Air Conditioning - Design Questions

I have a question about the HVAC system put into the house I built one year ago. It was designed by a licensed subcontractor, permit approved by county energy inspector, and also energy inspected/passed by local electric company.

The house is in South Florida. It is a two story CBS construction facing north. It was designed to meet maximum energy efficiency ratings here with R30 insulation in 1st floor ceilings, and in all 2nd floor interior walls and ceilings. Exterior walls of the structure are R11 (CBS w/radiant barrier & foam). Roof is full perimeter aluminum soffits with ridge venting and auxilliary pan venting on east/west gables. The 2nd floor is a 766sf open area built within the truss area of the roof and above living space on the 1st floor (i.e., surrounded with 120/140 degree heat all day). The 1st floor is 2565sf. There is open foyer space in the front between both floors. All windows are solar treated to reflect radiant heat.

All the HVAC equipment is Ruud brand. The 1st floor is UBHK-24J11SFC handler attached garage mounted and UAPC-048JAZ compressor outside. The 2nd floor is UBHK-21J11SFC truss crawlspace (no attics down here) mounted and UAPC-036JAZ outside. In an unusual design point worth mentioning the first floor master bedroom outlets are ducted to the 2nd floor handler (I was told is was done to not overload the 1st floor capacity). And the 2nd floor handler has three 13x13 filtered intakes, one in the 1st floor master bedroom and two upstairs (the mounting of the handler within the trusswork negated a more common single intake design). The 1st floor filtered inlet is a more usual 21x24.

The system has been checked by a second licensed HVAC Ruud dealer besides the one that installed the equipment. Both report the units are performing to specification and blame the house design (2nd floor within trusswork). Duct testing OK by yet a third company/opinion. Both claim the units are sized correctly for the sq footage.

Here is my issue. The 2nd floor system is on constantly from noon till after midnight (no cycling) or basically whenever outside temps are above 88. Yet the 2nd floor cannot be cooled better than 80 mid-day, 78 at night is the best can-do outside of Winter. Of course the master bedroom is low 70s due to the ducting design (though installation of a damper has helped here). Now the 1st floor unit rarely comes on (only in the summer) and when it does is on little more than 45 minutes. But it has no issues maintaining any temperature we want for the 1st floor. I have both 1st/2nd thermostats (electronic) set for 78 and desire nothing lower in temp.

My questions are, is it normal for a SEER15 unit to run constantly like the 2nd floor one is? And does it not sound like the first floor is oversized and the second floor is undersized given what I'm experiencing? I am not experiencing humidity issues - in fact just the opposite as one would expect. Very dry air conditions.

Any suggested options I could explore to improve?

Thanks,

So, 4 tons for 2565 SF (minus master) downstairs and 3 tons for 766 SF (plus master) upstairs. There isn't nearly enough information to determine if these are really sized correctly, but they seem reasonable. In fact, the upstairs unit seems a bit large.

Part of the problem is that the upstairs unit is actually cooling the downstairs too. Cool air always wants to stay lower than warm air so the cool air coming from the upstairs unit is shooting down the open foyer space almost as fast as it's pumped out. Start by setting the downstairs thermostat 2 degrees lower than the upstairs thermostat and see if it solves some of the problem. This will force the downstairs unit to take on some of the load and slow some of the down movement of cool air from the upper floor.

In addition, I understand the logic behind putting the master on the upstairs unit, and it was done with noble effort, but it's adding to the problem. How hard would it be to move the master bedroom to the downstairs unit? How hard would it be to put a duct from the downstairs unit in the upstairs open space? Often heat loss calculations don't take movement within the space into consideration.

Doug M.

The unit will run all day to maintain indoor temp when it's that hot outside. SEER has nothing to do with how the system is sized. That would be the tonnage. SEER is how efficient the unit is. If the unit runs all day the humidity should be low. I think you still have a problem because the unit should be able to pull the temp down to the set point (slowly).

Doug,
Well I tried setting the downstairs to 76 and it has stopped the upstairs from running constantly without cycling. Now things are more of what I consider "normal" runtime/cycle wise during the day for a 78 setpoint with no running at all after late afternoon. Downstairs has a longer cycle length more like what I was used to and it too does not come on after late afternoon. No running in the morning or middle of the night as I was looking for. So the HVAC is only on during the day which I would expect with the heavy insulation this construction utilized. An improvement for sure!

The downstairs master bedroom is 396sf BTW, that gets added to the upstairs sf if that helps in knowing if the sizing is correct.

But it's too chilly @ 76 so I think I will try getting that downstairs master moved to the downstairs air handler. Hopefully that will be the "fix" plus getting the two units working as a team now.

Thanx,

Well I tried setting the downstairs to 76 and it has stopped the upstairs from running constantly without cycling. Now things are more of what I consider "normal" runtime/cycle wise during the day for a 78 setpoint with no running at all after late afternoon. Downstairs has a longer cycle length more like what I was used to and it too does not come on after late afternoon. No running in the morning or middle of the night as I was looking for. So the HVAC is only on during the day which I would expect with the heavy insulation this construction utilized. An improvement for sure!
Great! You're almost there. Just remember, with the design of your house, you're probably never going to reach a point where you can keep both floors at the same temp. There are ways to accomplish that with doors, walls and insulation between the floors and designs that control circulation of air in an insulated envelope around the house to maintain balance between the floors, but you don't have any of those so you're stuck with a natural temperature gradient. Fighting nature is a futile effort. I also want to mention that variable speed air handlers could be very beneficial in your situation, but I don't know if Ruud makes a model that ramps the fan slowly up on a call for cooling and slowly down when it's been satisfied. Without that feature, I'm not sure it would be worth it to change.

Doug M