TREAT allows specifying main (primary) and back-up (secondary) heating systems. The same thermostat controls both systems. The secondary system is turned on when the primary system capacity is not sufficient to satisfy the building load.

Primary Heating System:

Click the check box to indicate that there is a primary heating system in the building. This is a required condition for running energy calculations.

  • Modeling Hydronic Systems In some cases TREAT may set hydronic piping area defaults to the calculated area for duct-work. This leads to unrealistic distribution losses which causes the heating plant not to be able to satisfy the heating load or thermostat setting. If you view the Base Building model in the Model Inspector you will see the warning about he hearing system not being able to meet the heating load. To correct this issue simply uncheck and recheck the primary heating system box. This will force TREAT to recalculate the correct default piping area which should be 19.6 Sq.Ft. of supply and return piping per 1q. Ft. of conditioned space.
  • Heating Type: This selection tailors the additional inputs needed to the selected type. Set type to Other if your system is not listed in the drop-down box; for example, if it is a wood stove.
  • Fuel: Selected from the list of fuels entered on Fuels/Rates screen.
  • Input Capacity: The Input Capacity must be entered in Btu/hour.


  • Year: This input is for your records only
  • Annual Efficiency: Often referred to as Annual Fuel Utilization Efficiency (AFUE); the annual efficiency represents heating equipment performance over an entire heating season. It includes performance during start-up, steady state, and cool-down operations. The AFUE is calculated from performance parameters that are measured experimentally using U.S. Department of Energy (DOE) test procedure. This test includes combustion efficiency, jacket loss, and off-cycle flue loss. Credit is given for design features such as flue dampers. AFUE does not account for electricity consumption and therefore does not include the circulating air (or water pump for boilers) and combustion fan power consumption.
  • Annual Efficiency of heating system may deteriorate overtime. Use the Calculate Efficiency button open the Heating System Efficiency screen and calculate the annual efficiency of the heat plant.
  • Steady-state efficiency is the maximum efficiency achieved after a heating system has been running long enough to reach its peak operating temperature. It is equal to the ratio of the heat actually available to the distribution system to the amount of heat potentially available in the fuel. Since Steady-State Efficiency takes into consideration the jacket losses, it is lower than the Combustion Efficiency, but higher than AFUE which accounts for start-up and cool-down losses.

Detailed Input: A detailed input may be designated to give more specificity to your model.

  • Listed Annual and Steady-state Efficiencies:

These two values may be obtained from equipment nameplate or manufacturer directory.

  • Measured Steady-state efficiency:

In order to find this number for furnaces one has to measure airflow and temperature rise, and for boilers one has to measure water flow and temperature rise, and for both furnaces and boilers one has to measure gas/oil input at the fuel meter. This measurement and the associated calculations are clearly more complicated than a combustion efficiency test. Actual and listed combustion efficiencies may be entered instead of steady state efficiencies. If, and only if, jacket losses are known or assumed to be very small, which is a reasonable assumption primarily for new furnaces and low-mass boilers, steady state efficiency is close to the combustion efficiency.

Combustion Efficiency is a measurement of efficiency based on the percentage of heat lost up the flue while operating at a steady state condition. Combustion efficiency is determined indirectly, based on measuring flue gas parameters such as temperature and percent carbon dioxide or oxygen. TREAT relies on the assumption that deterioration in AFUE is proportional to the deterioration in combustion (or steady-state) efficiency. Then, the listed AFUE and listed combustion (or steady state) efficiency and measured actual combustion (or steady state) efficiency may be used to calculate actual AFUE as follows:

AFUE actual = AFUE listed (CombustionEfficiency actual / CombustionEfficiency listed 

A rule of thumb may be used as a last resort when the listed efficiencies are not available. In such cases you may estimate AFUE by multiplying the measured combustion efficiency by 0.85. For example, if the measured combustion efficiency is 75%, the AFUE is around 75% x 0.85 = 64%.

  • Enter Supply and Return Temperatures and other parameters specific to your heating system.
  • Year and Location of heating system is recorded for record keeping purposes only.

Secondary Heating System:

Click the check box to indicate that there is a secondary heating system in the building. This will enable the secondary system inputs, which are similar to primary system inputs.

NoteHeat plant output capacity may affect heating consumption of the building due to the following:

  1. If there is no secondary heat plant in the project or if there is a secondary heat plant and secondary system control is set to operate the secondary system when primary system capacity is insufficient then model heating consumption displayed on the Feedback Panel and in reports is limited by the capacity of the heating system. In this case if the system output capacity is insufficient to satisfy building heating load, the displayed heating consumption will be lower because on cold days the temperature in the building will be less than the specified thermostat set point. Check the load sizing report to make sure that the building heating system is not undersized.
  2. Low heat plant capacity leads to longer heat plant run time, which may increase distribution loss and hence overall heating consumption.
  3. The current version of TREAT does not model the heat plant standby loss. Consequently, oversizing the heating system is not penalized.

Secondary System Control:

These inputs allow for describing how the heating load is allocated between primary and secondary system. Two control modes are supported:

  • Operate when primary capacity is insufficient: This mode allocates the energy between primary and secondary systems based on primary system output capacity. In this mode secondary system operates only when primary system capacity is insufficient to satisfy building heating load. The percentage of heating energy generated by secondary heating system is different for every month.
  • Fixed percentage of monthly energy usage: This mode allocates the energy between primary and secondary system for every month based on fixed percentage entered by user. In this mode energy consumption of primary and secondary heat plants is not limited by system capacities.

Note:  When secondary system control mode is changed, the energy consumption is not just shifted from one system to another; the total monthly energy will likely also be different due to different heat plant efficiencies and different calculated distribution efficiencies of primary and secondary systems. For example if overall primary system efficiency is 80% and overall secondary system efficiency is 70%, then the total building energy consumption increases as entered percentage of monthly energy usage by secondary system goes up. This is expected because larger a fraction of the heating load is now satisfied by less efficient system.

Air Conditioning:

Click the check box to indicate that there is a cooling system in the building.

Total Output Capacity, Btu/Hr:

Specify overall output capacity of the cooling system. If there are multiple room air conditioners in the building, add up their capacity and enter the total in the input field.


This measure represents the efficiency of the cooling system. Enter capacity-weighted average efficiency if you have multiple room air conditioners.


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 EER. Then the total output capacity Q=Q1+Q2+Q3. Capacity-weighted efficiency E= (Q1ÃE1+Q2ÃE2+Q3ÃE3)/ (Q1+Q2+Q3).


May be set to Central or Room Air Conditioner. The selection does not affect the calculation results.

Number of Units:

This input does not affect calculations. It is used only for the record keeping and reports.

Note: Cooling energy usage and load will not be calculated if you entered a cooling system but have not specified any spaces as cooled on Thermostats screen.

Use the Heating and Cooling libraries to obtain information on typical systems.

The Edit Primary Distribution System button allows the user to customize the distribution system description.

  • Shared with Cooling:This box should be checked for a forced air distribution system that is used for both heating and cooling. The checkbox is enabled only if a cooling system has been entered.
  • Estimated Total Distribution Efficiency:This value is used as a starting point for heating energy use calculations. This value is recalculated by the program depending on distribution location (heated/unheated areas), insulation, leakage, etc. The calculated value is displayed in the Design Heating and Cooling Load report.
  • Insulation R-Value:This is the R-value of pipe/duct insulation not including air film. Since distribution losses to conditioned space are usually minimal, we recommend entering average R-Value of distribution located in un-conditioned spaces.
  • Total Area of Piping/Ductwork:Represents the overall surface area of the piping/ductwork. The default is provided based on the total conditioned area in the building. Defaults should be overwritten with detailed site measurements, if available. Diameter and perimeter of the outer surface should be used. Surface area of circular pipe/duct can be calculated as the sum of 3.14 (Diameter)(Length) for all pipe/duct segments. Surface area of rectangular duct can be calculated as the sum of Cross Section Perimeter(Length) for all duct segments.
  • % Of Piping/Ductwork Running through Each Space:This selection allows allocating the distribution system to spaces entered on the Spaces screen. By default the allocation is done in proportion to floor area of each space at the time the distribution system was first entered. The distribution system allocation may be changed, or it may be reset to default by clicking the following selection
  • Reset % Piping/Ductwork to Default:The sum of values in each column should always be equal to 100%. The input is used along with distribution leakage and/or insulation to calculate distribution loss and allocate it to spaces through which distribution system is running.

Note: Specify heat pumps in both Primary Heating System and Air Conditioning sections. Entering a heat pump in the heating section only will not automatically account for cooling energy.