“Your house is this many square feet, so you need this size” leaves out much of the work. Windows, insulation, orientation, air leakage and ducts can affect heating and cooling needs.
You don't need to do the calculation. Ask the installer to explain how the house led to the equipment being recommended.
| Part | What it answers | ACCA method |
|---|---|---|
| Calculate the home's load | How much heating and cooling does the house need under the design conditions? | Manual J |
| Select equipment | Which equipment meets those needs at those conditions? | Manual S |
| Design air distribution | How will the ducts deliver the needed air? | Manual D |
These methods answer related questions. A load calculation doesn't show that the existing ducts can deliver the air, and a model's advertised capacity doesn't describe its performance in every condition.
Local submission requirements depend on the project. Check those with the building department rather than assuming every replacement requires the same documents.
Mention added insulation, replacement windows, additions and duct changes. Describe rooms that are uncomfortable and when you notice it.
Replacing equipment with the same size as the old system doesn't explain whether any of those changes were considered. Ask which house information was used, especially if a recommendation seems based only on what is installed now.
Capacity describes heating or cooling output. Efficiency describes output relative to energy used under the rating method. A more efficient unit isn't automatically the right size.
For cooling, a ton means 12,000 Btu per hour of capacity, not the equipment's weight. Don't use a tons per square foot shortcut to choose a system yourself.
For a heat pump, ask about heating output under the outdoor conditions used for selection and what supplemental heat contributes.
If the old ducts stay, ask how the contractor evaluates whether they're suitable and which changes are included. A larger unit alone doesn't explain how more conditioned air reaches a particular bedroom.
Request a summary of the calculated heating and cooling needs, design conditions, important house assumptions and selected equipment. It gives you a concrete basis for discussing another contractor's recommendation rather than comparing sizes without the reasoning behind them.