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The AC system (3 ton unit for 3000+ sqft house) seems too small per HVAC contractor. He said we need to replace with 5.5 ton at minimum and the duct work seems under sized as well. He gave me the quote of 8500~11k to upgrade it. Is his price reasonable? What if we just add another 2.5 ton unit, will that cost so much as well? This is one story house with walk out basement. The HVAC was in basement. Any information is appreciated.
Ultimately it will be more efficient purchase and running wise IMO to have one larger unit vs. two small ones. We have two, 2.5 ton units and the only benefit is when one goes down it's not an emergency to get it fixed.
If I had my way I would replace and run with one unit.
FWIW I would think a 3 ton is a little on the small side, we have two 2.5 ton for 2000 sq ft.
The new regs do require a calculation for proper sizing but that's no guarantee of accuracy either.
Since this is an existing situation - I'm curious what the OP's experience has been with the existing unit? If it's really a 5.5 ton house then a 3 ton unit would run out of capacity in a pretty obvious manner. Can you maintain a set point temp of say 72F summer and 70F in the winter? If you maintained 70F here over the last few weeks then its hard to argue the capacity (ton rating) is that far off.
Before looking at equipment, look at your home's thermal and vapor barriers (i.e. insulation).
Look for points of air infiltration (gaps or cracks). You can possibly seal them up with spray foam (i.e. Dow GREAT STUFFâ„¢).
That would be known as a blower door test which checks all of those things to identify envelope leakage areas of the home around windows, doors, electrical outlets, chimney, etc...typically done as part of an energy audit.
I would appear to me that he "ball parked" it! Just curious but how old is the house. Newer houses are more energy efficiency and the old rule of 600 sq ft/ton generally doesn't apply (not that it was ever that accurate). To get a properly sized system the calculations have to be performed! As noted, always get several quotes--you may be surprised at the differences.
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