VisualDOE Tech Support

In a VAV system, the minimum flow ratio (ZONE|AIR) may be too low. If the thermostat type is "proportional" then the flow will be at its minimum point in heating mode. This minimum flow may not be high enough to meet the heating loads of the zone. The solution is to increase the minimum ratio (leading to higher energy usage as well) or to specify a "reverse action" thermostat. The reverse action thermostat increases the zone air flow as the heating load increases (just like it does in cooling mode).

Another problem, especially in cold climates and heating dominated buildings, is that the total supply air flow is too low. DOE2 sizes the total supply air based on peak cooling loads. The solution is to enter the supply air flow in the HVAC system editor rather than allowing DOE2 to autosize the flow.

Zone reheat may be missing. In a multiple-zone system some form of reheat is generally necessary to control space temperature in zones that do not need much cooling. Make sure that reheat (or baseboard heating) is specified on the ZONE|AIR tab.

Zone Heating and Cooling Loads
If you request Standard DOE-2 report LS-A in Run Setup then you can view the zone peak loads in the *.o* output file after simulating but these loads do not account for minimum outside air load and are based on a constant zone temperature equal to the Cooling Design Temperature (set in Schedule Maker for the selected Zone Occupancy). The LS-A report also lists the building coincident peak load but it also does not account for minimum outside air load and is based on constant zone temperature.

Zone Air Flows
If you let the program size the supply fan (HVAC Systems Editor | Supply Fan), then the VisualDOE Zones Summary Report will list the peak air flow (CFM) for each zone. These flows are based on (a) the zone loads, (b) the Heating and Cooling Design Temperatures (set in Schedule Maker for the selected Zone Occupancy), and © the Cooling Supply Air Temperature (selected in the HVAC System Editor). Special sizing cases include zones with exhaust fans and/or high ventilation rates. Exhaust flow or ventilation requirements will override the calculation air flow if they are greater than the flow needed to satisfy zone loads.

System Air Flows
The supply fan CFM listed in Systems Summary Report is the sum of the zone peak cooling air flows (non-coincident), and does not necessarily correspond to the CFM required to meet the building peak load (coincident) since all zones are unlikely to peak at the same time. (DOE 2.1E restricts the use of coincident fan flow sizing to buildings with a single VAV system). For heating-dominated buildings, the supply air flow may be undersized because DOE-2 sums the peak cooling flow for each zone. Furthermore system air flow is based on the indoor design temperature, not the thermostat schedules.

System Sizes
System-level heating and cooling equipment sizes are listed in the Systems Summary Report in kBtuh for most system types. Unlike zone loads, system sizes do consider outside air load and both heating and cooling indoor design temperatures, but they are based on the non-coincident peak load, i.e. the sum of the zone peaks and not on the building peak load (coincident peak load). These sizes also account for fan heat. System sizes are adjusted to ARI conditions (ARI conditions are different for air-cooled DX equipment than for other types of equipment.) System sizes also include any oversizing ratios you may have specified. If you have only one system in your building and the fan is VAV (minimum flow ratio < 1) then you can override the default and set SIZING-OPTION = COINCIDENT in the DOE-2 input file (see Editing DOE2.1E files outside of VisualDOE). This will give the coincident system size, including fan heat.

System Loads
System-level heating and cooling peak loads are reported in Standard DOE-2 reports SS-A and SS-J in kBtuh. Unlike zone peak loads, system loads account for minimum outside air, fan heat, and thermostat schedules. Furthermore, system loads represent the coincident peak load of the zones served by the system.

Plant Sizes
If the building has central plant equipment (chiller, tower, boiler, etc.) and you let the program size the equipment, then the Plant Summary Report will list the plant cooling and heating equipment sizes in tons and MBtuh. These sizes are based on coincident peak loads and account for outside air and thermostat schedules and fan heat.

Design Day Load Calculations
It is important to realize that the peak loads calculated by VisualDOE are based on the selected annual weather file and not on ASHRAE Design Day climate data. The difference is not immediately obvious. The weather file may have extremes that are not typical while a design day calculation assumes design day conditions are constant for several days. DOE-2 does have the option of computing peak loads using a Design-Day Routine (see Editing DOE2.1E files outside of VisualDOE).

The following schedules from the System Occupancy take precedence over the Zone Occupancy: Fans, Outside Air. The Fans schedule also controls any pumps, chillers, and other mechanical equipment.

If multiple systems share a central plant then the central plant is controlled by the Fans schedule of the first system in the the list shown at the system tab. Therefore, the system with the longest operating hours should be first in the list.

The Lighting Power Density, Equipment Power Density, and Occupant Density specified under the zone tab takes precedence over the values in the Zone Occupancy and the System Occupancy.

1. Create ASCII file with full year of weather data (*.ft)
2. Run PKAFT batch file to create compressed weather file (*.bin)

STEPS TO PACK WEATHER DATA

1. Place all the files uncompressed from PACKWTHR.EXE into the directory where the weather files are located (*.BIN).
2. View the format of the *.ft file listed below. Create an ASCII(text) file with the same format using your weather data. Note that the format must be exactly the same – same number of spaces, 8760 hours…
3. Give the ASCII file the extension ".FT" and place it in the same directory with the uncompressed files.
4. Run the batch file PKAFT.BAT "pack an FT file". It packs an FT file into a BIN file. At the DOS prompt type> "PKAFT weatherfilename S" if your data includes solar info and "PKAFT weatherfilename NS" if it does not include solar.

STEPS TO UNPACK WEATHER FILES
MKAFT.BAT "make an FT file", the FT file is the ascii listing of weather variables. This batch file unpacks the BIN file. The file FT-FORMT.TXT contains a listing of variable names.

-- at DOS prompt type> MKAFT weatherfilename(without BIN extension)

TO CREATE WEATHER STATISTICS REPORT
STAT.BAT "create statistics report for BIN file". The report file gets an .STA extension.

-- at DOS prompt type> STAT weatherfilename

UNPACKED WEATHER FILE (*.FT) FORMAT
See the instructions in the download file for a detailed description of the ASCII weather file format.

For any design alternative, select "Use existing DOE2 input file" in the Alternative|Define Alternatives dialog box. Usually it makes sense to do this after running a similation at least once, so that VisualDOE has created an input file. When this button is selected, VisualDOE will not overwrite the existing name.i# file (DOE2 input file that VisualDOE creates) and manual text edits of the name.i# will be run instead. Once the simulation is complete, the output file (name.o#) results are extracted as usual.

To edit the file, choose the "View DOE2 files" command under the simulation menu. Alternatively, use the text editor of your choice and open the input file outside of VisualDOE.

Once a simulation has been done with "Use existing DOE2 input file" activated, any future changes for that alternative should not be done in VisualDOE. If the "Use existing DOE2 input file" button is deselected and another simulation is run, the manually entered keywords will be overwritten with the current VisualDOE settings. It is recommended that DOE2 documentation be consulted before doing direct DOE2 edits.