Understand what each number represents
A heating-load calculation estimates the heat your home needs under stated design conditions. ACCA's Manual S then uses loads and manufacturer performance information to select suitable equipment. ACCA equipment-selection guidance.
Ask whether the online result is a heating need, a suggested furnace fuel-input size, or something else. Ask the same of the quote. Numbers that look similar may describe different things.
The proposed furnace's delivered heat output matters. Its fuel input describes what goes into it. The old furnace label can be useful history, but it does not establish the home's current requirement.
Request the inputs, not just the total
Ask the contractor for a readable summary of the calculation. Review the home dimensions, areas included, insulation and window assumptions, outdoor design temperature, and indoor target used. Point out known changes such as a finished addition or improved insulation.
You do not need to become the designer. You need enough information to catch a wrong floor area, a missing room, or an assumption nobody checked. Ask which inputs were observed and which were estimated.
Then request the exact proposed model's output data and the selection explanation. If the home's elevation changes the equipment's applicable setup or capacity, ask how that was handled using its instructions. Do not assume every Colorado address requires the same adjustment.
Check fit beyond the heating number
Ask whether the selected furnace can provide the needed airflow with the existing arrangement. If AC is included, ask for the cooling equipment and airflow check separately. A suitable heat output alone does not complete the installation design.
Ask what will be checked after installation, including operation and settings against the exact model. A proposal should connect the design with a usable completed project.
If the contractor cannot provide the calculation or explain the size, request another qualified design review. A smaller number is not automatically careful sizing, just as a larger number is not automatically protection.
Your next action is to send the calculator result to the bidder and ask: “What home data and output requirement explain the difference?” Decide after that comparison. If the smaller furnace meets the documented need and installation requirements, the calculator's higher guess is not a reason by itself to reject it.