I need some advice. I have an addition that was not cooled or heated as well as the rest of the house. I found out that two of the three ducts were disconnected and when I reconnected them the addition cools to within one degree of the rest of the house on a 90 degree day. A contractor wishes to upsize two of the runs to the addition from 6 to 7 inches and enlarge the registers. One of the runs he wishes to upsize comes off near the end of the 10 foot long trunk line that is 10x8 inches and has two other 6 inch runs coming off it. I know this is simplistic but a 6 inch circle has an area of 28.27 inches and a 7 inch has an area of 38.4 inches. 2 six inch plus one 7 inch run is a total of 94.94 sq inches. Three 6 inch runs have a total area of 84.81 sq inches which is close to the 80 square inches the 8x10 trunk. I am concerned that the 7 inch duct will steal air from the other 2 ducts coming off this trunk?
The trunk feeding the 8x10 trunk is a 28 foot long 8x18 and has 3 six inch branch ducts coming of it as well a 2x17 register and a 10x 3.5 inch register that come off nearly directly from the trunk. The contractor wants to upsize one of the 6 in branchs to the addition to seven inches.
Using my simplistic method the 8x18 currently supplies a 8x10 trunk and 3 six inch ducts and two registers. This is an area of 84.81 for the tree 6 inch runs and 25 and 25 inch for the registers.[ I decreased the area of the registers to approximate the open space between the grills]
This is a total of 214 sq inches. The 8x18 only has an area of 144 sq inches.
My questions are:
Are the demands on the system greater in the winter than in the summer since the temp differential is greater in the winter. It freq gets into the teens in the winter and occasionally to below zero. So my addition with 3 sides to the weather may have more of a problem in the winter than in the summer.
Short of a manual D which I do not know how to do and it does not seem that the contractors around here do them either, does it seem that upsizing two of the runs will increase air flow without stealing significant air from the other runs? The contractor say he will keep dampers on the 7 inch runs in case there is a problem. If he enlarges the registers and we don't get more air flow I worry that this will decrease the "throw" of the a/c air.
Any comments would be appreciated.