Step 4. Evaluate and Select the Acceptable Candidates
These two cooling equipment candidates were evaluated throughout the example. However, an HVAC designer should evaluate and be prepared to offer multiple HVAC systems in order to meet different energy efficiency, IAQ, price points, or other needs. The evaluation of the candidates may look like this:

Figure 7: Cooling Equipment Comparison

How to Apply Manual S: Heating Example (Part II)
The cooling system has been selected and the proposed design airflow is 1,050 CFM, this airflow must also meet the furnace's requirements before it can be used for the Manual D duct design10. The XYZ model FR80-036 furnace has an OEM design temperature rise range of 35�‹F - 65�‹F (see Figure 2 XYZ Furnace Company Performance Data). The air temperature rise through the furnace depends on the airflow through the heat exchanger. If 1,050 CFM causes the temperature rise to fall outside the desired range then: the equipment may cycle off on safety limits, the furnace could suffer damage, or other unsafe conditions may be created.

ƒ¢T = Btu/h¹¹ �€ (CFM X 1.1 X ACF)
Btu/h = Thermal output capacity of the furnace
CFM = Cubic feet per minute; volume of air moved through the furnace by the blower assembly
1.1 = A physical air constant (derived from the laws of physics)
ACF = Altitude Correction Factor; 1.0 applies to sea level.

Conclusion
Equipment selection is a vital step in the HVAC system design process. But, equipment selection and HVAC system design are only ONE of the four pillars to a quality installation. The heating and cooling system may be meticulously designed, but if the equipment is improperly installed, or if the duct system is leaky, then the customer views the contractor as a bungling amateur. The solution: Design HVAC systems properly and then follow the other three pillars of a quality installation in the ACCA QI Specification; available for free download at www.acca.org/quality.

This simple example was quickly reviewed; read Manual S for much more instruction, examples, and useful information.

Notes:

1. Over sized equipment satisfies the control's set point faster, this leads to more starts and stops. Equipment startup creates extreme stress on mechanical components.
2. Manual S �˜3-10, page 3-4, Step 2 provides detailed instructions to determine the target airflow used in the equipment selection process.
3. In this case we use the homes sensible cooling requirement in the formula instead of the total cooling capacity.
4. This measurement considers the air's temperature and moisture content.
5. Air volume is measured in Cubic Feet per Minute (CFM). CFM is the amount of air in cubes (one foot by one foot by one foot) which move through a duct in one minute.
6. The XYZ model AC-36 must have 1,050 CFM over indoor air conditioner coil in order to deliver the specified cooling capacity, if the airflow value changes the equipment capacity changes as well.
7. Also a code requirement - IRC �˜1601.1, UMC �˜601.2
8. Eternal static pressure (ESP) is the pressure produced by ducts and accessories external to the rated blower assembly. The OEM provided pressure chart will define accessories that were included (e.g. a filter) and other pertinent information. Ensure you understand all of the items which affect the blower assembly's performance.
9. For more information see ACCA Manual D, section 3.
10. 1,050 CFM at 0.58 IWC is information needed to determine the friction rate for duct sizing
11. In this case we use the furnace's actual capacity in the formula instead of the Manual J heat loss. The actual furnace output capacity of 64,000 Btu/h is used, not the 56,000 Btu/h design capacity from the load calculation.