Many HVAC companies and or contractors will give you more than one option to consider as well so that no matter what you decide on some sort of indoor air comfort solution will be achieved to atleast improve the comfort situation. Let's go ahead and rip out the 4" diameter supply runs and install 6" diameter runs so that we can circulate more air into that sun room! No, just kidding, but the air will come out a touch harder, though as a result in sealing up the ductwork better and you will see a significant reduction in your utility bill because the system will only cycle half as often now. You know, the most cost effective thing that I can think of that will improve your indoor comfort tremendously is to simply just zone your current system so that 3 different thermostats will control 3 different temperatures in your home.
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