46 posts, read 182,329 times 3,191 posts, read 6,854,645 times well since hubby is in the HVAC biz and sitting right here, I'll try to pass on what he is saying... 2 AC systems in a 2 story home is used to provide the same heat & cooling temperatures on both levels. ( warm air rises, therefore more cooling may be needed there If you had one system without zoning, the upstairs will often be warmer than down. Some companies put in a zoning system to monitor each level ,with one AC system, to help maintain the same temperature throughout the whole house. 2 AC systems on a 1 story may be needed if the house is large or the length of the house is very long, or depending on the layout /locations of certain rooms. The largest residential AC system is 5 ton. There are some one story homes that need more 5 tons of cooling. To determine what size unit is needed, a qualified HVAC contractor will perform a manual J load calculation. hope that helps you some

684 posts, read 1,642,638 times Originally Posted by workingwifeandmom This is just supposition, but what if you wanted two different temperatures in different areas of the house, or for an addition to the house? You would need two units in that example, wouldn't you? 8,535 posts, read 12,566,730 times crazyma stole my thunder..... it is VERY important that the HVAC contractor perform the ACCA Manual J calculations to determine the heating / cooling loads of the house..... that way, the system(s) can be sized properly, giving you the best efficiency for the space and your buck. 385 posts, read 935,462 times 16,275 posts, read 44,896,608 times Originally Posted by latetotheparty You are VERY correct in restating this!! There is no eye balling it, there is no ton per every so many sq feet 'rule'. Every house is different!! ( and there are 'contractors' out there that have no idea what a manual J is scary thought ain't it) 4,010 posts, read 7,401,586 times

No membership or credit card requiredHvacElectricianNo thanks, I'm not quite ready. I have a Cape Cod with 1000 ft2 on the first floor and an apartment in the attic. In a few years I may get rid of the apartment and build up a second story + attic. Both the first floor and the attic currently have window AC units. The heat is hot water radiators and the boiler is on gas. Does it make sense to install high-velocity AC in the crawl space for the first floor and later, when I build up the second floor, add another AC system in the new attic to cool it?do portable ac units use a lot of electricity Or is that going to be too expensive?ac dc power supply schematic I'm thinking that a dual zone system would be nice anyway. air conditioner outside unit cost

Are two AC systems twice as expensive than a single dual-zone one? Or what is the ratio? Also, what are the prices of such systems? A dual-zone system is one heating element and/or AC coil serving two separate ductwork segments. The extra cost of a dual-zone system as opposed to a single-zone system is in the dual-zone temperature monitoring/control panels, and a system-controlled diverter that sends the air to the side of the system that needs it (or both). One interior and one exterior unit to buy/maintain. Drastically reduces most routine maintenance costs and generally increases MTBF. Easier to balance your home's ventilation; you don't have to worry about the relative heating/cooling capacity of two units versus the space they're expected to cover. Dual-zone systems usually have a "master/slave" control panel arrangement allowing the "master" panel to control both zones while the "slave" panel can either also control both or only control the secondary zone, possibly with additional restrictions like max/min temperature that can be set on the master panel.

if something on the one unit goes, there is no heated/cooled air anywhere in your home. Impossible to differentiate the electric costs of you versus an upstairs tenant. Longer ventilation runs to the second zone, depending on home design; the second zone needs vents and returns run from the main unit to a usually distant part of the home. These longer runs will not be as efficient. To supply ample heated/cooled air to both zones at once, the unit must usually be a bit beefier than a comparable single-zone system, to compensate for inefficiencies and maintain desired airflow when both zones are being ventilated. You cannot have the heat on in one zone and the A/C on in another. The entire system must be set to heat or cooling, meaning if your tenant likes it substantially warmer or cooler than you do, the ability of the system to provide the proper temperatures can be limited at times. A dual-unit system will have two separate single-zone HVAC units each controlling one area of the home (upstairs-downstairs is common in new construction, as is having a second HVAC for a new addition).

Having a backup system means at least some of your house can still be heated or cooled in the event of a failure of one unit. Vent runs can usually be shorter, as the units can be strategically placed in new construction for the most efficient ductwork layout to each zone. Each unit can be smaller than a single unit for the whole house would have to be, meaning the exterior units can be located in tight spaces or more easily hidden behind shrubs. By hooking the second HVAC along with all upstairs circuits to a sub-meter, you can easily determine (and sever) the electrical costs of an income property. More expensive to put in than a single unit, even a dual-zone. Two 1-ton units will cost more than a 2-ton. Maintenance costs also increase; with two units the MTBF of a single unit in the home is halved, meaning on average you'll have to call the repair guy twice as often.There are some nice thermostats that can communicate wirelessly (usually as part of a whole-house automation/alarm system), but basically each unit will be its own completely separate system and to balance the temperature in the whole house you must go upstairs and down to fiddle with settings.