I have just moved into a new House in December 2001. House has 2450 sq ft, of which 700 sq ft is on 2nd floor, and there is an open Stairwell. I have a single Nat. Gas Forced Air Heating + Cooling System with two Zones. The house has 2×6 walls with blown R-23 insulation and ceilings with R-48 insulation. As expected – – Hot air rises. Last winter, the 1rst floor thermostat essentially controlled the heat due to an increase of 1 degree from the 1rst to the 2nd floor due to the open stairwell. I was O.K. with this. Likewise, this summer, the 2nd floor thermostat controls the cooling cycle due to a 2 degree difference from up to down.
Since my Zoning system only allows one Zone to operate at a time, the 2nd floor calls for cooling first since it heats up first and runs until its cooling needs are complete. All the cooling power of my system seems to be cooling the whole house through just a few vents upstairs and then “flowing†down the steps to cool the downstairs until the whole house is cooled. This seems to be inefficient. Since it is cooling the downstairs anyway (open stairwell), would it be more efficient to turn off the zoning system so that all the vents operate at one time and therefore utilizing the whole system to cool the whole house?
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" This seems to be inefficient."
Now do you define EFFICIENT. As an engeering term it is the amount of energy output by a system vs the energy input. In the case of AC is would be the amount of cooling vs the electrical power. Is this how you are using the term?
You did not say anything about comfort problems.
If you have did have the lower vents open you will still the warm air from the 1st floor rising and the cool air from the 2nd floor falling so that you would probably over cool the first floor.
How I have a 2 story house with equal sq ft on each floor. The first floor is slab on grade, partial earth contac, with lots of glass. The 2nd is conventionaly framed and less glass. Since these are two completely different enviroments I have two separate systems (this was before zoning was "common"). But I only have AC on the 2nd floor.
The only way to effectively zone a forced air system of the size and type that are common to residential applications, is multiple, separate systems. You said nothing about the ductwork, location of supply and return registers.
Would it be more efficient to disable the zone controls and let the unit run? It would be as efficient. Depending on how the zones are setup and controlled and wether are not there are minimum flows maintainaed and if the system is sized properly for only one zone to be satisfied at a time.
If it were my house, I would never have bothered with zoning. As you can tell, it doesn't work. In a house, zoning is most often an expensive gizmo taht does little to improve the comfort or efficiency(energy use for a given effect) of the residence. The money is better spent in the ductwork and the equipment. That doesn't help you, though, does it. Putting enough cold air in the 700 sf to cool the remaining 1750 from the top down would not be the best way to go.
Do you have programmable thermostats? Can you get them? I would let the system operate as good as it can, but program the zones to be the way you want them when you use them, i.e. Upstairs (bed and bath), cool in the evening and night; downstairs (kitchen, den, dining, family) cool during the day when people are downstairs.