump unit. The choices are: Reheat coils Water heating
Units	List (see above)
Input Restrictions	As designed. The software must indicate that supporting documentation is required on the output forms if heat recovery is specified.
Baseline Rules	Not applicable for most conditions. The end use will be water heating if required for 24-hour facility operation.

90.1-2019

For baseline systems requiring energy recovery, the heat exchanger is assumed to be integral with the AHU. The system fan power or pressure drop will be adjusted according to the methods in Section 3.7.3.1 of this document.

For proposed systems with heat recovery, the analyst must be careful to set all descriptors, particularly those for control, parasitic energy, and exhaust airflows, to realistically represent the equipment components, operation, and maintenance of building pressurization.

When exhaust air energy recovery systems are installed in cold climates, frost control may significantly affect total recovered energy during subfreezing conditions. Pumps and dedicated fans will consume parasitic energy. Simulation program inputs and hourly reports should be reviewed to ensure all items are represented as close as possible to the actual result of the proposed control sequences

Requirements related to condenser heat recovery are documented in Section 3.8.7 of this manual.

Exhaust Air Energy Recovery
Applicability	Any system with outside air heat recovery
Definition	Provision of exhaust air energy recovery system. Provisions shall be made to bypass heat recovery system to permit air-side economizer operation as specified in Section 3.7.4.2 of this document.
Units	Unitless
Input Restrictions	As designed
Baseline Building	Required for fan systems with a design supply air flow rate of 5,000 cfm or greater, if the minimum outside air quantity is 70% of the design air flow rate. Energy recovery is not required for the following situations: Systems serving spaces that are not cooled and that are heated to less than 60°F. Systems exhausting toxic, flammable, paint, or corrosive fumes or dust. This exception shall only be used if heat recovery is not used in the proposed design. Commercial kitchen hoods used for collecting and removing grease vapors and smoke classified as Type 1 my NFPA 96. This exception shall only be used if heat recovery is not used in the proposed design. Heating systems in climate zones 1 through 3 Cooling systems in climate zones 3c, 4c, 5b, 5c, 6b, 7 and 8 Where the largest source of air exhausted at a single location at the building exterior is less than 75% of the design outdoor air flow rate. This exception shall only be used if heat recovery is not used in the proposed design. Systems requiring dehumidification that employ energy recovery in series with the cooling coil. This exception shall only be used if heat recovery is not used in the proposed design.

Enthalpy Recovery Ratio

Applicability

Any system with outside air heat recovery

Definition

The general effectiveness of an air-to-air heat exchanger as characterized by the reduction in heating or cooling load between the building exhaust and entering outside air streams. Enthalpy Recovery Ratio is defined as follows:

ERRH=OSAH-OSLAHOSAH-ELAH

(43)

Where:

ERR_H = The air-to-air heat exchanger enthalpy recovery

OSA_H = The total enthalpy of the outside air entering the exchanger

OSLA_H = The total enthalpy of the outside air leaving the heat exchanger

ELA_H = The total enthalpy of the exhaust air entering the heat exchanger

Units

Fraction (between 0 and 1)

Input Restrictions

As designed in accordance to the formula above

Baseline Building

The baseline would have 50% energy recovery effectiveness based on design conditions. Fifty percent energy recovery effectiveness shall mean a change in the enthalpy of the outdoor air supply equal to 50% of the difference between the outdoor air and return air enthalpies at design conditions.

Exhaust Air Energy Recovery Economizer Interaction
Applicability	Any system with outside air enthalpy heat recovery
Definition	Energy recovery control during economizer operation
Units	Lockout, no lockout
Input Restrictions	As designed
Baseline Building	Lockout. The baseline system should bypass the energy recovery device during economizer operation. Refer to Section 3.7.4 of this document for baseline economizer requirements. During economizer operation, parasitic losses of energy recovery device and fan energy impact of pressure drop through energy recovery device should not occur.

Heat Exchanger Parasitic Energy
Applicability	Systems that use heat recovery
Definition	This input is used to model electric power consumption by controls (transformers, relays, etc.) and/or a motor for a rotary heat exchanger. None of this electric power contributes thermal load to the supply or exhaust air streams.
Units	Watts (W)
Input Restrictions	As designed. A default of 50W is assumed which can be overridden by the user.
Baseline Building	50W

Heat Exchanger Fan Energy Consumption
Applicability	Systems that use heat recovery
Definition	The additional fan energy needed for the energy recovery device. For all energy recovery ventilator (ERV) systems that include a bypass during economizer operations, the fan energy consumption for ERV systems should only be modeled when the ERV runs and should not be considered when the ERV is bypassed for economizer operation.
Units	Watts (W)
Input Restrictions	As designed
Baseline Building	The ERV fan power for the baseline system can be calculated as follows: Bhp = [ (0.6 × OAcfm) + (0.6 × 0.9 × OAcfm)] /4131 W = bhp x 746/nm This has been calculated from: Bhp = [((2.2 × HREFF) - dp_oa) ´ OAcfm]/4131 + [ ((2.2 × HREFF × E_f) – dp_ex) × OAcfm ]/4131 Where: Bhp = Fan brake horse power HREFF = Heat exchanger effectiveness dp_oa= ERV pressure drop on the outdoor air side (is assumed to be 0.5 in. w.c.) dp_ex = ERV pressure drop on the exhaust air side (is assumed to be 0.5 in. w.c.) E_f = Exhaust airflow fraction (exhaust airflow is 90% of outdoor airflow after considering leakage and zone exhaust) W = Fan power nm = Fan motor efficiency of supply fan. For EnergyPlus fan, energy for an ERV is not an input of the ERV module and the ERV fan energy should not simply be added to the system supply fan if the ERV includes a bypass during economizer operations as required in the baseline. The following workaround should be used instead. The fan energy associated with energy recovery is modeled as additional ERV parasitic power. This results in the ERV fan energy occurring only when the ERV runs, which is the desired behavior. If there is not a bypass in the proposed design, ERV fan energy shall be included in the HVAC system fan so that its impact is accounted for whenever the fans are running.