Design-build puts the designer and the installer under one contract. That can simplify your life, since one company answers for the drawings, the equipment, and the startup. It also changes what a bid comparison means. When two proposals arrive with different totals, the gap may sit in the equipment, in the duct work, or in engineering hours that one company folded into another line.
ACHR News's piece on the consulting engineer's role in design-build makes the point that design-build is "reshaping, not eliminating" that role. As a homeowner, read that phrase literally. The engineering still happens. Your job is to make sure you can see it on paper.
Find the sizing number in the paperwork
Ask every bidder for the load calculation behind the equipment size. A usable calc lists the indoor design temperature, the outdoor design condition, the assumptions about the building envelope, and the resulting heating and cooling loads in Btuh. If a bid shows a tonnage or an input rating and never mentions a load, you are looking at a product choice, not a sizing decision. That may still be a fine outcome, but you should know which one you are buying.
Check whether the calc exists in the package at all. In a design-build proposal it may appear as an attachment, a single line on the quote, or nowhere. Missing paperwork is information. It does not prove the work is wrong, but it does mean the design hours were either absorbed into the install price or never priced.
Put every bid on the same assumptions
Two bids can look far apart because they assumed different things. One may size for a hotter design day than the other. One may count the attic insulation as existing while the other plans an upgrade. One may keep the current duct layout while the other adds new returns. Before you compare totals, write each bidder's assumptions down and line them up side by side. If the numbers do not share a starting point, the comparison is noise.
Ask each bidder to state the indoor setpoint, the outdoor design condition, the infiltration assumption, and the duct plan. Ask whether the duct design was done for the new equipment or carried over from the old system. Those four answers tell you more than the headline price does.
Read the low bid for missing hours
A low design-build number is not automatically a deal. Sometimes the efficiency is real, the crew is good, and the margin is simply thin. Other times the lower bid has no engineering line and no load calc, and the design work was never priced at all. That gap tends to show up later as oversized equipment, short cycling, or a duct run that cannot move the air the equipment needs.
Test the low bid with three questions. Who did the load calculation, and can you see it? Does the equipment capacity match that calculation within a sensible range? If the installer changes the duct layout during the job, who approves the change and who updates the drawing? A clear answer to each one is a good sign. A shrug is a reason to slow down and ask again in writing.
Keep the design record with the house
Once the job is done, file the load calc, the equipment submittal sheet, and the final duct drawing together. Those documents are the only way a future technician can tell what the system was built to do. Without them, the next service call starts from guesswork, and the person paying for it is you.
If the project will happen while someone in the house needs steady help, plan that schedule too. A crew working inside an occupied home can disrupt daily routines, and you may need to decide in advance who is present, when the equipment is off, and how the house stays comfortable during the changeover. If that is your situation, contact a locally owned St. Louis home care agency to talk through the timing before the crew arrives.