1. Drafts and Duct System Sizing
HVACR416 - Design
Drafts and
Duct System Sizing

2. Noise
23.5.1
Two common problems with air distribution systems are noise, produced by movement or vibration, and drafts.
Three factors that affect noise are the noise source, carriers, and noise amplifiers or reflectors.
Noises may be:
High pitch sound. Usually caused by air velocity that is too high. Possibly due to sharp metal edges.
Low pitch rumble. Usually caused by fan and motor sounds traveling along duct system.
Popping sound as unit starts or stops. Caused by expansion or contraction of the duct.

3. Noise
23.5.1
To locate the source of a high-pitched sound, remove the grille or diffuser. If the noise stops, it was due to sharp edges. Locate and correct the problem.
Decibel meters measure noise level.
Decibels (dB) refer to the frequency of pressure fluctuations in the air and the amplitude, or size, of these vibrations.
Airborne sound is expressed in cycles per second (cps).

4. Noise Sources of noise: Fans and motors.
High-velocity air traveling through ducts and causing turbulence.
Noises produced by compressors.
High-velocity refrigerant flow, especially at sharp bends in the piping.
Noise caused by high-speed air is often the result of an undersize unit or duct.

5. Noise Rigid structures are noise or vibration carriers.
23.5.1
Rigid structures are noise or vibration carriers.
Hard, smooth surfaces in the conditioned space may reflect or amplify sound.
Soft fabrics, such as drapes, curtains, and fabric-covered furniture, are noise absorbers. Felt-lined, soft-insulation-lined, and covered ducts absorb noise.

6. Drafts
23.5.2
Ducts and fans must be large enough to provide correct amount of air for conditioning.
Air must enter and circulate to all parts of room without interfering with flow of air to air return. There should be no objectionable noise or drafts.
Air moving past people faster than 25 ft/min. (7.6 m/min), or about 1/4 mile per hour (.4 kph), creates an annoying draft.
Air should not flow faster than 1/4 mph (2.25 kph) through the length of a 25' (7.6m) room.

7. Drafts
23.5.2
For a grille outlet designed to throw air into room a distance of 8'-13' ( m), a velocity of 500 ft/min. (152 m/min.) is needed. Therefore, the grille or outlet locations must be carefully selected to avoid drafts.
The location of air returns is important when moving air across long space at reasonable velocity. Air returns for long rooms should be on the side opposite where air enters the space.
Air returns should be located high on the wall for warm air return (cooling season) and low on the wall (or in the floor) for cold air return (heating season).

8. Drafts
23.5.2
The location of outlets can minimize drafts in the living areas of a room.

9. Drafts
Locations of return air grilles for residences.

10. Duct Sizing
Duct sizing is determined by the air volume to be delivered.
Air volume depends on the amount of heat that the air must deliver and remove.
The amount of air delivered must always equal or exceed the minimum fresh air ventilation requirements.
Smaller ducts are now being used to reduce duct size and save space.
Operate with about twice the normal air velocity.
The higher air pressure and velocity require more powerful fans and create more noise.
23.6

11. Duct Sizing In colder climates, duct size is based on heating needs.
23.6
In colder climates, duct size is based on heating needs.
In warmer climates, duct size is based on cooling needs.
Where both heating and cooling are needed, one duct system serves both.
Higher air volumes are required for air conditioning.
System capacity is increased by increasing the fan speed by about 20%.

12. Air Volumes for Heating
23.6.1
When the furnace is the only source of heat for a room, three factors must be known to calculate the air volume:
Heat load.
Room temperature.
Duct temperature.
Then, use the formula:
heat load = specific heat X wt. of air X temp. difference

13. Air Volumes for Heating
Room temperature is decided by the designer. Normally, the temperature is 72°F (22°C).
The duct temperature is more difficult to decide.
Low duct temperature used = large air volumes necessary to carry enough heat.
High duct temperatures used = furnace must operate with higher chimney (stack) temperatures. Ducts may need to be insulated.
For heating, engineers recommend the grille temperature be at least 125°F (52°C); duct air temperature should be near 140°F (60°C).
Specific heat of air is 0.24 Btu/lb.°F (1.004 kJ/kgK).
23.6.1

14. Air Volumes for Heating
Since the heat load, specific heat, and the temperature difference are known, use the formula to determine the weight of the required air per minute.
Now, air volume must be determined. To find the total volume, first find the volume of one pound of air at room and duct temperatures.
From a psychrometric chart, the volume of one pound of air at room temperature (72°F) is ft3.
Use Charles’s law to find the volume of one pound of air at duct temperature (140°F), which is calculated to be ft3.
Finally, multiply the weight per minute by the volume per pound to determine the total volume per minute (cfm).
23.6.1

15. Air Volumes for Cooling
23.6.2
To obtain air volume needed, large ducts and lower air pressure (velocity) should be used.
Less power will be needed.
Duct cost is one-time cost.
A larger fan and motor mean continuous higher power costs.
The higher velocity needed with smaller ducts creates more noise.
Short method for determining air volume:
For each square foot of floor space, excluding basement, use 1 cfm. If a home has 1500 ft2 based on outside dimensions, the fan capacity should be 1500 cfm.

16. Air Volumes for Cooling
23.6.2
A typical uninsulated home needs 12,000 Btus of cooling per hour (1 ton) for each 400 ft2 of floor space. To determine the cooling load for a 1500 ft2 house:
1500/400 X 12,000 Btu = 45,000 Btu/hr (4 ton).
There should be six to ten air changes per hour.
Return air is warmer and at a lower pressure, so it occupies more volume.
Return ductwork should be about 20% larger in cross-section area than the delivery duct.

17. Air Volumes for Cooling
23.6.2
Air volume for cooling is calculated in same way as for heating.
Knowing heat load, specific heat of air, and temperature difference, the weight of air needed can be determined.
If weight of air is known, air volume can be determined and duct size selected.
Formula:
heat load = specific heat of air X wt. of air X temp. difference

18. Duct Calculations In some systems, a duct serves more than one room.
Each room served must receive the correct amount of air.
Distribution must be balanced.
There are two methods for calculating the proper size for plenum chambers, main ducts, branch ducts, and grilles.
Unit pressure drop system.
Total pressure drop system (most accurate).
23.6.3

19. Unit Pressure Drop System
23.6.3
Air forced through a duct follows the path of least resistance.
Many duct systems have several openings (grilles) for air to escape from the duct.
A duct with low resistance will allow the most air to flow through it.
Ducts with higher resistance will not carry the correct amount of air.
In the past, duct installations often fed too much air to some rooms and not enough to others.

20. Unit Pressure Drop System
23.6.3
Unit pressure drop calculating system uses the same pressure drop for each length of duct throughout system.
To determine the air volume handled by a duct size, airflow data are needed.

21. Unit Pressure Drop System
A friction chart for airflow in straight ducts. Values were obtained by research.

22. Unit Pressure Drop System
23.6.3
Friction charts have four variables:
Friction loss in inches of water on the vertical scale (equal-value lines are horizontal).
Cubic feet of air/min on the horizontal scale (equal-value lines are vertical).
Velocity on scale lines that slant down to the right.
Round duct diameter on scale lines that slant down to left.

23. Total Pressure Drop System
23.6.3
More accurate method of calculating proper sizes of ducts.
Based on having the same total pressure drop from the fan to each outlet.