low air flow

[mishenkler]

My daughter's home is about 3600 sf on 3 levels located in suburban Washington, DC and is about 14 years old. The previous owner finished the basement where the furnace is located and installed flex duct and commercial ceiling diffusers in an ACT ceiling. The existing HVAC system is a 3.5 ton Trane unit with a .5 HP motor which I estimate is providing about 12-1400 CFM from the blower motor. Even after shutting down the diffusers in the basement, which is still comfortable, there is very little air flow in the upstairs bedrooms. As a result its hot in the summer and cold in the winter. While I don't believe the condenser unit is undersized, I am concerned that if I replace the furnace and blower motor with a multi speed motor with a 2000 cfm capacity I will be introducing other problems into the system. For example, at 2000 cfm there will probably be more noise plus if the A coil isn't increased to provide more surface area the air will be flying past the coils in the summer and if I increase the A coil should I also upsize the condenser unit. Any suggestions? Also if I keep the furnace output the same do I need to worry about the increased blower capacity or just set it to 1400 cfm for heating operation. Also any input on the quality of manufacturers equipment would be appreciated.

[dash]

You don't want more than 1400 cfms for a 3.5 ton system.

The duct system may need a serious overhaul,furnace with variable speed blower,may help some or take care of the problem,but more likely it's the ducts to the second floor that are just to small.

Also look at the return air path from there,as a possile problem.

[honglo]

dash, a heat load sounds in order. For a quickie, you might turn the unit off in mid-day and observe the average temperature increase in each room. If you have 8 foot ceilings and an avg temp increase of 2.5 deg/room per hour, your going to need 7.2 tons of ac at 2880 cfm! Otherwise, you'd better plan on zoning and not try to cool the whole house at one time.

[mattm]

Change the filter and clean the coil.