Simple HVAC Calculator

Enter the size, either for a whole room, the whole house, or an entire floor.
Add more rooms/floors as needed.
Room Volume Width Length Height Total Area
ft. ft. ft. cu.ft.
Total Volume  1521 cu.ft.
Desired ACH (Air Changes Per Hour)
literally how many times the entire room’s air is replaced.
3-5 ACH is ideal.
 ACH
CFM (Cubic Feet per Minute) needed for area
CFM per room =
(desired ACH * Room Volume [L x W x H in cu. ft.]) / 60
102 CFM
You need AT LEAST this size A/C unit for this area
400 CFM = 1 ton of air supply
0.255 tons
BTU (British Thermal Units) produced by the above
1 ton of air supply = 12,000BTU
3060 BTU
Friction Loss you are aiming for
HVAC Pros recommend between 0.05 and 0.2
 in. water/100ft.
Your starting duct size should be about
duct diameter5.02 = (0.109136 * cfm1.9) /
friction loss
7 in. diameter
In rectangular duct, this would be about 8 x 4 or 12 x
Your return grille opening should be at least
return grill size = 200 sq.in. per Ton of air
51 sq.in.
 
a 51 sq. in. grille is:
8 x 6 in. or 10 x 5 in. or 12 x 4 in.
Note: to use MULTIPLE air returns, simply subtract one of the 2 numbers.
For example, for 8 x 20, you can use two 8 x 10 returns, or an 8 x 8 and a 8 x 12.
 

How To Use The Above Charts

  1. Pick a desired CFM (e.g., 400 CFM/Ton).
  2. Look at the airflow table for your air handler (check its owner's manual) and pick an ESP that will give you that CFM (e.g., 0.5 iwc).
  3. Calculate a friction rate based on that static pressure (e.g., 0.1 iwc/100’).
  4. Use that friction rate to size the ducts.
  5. Run the ducts across and through the house as STRAIGHT as possible, branching off when needed.
  6. Install the system, turn it on and you get the TESP you wanted (or less) and the CFM you wanted (or more).

Terminology

External Static Pressure: Power of the fan blower in your air handler.

Component Pressure Loss: Power loss because of things like elbows in ductwork, registers/grilles, dampers, evaporator coil, and air filters. Note: When you double the CFMs, you quadruple the Pressure Loss of an air filter.

Available Static Pressure: ESP – CPL

Total Equivalent Loss: Actual length of longest run of duct + Equivalent Lengths for elbows and fittings

Friction rate [pressure lost as air moves down the pipe]: (Available Static Pressure * 100 ft.) / Total Equivalent Loss

Static Pressure at the end of the ducts, once the air leaves the register, is always ZERO. ESP is how much pressure you have to burn.
Friction Rate is how fast you can burn it and have nothing left at the end.

Dry Bulb: the standard temperature as measured by a dry thermometer.

Indoor Dry Bulb: what you WANT your house temperature to be. should be 70 degrees in winter, 75 degrees in summer.

Wet Bulb: the temperature measured by a thermometer wrapped in a wet fabric with air running over it for a minute or so. Used to measure humidity.

Relative Humidity: the difference in temperature between Dry Bulb and Wet Bulb, expressed as the percentage of humidity in the air.
These are the basic steps to design a duct system that will make the fan feel a certain static pressure:

Equivalent Resistances per Elbow in Ft (for TEL)
Duct
Diameter		90˚
Elbow		60˚
Elbow		45˚
Elbow		30˚
Elbow
6”12864
8”151085
9”181296
10”2013107
12”2517138
16”36241812
18”41272114
20”46312315

ACCA HVAC Manuals And Their Purposes

Manual NameMeaningSubject of BookTitle
Manual JJoule (unit of heat)to calculate heat loss & gain, and moisture gain in the home, and what you need to replace or accomodate that situation.Residential Load Calculation, Eighth Edition
Manual DDuctworkto help choose proper sizing for ductwork, using the information from Manual J.Residential Duct Systems 3rd Edition
Manual SSelection/Sizingto determine which HVAC equipment you need for your Manual J calculated load based on your local environment.Residential Equipment Selection 2nd Edition
Manual TTerminationshow to prevent drafts and stagnant air caused by improper sizing or incorrect equipment, using diffusers, grilles, and registers.Air Distribution Basics for Residential & Small Commercial Buildings 1st Edition
Manual BBalancetesting adjusting balancing HVAC systems, measuring and adjusting air and water flows to meet design requirements.Balancing and Testing Air and Hydronic Systems
Amazon page: https://www.amazon.com/stores/Air-Conditioning-Contractors-of-America/author/B0798SDJNZ

Head Loss (inches water gauge per 100 feet duct) [recommended 0.08]

Air velocity (ft/min) [recommended 700 - 900]

 

 

Duct Size (inches) 

 

CFM (Air Volume)

4

5

6

8

10

12

14

16

18

20

22

24

HL

100

0.65

0.21

0.09

0.02

0.01

 

 

 

 

 

 

 

AV

1146

733

509

286

183

 

 

 

 

 

 

 

HL

200

 

0.8

0.32

0.08

0.02

0.01

 

 

 

 

 

 

AV

 

1467

1019

573

367

255

 

 

 

 

 

 

HL

300

 

 

0.69

0.16

0.05

0.02

0.01

 

 

 

 

 

AV

 

 

1528

859

550

382

281

 

 

 

 

 

HL

400

 

 

1.19

0.28

0.09

0.04

0.02

0.01

 

 

 

 

AV

 

 

2037

1146

733

509

374

286

 

 

 

 

HL

600

 

 

 

0.61

0.2

0.08

0.04

0.02

0.01

 

 

 

AV

 

 

 

1719

1100

764

561

430

340

 

 

 

HL

800

 

 

 

 

0.34

0.14

0.06

0.03

0.02

0.01

 

 

AV

 

 

 

 

1467

1019

748

573

453

367

 

 

HL

1000

 

 

 

 

 

0.21

0.1

0.05

0.03

0.02

0.01

0.01

AV

 

 

 

 

 

1273

935

716

566

458

379

318

HL

1200

 

 

 

 

 

0.3

0.14

0.07

0.04

0.02

0.01

0.01

AV

 

 

 

 

 

1528

1123

859

679

550

455

382

HL

1400

 

 

 

 

 

 

0.18

0.09

0.05

0.03

0.02

0.01

AV

 

 

 

 

 

 

1310

1003

792

642

530

446

HL

1600

 

 

 

 

 

 

 

0.12

0.07

0.04

0.02

0.02

AV

 

 

 

 

 

 

 

1146

905

733

606

509