Before modeling any type of HVAC equipment that uses a heat pump, read through the General Settings for Heat Pumps section as well as the Modeling Instructions for Specific Types of Heat Pump Types section below.

General Settings for Heat Pumps

This section gives information on some of the settings that should be used to model heat pumps, and detailed information on what inputs to enter into TREAT.

Account for Climate Impacts on HSPF and SEER

Heating Season Performance Factors (HSPF) and Seasonal Energy Efficiency Rating (SEER) are seasonal efficiencies often used to predict performance over an entire year. The actual performance of the equipment depends on a number of factors including the climate in which the equipment is being operated.

When modeling heat pumps it is recommended that the user checks Account for Climate Impact on HSPF and SEER. This option is on the Weather/Defaults page within the Advanced section.  This option enables the TREAT climate efficiency degradation algorithms in order to account for the effects of the local climate on the rated efficiency.  Please see the “General Information Screens” page in the manual for more details.

Input vs. Output Capacity

Heat pump specifications typically list the rated output capacity for theheating and/or cooling, however, TREAT requires input capacity.

Please note that while Account for Climate Impact on HSPF and SEER algorithm accounts for the local climate impact on the rated efficiency, it is up to the modeler to enter the SEER or HSPF that accurately describes the efficiency at which the equipment operates.  Due to age, improper charge, improper sizing, and operating conditions, this may be different from the rated efficiency of the unit.  Additionally, the efficiency should be bounded by the calibration process and/or real world performance constraints such as those listed in the ANSI/BPI-2400 standard.

To convert rated output capacity to input capacity:

  1. Convert the HSPF to a COP:    COP = HSPF / 3.412
  2. Divide the output capacity by the COP.    Input Capacity = Rated Output Capacity / COP
  3. Enter the calculated input capacity and HSPF into TREAT.

Example:  

To enter a 30,000 Btu/hr (2.5 ton), 10 HSPF ASHP system into TREAT:

  1. COP = 10 / 3.412 = 2.93
  2. Input Capacity = 30,000 / 2.93 = 10,239 Btu/hr.
  3. Select Air Source Heat Pump and enter the capacity as a 10,239 Btu/hr with a 10 HSPF
  4. Double check your calculations by viewing the Design Heating and Cooling Loads Report after running the model with these inputs. The Available Heating Equipment Output Capacity should match the output capacity with which the calculations for this step began.

Converting EER to SEER

Seasonal Energy Efficiency Rating (SEER) efficiency values must be entered for all cooling systems in TREAT.  Packaged AC units are typically rated in Energy Efficiency Rating (EER).

To convert EER to SEER:

SEER = EER / 0.875

Modeling Instructions for Specific Heat Pump Types

This section gives specific instructions for modeling each of the most common types of heat pumps. Please follow the instructions in the General Settings for Heat Pumps section to understand how to convert output to input capacity, SEER versus EER, and rated efficiency versus in service efficiency.  If the equipment being modeled has a backup heating system, also follow instructions in the Modeling Backup Heat section.

Ductless / Mini-Split Heat Pump (MSHP)

  1. Create an air conditioning system by checking the appropriate box on the Heating/Cooling screen.
  2. Select:  Room Air Conditioner
  3. Enter the cooling output capacity in Btu/hr and efficiency in SEER.
  4. If the equipment provides heating, create a primary or secondary heating plant, as appropriate and select type Air Source Heat Pump.
  5. Enter the calculated heating input capacity in Btu/hr and efficiency in HSPF.
  6. Click the Edit Primary/Secondary Distribution System button and change the Duct Test Leakage and Total Duct Surface Area to zero in order to account for no ductwork.
  7. If applicable, set the secondary system control to Fixed Percentage of Monthly Energy Use.  Use the best information available to estimate the percentage contribution of the secondary system.
  8. For multiple mini-splits, please see the section on Combining Multiple HVAC Equipment for Entry into TREAT

Central Air Source Heat Pump (ASHP)

  1. Create an air conditioning system by checking the appropriate box on the Heating/Cooling screen.
  2. Select:  Central Air Conditioner
  3. Enter the cooling output capacity in Btu/hr and efficiency in SEER.
  4. Create a primary or secondary heat plant, as appropriate for the building systems, and select type Air Source Heat Pump.
  5. Enter the calculated heating input capacity in Btu/hr and efficiency in HSPF.
  6. Click the Edit Primary/Secondary Distribution System button and change the settings to ensure that the entries match audit and duct testing results.  Check the Shared with Cooling box to apply these setting to the cooling system.
  7. If the ASHP is the primary heating system and includes a backup heating system (integrated or separate), model the backup as the secondary heating system.

Ground Source Heat Pump (Geothermal)

  1. Create an air conditioning system by checking the appropriate box on the Heating/Cooling screen.
  2. Select:  Central Air Conditioner
  3. Enter the cooling output capacity in Btu/hr and efficiency in SEER.
  4. Create a primary or secondary heat plant, as appropriate for the building systems, and select type Ground Source Heat Pump.
  5. Enter the calculated heating input capacity in Btu/hr and efficiency in HSPF.
  6. Click the Edit Primary/Secondary Distribution System button and change the settings to ensure that the entries match audit and duct testing results.  Check the Shared with Cooling box to apply these setting to the cooling system.

Ground Water Source Heat Pump (Open Loop System)

Follow the instructions for the Ground Source Heat Pump, except select Groundwater Source Heat Pump as the equipment type.

Window / Sleeve AC Unit or Heat Pump

  1. Create an air conditioning system by checking the appropriate box on the Heating/Cooling screen
  2. Select: Room Air Conditioner
  3. Enter the cooling output capacity in Btu/hr and efficiency in SEER.
  4. If the equipment provides heating, create a primary or secondary heat plant, as appropriate and select type Air Source Heat Pump.
  5. Enter the calculated heating input capacity in Btu/hr and efficiency in HSPF.
  6. Click the Edit Primary/Secondary Distribution System button and change the Duct Test Leakage and Total Duct Surface Area to zero in order to account for no ductwork.

Modeling Back-Up Heating System

This section describes how to model the back-up heating system(s) that operate with the heat pump system.

Secondary System Control

As a starting point, set this to Operate when primary capacity insufficient.  During calibration process,  if the primary and secondary heating energy consumption between the Building Model and Billing Data are not in agreement in the feedback pane, change the secondary system control to Fixed Percentage of Monthly Energy Use.  Then iteratively change this percentage and re-calculate results to calibrate the model within your desired bounds.

If the secondary heating system runs in tandem with the primary, set the Secondary System Control to Fixed Percentage of Monthly Energy Use and use the best available information to estimate the percentage of monthly heating energy consumption.

The capacity and efficiency of the backup systems should be entered based on the real specifications of the equipment. The following are some of the most common backup heating system and how they should be entered.

Ducted Gas Backup: Create a natural gas furnace as a secondary heating system.

Ducted Electric Backup: Create an electric furnace and enter an efficiency of 100% as the secondary heating system.

Electric Baseboard Backup: Create an electric baseboard backup heating system.

Ductless Mini-Splits: A mini-split that provides heating and cooling in addition to the primary heating plant should be modeled both as a secondary ASHP heating and cooling system.

Combining Multiple Pieces of HVAC Equipment for Entry into TREAT

Modeling buildings with multiple distributed HVAC systems, such as multifamily buildings with in-unit heating systems, requires that multiple pieces of equipment must be combined for entry into TREAT.

  1. Sum the capacities of the equipment.
  2. Create an average of the efficiencies. If possible, create a weighted average of the efficiencies using the best data available. Assuming the runtimes of all equipment are similar, use the output capacity for determining this weighted average.

Example:

There are three room air conditioners in the building with output capacities of Q1, Q2 and Q3 Btu/hr and efficiencies of E1, E2 and E3.

The total output capacity Q = Q1+Q2+Q3

The capacity-weighted efficiency E = (Q1xE1 + Q2xE2 + Q3xE3) / (Q1+Q2+Q3)