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View Full Version : Sizing central AC, zones and other issues



Augusto Orosco
06-07-2012, 4:29 PM
I live in a 5000 sf house, three floors plus a partially finished basement. We have a 6 year old 5ton AC unit and three zones. Zones #1 and #2 control the east and west halves of all the floors but the third, which is controlled by Zone #3. I find this east/west zoning odd (as opposed to zoning by floor), but what do I know.

The 5ton unit seems undersized for the house and while the first floor and basement can achieve pretty cool temperatures quickly, the second and third floor struggle to even break 74F in 90F weather. At one point, the first floor reached 67F so that the second floor could be at 74F. I also suspect that the zones are not well isolated (for lack of a better term): When zone #3 works, zone #1 seems to keep cooling, even though it is well below the thermostat setting. I don't know if this is because the dampers are not closing properly, but I can clearly feel cool air flowing through the registers in zone 1, even stronger than in zone 3. Finally, there seems to be a fair amount of air leakage in the unfinished part of the basement. I can feel cool air escaping from the encasing where the evaporator and furnace are located and that area gets freezing cold, even though there are no registers there (it's unfinished).

To top it off, the system hasn't been maintained in 2 years (and I don't even know how old the filters are). I am having an HVAC specialist come next week to tune up and service the system, asses the situation and propose solutions, but I want to poke the collective brain here before hand so as to be prepared with the right questions and know what to look for.

Thanks!

Brian Elfert
06-08-2012, 11:23 AM
One of the issues with zoning is physically isolating the thermostat so cold or hot air from another zone won't affect that zone. I have two zones and the 2nd floor thermostat is in the hallway (logical location). If I reduce the air conditioning on the first floor at night the bedrooms will end up freezing because the hot air from the 1st floor rises up the stairwell. The builder should have included a door on the stairway. Reducing the heat on 1st floor in the winter really is not an issue.

It would takes years of energy savings to make up the cost of adding a door now. My cooling costs are pretty cheap as it is.

If you are feeling air leaks those need to be fixed to get the system working right. My HVAC system was installed by some real professionals who sealed every joint in the ductwork. No leaks anywhere.

Joe Leigh
06-08-2012, 1:27 PM
The bottom line is that residential zoning in a multi level home simply doesn't work. It's a cheap try at a solution that requires real engineering to solve.
In addition a 5 ton unit for a 5000 sq/ft house is woefully undersized. A basic rule of thumb for residential installation is approx. 500 sq/ft per ton. This number is based on a standard 8' ceiling height. Keep in mind this number can vary depending on a multitude of variables like square footage and type of windows, building insulation R-value, house orientation to solar exposure, climate, etc.
That would mean you need to double your current unit size for a start. Even then it would be very difficult to get proper air distribution to three different levels with three different heat loads from a single unit, and even if you could your controlling thermostat can only be in one location at a time. Not ideal.
The correct solution is a separate system for each floor of the home, whether it be conventional systems or ductless split systems.
This way the units could be properly sized for each floor load andcan be controlled independently for maximum comfort.

Augusto Orosco
06-08-2012, 1:43 PM
The bottom line is that residential zoning in a multi level home simply doesn't work. It's a cheap try at a solution that requires real engineering to solve.
In addition a 5 ton unit for a 5000 sq/ft house is woefully undersized. A basic rule of thumb for residential installation is approx. 500 sq/ft per ton. This number is based on a standard 8' ceiling height. Keep in mind this number can vary depending on a multitude of variables like square footage and type of windows, building insulation R-value, house orientation to solar exposure, climate, etc.
That would mean you need to double your current unit size for a start. Even then it would be very difficult to get proper air distribution to three different levels with three different heat loads from a single unit, and even if you could your controlling thermostat can only be in one location at a time. Not ideal.
The correct solution is a separate system for each floor of the home, whether it be conventional systems or ductless split systems.
This way the units could be properly sized for each floor load andcan be controlled independently for maximum comfort.

Yeah, I am pretty much resigned to get at least a second 5ton unit to supplement the first one. I just hope it's not too complicated to split the ducting to make the two floors independent, particularly when they have the zones currently split east and west instead of upper and lower.

Thanks for the input!

Greg Portland
06-08-2012, 1:46 PM
You need to perform a manual J calculation to correctly size the A/C. Here is a free option (http://www.loadcalc.net) that does a reasonable job. There are also residential (non-contractor) options available for <$50.

Brian Elfert
06-09-2012, 2:26 AM
The bottom line is that residential zoning in a multi level home simply doesn't work. It's a cheap try at a solution that requires real engineering to solve.


Zoning works great in my two story home and the cost was only about $700 additional when the house was built in 2001. I don't the problems typical to many two story houses without zoning with the two floors having differing temperatures. In the winter I can lower the 1st floor temp at night to save money on heat.

Mark Paavola
06-09-2012, 10:03 PM
Zoning can work very well when setup correctly. 500 sq ft per ton is an old outdated rule of thumb. Where I live, 30 miles East of Sacramento, CA. houses will be sized anywhere from 600 to 1200 sq ft per ton. It all depends on your climate and house construction. I have a 2700 sq ft house with 4 different levels with 5 zones. The key is finding a competent contractor who will perform a manual "J" at least and is knowledgeable with zoning. I have 30 yrs experience in the service and control side of HVAC.

Myk Rian
06-09-2012, 11:14 PM
I live in a 5000 sf house, three floors plus a partially finished basement. We have a 6 year old 5ton AC unit and three zones. Zones #1 and #2 control the east and west halves of all the floors but the third, which is controlled by Zone #3. I find this east/west zoning odd (as opposed to zoning by floor), but what do I know.
It has to do with the sun heating the east in the morning, the west during the afternoon.