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Quick Visitor Observations

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There are 3 ways to identify the ACH ventilation level in a room:

1. General observations provided by building visitors
2. A site survey using tools to measure the size of the room the and the ventilation rates from each vent
3. The system reports real time ventilation rates. (future)

This will walk building visitors through the process to enter observations that will provide a rough estimate of the ACH level in a room. It is amazing how close the rough estimates are once compared to site survys.

The are 3 steps used for the general observations:

1. Estimate the room size just by observations
2. Estimate the ventilation rate provided just by observations
3. Answer some questions

Room Size Guidance

Use multiple approaches to do a sanity check of the observed estimate.

Determine Room Size

The first step is to enter the room size. These are some general rules to estimate the rooms size.

Occupancy based Estimate

• North American office is 150-175 square feet per person.
• Open office spaces for tech companies typically is 125-175 square feet per person.
• Classrooms are typically 800 to 1,100 square feet, sized to accommodate 20 to 30 students.
• Common classroom sizes include 28 by 30 feet, 30 by 30 feet, and 32 by 32 feet.
• Full service restaurant is 12 to 15 square feet per person
• Fine dining 18 to 20 square feet per person
• Counter service: 18-20 square feet per guest
• Fast food dining: 11-14 square feet per guest
• School lunchroom/cafeteria: 9-12 square feet per guest
• Estimate occupancy based on the tables with 2 or 4 or 6 people per table
• 50 people x 15 sqft = 750 = 27 ft x 27 ft

Ceiling Height Estimate

• Standards and Spaces based
  • Retailers consider ceiling heights of 16 to 24 feet essential to the success of the stores.
  • Office building 9 feet with 10 to 13 feet between floors
  • School minimum ceiling height is typically 9 feet
  • For suspended ceilings the minimum standard ceiling height is 8 feet
• Observations based
  • Residential doors are typcally 7 feet. The space above the door is typically 2 feet. Ceilings are typically 9 feet.
  • Use 8, 9, 10, 12, 15, 20 and 24 feet with 10 feet being a resonable estimate in most cases.
  • For the height visualize how many people can be stacked until the ceiling is reached. Use 6 feet for each person.
  • Alternatively think in terms of 10 foot sections stacked up.
  • Look at the doors. They are typcally 7 feet high, how many doors can be stacked to reach the ceiling.
  • Ignore any sections that are higher or lower, just get most of the ceiling height.
  • If there is a cathedral ceiling just estimate to the point where the wall starts to diverge to the cathedral ceiling.
  • You can add the cathedral ceiling to the height by estimating the addition of the highest point, dividing by 2 and adding it to the wall height.
  • (e.g. 12 ft wall height, ceiling top addiitional 12 ft, average height = 12 + 12/2 = 18 ft).
• Classroom minimum ceiling height is typically 9 feet, with a floor-to-floor height of about 13 feet.
• Building construction typically uses 4x12 foot sheets. Visualize how many 4x12 sheets make the height.

Room Length and Width Estimate

• Windows
  • Look at the outside windows.
  • Count the number of windows along the wall.
  • Approximate the width of each window using 3, 4, 6, 8, 10 feet.
  • For walls where there are no windows visualize how many windows might be along the wall.
• Tables
  • Look at the number of tables.
  • Estimate the size of the  typcal table table.
  • Tables are 3x3 ft, 4x4 ft, 3x6 ft.
  • Visualize the space filled with tables.
• Ceiling Tiles
  • Look at the ceiling.
  • If there are ceiling tiles they come in 2x4 ft and 2x2 ft sections.
  • Count the number of tiles.
• Common classroom sizes include 28 by 30 feet, 30 by 30 feet, and 32 by 32 feet.
• Building construction typically uses 4x12 foot sheets.
• Visualize how many 4x12 sheets make the walls.
• Ignore the bumpouts.
• Just get the rectangle or square dimensions of the room.

Back to Room Size Observations

Ventilation Rate Guidance

If you feel air across your face the facility ventilation is not bad.

In the calculations you can assume 100, 200, 300, 400 linear feet per minute per vent.

If you don't feel air across your face you can assume 0, 50, 150, and 200 linear feet per minute.

Typically the designs are in the 350 FPM range.

Back to Ventilation Rate Observations

Vent Size Guidance

There are basically 3 types of vents:

1. Vents
2. Square, Round, Linear Diffusers
3. Return Grills

Look at the ceiling and look for the most common vent. Usually it is always the same type of vent but there may be a mix near fringe spaces in the room. The following tables list differnt vent, diffuser, and return grill sizes that can be used to estimate the size of the vents in a facility.

Vent, Diffuser, and Return Grill Sizes

When looking at a vent, diffuser, or return grill the outside size is larger than the actual vent opening. The following tables identify the vent, diffuser, and return grill openings. The vent, diffuser, and return grill size exceeds the opening on each side by 1 inch. So a vent that looks like and measures from  the outside as a 4 x 12 inch vent is really a 2 x 10 inch vent because that is the size of the opening. Also, the opening might be partially blocked around the perimeter and with reinforced strips down the middle. So a 4 x 10 vent might scale down to a 3 x 8 vent opening.

Back to Vent Size Observations

Vents

If applicable pick a vent size (Length and Width). Keep in mind the FPM level for when the Vent Air FPM is requested.

Vent Sizes Vent Size (in) Vent Size (in) Vent Size (in) Vent Size (in) Vent Size (in) Vent Size (in) 2 x 10 2 x 12 2 x 14 2 x 48 3 x 8 3 x 10 3 x 12 3 x 14 3 x 48 4 x 8 4 x 10 4 x 12 4 x 14 4 x 48 6 x 8 6 x 10 6 x 12 8 x 10 8 x 12 8 x 14 8 x 8 10 x 10 12 x 12 14 x 14 18 x 18 24 x 24 Note: Most likely near 350 FPM

1. Look at the vent and count the number of slots.
2. Each slot is 1/2 inch appart.
3. A vent with 2 groups of 9 slot openings is 4.5+4.5 = 9 inches.
4. The opening is smaller than the vent size.
   1. A 10 inch vent has a 9 inch opening.
   2. Center seperator is 1/2 inch.
   3. Edge borders are 1/4 inch each.
   4. Vent size = 10 = 9 + 1/2 + 1/4 + 1/4
5. Use your camera and take a picture of the vent and count the slots.
6. Estimate the other side size by comparing it with the slot count size.

Back to Vent Size Observations

Diffusers

The FPM numbers in the table are the upper limit and the airflow will Most likely be near 350 FPM.

If applicable pick a Diffuser size and enter the neck diameter (black hole). Keep in mind the FPM level for when the Vent Air FPM is requested.

4-Way Diffuser Sizes Airflow (cfm) Diffuser Size Neck Size (in) Airflow (FPM) 150 2 ft x 2 ft 8 430 200 2 ft x 2 ft 8 573 250 2 ft x 2 ft 10 458 300 2 ft x 2 ft 10 550 350 2 ft x 2 ft 12 445 400 2 ft x 2 ft 12 509 450 2 ft x 2 ft 14 421 500 2 ft x 2 ft 14 468 Note: Most likely near 350 FPM Round Diffusers Neck Sizes are: 8, 10, 12, 14 inches Note: Most likely near 350 FPM

Back to Vent Size Observations

Linear Diffuser Sizes Airflow (cfm) Diffuser Size Neck Size (in) Airflow (FPM) 50 1-slot, 2 ft 6 255 100 2-slot, 2 ft 8 286 150 2-slot, 4 ft 8 430 200 2-slot, 4 ft 10 367 250 3-slot, 4 ft 8 716 300 3-slot, 4 ft 10 550 350 4-slot, 4 ft 10 641 400 4-slot, 4 ft 12 509 Note 1: 50mm/slot or 1.95 inches/slot Note 2: Most are 1 or 2 inches/slot Note 3: Most likely near 350 FPM

For sizing use the length and width. For example 2 x 48 inches. The neck size does not apply.

The following table can be used to deal with mixed sized vents in a room.

Neck Size (in)	Area sq-in
6	28
8	50
10	79
12	113

Back to Vent Size Observations

Return Grills

You can measure the ventilation level using the vents / diffusers or return grills. The system sucks in the air through the return grills and then expels the air out the vents or diffusers. Measuring the return grills will provide an indication of the percent of outside air being introduced into the space. It is also a good sanity check of the vent or diffuser measurements.

• percent outside air = 100 - (grills total FPM * 100 / vents total FPM)

The following are return air grills. The return air grills are similar in size to vents. The data in the following tables are based on 350 linear feet per minute (FPM) as measured at the grill.

If applicable pick a vent size (Length and Width).

Horizontal Return Air Grilles Grille Area (sq.in) Grille Size (in) Airflow (cfm) Airflow (FPM) 24 4 x 6 58 348 48 4 x 12 117 351 60 6 x 10 146 350 all others 180 6 x 30 438 128 8 x 16 311 144 8 x 18 350 192 8 x 24 467 240 8 x 30 583 256 8 x 32 622 200 10 x 20 486 220 10 x 22 535 240 10 x 24 583 300 10 x 30 729 216 12 x 18 525 240 12 x 20 583 288 12 x 24 700 360 12 x 30 875 238 14 x 17 578 280 14 x 20 681 350 14 x 25 851 364 14 x 26 885 288 16 x 18 700 320 16 x 20 778 400 16 x 25 972 416 16 x 26 1011 432 18 x 24 1050 468 18 x 26 1138 540 18 x 30 1313 576 18 x 32 1400 480 20 x 24 1167 500 20 x 25 1215 600 20 x 30 1458 Note: Most likely near 350 FPM

Back to Vent Size Observations

Square Return Air Grilles Grille Area (sq.in) Grille Size (in) Airflow (cfm) Airflow (FPM) 36 6 x 6 88 352 64 8 x 8 156 351 100 10 x 10 243 350 all others 144 12 x 12 350 196 14 x 14 476 256 16 x 16 622 324 18 x 18 788 400 20 x 20 972 484 22 x 22 1176 576 24 x 24 1400 676 26 x 26 1643 900 30 x 30 2188 Note: Most likely near 350 FPM

Back to Vent Size Observations

Verticle Return Air Grilles Grille Area (sq.in) Grille Size (in) Airflow (cfm) Airflow (FPM) 48 12 x 4 117 351 72 12 x 6 175 350 all others 120 12 x 10 292 84 14 x 6 204 112 14 x 8 272 140 14 x 10 340 96 16 x 6 233 128 16 x 8 311 192 16 x 12 467 180 18 x 10 438 216 18 x 12 525 252 18 x 14 613 160 20 x 8 389 200 20 x 10 486 240 20 x 12 583 280 20 x 14 681 320 20 x 16 778 360 20 x 18 875 176 22 x 8 428 220 22 x 10 535 96 24 x 4 233 144 24 x 6 350 192 24 x 8 467 288 24 x 12 700 336 24 x 14 817 384 24 x 16 922 432 24 x 18 1050 364 26 x 14 885 416 26 x 16 1011 468 26 x 18 1138 224 28 x 8 544 644 28 x 23 1565 180 30 x 6 438 240 30 x 8 583 300 30 x 10 729 360 30 x 12 875 420 30 x 14 1021 480 30 x 16 1167 128 32 x 4 311 256 32 x 8 622 384 32 x 12 933 640 32 x 20 1556 272 34 x 8 661 360 36 x 10 875 432 36 x 12 1050 504 36 x 14 1225 608 38 x 16 1478 800 40 x 20 1944 Note: Most likely near 350 FPM

If you can't measure the ventilation rates from the vents then just count the number of vents and use the guidelines at the start of this page to determine the ventilation rate from each vent. Pick the vent size and estimate the ventilation rate. If you feel air across your face the facility ventilation is not bad. In the calculations you can assume 100, 200, 300, 400 linear feet per minute per vent. If you don't feel air across your face you can assume 0, 50, 150, and 200 linear feet per minute.

Back to Vent Size Observations

The procedure and process is very simple. Do not be side tracked with irrelevant details that some may claim to stop the measurements. Just roll up your sleeves and do the work and then present it to the community.

Copyright © 2021 Cassbeth Inc. All Rights Reserved. Proprietary. This information can be only used for education, research, or news media organizations. Any other use of this information by any entity where there is a financial transaction such as for products or services by for profit, nonprofit, and government organizations must contact Cassbeth for release.

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