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 27,236 posts, read 33,636,824 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? 2 ton ac unit powerYou would need two units in that example, wouldn't you?4 ton ac package unit 8,535 posts, read 12,566,730 timeshide ac window unit 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. 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 3,750 posts, read 15,267,568 timesUnless you have a zoned system, a single AC unit can have you changing the thermostat constantly. It can be challenging to maintain a comfortable temperature in an older two-story home with only one central air-conditioning unit. Using two systems has many advantages over a single unit, but it is not always the best choice. The age and design of your house may affect your decision, along with the costs of purchasing and running a second central air-conditioning unit. Cost Considerations Purchasing two central air-conditioning units can be more expensive initially than purchasing a single unit, but the cost difference may not be as much as you think. You need either two smaller units or one larger unit to run the entire house.

The added cost of the second unit may be offset by the energy savings. Much depends on your house and how you use the air conditioner. Temperature Balance In many older homes, it is difficult to keep the entire house at the same temperature. The second floor is often much warmer than the first, so one area of the house often seems too hot or two cold. Using separate central air-conditioning units for each floor allows you to maintain a comfortable temperature throughout the entire house. Energy Savings A single non-zoned central air-conditioning unit requires you to keep all areas cool, even when some are not in use. In many homes, the first floor can be cooled during the daytime and turned off at night, while the upstairs bedrooms may only need to be cooled at night. In this situation, separate central air-conditioning units can result in significant energy savings. This situation can also be solved with a zoned air-conditioning unit. System Failure If one of the central air-conditioning units goes out, you may be able to keep the house cool enough with the other unit until repairs are made, though this is only important in areas where summer temperatures are extreme.

Having two units also doubles the chance of system failure and increases the cost of repairs overall. Advantages of a Zoned System It is possible to install a single central air-conditioning unit, with two or more thermostats, that can cool the house evenly. A zoned system includes dampers within the ductwork that open and close to regulate the flow of air and temperature in each zone. A regular central air-conditioning system, by contrast, pushes cool air to all parts of the house. A single zoned system has many of the advantages of a two-unit system: The house is divided into two or more zones -- in this case, upstairs and downstairs. Each zone of the house can be turned down or closed off to save energy. The thermostat in each zone controls the temperature in the zone. A zoned system costs more to install than a regular single air-conditioning unit, but the energy savings may make up the difference over time. References Green Building Advisor: Keeping Cool in a Two-Story HousePurchase HVAC: What if I Have a Two-Story Home?

Austin Air Companie: What Is a Zoned Air Conditioning and Heating System? Photo Credits Image Source/Photodisc/Getty Images Suggest a CorrectionNo 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? Or is that going to be too expensive? I'm thinking that a dual zone system would be nice anyway. 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.