Hi new member with a question. May sound anal but I want to change out round ducts with rectangular. HVAC contractor my builder hired ( I was still asking for estimates and pricing out different systems at the time) came in and without my consent and ran 14 -16 inch round ducts down the middle of the 10 feet garage ceiling effectively making them just over 8 feet after being framed out and covered with sheetrock (firecode). This is a small project. We are building a three car garage with an apartment above (approx. 900 square feet). I really don’t like losing all this headspace. Because my builder allowed HVAC guy to do the work and HVAC guy won't change out the round to rectangular ducts without additional expense to me, I paid him but have told him I don't think I will use him for the rest of the installation. PS not being difficult, this HVAC contractor also said we didn't need to do a heating or cooling load calculation. He said has been in the business long enough and can size systems with out all those calculations. He said the old "rule-of-thumb" of 800 SQ FT per ton works good enough. I had to demand he do the calculations and they proved the unit he was planning to install was oversized. It was a high end 2 stage Heat Pump unit and even in the worst weather the chances were very remote I would ever use the second stage. So my question is how I can find a table or formula that can help me see what sizes I need to use in rectangular ducts to replace the round ones, and any issues or concerns anyone has regarding this. Would prefer the programs or software or help be reasonably priced and easy to use. Thanks for any help or suggestions you can offer.