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Heavent Software Features

 Steven E. Guffey, PhD, CIH
(304) 685-1298

1403 Far Meadows
Morgantown, WV 26508
prefer email or text message to voice mail


Overview of Program Features

  • Designs new ventilation systems for contaminant control
  • Simplifies design process by minimizing user inputs and doing virtually all computations
  • Enables users to operate at an expert level even with a beginner's expertise
  • Modifies working ventilation systems to meet new requirements
  • Employs a markedly easier and more accurate way to balance with dampers
  • With included additional software, makes measuring pressures and flows much easier and employs a troubleshooting method that is markedly more accurate and reliable.
  • Makes creating professional reports much easier.
  • Much, much easier, faster and more reliable and accurate than spreadsheets and manual calculations


Aspect of design

Features for new design

Non-modal operation of software
  • Program re-computes every calculation after every user input, including re-establishing how the system is laid out. As a result, you can change any input at any time in the process. You never have to go back and start over.
  • You can add a branch at any time and remove any branch at any time.
  • You can add an air-cleaning device at any time and remove one at any time.
  • You can change from U.S. to metric units or back again at any time in a matter of seconds. The program displays all inputs and results in whatever unit system you are in currently.
  • Program assumes an editable default value which remains until the user inputs a different value.
  • All inputs are check for physical impossibility and for likelihood. The former are rejected and the latter merit a warning (inputed value appears in red). Some checks are based on complex assessment of the affected portion of the system.

Schematic/ Duct layout
  • Layout of the system can be done quickly and easily on a simple schematic screen.
  • Can create a system of any complexity.
  • Detemines for itself whether a duct is a branch, submain, or main and whether it is part of a subsystem.
  • Simple to change layout: just change the schematic
    Layout can be changed at any time without re-inputting other information (program automatically re-computes based on the new layout).
  • Allows two laterals on a junction fitting (a real boon to rational layout).
  • Can have no more than about 480 branches in the system
  • The program creates a schematic view, not a scale drawing
  • Schematic cannot be printed as a graphics image

Hood airflow
  • Computes suggested airflows for common simple hood designs (e.g., table-top capturing hoods and enclosing hoods) using the author's best guess of the effects of toxicity, exposure conditions, cross-draft velocity, and distance of the source from the hood and from the person protected by the hood (see Estimate Hood Airflows ).
  • User also can simply input the desired hood airflow.
  • Many of the airflow recommendations are based on "educated guesses" since published research offers little guidance

Duct sizing
  • Allows use of round, rectangular, or flat oval ducts
  • Selects intial round duct sizes from user-editable files of available duct sizes
  • Allows user to over-ride program choices
  • Can display duct dimensions on schematic
  • Makes optimal re-sizing relatively easy.
  • Automatic sizing for round ducts, only

Static pressure calculations
  • Extensive user-editable defaults drastically reduce number of required inputs
  • If you are experienced, you may need little help in inputing coefficients. If so, you should use the Quick Inputs screen. Help screens show tables for each type of input.
  • If you are not that experienced, you may want to start with the screens created for each type of input, which has the help right in front of you for each one. See Airflow and Pyschrometrics, Duct Dimensions, Hood loss coefficients, Elbows, Fricton Loss, Lateral Entry Angle, Contractions and Expansions.
  • Loss coefficients choices shown when and where needed. Progam computes many for itself based on simple user inputs.
  • Entire run of ducts described at once (e.g., number of 90 degree elbows in this branch), which is much faster than describing each and every component individually as some other programs require
  • Pressures and flows re-computed instantly with every input you make, giving helpful feedback on effects of design choices
  • Makes psychrometrics easy. Requires only that you input the dry bulb temperature and one other parameter (e.g., relative humidity). Elevation effects done automatically (table of elevations for most U.S. cities included)
  • All computations done the most robust way possible -- not by using the approximations necessary for hand calculations.
  • The pressures computed are what you should expect to measure in the system. Oddly enough, the values computed in manuals, text books, spreadsheets and other programs are not. They fail to correct for the complex interactions between branch pressure requirements and are computed for locations that are inimical to accurate measurements.

  • The schematic can display several parameters that are helpful in optimizing system design. The most important are Percentage of Ideal Airflow and Percentage of Target Velocity. The values appear as a label on each duct on the schematic, giving a easily understood view of which ducts should be re-sized or otherwise modified.
  • Just as importantly, it is so easy to make changes to the system that you can quickly try a change, then look at the flow at the fan and see if your change was a good idea. If not, change it back. It takes only a few key strokes or mouse clicks, so just try different combinations until you are completely satisfied you have done the best that can be done (i.e., ideal airflow at the fan approaches 100%).
  • In that regard, note that the author's damper balancing procedure produces the least possible energy costs and involves the least possible time to adjust dampers (second rounds rarely necessary)
Balancing with dampers
  • Method developed by author (see Resume) is extremely robust and typically is much easier, faster and more accurate than other methods.
  • Should require only one round of damper adjustments
  • Works even when the fan is set to the wrong speed. Even tells you how much the fan speed should be changed.

Fan calculations

  • Allows booster fans on branch ducts.
  • Computes FanSP and FanTP properly corrected for air density.
  • Simplifies computation of effects of poor inlet and outlet condition on fan performance.
  • Table and printout provide the pressure and airflow information you need to provide to a fan vendor.
  • One main fan allowed per system (but does allow "booster fans" on ducts)
Air cleaning devices
  • Computes pressures and flow for nearly all types of air-cleaning devices.
  • Pressure calculations require a known pressure for a known airflow, values the vendor should be able to provide
  • Can have no more than 10 air-cleaning devices on the same systems

List of materials
  • Lists numbers of 15, 30, 45, 60, and 90 degree elbows for each combination of duct diameter, duct roughness, and duct gauge.
    lists total length of straight duct for each combination of diameter, duct roughness, and duct gauge


Evaluating Newly Installed Systems


Note that these two procedures do not require that the fan be running at the correct speed and both have little difficulty in separating out multiple problems in the system.

  1. Program predicts ratio of hood static pressure (SPH) to pressure at the end of the duct (SPend) for each branch duct.
  2. You measure SPH and SPend for each branch in the installed system.
  3. You compare ratio of measured values to predicted ratio for each branch
  4. A deviation greater than 15% indicates a substantial deviation from expected values.
  5. Does not consider substantial deviations in ducts other than branches.

Same as "Quick", except that you measure SPend for every duct in the system and do a Pitot traverse on each branch. The included Hv_measurement data acquisition program can speed that up enormously.
  1. Heavent computes the equivalent loss coefficient for each duct and compares it to its expected value.
  2. A deviation greater than 30% indicates a substantial deviation from expected values that warrants close inspection. First, you should review tables of inputed information (diameter, number of elbows, etc.) and visually compare to the installed duct. If that checks out, you should consider viewing the inside of the duct with a borescope or by disconnecting it.
  3. If the deviations at the fan inlet and outlet are large, then fan performance will be affected. Conversely, if they are small, then deviations from expected airflow from the fan are due to the fan or its inlet or outlet conditions.


Redesigning or Re-balancing Installed Systems
to Meet Current Needs



Evaluate Newly Installed System
  1. Using Heavent, create a schematic of the system that matches your sketch of the system.
  2. In Heavent, input the correct duct length, number of elbows, and roughness for each duct you may wish to alter. For example, if you want to remove and elbow and 10 ft of duct from branch 3, you must enter the length and roughness of the duct and the number of elbows in it.
  3. Using Hv_measurement (an included program) to do data acquistion. You measure hood static pressure (SPH) and complete a Pitot traverse for each branch and measure SPend for each duct in the installed system.
  4. HvMeasurement will compute BrRatio=SPh/SPend for each branch as well as the resistances to flow at the terminus of each duct, X=-[SP+VP]/VP).
  5. Using Heavent, compute pressures and flows for every duct. Heavent automatically computes values of BrRatio for each branch and X for each duct terminus.
  6. Compare values of BrRatio and X based on measurements to corresponding values based on modeling as a new system by Heavent.
  7. A difference of more than 10% in BrRatios or a difference of more than 25% in X values indicates a substantial deviation between expected and likely values. Each case should be investigated to determine whether there is an obstruction, leak or other problem.
  8. Make any changes to the schematic to reflect the changes you wish to make to the system.
  9. Looking at the predicted effects on the system, tweak the system for optimal performance by changing duct diameters as needed. If a substantial fraction of ducts must be replaced, you should consider rebuilding the system from scratch.
Troubleshoot a long-installed system
  1. Using HvMeasurement (an included program) to do data acquistion. Measure hood static pressure (SPH) and complete a Pitot traverse for each branch and measure SPend for each duct in the installed system.
  2. HvMeasurement will compute BrRatio=SPh/SPend for each branch as well as the resistances to flow at the terminus of each duct, X=-[SP+VP]/VP).
  3. If you have taken similar measurements in the past, you can compare the old and new values of BrRatio and X values.
  4. A difference of more than 10% in BrRatios or a difference of more than 25% in X values indicates a substantial deviation between expected and likely values. Each case should be investigated to determine whether there is an obstruction, leak or other problem.

Balancing with dampers
When designing a new sysem:
  1. In Heavent, set to "balance with dampers" in Toggles.
  2. Lay out the schematic of the system and input all required airflows, duct lengths, elbows, contraction, expansions, contractions, and air-cleaning devices.
  3. Heavent automatically predicts BrRatio values that would exist if the dampers are each adjusted correctly.
  4. Adjusting dampers in the field: Adjust each damper in turn until the observed value of BrRatio is nearly the same as the predicted value. One round of adjustements should be sufficient.
  5. Measure airflow at fan inlet, then adjust fan rotation rate (w) so that the fan produces the correct rate of airflow.
  6. If you were to measure the airflows at each branch after the dampers are adjusted and the fan speed is corrected, you should find that each is within roughly 5% of the desired value. There is probably no need to make a second round of adjustments.
  7. Note: this procedure should give the best possible distribution at the lowest possible energy use and take far less time than other methods.


Over 200 pages of guidance, step-by-step procedures for common problems, and example problems. All input and design choices are explained -- including the ventilation issues behind them.
Title of Section




Heavent Screens


Step by Step Detailed Solution of an Example Problem
Use of HvMeasurement, an included program
Use of Dilution, an included DOS program


Additional Applications
(note: all programs toggle freely between S.I. and inch-pound units)


Purpose of each component of the software package


 Computer data acquisition of pressures and flows .

 Aids in dilution ventilation calculations employing the commonly used one-cell, mass-balance model. That model is seriously flawed but usually much better than guessing.

Components of the Heavent Software Package 

Main Program
Program Manual
Auxiliary Programs in the Heavent Package
Design, balancing and redesign of ventilation systems

Covers use of Heavent & use of the auxiliary programs

Measurement of pressures and flows in the field & troubleshooting

Dilution ventilation using the one-cell mass-balance model

Calibration of pressure measuring devices




 Steven E. Guffey, PhD, CIH
(304) 685-1298

1403 Far Meadows
Morgantown, WV 26508

prefer email or text message to voice mail