Heat Pump Upgrades
The following information is relevant for when a heat pump -related upgrade is defined in a project definition file.
Types of Backup
The ResStock workflow allows modeling heat pumps with either “integrated” or “separate” backup heating. Definitions for each are given below. See HPXML Heat Pumps for more information.
Integrated
The heat pump’s distribution system and blower fan power applies to the backup heating (e.g., built-in electric strip heat or an integrated backup furnace, i.e., a dual-fuel heat pump).
Separate
The backup system has its own distribution system (e.g., electric baseboard or a boiler).
Lockout Temperatures
The ResStock workflow allows for controlling the compressor and/or backup heating lockout temperatures. Definitions for each are given below. See the Backup section of the OpenStudio-HPXML documentation for more information.
For example, a heat pump upgrade option could be defined with a compressor lockout temperature of 5F and a backup heating lockout temperature of 40F (i.e., a 5F - 40F switchover band).
See the argument assignments below that would need to be added to the options_lookup.tsv file.
These values would override the OpenStudio-HPXML defaults.
heat_pump_compressor_lockout_temp=5
heat_pump_backup_heating_lockout_temp=40
Compressor
Minimum outdoor temperature for compressor operation.
Backup Heating
Maximum outdoor temperature for backup operation.
Replacement Scenarios
When defining a heat pump upgrade, the new heat pump can:
Replace the Existing Primary Heating/Cooling System
Remove any existing heating or cooling systems, and replace with a heat pump.
Heat pump backup type, fuel type, efficiency, capacity, etc. may be specified using heat pump arguments named with the heat_pump_backup prefix.
For example:
- upgrade_name: ASHP
options:
- option: HVAC Heating Efficiency|ASHP, SEER 22, 10 HSPF
apply_logic:
- HVAC Has Ducts|Yes
costs:
- value: 50.0
multiplier: Size, Heating System Primary (kBtu/h)
lifetime: 30
- option: HVAC Cooling Efficiency|Ducted Heat Pump
Retain the Existing Primary Heating System as Backup
Set heat_pump_backup_use_existing_system=true for HVAC Heating Efficiency options in the lookup to retain the primary (existing) heating system as backup to the heat pump.
In this case, all properties of the existing primary system are retained as properties of the heat pump backup heating system.
The following properties of the existing heating system are retained:
fuel type
efficiency
capacity
autosizing factor
For example:
- upgrade_name: ASHP
options:
- option: HVAC Heating Efficiency|ASHP, SEER 22, 10 HSPF
apply_logic:
- HVAC Has Ducts|Yes
costs:
- value: 50.0
multiplier: Size, Heating System Primary (kBtu/h)
lifetime: 30
- option: HVAC Cooling Efficiency|Ducted Heat Pump
- option: Heat Pump Backup|Use Existing System
For this scenario, the type of the backup is automatically determined based on information in the table below:
New Heat Pump
Backup System
Backup Type
Example
ducted
ducted
integrated
ASHP w/Furnace [1]
ducted
ductless
separate
ASHP w/Boiler
ductless
ducted
separate
Ductless MSHP w/Furnace
ductless
ductless
separate
Ductless MSHP w/Boiler
Other situations and considerations:
The existing primary system does not become backup to the heat pump when:
the primary system is a heat pump
the primary system is a shared system
When an existing secondary system exists:
it remains secondary if the heat pump upgrade is integrated backup
it is removed if the heat pump upgrade is separate backup
Detailed Performance Data
Use the HVAC Detailed Performance Data option from the lookup to add specific performance data coefficients for a variable-speed heat pump.
Detailed performance data (i.e., capacity and COP coefficients) for minimum and maximum compressor speeds can be defined for a set of 5 outdoor temperatures.
See below the argument assignments that would need to be added to the options_lookup.tsv file.
These values would override the OpenStudio-HPXML defaults.
See HPXML HVAC Detailed Perf. Data for more information.
hvac_perf_data_heating_outdoor_temperatures=67.0, 47.0, 17.0, 5.0, -15.0
hvac_perf_data_heating_min_speed_capacities=0.423, 0.308, 0.353, 0.371, 0.331
hvac_perf_data_heating_max_speed_capacities=1.387, 1.028, 0.766, 0.688, 0.507
hvac_perf_data_heating_min_speed_cops=5.150, 4.390, 2.550, 2.050, 1.660
hvac_perf_data_heating_max_speed_cops=3.684, 3.370, 2.330, 1.980, 1.490
hvac_perf_data_cooling_outdoor_temperatures=125.0, 95.0, 82.0, 70.0
hvac_perf_data_cooling_min_speed_capacities=0.267, 0.333, 0.368, 0.391
hvac_perf_data_cooling_max_speed_capacities=0.847, 1.017, 1.090, 1.148
hvac_perf_data_cooling_min_speed_cops=1.918, 3.870, 5.860, 7.126
hvac_perf_data_cooling_max_speed_cops=2.017, 3.220, 4.140, 4.900
This HVAC Detailed Performance Data option would be called in the yaml file along with an HVAC Heating Efficiency heat pump option.
For example, to include HVAC Detailed Performance Data with a cold climate heat pump, you could include the following:
- upgrade_name: Typical Cold Climate with Detailed Performance Data
options:
- option: HVAC Heating Efficiency|ASHP, SEER2 17.5, 8.5 HSPF2, Typical Cold Climate
apply_logic:
- HVAC Has Ducts|Yes
- not:
- or:
- HVAC Shared Efficiencies|Boiler Baseboards Heating Only, Electricity
- HVAC Shared Efficiencies|Boiler Baseboards Heating Only, Fuel
- HVAC Shared Efficiencies|Fan Coil Heating and Cooling, Electricity
- HVAC Shared Efficiencies|Fan Coil Heating and Cooling, Fuel
- HVAC Shared Efficiencies|Fan Coil Cooling Only
- option: HVAC Detailed Performance Data|Cold Climate Heat Pump Ducted
apply_logic:
- HVAC Has Ducts|Yes
- not:
- or:
- HVAC Shared Efficiencies|Boiler Baseboards Heating Only, Electricity
- HVAC Shared Efficiencies|Boiler Baseboards Heating Only, Fuel
- HVAC Shared Efficiencies|Fan Coil Heating and Cooling, Electricity
- HVAC Shared Efficiencies|Fan Coil Heating and Cooling, Fuel
- HVAC Shared Efficiencies|Fan Coil Cooling Only
Sizing Methodologies
When defining a heat pump upgrade, the new heat pump can be sized according to:
See HPXML HVAC Sizing Control for more information.
ResStock currently assumes the existing home’s duct system for HVAC equipment upgrades. Without restricting the upgraded equipment’s capacity to the maximum of the existing duct system’s heating/cooling airflow rate divided by 400 cfm/ton, fan pressure rise increases and results in higher power use. Based on the existing duct system, this duct restriction can be avoided by:
Limiting the upgraded equipment’s capacity
Adjusting the blower fan efficiency (W/cfm)
Set heat_pump_sizing_is_duct_limited=true for HVAC Heating Efficiency options in the lookup to use autosizing limits and maintain the existing duct system curve.
For example:
- upgrade_name: ASHP
options:
- option: HVAC Heating Efficiency|ASHP, SEER 16, 9.2 HSPF, Duct Limited
apply_logic:
- HVAC Has Ducts|Yes
costs:
- value: 50.0
multiplier: Size, Heating System Primary (kBtu/h)
lifetime: 30
- option: HVAC Cooling Efficiency|Ducted Heat Pump
ACCA Manual J/S
Autosized heat pumps have their nominal capacity sized per ACCA Manual J/S based on cooling design loads, with some oversizing allowances for larger heating design loads.
Set heat_pump_sizing_methodology=ACCA for HVAC Heating Efficiency options in the lookup to size based on ACCA Manual J/S.
HERS
Same as ACCA except autosized heat pumps have their nominal capacity sized equal to at least the larger of heating and sensible cooling design loads.
Set heat_pump_sizing_methodology=HERS (or heat_pump_sizing_methodology=auto) for HVAC Heating Efficiency options in the lookup to size based on HERS.
Max Load
Autosized heat pumps have their nominal capacity sized based on the larger of heating/cooling design loads, while taking into account the heat pump’s reduced capacity at the design temperature, such that no backup heating should be necessary.
Set heat_pump_sizing_methodology=MaxLoad for HVAC Heating Efficiency options in the lookup to size based on Max Load.