First, I owe a big thanks to all of you who have helped me in the past. During the months of April and May I made several posts while hunting for a 5 ton heat pump that would qualify for the federal tax credit. Thanks to the suggestions on this board I found such a unit and it is now installed. I went with a Lennox XP14-060 condenser, CBX27-060 fan/coil, Comfortsense 7000 thermostats, the 26W62 fan relay, and 2 zone dampers with a LZP-4 zone control panel. Also, a barometric bypass was installed. Previously I only had one zone and the upstairs was always a little uncomfortable. The upstairs is now great. The downstairs however, will not maintain a temperature lower than 77 during the day. The outside temperature has been in the high 90's or as high as 101. The problem seems to be that the discharge air sensor reaches it's 40 degree cutoff before the unit can cool the house down below 77 or 78. This results in the unit not running long enough to cool the house down. What are the most likely causes? Thanks.