Put the calculations next to each other
ACCA's Manual J is the residential load-calculation method. Its published contents cover design conditions and home characteristics. ACCA Manual J information.
Request the calculated heating need and the important inputs from each bidder. Compare which spaces are included, dimensions, insulation and window assumptions, indoor target, outdoor design conditions, and how air leakage was addressed. Ask which details were checked at the home.
A different outdoor design condition or a missing space can explain a meaningful difference. Ask the designer to resolve it rather than adding an unexplained cushion to the total. A calculation report is useful only if its inputs fit your home.
If one contractor did not calculate the load, ask what basis supports the recommendation. Do not equate a walkthrough or square-foot rule with a documented calculation.
Compare output with output
Request the exact model number and rated heat output for each proposed size. Confirm whether the number on the sales sheet is fuel input or delivered heat. Ask how applicable model instructions and site conditions affect selection.
For staged equipment, request both lower and higher output levels. The maximum size matters, but so does how the furnace will operate during more ordinary conditions. Do not assume a lower stage makes any maximum size suitable.
ACCA distinguishes calculating the home load from selecting equipment using performance data. ACCA Manual S overview.
Check the rest of the fit
Ask each bidder how the furnace fits the required airflow and controls. If cooling equipment is present or being added, compare that match too. A heat-output number alone does not show that the blower and air path suit the complete project.
Request the startup checks and handoff record that will confirm the installed setup. If a bidder says the current air distribution will work, ask what supports that conclusion. Keep any other-trade work and its responsibility visible.
If the differences remain unexplained, obtain an independent qualified design assessment. Tell that reviewer about the conflicting proposals and supply the reports, rather than asking only which brand they prefer.
Your next action is to request a one-page explanation from each bidder showing the home heating need, chosen output, and reason for the match. The right size is the one supported by the home's design and the equipment requirements, not the most confident presentation.