FEDS was originally designed to model buildings with single, homogeneous heating and cooling technologies within each individual building. The portion of building set served inputs are available to specify whole buildings within a building set that are served by a given technology. If your building has more than one type of heating or cooling technology, there are a couple of options. If the majority of service is provided by one system, users might simply model that one as if it were the only system serving the building. If the occupants use portable space heaters, users could account for the energy consumed and heating service provided by representing them as a miscellaneous equipment record. If, on the other hand, one system does not dominate, it would be best to model the building as a pair of linked buildings, with one technology serving each portion. Or, if a major renovation is being contemplated, users might wish to model the building once as if it were served by one technology, and once by the other. Optimizing each case separately, the results will provide insights into which system type would be best for the building.
An option is also available that makes it possible to model multiple heating or cooling technologies serving the same building(s). To enable this feature, select the percentage of each building served option from the heating or cooling end use inputs. When this option is specified, FEDS will model the defined HVAC technologies as serving the specified portion served of each building in the building set.
If the boiler serves only one building, select single building boiler as the equipment type and the fuel type that fires the boiler (natural gas, distillate oil, etc.). If the steam is piped in from a central boiler plant or purchased from offsite, select central steam as the fuel type and specify the equipment type as either a radiator, fan coil, or air handling unit using central steam or hot water (in-building equipment is a heat exchanger). For steam purchased from an off-site supplier, input a price for purchased central steam in the non-electric energy price inputs. For self-generated steam, create a central plant record, associated conversion equipment, and thermal loops within the central plant and thermal loops inputs.
inking two distinct building sets together allows greater flexibility in modeling complex building geometries or uses. Linked buildings are designed to model two buildings that share a common wall or are stacked on top of one another. Specifying that the buildings are linked directs FEDS to automatically (based on the geometry information for each building) determine the wall area (or roof/ceiling area) that is shared, and thus not exposed to exterior conditions. It essentially calculates the portion of each buildings shell that is an adiabatic surface (i.e., does not experience conductive heat transfer) and does not receive solar gains. It uses this information in load calculations to appropriately account for the impact of the buildings being connected. There are some rules, however, that must be satisfied in order to link building sets. First, both sets must contain the same number of buildings so that a direct one-to-one linking is achieved. Second, both sets must have the solar normalization turned off (calculate solar gains by facing direction). Also, FEDS currently does not model cantilevered buildings so for top/bottom linking, the N/S and E/W lengths of the top building must not be greater than the corresponding lengths of the bottom building.
The number of heat or cooling equipment should be specified. For boilers or chillers, enter the number of boilers or chillers and not the number of air handling units or fan coil units. Similarly, for furnaces, packaged cooling units, and heat pumps specify the number of those devices. If the building is served by a fuel generated at a central plant (not within the building), specify the number of heat exchangers that transfers heat from the central distribution loop to the building loop.
This is an error message that comes up when there are incompatible heating or cooling systems defined in a building set. This can occur when one building is modeled with linked heating (a heating system requiring a fan to deliver the heat; e.g., furnace, fan coil, or AHU) with one served by an unlinked heating system (no fan required ; e.g., radiator or baseboard system) together in the same building set. The situation can be remedied by separating the buildings into distinct building sets. For more information about allowable HVAC combinations, refer to Appendix I of the FEDS User's Guide.
Prototype buildings in FEDS are modeled as basic rectangular blocks, with the actual geometry calculated based on the total floor area, number of floors, floor-to-floor height, and aspect ratio. However, additional geometries can be modeled by using the linked building approach or through the advanced geometry inputs, which allow modification to underlying parameters including window/wall/roof/floor areas and conditioned air volumes.
A dual-fuel heat pump is an electric air-source heat pump that uses another fuel source (natural gas, LPG, oil) for the auxiliary or backup heat (instead of electric resistance coils). It can be accomplished within a single integrated unit or pieced together by mating a standard air source heat pump with a furnace via a controller. The controller determines which unit to operate based on outside temperature, relative efficiencies, and cost of each fuel.
The heat/cool pair is a concept added to FEDS with the advent of considering heat pumps as replacements to conventional heating and cooling technologies. A heat/cool pair identifies to the model which heating and cooling technologies jointly serve a particular building or group of buildings in the building set and may be considered for joint replacement by a heat pump technology. In order to consider heat pumps or any other integrated heating and cooling technology as replacements for existing heating and cooling technologies, the heat/cool pairs must be defined. Baseline heat pump records are automatically paired (as long as their fuel type, equipment type, number of units, and vintages match) upon updating inferences, while all non-heat pump technologies must be paired manually.
The advanced geometry inputs allow for more flexibility in modeling non-standard building geometries compared to the linked building approach. When accessing the advanced geometry inputs, the user may specify or alter a number of geometric parameters for each zone of the building to customize the resulting model. For example, the exterior wall areas and window areas can be specified for the north, east, south, and west sides of each zone. Additionally, roof, floor, footprint areas, exterior perimeter length, and conditioned air volume can be specified for each zone. These adjustments provide users with the ability to model a number of more complex geometries, such as individual parts of a strip mall complex or varying window fractions for different sides of a building, with greater accuracy than through other means. The option can be accessed via the button on the regular geometry inputs screen.
A linked heating and cooling system is when the heating and/or cooling coil is integrated with the ventilation system, employing air as the distribution fluid (air handler, fan coil, packaged unit, furnace, etc.).
An unlinked heating and cooling system is when the ventilation system (if present) is separate from the heating coil, and heat is provided without requiring fan-powered air delivery. Unlinked heating technologies include radiators, baseboard electric, or infrared heaters.
Currently, FEDS assumes that all cooling is linked, with the exception of evaporative coolers, which are assigned a separate, special ventilation scenario. For more detailed information see Section 4.4.2 of the FEDS User's Guide.
Solar normalization is used when the orientation of a single building is unknown, does not align with N/S/E/W directions, or when there are multiple buildings of differing orientations in a building set. It can be used to avoid biasing the solar gains calculation by normalizing the exterior wall, window, and roof areas, such that the resultant loads are roughly the average of two buildings: one with an east/west orientation and one with a north/south orientation. FEDS can be set to "ignore facing directions" to use solar normalization.
The aspect ratio is used to define the geometric orientation of the buildings in a building set. It is a ratio of length to width and is calculated by dividing the typical north-facing length by the typical east-facing length.
The crossover temperature is the outdoor air temperature at which a dual-fuel heat pump switches operation from the heat pump to the backup technology. This is typically the control methodology for these systems and can be entered or determined by FEDS. FEDS will calculate the optimal crossover temperature based on electric and backup fuel prices, heat pump performance and capacity vs. temperature, and furnace efficiency.
A separate heat/cool pair is a pair of heating and cooling technologies that are completely separate units, yet serve the same area/building (e.g., a furnace and separate package unit, or a boiler and a chiller). An integrated heat/cool pair is one in which the heating and cooling sources are packaged together in the same unit (e.g., a packaged cooling unit with integral gas burner or 'gas-pack' system). Identifying a heat/cool pair as integrated tells FEDS that individual heating and cooling replacements cannot be considered as a direct replacement.
Similar to the separate vs. integrated discussion for heat/cool pairs, a separate backup source for a dual-fuel heat pump indicates there is a separate furnace that is connected to the heat pump via a controller. A dual-fuel heat pump with integrated backup is a heat pump unit with a built-in gas or LPG auxiliary heat source. For modeling purposes, the only real difference is that FEDS will consider replacing the individual components (heat pump or backup furnace) of a separate dual-fuel heat pump in addition to replacing the entire system.
Thermostat dead band for heating represents the range of temperatures below the set point at which the thermostat does not call for heat. For example, at a 70-degree set point and a 2-degree dead band, the temperature will drop to 68 degrees before heating is activated, raising the temperature back to 70. It may also be referred to as the throttling range or differential. It operates similarly for cooling, allowing the temperature to rise a number of degrees equal to the dead band before cooling is activated.