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

General observations provided by building visitors

A site survey using tools to measure the size of the room the and the ventilation
rates from each vent

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:

Estimate the room size just by observations

Estimate the ventilation rate provided just by observations

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 150175 square feet per person.

Open office spaces for tech companies typically is 125175 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: 1820 square feet per guest

Fast food dining: 1114 square feet per guest

School lunchroom/cafeteria: 912 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 floortofloor
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:

Vents

Square, Round, Linear Diffusers

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 
 Look at the vent and count the number of slots.
 Each slot is 1/2 inch appart.
 A vent with 2 groups of 9 slot openings is 4.5+4.5 = 9 inches.
 The opening is smaller than the vent size.
 A 10 inch vent has a 9 inch opening.
 Center seperator is 1/2 inch.
 Edge borders are 1/4 inch each.
 Vent size = 10 = 9 + 1/2 + 1/4 + 1/4
 Use your camera and take a picture of the vent and count the slots.
 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.
4Way 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 
1slot, 2 ft 
6 
255 
100 
2slot, 2 ft 
8 
286 
150 
2slot, 4 ft 
8 
430 
200 
2slot, 4 ft 
10 
367 
250 
3slot, 4 ft 
8 
716 
300 
3slot, 4 ft 
10 
550 
350 
4slot, 4 ft 
10 
641 
400 
4slot, 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 sqin 
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.
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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