Bidding on the job now. Did a calc. on the house and it comes up to 3 tons. That's what the HO has now and he said it didn't do the job. Looked at his furnace,75,000 btu input. Problem...supply only 14X8. Same size trunk all the way to end. His upstairs is cold in the winter. Wants to replace furnace also. Did a heat loss on the house,said unit should be a 60,000. HO added a big addition to the house and it has its own heating and cooling system. Problem is that addition has big entry ways on both floors. Opens up old part of house. He is kind of tight with the money. I am going to suggest fixing main supply duct to proper size,replacing furnace with 80,000btu 90%. I'm thinking keeping the 3 ton cond. unit and throwing in a 3.5 ton coil. If it still doesn't cool I could replace cond. unit later.
Sounds to me like your on the right track for sizing purposes. I like the idea of 3.5 ton coil with existing 3 ton condenor. The 14x8 supply is way to small and sounds like his #1 problem. 26x8 supply would be better. Is the return also undersized?