3.7.6.5 Electric Heat Pump

Electric Heat Pump Heating Capacity
Applicability	All heat pumps
Definition	The full load heating capacity of the unit, excluding supplemental heating capacity at ARI rated conditions
Units	Btu/h
Input Restrictions	As designed
Baseline Rules	Autosize and use an oversizing factor of 25% (let the software determine heating capacity based on the building loads). The equipment capacity may need to be increased to ensure that the number of unmet load hours do not exceed 300.

Electric Heat Pump Supplemental Heating Source
Applicability	All heat pumps
Definition	The auxiliary heating source for a heat pump heating system. The common control sequence is to lock out the heat pump compressor when the supplemental heat is activated. Other building descriptors may be needed if this is not the case. Choices for supplemental heat include: Electric resistance Gas furnace Oil furnace Hot water Other
Units	List (see above)
Input Restrictions	As designed
Baseline Rules	Baseline electric air-source heat pumps shall be modeled with electric auxiliary heat and an outdoor air thermostat. The systems shall be controlled to energize auxiliary heat only when the outdoor air temperature is less than 40°F. The air-source heat pump shall be modeled to continue to operate while auxiliary heat is energized.

Electric Heat Pump Heating Efficiency

Applicability

All heat pumps

Definition

The heating efficiency of a heat pump at ARI rated conditions as a dimensionless ratio of output over input. The software must accommodate user input in terms of either the Coefficient of Performance (COP) or the Heating Season Performance Factor (HSPF). The ARI ratings include the energy of the supply air fan. For all unitary and applied equipment where the fan energy is part of the equipment efficiency rating, the COP shall be adjusted as follows to remove the fan energy:

When COP₄₇ is given

(Equation 3.7.6-4)

$$COP_{nfheating}=1.48\times 10^{-7}\cdot COP_{47} \cdot Q +1.062 \cdot COP_{47}$$When HSPF is given

(Equation 3.7.6-5)

$$COP_{nfheating}=-0.0296\cdot HSPF^{2} + 0.7134 \cdot HSPF$$

where

COPnfheating	The heat pump efficiency adjusted to exclude the fan
COP₄₇	The COP of the heat pump determined at an outdoor temperature of 47 °F
Q	The ARI rated heating capacity in Btu/h
BHPsupply	The supply fan brake horsepower (bhp).

Units

Unitless

Input Restrictions

As designed

Baseline Rules

The heat pump efficiency for baseline systems 2 and 4 shall be determined from the data below.

Size (based on cooling capacity)	Outdoor Conditions	Efficiency	Test Procedure
<65,000 Btu/h	n.a.	6.6 HSPF	ARI 210/240
≥65,000 Btu/h and <135,000 Btu/h	47 °F db, 43 °F wb	3.2 COP	ARI 340/360
≥65,000 Btu/h and <135,000 Btu/h	17 °F db, 15 °F wb	2.2 COP
≥135,000 Btu/h	47 °F db, 43 °F wb	3.1 COP
≥135,000 Btu/h	17 °F db, 15 °F wb	2.0 COP

Electric Heat Pump Heating Capacity Adjustment Curve(s)

Applicability

All heat pumps

Definition

A curve or group of curves that represent the available heat-pump heating capacity as a function of evaporator and condenser conditions. The default curves are given as follows:

(Equation 3.7.6-6)

$$Q_{available} = CAP\_FT \times Q_{rated}$$

For air-cooled heat pumps:

(Equation 3.7.6-7)

$$CAP\_FT = a+b \cdot t_{odb} + c \cdot t_{odb}^{2} + c \cdot t_{odb}^{3}$$

For water-cooled heat pumps:

(Equation 3.7.6-8)

$$CAP\_FT = a + b \cdot t_{db} + d \cdot t_{wt}$$

where

Q_available	Available heating capacity at present evaporator and condenser conditions (kBtu/h)
t_db	The entering coil dry-bulb temperature (°F)
t_wt	The water supply temperature (°F)
t_odb	The outside-air dry-bulb temperature (°F)
Q_rated	Rated capacity at ARI conditions (in kBtu/h
a, b, c, d	Coefficients taken from Appendix H

Units

Data structure

Input Restrictions

User may input curves or use default curves. If defaults are overridden, supporting documentation shall be provided.

Baseline Rules

Use default curves.

Electric Heat Pump Heating Efficiency Adjustment Curve(s)

Applicability

All heat pumps

Definition

A curve or group of curves that varies the heat-pump heating efficiency as a function of evaporator conditions, condenser conditions and part-load ratio. The default curves are given as follows:

(Equation 3.7.6-9)

$$PLR - \frac{Q_{operating}}{Q_{available, (t_{db},t_{odb/wt})}}$$

(Equation 3.7.6-10)

$$EIR\_FPLR = a + b \cdot PLR + c \cdot PLR^{2}+ d \cdot PLR^{3}$$

Air Source Heat Pumps:

(Equation 3.7.6-11)

$$EIR\_FT = a + b \cdot t_{odb} + c \cdot t_{odb}^{2}+ d \cdot t_{odb}^{3}$$

Water Source Heat Pumps:

(Equation 3.7.6-12)

$$EIR\_FT = a + b \cdot t_{wt} + d \cdot t_{db}$$

(Equation 3.7.6-13)

$$P_{operating} = P_{rated} \times EIR\_FPLR \times EIR\_FT \times CAP\_FT$$

where

PLR	Part load ratio based on available capacity (not rated capacity)
EIR-FPLR	A multiplier on the EIR of the heat pump as a function of part load ratio
EIR-FT	A multiplier on the EIR of the heat pump as a function of the wet-bulb temperature entering the coil and the outdoor dry-bulb temperature
Q_operating	Present load on heat pump (Btu/h)
Q_available	Heat pump available capacity at present evaporator and condenser conditions (Btu/h).
t_db	The entering coil dry-bulb temperature (°F)
t_wt	The water supply temperature (°F)
t_odb	The outside air dry-bulb temperature (°F)
P_rated	Rated power draw at ARI conditions (kW)
P_operating	Power draw at specified operating conditions (kW)
a, b, c, d	Coefficients from Appendix H

Units

Data structure

Input Restrictions

User may input curves or use default curves. If defaults are overridden, documentation shall be provided.

Baseline Rules

Use default curves

Electric Heat Pump Supplemental Heating Capacity
Applicability	All heat pumps
Definition	The design heating capacity of a heat pump supplemental heating coil at ARI conditions
Units	Btu/h
Input Restrictions	As designed
Baseline Rules	Autosize

Electric Supplemental Heating Control Temp
Applicability	All heat pumps
Definition	The outside dry-bulb temperature below which the heat pump supplemental heating is allowed to operate
Units	Degrees Fahrenheit (°F)
Input Restrictions	As designed. Default to 40°F
Baseline Rules	The space is required to be controlled with multistage space thermostats and an outdoor air thermostat which would energize the auxiliary heat on the last thermostat stage and when the outdoor air temperature is less than 40°F

Coil Defrost
Applicability	Air-cooled electric heat pump
Definition	The defrost control mechanism for an air-cooled heat pump. The choices are: Hot-gas defrost, on-demand Hot-gas defrost, timed 3.5 minute cycle Electric resistance defrost, on-demand Electric resistance defrost, timed 3.5 minute cycle Defrost shall be enabled whenever the outside air dry-bulb temperature drops below 40°F.
Units	List (see above)
Input Restrictions	Default is to use hot-gas defrost, timed 3.5 minute cycle. User may select any of the above.
Baseline Rules	The baseline building uses the default.

Coil Defrost kW
Applicability	Heat pumps with electric resistance defrost
Definition	The capacity of the electric resistance defrost heater
Units	Kilowatts (kW)
Input Restrictions	As designed. This descriptor defaults to 0 if nothing is entered.
Baseline Rules	Not applicable.

Crank Case Heater kW
Applicability	All heat pumps
Definition	The capacity of the electric resistance heater in the crank case of a direct expansion (DX) compressor. The crank case heater operates only when the compressor is off.
Units	Kilowatts (kW)
Input Restrictions	As designed. This descriptor defaults to 0 if nothing is entered.
Baseline Rules	Zero (0)

Crank Case Heater Shutoff Temperature
Applicability	All heat pumps
Definition	The outdoor air dry-bulb temperature above which the crank case heater is not permitted to operate.
Units	Degrees Fahrenheit (°F)
Input Restrictions	As designed. This descriptor defaults to 50°F.
Baseline Rules	50°F

90.1-2019

*Electric Heat Pump Heating Capacity*
Applicability	All heat pumps
Definition	The full load heating capacity of the unit, excluding supplemental heating capacity at ARI rated conditions
Units	Btu/h
Input Restrictions	As designed
Baseline Building	Autosized, with a heating oversizing factor of 25%. Sizing calculations shall be based on the heating design day and cooling design day conditions, as defined in Section 3.2.3 of this document. Oversizing would be carried out at zone level where the sizing parameters would be applied to the zone design heating loads, but not the airflow rates. If the number of UMLH for the baseline exceeds 300, heating coil capacity may need to be increased along with system airflow as described in Section 2.7.2 of this document.

*Electric Heat Pump Supplemental Heating Source*
Applicability	All heat pumps
Definition	The auxiliary heating source for a heat pump heating system. The common control sequence is to operate the heat pump when the auxiliary heat is activated, until the low temperature limit, at which the compressor is turned off. Other building descriptors may be needed if this is not the case. Choices for supplemental heat include: Electric resistance Gas furnace Oil furnace Hot water Other
Units	List (see above)
Input Restrictions	As designed
Baseline Building	Electric resistance

Electric Heat Pump Heating Efficiency

Applicability

All heat pumps

Definition

The heating efficiency of a heat pump at ARI rated conditions as a dimensionless ratio of output over input. The abbreviation used for this full-load efficiency is COP_{nf heating}. Fan energy shall be modeled separately according to Section 3.7.3 of this document.

Units

Unitless

Input Restrictions

As designed

COP_{nf heating}=GrossHeatingBtuhTotalInputPowerW-W_fan×3.412 BtuhW

(34)

Where:

Q_{t,gross,rated} = The AHRI rated total cooling capacity of a packaged unit (Btu/h)

Baseline Building

For Baseline System 2:

The heat pump efficiency for baseline system 2 shall be determined using the heating mode COP from Table 64 (Standard 90.1-2019 Table G3.5.2 and G3.5.4). Use the total net heating capacity of the baseline design to determine the size. COP_nfcoolingand COP_nfheatingare the packaged HVAC equipment cooling and heating energy efficiency, respectively, to be used in the baseline building design, which excludes supply fan power. Fan energy shall be modeled separately according to Section 3.7.3 of this document.

Where multiple HVAC zones or residential spaces are combined into a single thermal block, the efficiencies for baseline HVAC Systems shall be based on the equipment capacity of the thermal block divided by the number of HVAC zones or residential spaces.

For Baseline System 4:

Equipment heating efficiencies for heat pumps shall be modeled in accordance to Table 66, which specify COP_nfheatingfor unitary heat pumps.

Electric Heat Pump Heating Capacity Adjustment Curve(s)

Applicability

All heat pumps

Definition

A curve or group of curves that represent the available heat-pump heating capacity as a function of evaporator and condenser conditions. The default curves are given as follows:

Q_available=CAP_FT×Q_rated

(35)

(applies to heat-pump heating efficiency only)

For air cooled heat pumps:

CAP_FT=a+b×t_odb+c×t_odb²+d×t_odb³

(36)

For water cooled heat pumps:

CAP_FT=a+b×t_db+d×t_wt

(37)

Where:

Qavailable = Available heating capacity at present evaporator and condenser conditions (kBtu/h)

tdb = The entering coil dry-bulb temperature (°F)

twt = The water supply temperature (°F)

todb = The outside-air dry-bulb temperature (°F)

Qrated = Rated capacity at ARI conditions (in kBtu/h)

Table 76. Heat Pump Capacity Adjustment Curves (CAP-FT)

Coefficient	Water-Source	Air-Source
a	0.4886534	0.2536714
b	-0.0067774	0.0104351
c	N/A	0.0001861
d	0.0140823	-0.0000015
Source: COMNET 2017

Units

Data structure

Input Restrictions

User may input curves or use default curves. If defaults are overridden, supporting documentation shall be provided.

Baseline Building

Use default curves

Electric Heat Pump Heating Efficiency Adjustment Curve(s)

Applicability

All heat pumps

Definition

A curve or group of curves that varies the heat-pump heating efficiency as a function of evaporator conditions, condenser conditions and part-load ratio. The default curves are given as follows:

PLR=Q_operatingQ_availablet_db,t_odb/wt

(38)

EIR_FPLR=a+b×PLR+c×PLR²+d×PLR³

(39)

Air source heat pumps:

EIR_FT=a+b×t_odb+c×t_odb²+d×t_odb³

(40)

Water source heat pumps:

P_operating=P_rated×EIR_FPLR×EIR_FT×CAP_FT

(41)

EIR_FT=a+b×t_odb+c×t_odb²+d×t_odb³

(42)

Source: PRM-RM 2010

Where:

PLR = Part load ratio based on available capacity (not rated capacity)

EIR-FPLR = A multiplier on the EIR of the heat pump as a function of part load ratio

EIR-FT = A multiplier on the EIR of the heat pump as a function of the wet-bulb temperature entering the coil and the outdoor dry-bulb temperature

Q_operating = Present load on heat pump (Btu/h)

Q_available = Heat pump available capacity at present evaporator and condenser conditions (Btu/h)

t_db = The entering coil dry-bulb temperature (°F)

t_wt = The water supply temperature (°F)

t_odb = The outside air dry-bulb temperature (°F)

P_rated = Rated power draw at ARI conditions (kW)

P_operating = Power draw at specified operating conditions (kW)

Table 77. Heat Pump Heating Efficiency Adjustment Curves

Coefficient	Air-and Water-Source EIR-FPLR	Water-Source EIR-FT	Air-Source EIR-FT
a	0.0856522	1.3876102	2.4600298
b	0.9388137	0.0060479	-0.0622539
c	-0.1834361	N/A	0.0008800
d	0.1589702	-0.0115852	-0.0000046
Rated T_odb			Max = 50°F, Min = -10 °F
Rated T_wt			NA
Source: CEC 2013

Units

None

Input Restrictions

User may input curves or use default curves. If defaults are overridden, documentation shall be provided.

Baseline Building

Use default curves

*Electric Heat Pump Supplemental Heating Capacity*
Applicability	All heat pumps
Definition	The design heating capacity of a heat pump supplemental heating coil at ARI conditions
Units	Btu/h
Input Restrictions	As designed
Baseline Building	Autosize

*Electric Supplemental Heating Control Temp*
Applicability	All heat pumps
Definition	The outside dry-bulb temperature below which the heat pump supplemental heating is allowed to operate
Units	Degrees Fahrenheit (°F)
Input Restrictions	As designed. Default to 40°F.
Baseline Building	The space is required to be controlled with multistage space thermostats and an outdoor air thermostat that would energize the auxiliary heat on the last thermostat stage and when the OAT is less than 40°F. The air-source heat pump shall be modeled to continue to operate when auxiliary heat is energized below 40°F.

*Heat Pump Compressor Minimum Operating Temp*
Applicability	All heat pumps
Definition	The outside dry-bulb temperature below which the heat pump compressor is disabled
Units	Degrees Fahrenheit (°F)
Input Restrictions	As designed. Default to 35°F.
Baseline Building	For baseline system 4, the compressor minimum operating temperature is 0°F, and for system 2 it is 35°F

*Coil Defrost*
Applicability	Air cooled electric heat pump
Definition	The defrost control mechanism for an air cooled heat pump. The choices are: Hot-gas defrost, on-demand Hot-gas defrost, timed 3.5 minute cycle Electric resistance defrost, on-demand Electric resistance defrost, timed 3.5 minute cycle Defrost shall be enabled whenever the outside air dry-bulb temperature drops below 40°F.
Units	List (see above)
Input Restrictions	Default to use hot-gas defrost, timed 3.5 minute cycle. User may select any of the above.
Baseline Building	The baseline building uses the default

*Coil Defrost kW*
Applicability	Heat pumps with electric resistance defrost
Definition	The capacity of the electric resistance defrost heater
Units	kilowatts (kW)
Input Restrictions	As designed. This descriptor defaults to 0 if nothing is entered.
Baseline Building	Not applicable

*Crank Case Heater kW*
Applicability	All heat pumps
Definition	The capacity of the electric resistance heater in the crank case of a DX compressor. The crank case heater operates only when the compressor is off.
Units	kilowatts (kW)
Input Restrictions	As designed. This descriptor defaults to 0 if nothing is entered.
Baseline Building	Zero (0)

*Crank Case Heater Shutoff Temperature*
Applicability	All heat pumps
Definition	The outdoor air dry-bulb temperature above which the crank case heater is not permitted to operate
Units	Degrees Fahrenheit (°F)
Input Restrictions	As designed. This descriptor defaults to 50°F.
Baseline Building	50°F