Have the assumptions explained
We recommend asking for the load result, the local outdoor design temperature used, the desired indoor temperature, and the home information included. Ask whether insulation, windows, air leakage, room needs, and any planned changes have been considered.
A square-foot shortcut cannot show those details. Neither can copying the old furnace's nameplate. The old installation might have been chosen under different assumptions, and the home may have changed since then.
Larger has tradeoffs
A bigger furnace can reach the thermostat target quickly without fixing a cold room. ACCA's sizing discussion explains why closer alignment with heat loss is usually preferable for comfort, while noting that equipment selection can involve blower requirements and allowed sizing ranges. The conclusion needs the actual design, not a rule that every extra size is forbidden. ACCA furnace-sizing explanation.
If the proposed furnace has more than one heating stage, ask how its lower and higher output fit ordinary weather and the design need. More heating capacity is useful only within an installation that can operate as intended. Request an explanation of airflow fit and any other-trade dependency before approving the size.
Colorado elevation needs its own check
Elevation can change the approved fuel setup for particular furnaces. For example, Lennox's March 2026 instructions specify kit 11K46 for converting the listed EL280, SL280, EL297, and ML296 models to propane at 7,501 to 10,000 feet. This is a model, fuel, and elevation example, not a rule for your home. Have the qualified installer use your furnace's exact instructions. Do not apply a statewide percentage or turn up a gas valve. Lennox altitude-kit instructions.
Ask whether the quoted heating output reflects the approved configuration at your home's elevation. Input is the energy from fuel the furnace uses. Delivered heating output is the useful heat it supplies to the home. Those ratings and the home's calculated heating need are different numbers. Have the designer connect them for the proposed equipment.
If the proposal also includes a heat pump
Ask for its delivered heating capacity at your local cold-weather design temperature and the intended backup arrangement. A heat pump’s output can change as outdoor temperature falls, so its nominal size cannot be compared with a gas furnace’s size without the relevant performance data. PNNL cold-climate sizing guidance.
Plan for weather beyond the design point
Ask what the system is expected to do during a colder-than-design event, whether the home may temporarily fall short of the selected target, and which safe contingency is appropriate. That discussion does not justify indiscriminately adding capacity. It explains the limitations of the design you are choosing.
Your next step is a load-based proposal with stated local conditions and elevation assumptions. Compare sizes through that record, and ask for an explicit extreme-weather plan before deciding.