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Please check this list of Frequently Asked Questions for your HVAC-Calc support needs.

  1. How do I register the program?
  2. "Register program" does not appear, what do I do?
  3. I bought the program, but it still says trial version.
  4. I lost my hard drive and re-installed the program, now it won't let me change the Design Conditions.
  5. When I go to print a report, it says "Results are not available until the error conditions are cleared.
  6. I have entered a Wall in my room but it won't let me enter another Wall.
  7. The cooling load is smaller than I expected.
  8. I bought a new computer, how do I put the program on my new computer?
  9. You discuss using safety factors in sizing furnaces. Do you recommend using safety factors in sizing air conditioners?
  10. I work in a dry area, so matching sensible loads requires me to choose larger units than the reports say. This is confusing to the end user.
  11. How do I do cathedral ceilings?
  12. Should I include closets, stairwells and pantries?
  13. Should I include unheated basements?
  14. The house has an attached garage, what do I do?
  15. I have rooms in the attic with low walls, attic space on the other side, what do I do?
  16. I lost my Registration Number, do I have to buy the program again?
  17. I want to copy files from one computer to another.
  18. I am using an ERV (or HRV), how do I enter it?
  19. This house is using spray foam, what do I do?
  20. Do I put the people in every room?
  21. I have Windows 8 or 10 with a touch screen and the Tree keeps disappearing.



  1. How do I register the program?

    To register the program(s),

    1. Start the program running (in trial mode) and load in a new job (or an existing job, it doesn't matter) so you can see the main screen.
    2. On the Menu (at the top of the program screen, where it says File Reports ) click on Settings.
    3. Then click on Register Program. Go to Step 4.
      If "Register Program" does NOT appear:
      1. Click on File
      2. Click on Open Existing Job
      3. Press the Function Key F5 on your keyboard
      4. If your Function Keys are turned off, then use Shift H instead.
    4. Carefully enter your registration name as shown in your registration e-mail. The name must be exactly as shown, same capitals, same punctuation.
    5. Registraion name, line 2 is usually left blank.
    6. Enter your Registration Number.
    7. Click on OK. If you have entered the information properly, you will see a "Success!" box. Click OK.
    Each time you install the program on a new computer or re-formatted hard disk, it needs to be registered again.

    If you have lost your registration number, please see #17

    Mistakes that people make

    1. The Residential program has one registration number, the Commercial program has a different one. Be sure you have started up the program that you meant to.
    2. Delete the text "Unregistered Trial Version". Line 1 must be completely cleaned out before typing your Registration Name in. Do NOT delete by pressing the space bar, that is not deleting, that is replacing with a space.
    3. Copying and pasting. Sometimes this does not work, depending on your email software. Some software puts in characters that you cannot see but make the registration name different than the simple text one the program is looking for.


  2. "Register program" does not appear, what do I do?
    If "Register Program" does NOT appear under Settings on the menu:
      1. Click on File
      2. Click on Open Existing Job
      3. Press the Function Key F5 on your keyboard
      4. If your Function Keys are turned off, then use Shift H instead.
    That should bring up the registration screen, now:
    1. Carefully enter your registration name as shown on the CD sleeve or in your registration e-mail. The name must be exactly as shown, same capitals, same punctuation.
    2. Registration name, line 2 is usually left blank.
    3. Enter your Registration Number.
    4. Click on OK. If you have entered the information properly, you will see a "Success!" box. Click OK.

  3. I bought the program, but it still says trial version.
    The program will run as a Trial Version until it has been registered on that particular computer. Each time it is installed on a new hard drive, it needs to be registered. See Item 1 for registration instructions.

    If you have lost your registration number, please see #17

  4. I lost my hard drive and re-installed the program, now it won't let me change the Design Conditions.
    It won't let you change the Design Conditions or print the Reports because it thinks it is just a Trial Version. You need to register the program so that it knows that you have purchased it. See Item 1 for registration instructions.


  5. When I go to print a report, it says "Results are not available until the error conditions are cleared"
    You have entered something incorrectly. Instead of giving you incorrect results, the program is informing you of the error(s). Look in the Tree. There will be at least one item that has a red circle with a line through it. This is the ERROR sign. Select that item and read the error message at the bottom of the screen. It will tell you what you did wrong and how to fix it. Once all the errors are fixed, you will be able to see the results.


  6. I have entered a Wall in my room but it won't let me enter another Wall.
    Sure it will, the number of walls you can add is unlimited. What you are forgetting is Rule Number 2: To add a new component, select the owner of that component in the Tree. Please see page 19 of the manual for a more complete discussion.
    In a nutshell, Walls are attached to Rooms, so select the Room name in the Tree, then click on the Wall button at the top. Now you can fill out the info about the Wall, then click on Add.


  7. The cooling load is smaller than I expected.
    This often happens, as over the years many contractors have been sizing air conditioners larger than necessary. An A/C that is too big often does a poor job of humidity removal. Please see the complete discussion of sizing on page 7 of the manual.
    However, a too low cooling load can also be caused by a couple of other things so check these:
    • Windows and Glassdoors - Be sure that "No Inside Shading" and "0% (no external shading)" are selected. (You don't want the drapes closed in the middle of a nice summer day.) If you have found that you have selected "Draperies or blinds", then select "No inside shading" and click on Change. Then click on Change All to instantly change all of the windows to this selection.
    • If you do not have Tinted or low-e glass, then do not select them, select clear glass instead.
    • Check the Infiltration component. Most American homes do not have a vapor barrier, so would be "Avg. (loose construction, poor vapor barrier)"
      Also check Step 3 of the Infiltration component. You will see there the floor area of the house. If you know that the house is 2,450 sq.ft. yet HVAC-Calc is saying 1,875, well, you must have forgotten something.
    If you enter the correct information, which is easy to do, HVAC-Calc Residential will give you the same answers as a manual Manual J calculation. If you have input the information correctly, then you have the cooling load as recommended by Manual J. If you are in a dry area (such as Arizona) it is important you read the discussion on page 7 about Sensible Gain.

    If you live in a dry area, you need to be sure to understand how that affects your equipment selection. It is covered in the manual and also in this article on Heat Loss and Heat Gain.


  8. I bought a new computer, how do I put the program on my new computer?
    You can install the program(s) in Trial version from our website HVACComputer.com.

    Once you have the trial version running on your new computer, register it with your registration name and number, as explained in questions 1 and 2. This tells your new computer that you own the program. Your registration name and registration number were sent to you by email and on paper with the disk when you purchased the program.

    If you have lost them, you need to purchase a replacement for $99.
    Click here to Order a Replacement and get the Registration Numbers by email


  9. You discuss using safety factors in sizing furnaces. Do you recommend using safety factors in sizing air conditioners?

    No, you want to match the air conditioner capacity as closely as possible to the load of the house. Even then, the air conditioner will be over sized whenever the outdoor temp is below the design temperature, so most of the time. As long as you understand that air conditioners work best when they run a lot and as long as you remember that an air conditioner is supposed to remove humidity as well as cool, you or your customer will be more comfortable with a unit that is sized as close as possible to the calculated load.

    I repeat, do NOT add a safety factor to the cooling load.


  10. I work in a dry area, so matching sensible loads requires me to choose larger units than the reports say. This is confusing to the end user.
    Yes, matching the sensible capacity of the unit to the sensible load of the house in a dry area means choosing one with a larger tonnage then shown on the report. You can stop "tonnage" from showing on the report by clicking on Settings --> "Show Tonnage on Reports" to turn it off.

    If you live in a dry area, you need to be sure to understand how that affects your equipment selection. It is covered in the manual and also in this article on Heat Loss and Heat Gain.


  11. How do I do cathedral ceilings
    There is nothing difficult about calculating the heat loss and gain of cathedral ceilings. Because the ceiling is on a slant, its area will be larger than the floors area. This makes HVAC-Calc's default ceiling area incorrect, so enter the correct area in the blue area box. To do this, you can measure it, or, what I do is just add 25% to the floor area.
    The selection for Ceiling Type in Step 2 would probably be "Roof on exposed beams/rafters" or "Roof-Ceiling combination".
    In a room with a cathedral ceiling there often is an exterior wall that starts at 8 ft and ends up at say, 12 feet. You would enter the wall height as 10 ft, the average.


  12. Should I include closets, stairwells and pantries?
    Yes, you include all parts of the house, including the basement. In a room-by-room calculation you can usually include the closet as part of the room. For the upper part of stairwells, just pretend it is covered with a floor.


  13. Should I include unheated basements?
    Yes, I recommend that you include the basement as if it was a fully heated and air conditioned space. It is almost fully heated anyway, otherwise the pipes would freeze. Cooling loads for basements are usually quite low. A basement does get cooled whether you want it to or not because the cool air from above leaks down there.
    So, include the basement as heated space and the floor of rooms in the first floor as over a conditioned space.


  14. The house has an attached garage, what do I do?
    Pretend the garage is not there. Thus, the wall that separates the garage and house is then treated as an outside wall.


  15. I have rooms in the attic with low walls, attic space on the other side, what do I do?
    Consider those walls as outside walls.


  16. I lost my Registration Number, do I have to buy the program again?

    No, unlike most software companies, you will not have to buy the program again. However you will need to purchase a replacement at a cost of $99. When you order the replacement, we will send you an email with everything you need to get back to work today. Please go here to order:
    Order Replacement Numbers


  17. I want to copy files from one computer to another.

    The job files can be copied from one computer to another by using any standard Windows method such as Windows Explorer. The files are stored in their own folder, usually C:\HVAC-Calc Jobs\Residential 4 and C:\HVAC-Calc Jobs\Commercial 4.

    The Residential job files end in .rld and the Commercial files in .hdb. Locate the files on your old computer ( if you can't find them, do a search for *.rld) and copy them to the folder C:\HVAC-Calc Jobs\Residential 4 on the new computer.

    When you start up HVAC-Calc, it searches this folder C:\HVAC-Calc Jobs\Residential 4 for all the jobs ( .rld files) and lists them on the Active Job Screen.


  18. I am using an ERV (or HRV), how do I enter it?

    An HRV/ERV is an improvement on a fresh air intake fan because they bring in fresh air and they exhaust air at the same time. They also recover heat from the exhaust air in the winter time or exhaust it in the summer time. They have an efficiency rating, usually around 80%. So, this means they are 20% inefficient.

    If the HRV moves 200 CFM (cubic feet per minute) of air, then 20% of that 200 CFM, 40 CFM, needs to be accounted for in the heat loss/heat gain calculation. You do this on the Infiltration Screen. In Number "2. Enter any forced ventilation here:" you would enter 40 CFM, the air that would actually affect the loads.


  19. This house is using spray foam, what do I do?

    Spray foams are just insulations, with R-values. Open cell foams have an R-value of about 3.5 per inch and closed cell foams have an R-value of about 6 per inch, according to various sources. The foams have the additional benefit of sealing a house tighter, thus reducing infiltration.

    If the house is completely insulated with spray foam, you should select 'Best" for the tightness in the infiltration screen. For the walls and ceilings, you will need to determine the R-value of the foam and select the appropriate value from the lists provided.

    If the ceiling of the attic space as been sprayed, then the attic space becomes an inside conditioned space. However, because there are no people, windows or doors in the attic space, and because it is an awkward area to enter walls etc, you can simplify things by treating the ceiling of the upper zone rooms as below a standard attic and selecting "under ventilated attic" and selecting the appropriate R-value for the insulation.


  20. Do I put the people in every room?

    When calculating the load, you should place the people in the room where they may be found during the middle of the afternoon. Generally, you would choose the room where the TV is.

    You do NOT put them in every room.


  21. I have Windows 8 or 10 or 11 with a touch screen and the Tree keeps disappearing.

    This is a bug in Windows, that is why we did an update to 4.0.70a
    Please go here: Order Update