Correct Sizing of Residential Air Conditioning Systems & Ductwork Systems 

Procedures for Proper Ductwork Sizing for Residential Air Conditioning Systems - TEL ASP FR 
- with Darrell Udelhoven Ductwork Retrofitting Graphed Blower-Curve-Chart - Opportunity

ROOMDUCT CFM - Formula for finding CFM  | DUCT SIZING CHARTS | Use "Manual D" |  MAIN DUCT CHART | Square Inches Round Duct | Air Filter Rack Sizing  DUCTWORK BASICS  Basic AC Overview - Specifications VS. Reality  | TEL - Solving for ASP Available Static Pressure |  SA - RA | Gurgling@TXV 
First & foremost of importance is the correct order of proper procedures to follow for optimal efficiency of operation, & long trouble-free equipment performance.

Before you do anything else, educate yourself enough to ensure that you request the proper things be done in the proper order of sequence.

The first procedure is to do everything possible to reduce all sources of heat-gain & heat-loss.
Then have a manual "J" heat-gain load calc done, followed by a Manual "S" for equipment sizing & then a manual "D" for proper ductwork sizing.  All registers, grilles, filter racks, & diffusers must be located & sized for optimal efficient performance of the system.

The correct sizing of residential air conditioning systems & ductwork is crucial to ensuring proper indoor space conditioning, equipment performance, and economical operation. Unfortunately, many A/C contractors measure “correct sizing” by the system's ability to meet any indoor thermostat setting at any outdoor temperature. That method of sizing will lead to inefficient operation for the vast majority of conditioning time. Air conditioning systems “must be sized to meet typical or average indoor and outdoor conditions to ensure proper air mixing, filtration and dehumidification of indoor air across seasonal variations.”

For cooling, size the equipment and system based on 100% of the Total Cooling Load (both sensible and latent loads) at actual outdoor design conditions. Size the duct system properly & make certain that he proper airflow & optimal heatload is passing through the evaporator coil during most of the operating runtime. To Optimize Payback and lower operating costs always do as many things as you can to Reduce the heatgain-heatloss "BEFORE doing the manual J load calc & manual S for sizing the equipment."

To select the “proper size” heating, air conditioning, and duct system for a home, seven factors must be considered and all changes made prior to sizing equipment:

1. Improving Insulation Values and Reducing Infiltration, including e-windows, doors, etc.

2. Reducing Air leakage - air leakage accounts for between 25 percent and 50 percent of the energy used for heating and cooling in a typical older residence.

3. Solar orientation - and ways to reduce the radiant heatload should be considered.

4. The Internal BTU heat generation of appliances and people must be added.

5. Design conditions (typical outdoor and indoor weather conditions, humidity levels, etc.) A scientific calculation (manual J) called a Heat Loss/Heat Gain Calc, tabulates these factors into a load scenario for heating and cooling based on summer & winter outdoor design conditions for the climate where the home is located.

By comparing the heat loss/gain to manufacturer's equipment performance data, a properly sized heating and cooling system is selected. Use indoor design of 75-F dry bulb and 63-F wet bulb, around 50% Relative Humidity.

6. Proper & thorough ductwork testing and design Is Extremely Important for efficiency & BTUH performance - The evaporator coil needs to have an optimal heatload passing through it most of the time in order to approach achieving its Rated BTUH Capacities & its EER & SEER Ratings.

7. For optimal comfort in the Air Conditioning mode diffusers should be at or near the ceiling. Study the diffuser data in respect to room CFM, the required throw & a diffuser Face Velocity of around 500-fpm, & a Terminal Velocity at the human occupant level area of 50 to 75-fpm. This is critical toward achieving an optimal human comfort zone.

What should you expect from the average heating and cooling contractor?
When a typical heating and cooling contractor quotes the efficiency of the equipment (SEER or AFUE) and leads you to believe the new equipment will automatically deliver that efficiency, think again. Typical installed equipment only operates at 55% to 70% of rated capacity.

It is important to understand that equipment ratings are only "the potential efficiency of that component of the system under perfect conditions." "Over half of the system's efficiency depends on the duct system and the field-installation." Check for Return Air drawing Hot Air from attic areas, etc.!

An A/C's system efficiency can often be increased by a skilled tech from 10% to 50%. The biggest benefit is the increase in comfort and lower utility bills!

The heat loss/heat gain calculates the amount of heat transfer by component, based on the surface area, and then tabulates the total transfer for all of the components.

Air leakage/air infiltration: Up to 50% of an average home's heat loss/heat gain is attributable to air leakage/air infiltration. Therefore, determining the proper leakage/air infiltration rate for a specific home is paramount. Design leakage/air infiltration rates are based on dwelling size and projected efficiency or actual measured performance.

Solar orientation, or the amount of window surface area and the direction a home faces can have tremendous impact on the cooling needs (heat gain). Similar houses with different solar orientation will have different cooling loads. Glass facing east/west has more heat gain than glass facing south or North.

The service techs should use a manual D to properly size the ductwork supply air mains and runs to outlet diffusers, as well as the “sizing of the return air ducting in relationship to the btuh/cfm requirements of the various rooms.”

Qualified personnel must perform all installations and services. The duct system sizing and load sizing calculation should follow the design standards of Air Conditioning Contractors of America (ACCA) - Manuals D & J -or the American Society of Heating, Refrigeration & Air Conditioning Engineers, Inc. (ASHRAE) Fundamentals Volume (latest edition).
----------
When sizing ducts, the use of one value throughout will assure incorrect duct size for many branch runout duct segments. If 0.05 is used, nearly half of the runouts of the system will be oversized, resulting in those zones being too cold in summer, resulting in the remaining runouts furthermost from the air handler being undersized, creating zones that are too warm in summer.

The result is a human comfort design failure. The modified equal friction method of Manual D, "requires that the available static pressure from the fan be 'consumed' by the duct through its run from fan to outlet/inlet, with no shortage or excess at the end."

Pressure drop per 100 ft is not an input, it is a calculated intermediate value. A contractor who knows how to do the calculations to determine the available static pressure, and correctly allocate it to the supply runouts and return, is a rare tech.

This page does not explain everything you need to know about proper duct sizing a system for optimal comfort, but provides some general guidelines and concepts.
--------------------
Supply Air (SA) and Return Air (RA) -- near the ceiling or near the floor
For cooling both SA and RA work better near an 8 foot high ceiling. The SA diffusers should distribute the air near the ceiling to the walls on all sides of each of the rooms. Whether a single return system, or a multiple return system is used, there must be a low resistance path between every room and the nearest return air opening. This can be done by using wall transfer grilles, door grilles, or jump ducts.

To quickly aid you in evaluating existing duct systems, review the chart below. To insure the necessary air handling capacity of a duct system, each of the system's components (trunk lines, takeoffs, runs, diffusers, registers, and grill free areas) must be properly sized and matched together. A 12x8 duct with a 350-CFM capacity, for example, WILL NOT flow 350-CFM if the boot & diffuser(s) combo can only flow a total of 200-CFM within specs.

The boot size & register size is determined by the throw, spread, drop, Terminal Velocity of 50 to 75-fpm at the occupant level, & noise class according to the CFM that must be delivered to the room. Those specificatins are located in the technical engineering pages that the diffuser mfg'er has available to the dealer distributors. 

When sizing the return air duct system, the air handling capacity MUST BE EQUAL TO the SUPPLY SYSTEM at a minimum, I would oversize the return ductwork. It is recommended that contractors follow design parameters established by ACCA or ASHRAE on the return air duct systems.

If at all possible, use insulated metal ductwork.  For Heat pumps, Figure 450-cfm per btuh ton CFM | Round Duct | Square Inches | Rectangular Ducts | SA use 0.06" Friction Rate (FR) per 100 foot, 0.05" FR for Return  Air.

FIGURING ROOM CFM - DUCT SIZES
First, do the individual room-by-room heat-gain Manual J, to get each room's cooling BTUH.
Formula: (Heat Gain Room [3000] / Heat Gain System [32000]) = .09375) (1200-cfm *X's .09375 = 112.5-cfm. That will always provide the percentages of the total 1200-cfm that you want to go to each room. The distance from the airhandler and the velocity & friction rate you want to size for, can be entered into a ductulator or PC program. [Ten (10) rooms averaging 3,000-Btuh to equal 30,000-Btuh total cooling. [Most 3-ton rated systems would average 31,000, or less Btuh.]

Quick method: The heat gain and Btu/hr of cooling is done for each room.
Use 30-Btu/hr for each Cubic Foot per Minute (CFM) of Airflow.
Then Select Supply duct size by cfm, velocity, & optimal Supply Air friction rate down to 0.06".
A Room requires 3000-Btu/hr divided / by 30 equals 100-CFM, or around a 6" dia. RD metal duct.

For Room Return Air balancing, i.e., -.01" IWC = approximately -2.48 Pascals, which is a more precise easier incremental scale to read.  One inch water column (IWC) equals 248 pascals, 0.5" IWC is about 124 pascals; 0.25" WC = 62 pascals; 0.125 = 31 pa.; 248 pascals X- 0.01" WC  -2.48 pascals for low Return Air room pressure differentials.

Formula for finding CFM Airflow
If you can measure the air velocity coming from a duct, here is a rough ballpark formula to get the CFM:
CFM = (velocity in (FPM) Feet per Minute times the square footage of the duct area)
I.E., 16" Rd duct 201-sq.ins. / 144 = 1.3958333-sq.ft. X's Velocity of 800-fpm = 1116-CFM
Times 1000-FPM = 1395-CFM. Branch ducts: 7" Rd duct 38.48-sq. ins. = 0.2672222-sq.ft. X's 500-fpm=133-cfm

It is better to use a ductulator to enable the use of the appropriate Velocity Friction Rate balances to achieve the correct CFM on each Branch Run, etc.
This chart is in accordance with the tonnage of the unit.
Residential metal Duct Design varies from as low as .02" on Returns, to -
Supply Trunk runs down to 0.05" Friction Rate pressure drop per 100 feet of duct.

Refrain from using Flex duct, if unavoidable, use .03 to .05 pressure drop or lower per 100 feet of duct run.
A 3.5-Ton main run at 450-cfm Per Cooling Ton would require an 20" rd main trunk run, which will also work for heat pumps.

1575-CFM | 20" rd duct | Gross sizing of Return Air (RA) filter grilles: 200 sq. ins. Per Ton of cooling.

A 5-ton system should have two 500 sq. in. Return Air Filter Grilles. (Or,  two 25" x 20" Return Air filter grilles for 5-ton.)
 
This is to try to reduce the air velocity through a clean filter to 350-FPM and to reduce the resultant pressure drops as the filter loads. The free air area of a filter should be stated on the edge of the filter by the mfg'er.

It is very important to size each duct to the cfm and velocity needed for the room served in accordance with the manual J load and manual D duct design. Dampers’ on too large a duct, if dampered too much, would result in too much velocity loss.

Once you know the manual J loads, number of duct-runs and the Velocities, Friction Rates, & CFMs for each room, you can use the register/diffuser data to get required throws, etc. (i.e., Hart Cooley from your supplier).

Example: One-Way - Adjustable 10x6 diffuser; select a duct size that provides 80-cfm @ 600-fpm, which will provide an 8.5-foot throw. Hart & Cooley lists a Pressure Loss through the diffuser at .022” of an inch.

Consider the register/diffusers you are going to use and the various CFMs’ & Throws’ you need according to your design layout. – Darrell 

DUCT SIZING CHARTS Main Trunk Runs for Approximately 450-CFM Per Ton - USE Manual D!
1.5 ton 700-cfm
 13"metal Main133"SA 760-fpm FR .07 | RA 16" 201" 500-cfm  FR 0.01" (FR. 25' run)
1.5 ton 750-cfm
 14" Branch 154-sq"  702-fpm FR .05 | RA  16" 500-cfm per ton SA/RA floor level
2-ton 900-cfm
14-SA Rd 154-sq" 854-fpm | 18"-RA 254 @ 500-fpm
2.5-ton 1125-cfm
 16" metal  Rd 201-sq"  806-fpm | RA 20" RA 500fpm 
3-ton 1350-cfm
 18" metal 254-sq" 764-fpm | RA 22"  500-FPM
3.5-ton 1575-cfm
 18"metal 254-sq" 892-cfm | RA 23" Rd 314-sq" 500-FPM 
4-ton 1800-cfm
 20"-SA 314-sq" 825-fpm | RA 24" Rd 380-sq" 573-fpm
5-ton 2000-cfm
 22" SA 380-sq" 758-fpm  | RA 24" Rd 452-sq" 637-fpm
Tonnage Charts above for Main Run Sizing, check chart below
450-cfm per ton | Maximum 900-fpm Trunk Supply Air (SA)
Runouts 500 to 600-fpm, 500-fpm or less on Returns |
Return Air 350 to 600-fpm Lower is better - use large ducts
Friction Rates will be very low!
Main Trunks 0.03 to 0.1" Supply Air (SA) Runs.
Figuring duct size - cfm from required btuh of each room. Using 450-cfm per 12,000-btuh (one ton) 12,000 /  450 = 26.66-BTUH per CFM

A room  requiring 4,000-btuh / 26.66 = 150-cfm | Chart 150-cfm = 8" round metal duct or 50 Sq.In. Square duct.
I would use a 8" duct, or a 50 sq.in. duct. Using 425-cfm per ton 12,000-btuh 4,000-btuh is a third of 12,000-btuh.
Therefore, 4,000-Btuh room requirement, using 450-cfm per ton (airflow 1/3 of a  ton) X's .3333 = 150-cfm or  8" duct | Vel. 430-FPM  FR  0.05"

Converting square duct inches to round duct size, Figuring the Square Inches of Round Ducts, an 8" x 8" duct = 64-sq.ins. x .7854 = 50.26 sq. ins. You round off to 50 sq. ins. for an 8" duct.  Or, simply getting the square inches of round ducts: a 7" duct; 7" x 7" = 49 x .7854 = 38.48-sq.ins. / by 144 = .2672222-sq.ft. X's a velocity of 500-fpm = 133.6-cubic feet per minute delivered to the room; 133.6-cfm x 30 = 4,008-BTUH.

Sized for in the chart below - BTU/hr per CFM figures "are figured for heatpumps at 450-CFM per ton of cooling."
Use 800 to 900-FPM MAINS' VEL. Use an optimum of 500-FPM VEL for Supply Branch Runs | Air speed Face of Return. 

Air Filter Rack Sizing
I realize this will never happen if you use the furnace filter size; however, Air Filter Rack Sizing for efficient operation - Size Gross Return Air filter grille area for 200-sq. ins. per ton. For a 5-Ton system, that would mean Two filter racks 25X20's each, I would go with Two racks & 25X25's each.

Keep air velocities through the filter(s) as low as possible. It is a good idea to Size the Filter rack for a Free Air Area with a Velocity of 350-FPM or less.

A 5-Ton system requires two filter racks to achieve a low enough air velocity through those filters. Most 5-Ton systems nearly always have too much air velocity through initial clean filters, let alone when they begin loading.

ACCA Manual D specifies a maximum Return grille velocity of less than 500 ft per minute and a maximum Supply outlet of 700 ft per minute (fpm) or less.

All filter mfg'ers should print the free air area of the clean filter on the edge of the filter along with the pressure drop data. Divide the rated CFM the duct is carrying by the free area sq.ft. of the filter  for airflow velocity in FPM. (Or use above forumula with the duct's sq.ft. area for duct airflow velocity)

Velocity in FPM =  Known designed CFM to room divided / by Sq. feet of duct area.
 I.E., 8" duct 8x8 = 64 x .7854 = 50.26-sq. In. area / 144 = 0.3490666-sq.feet | designed CFM to room is 173-CFM X's  .3490666 = Velocity of 495.6-FPM, you can use a ductulator to get the actual Friction Rates (FR).

Formula for finding CFM Airflow and/or Velocity in FPM
If you can measure the air velocity coming from a duct, here is a rough ballpark formula to get the CFM:
CFM = (velocity in (FPM) Feet per Minute times the square footage of the duct area)
I.E., 16" Rd duct 201-sq.ins. / 144 = 1.3958333-sq.ft. X's Velocity of 800-fpm = 1116-CFM
Times 1000-FPM = 1395-CFM. Branch ducts: 7" Rd duct 38.48-sq. ins. = 0.2672222-sq.ft. X's 500-fpm=133-cfm
All filter mfg'ers should print the free air area of the clean filter on the edge of the filter along with the pressure drop data. Divide the rated CFM the duct is carrying by the free area sq.ft. of the filter for airflow velocity in FPM.
 
 Remember:
When sizing ducts, the use of one Fiction Rate value throughout will usually guarantee incorrect duct size, velocities & CFMs for some duct run segments. If Supply Side is 0.05" per 100 ft of duct run, or less is used, some runouts of the system may be oversized, creating zones that are too cold in summer. Usually the furthest from the air handler will be undersized creating zones that are too warm in summer. Install dampers in all Supply Air ducts for some balancing.

That is an incorrect design. The modified equal friction method of Manual D requires that the Available Static Pressure (ASP) from the fan be "consumed" by the duct through its run from fan to outlet/inlet, with no shortage or excess at the end. Also, pressure drop per 100 ft is not an input--it is a calculated intermediate value.

A contractor or Tech who knows how to do the proper calculations to determine the available static pressure - ASP and correctly allocate it to supply and return, is a very rare Tech indeed. First, select the CFM & velocities you want, that may result in small Friction Rate variations on the various Branch Runs & run lengths.

Always use "Manual D" for proper FRs & duct sizing for required CFMs to each room.
I used figures & formulas below for the table, & wanted 450-Btuh wet coil per ton airflow
Converting square duct inches to round duct size, i.e., an 8" x 8" duct = 64  -sq.ins. x .7854 = 50.26 sq. ins. You round off to 50 sq. ins. for an 8" duct.
Round duct diameter to Sq. Ins., duct diameter 6"x6" = 36 X's .7854 = 28.27-sq. ins.
CFM = (velocity in (FPM) Feet per Minute times the square footage of the duct area)
Quick method: The heat gain and Btu/hr of cooling is done for each room.
Use 30-Btu/hr for each Cubic Foot per Minute (CFM) of Airflow.
 
 Duct Sizing Chart for Approximately 425 to 450-CFM Wet Coil Per Ton on Main Runs 
BTUH
 CFM
 Rd. DIA.
 Sq"
 Vel/FPM
1290
 43
 4" BR. Outlets
 12.5
 493
  



2031
 68
 5
 19.6
 499




2910
 97
 6"
 28
 489
10x8
 12x7
 9x9
3200
 106
 6" Br
 28
 540
14x2
 7x4

4008
 134
 7" Br
 38.48
 502
10x4
 8x6
 7x6
5200
 173
 8" Br
 50
 496
7x7
 10x6 8x7
6666
  222
 9" Br
64
 505
8x8
 10x7
 12x6
9812
 327
 10" Br
78.5
 600
14x6 12x8
 16x5
18000
 700
 13"Metal SA|RA 14" 154-sq"
 201 RA
 RA 502
 RA 14x11
 12x12 18X8
 16X9
24000
 900
 14" Metal SA/RA 16" 201 RA
 RA 645
 RA 14x14
 18x10
 20x8
 14x12
32000 1125
 16" metal SA/RA 20"
 254 RA
 RA 20" 516 RA 14x18
 20x10 21x11

36000
 1350
 18" SA  RA 22" 380"
 254 SA
 RA 22" 512 24X11
 16X16
 18x14

42000
 1575 18" metal SA/RA 22"
 380 RA
 RA 597
 14x22
16x16

48000
 1800
 20" SA/RA 24"
 380 RA
 RA 573
 14x27
 20X19


60000
 2000 22" SA/RA 24" 452-sq.ins. 452 RA
 RA 637 RA 14x32
 16x30
 22x21


Solving for Available Static Pressure (ASP) - When Designing or Redesigning Duct Systems, TEL, FR:
Find the Total Return and Supply Lengths by measuring duct, runouts, and adding transitions, turning elbows with or without turning vanes, trunk take-offs, and boots, diffuser pressure drops, filters, etc.

Once you have all the correct Pressure Drops (PDs) for those lengths & devices, use the manufacturer's nameplate pressure (IWC) or .5" ESP and subtract all airstream device pressure losses (filters, wet coil, etc. all available in manual D) from that given value. That will leave the "Available Static Pressure" - ASP for duct & blower design purposes.

You can figure the Total Equivalent Length (TEL) by using the Manual D length additives for the various fittings, then use your Duct Designer ductulator to properly size the duct system to meet the velocities and CFMs required in respect to the blower's Nameplate ESP and its performance graph data.
There are charts available to determine what the total pressure drop will be when you figure the "Total Equivalent Length" run of the longest Supply duct runs; ALL lengths of duct and ALL fittings for turning, reductions, etc., have a Total Effective Length (TEL) additive to the duct length - that must be added to the regular length of that duct run!

The best thing to do is figure your available static pressure. using .5” ESP is good, because most furnaces are designed for .5" ESP to get desired CFM, if needed, you can always use a lower blower speed. You subtract any external pressure drops to the furnace evap coil, filter, registers, dampers, anything outside of furnace cabinet.

Then get a total equivalent length of your ductwork most ductulators have this on the back of them.  Friction Rate = Available Static Pressure times X’s 100; divided by the TEL, that is the friction rate per 100 ft of SA & RA duct. 

TEL ASP FR Chart Graph  Too large a file loads slow using dailup - Save the pdf to a quick access PC folder for review.

It will deliver the required CFM with some blowers, but at a higher operating cost.
If there are any ductwork air leaks, it will throw everything off.
Nearly all duct systems have a percentage of air leaks.
Check for & minimize air leaks.
Blower wheels, evaporator coils, etc., everything has to be clean.

Get copies of the ACCA Worksheet; below is my math involved with the linked Graph:
On older furnaces, the .45-Device Pressure Loss (DPL) should be subtracted from a furnace Nameplate Max ESP .50-ESP, leaving only .05-ASP (Available Static Pressure).
According to the graph, (using a .45-DPL), ASP only .05 X 100 = 5 / by 330 = .015 Friction Rate, way too low & off the graph resulting in Inadequate Airflow!

It would need a much shorter TEL or less DPL's to meet design functionality!
Air Turning vanes in 90-ELLs greatly reduce the TEL.
Other changes could also help reduce the TEL.

First find 330-TEL graph, then look at the ASP at the bottom the the FR shown.
Also, a .06-FR is the lowest Friction Rate shown on that graph; though many use .05-FR on the Return Side.
Once the ESP has been determined, look at the fan curve for that particular blower and determine the CFM from that chart.

Looking at the product data chart, what temp-rise does the mfg'er recommend?
Measure the temp rise & see what you get.

Unless the proper CFM heatload goes through the evaporator coil it is nearly impossible to achieve an accurate & proper refrigerant charge, and BTU/HR along with efficiency will be way below Ratings! 
Take the static pressure measurements on both the Return and Supply Plenums of the furnace (with filter(s) in place.  

Drill two holes large enough to insert the static pressure tip, one on the supply side and one on the return.  Pressure measurements are then taken at each location.  The measurement on the return side will be negative with a positive reading on the supply but you disregard the positive/negative and just add the two numbers together.

When using two tubes (neg. & Pos.) on a modern gauge you will read what the gauge indicates!

Once the ESP has been determined, look at the fan curve for that particular blower and determine the CFM from that chart.

If the air flow is not per manufacturers' recommendations, it is near impossible to get the refrigerant charge correct.

My Scan of My ThermoPride OL 11 Graphed Blower-Curve-Chart
Thermopride OL 11 Graph ipg image - Thank you Dave Staso, CA. for the better expandable image!
"After it loads Right click "Show Original Images" - Move cursor arrow over graph - Click + when 'over graph' for expanded image," then print on the highest quality setting.

Every manufacturer should furnish blower curve charts with their units and also put them on the Internet for service tech's to download and print. Also, air conditioning codes should be updated in respect to proper sizing of the duct work which must include all the pressure inducing factors when sizing the supply and return ducts. Also, illustrate best furnace to evaporator coil transitions, especially on oil furnaces!

The evaporator must be mounted 4 to 6 inches above this model oil furnace to achieve adequate airflow!

Service techs' put your Magnehelic gages' and Digital Micromanometer to good use to measure the static pressure and then get and apply the blower curve charts on each system you are working on, then you know you're getting the proper evaporator airflow and temperature and heat-load to meet the customer's desired humidity and temperature comfort zone.  It is always very good practice to measure the Total Static Pressure (TSP) on all systems each side separate for comparison, then add-to + for the totlal SP; you can do this with a simple magnehelic gauge. In any case, static pressures above 0.5"-IWC should be investigated and reduced to specifications.

DUCTWORK BASICS - Solving the Mysteries of ESP - External Static Pressure:
If you leave out the area up to & including the A-Coil where does that leave you? The area to the coil & including the coil can represent major Velocity -FPM losses & huge Static Pressure problems in Oil furnace applications. If there are any existing heating or cooling airflow problems, you can & should also measure & record each branch run static pressure, & velocity, then use the formula, CFM = (velocity in (FPM) Feet per Minute times the square footage of the duct area)
I.E., 16" Rd duct 201-sq.ins. / 144 = 1.3958333-sq.ft. X's Velocity of 800-fpm = 1116-CFM
IF Times 1000-FPM = 1395-CFM. Branch ducts: 7" Rd duct 38.48-sq. ins. = 0.2672222-sq.ft. X's 500-fpm=133-cfm Deliver the exact CFMs to & through each diffuser that the application calls for!
Always look at the amount of CFM the diffuser's will actually pass at a specific velocity!
DUCTWORK BASICS
In the linked pdf above: from pages 8 through 11 do NOT use those rules of thumb for sizing equipment & airflow to rooms! Use Manual J for heat-gain/heat-loss for each room & thus the total. Use the Manual S for heating & A/C equipment sizing! Then figure the airflow & ductwork sizing required for each room according to the airflow requirement for the cooling load, or Heat Pump load using Manual D.

Ductwork Retrofitting - An Excellent Economic Opportunity - Don't Miss IT! 

"More than 80% of the duct systems in residential and light commercial applications 'do not' work as designed." Do your service agreements include the duct system? If not, this is an important & significant business opportunity that you are missing!

Your best access into the duct renovation market is to include the duct system in your service agreements. What this includes is having the service tech measure static pressure on each service visit. Remember, this takes five minutes or less. If pressures are very high, or very low, send out a tech competent salesperson with a flow-hood and a manometer to identify the problem, and propose a bid to do the necessary ductwork fix.

One reason we have service agreements is to gain additional income from repairs, "so start repairing the real problem with the system," and not just the equipment. In most areas of the country there is very little competition for quality duct renovation and air balancing.

Prescribing HVAC repairs without competition from other contractors in a way that will greatly improve performance and efficiency, will delight your customers - that's our definition of opportunity.

Below is an outstanding PDF "Basic AC Overview - Specifications VS. Reality"
by John Proctor, P.E., Proctor Engineering Group, LTD:

HVAC TECH PERFORMANCE RATINGS 

"AC Specs vs Reality" PDF - It's Worth Your Time
--------------------------------------------------------

EFFICIENT INDOOR COMFORT  - An EXCELLENT SITE  for You
FREE HVAC Resources for Professionals
===================
Quotes from linked PDF:
"Proper sizing, installation and maintenance of HVAC equipment are major
factors in operating efficiency. In fact, the potential energy savings from a quality installation are greater than those gained from the installation of high efficiency equipment.

Proper sizing and installation can result in energy savings of up to 35 percent for air conditioners and 16 percent or more for furnaces. Moreover, energy-efficient installation and proper maintenance practices also provide substantial non-energy benefits, such as greater comfort, lower maintenance cost and longer equipment life." Specification_of_Efficient_Installation1-36.pdf

==============
Darrell Udelhoven |  udarrell.com 

Gurgling sounds at TEV: Low evaporator heat-loads lead to reduced liquid line mass and increased evaporator mass could be due to airflow problems. Eliminate low evaporator heat-loads before looking into adjusting the refrigerant charge.
Gurgling and pulsation noises at the expansion device can be caused by low charge, and/or non-condensibles and moisture in the system. Unbalanced airflow through the various distributor circuits of the evaporator coil will cause the TEV to close down refrigerant flow starving the coil. Piston-flow-rators will make it impossible to properly charge the system and cooling will be greatly compromised unless you eliminate the cause!

On every Rheem condenser cover it lists "non-condensibles and or moisture" as causes for a gurgling or pulsating noise at the expansion device. The entire evaporator circuits, may not become active for various reasons, - "the entire coil must become fully active for efficient performance."

The purpose of these recommendations is to provide liquid refrigerant at the expansion device and provide efficient operation. Hopefully, this will aid your research.  If I can be of additional assistance, contact me.
-----------------------------------------------------------

Too many do not properly purge & evacuate contaminated central air conditioning systems.

The Triple Evacuation Method is normally done on central air conditioning systems:

First, remove any valve cores with a special  valve core remover this will speed up the evacuation time. Back service valves two turns off their back seat.

1) Re-claim unit charge (Recover all the refrigerant)

2) Charge system to 150 PSIG with dry nitrogen and leak test

3) On contaminated systems replace the filter dryers. Then Repair all leak(s)

4) Evacuate system to 500 microns valve off & see if it holds 500 microns for ten minutes, if it holds, break the vacuum with dry nitrogen

5) Evacuate system to a deeper 300 microns, valve off vac pump, & again break the vacuum with dry nitrogen

6) Evacuate system to 300 microns and charge unit (Recharge with fresh clean refrigerant)

7) Check to see if the Supply and Return air ducts were correctly sized & sealed by the original installer.

Many HVAC contractors will consider this excessive time & effort for contaminated residential air conditioning systems, however it is a must for low temp applications.

The “micron” is a metric unit of measure for distance. The micron is a unit of linear measure; one micron equals 1/25,400ths of an inch. Modern high capacity vacuum pumps help speed up the evacuation process.
===============================================================
DISCLAIMER:
I assume NO responsibility for the USE of any information I post on any of my Web pages,in E-Mails or News Groups.
All HVAC/R work should always be done by a licensed Contractor & properly licensed Techs! This information is only placed on these pages primarily for your understanding & communication with contractors & techs. This information is also for the edification of Contractors and Techs. Never attempt anything that you are NOT competent to do in a SAFE manner! I am NOT liable for your screw-ups, you are liable for what you do! - Darrell Udelhoven
  • Air Conditioning Installations Now Produce 95% Failure Rate
    • Study the Failure Rate Graph - Great Information!

    ACH - Air Changes per Hour - AVERAGING INFILTRATION RATES  - New
    The Honeywell A/C Service Assistant -
    SA_Tech_Report.pdf May have promise

    RESIDENTIAL QUALITY A/C INSTALLATION NEW

       HVAC TALK #1 HVAC Talk Community
    GARDEN WEB - Heating & Air Conditioning Forum - new

    Empowerment Communications
    Darrell Udelhoven - (U-dl-hoven)
    udarrell.com
    Edited: 05/02/08
    Copyright ©1997-2008 All rights reserved
    Darrell - Bloomington