Air Conditioning Sizing Help

Correct Sizing of Residential Air Conditioning Systems & Ductwork Systems
Procedures for Proper Ductwork Sizing for Residential Air Conditioning Systems 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).
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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.