Unless 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 Correction There is no air barrier on the attic side of this sloping insulation.

I was recently a weekend guest at the house of some friends who live in Climate Zone 5 (a zone which includes Nebraska and Massachusetts). how to turn off power to outside ac unitSince I have no interest in embarrassing anyone, I won’t mention any names or the home’s precise location. residential ac unit repairThe story, however, is true.compare carrier ac units The weather was hot. The two-story house was built in the 1980s; it included a single-zone split-system air conditioner that delivered cool air through ductwork to every room in the house. My hosts complained that the second floor was always hot, even when the air conditioner was running full blast. Curious, I poked around a bit. There was an access door to a kneewall area upstairs, and I saw that the house had a crazy, chopped-up roofline with several distinct attic areas.

The second floor had varying ceiling heights, and the first floor included a “great room” with a double-height ceiling. Most of the second-floor ceilings were sloping ceilings that followed the roof plane. Houses with these features have complicated thermal boundaries, and rarely have an uninterrupted air barrier. Snooping around in the attic, I saw blown-in fiberglass installed above horizontal ceilings, blown-in fiberglass installed above sloping ceilings, and fiberglass batts installed in vertical kneewalls. Although the insulation job was neat and the insulation was undisturbed, none of the sloping or vertical insulation was protected by an attic-side air barrier. My House is Too Hot Solving Comfort Problems Caused by Attic Kneewalls How to Build an Insulated Cathedral Ceiling Questions and Answers About Air BarriersThe second-floor ceiling was peppered with recessed can lights. And there were two skylights upstairs. Needless to say, I wasn’t traveling with a Duct BlasterCalibrated air-flow measurement system developed to test the airtightness of forced-air duct systems.

All outlets for the duct system, except for the one attached to the duct blaster, are sealed off and the system is either pressurized or depressurized; the work needed by the fan to maintain a given pressure difference provides a measure of duct leakage. or a blower door, so I didn’t have a chance to test the home’s duct system or envelope leakage. But anyone familiar with homes built in the 1980s probably has a pretty good idea of what was wrong with this house: the fiberglass insulation is thin; air is leaking in and out of the building shell at a fast clip; the air leaks degrade the performance of the insulation; and the ducts are probably undersized and leaky. With a home like this, it’s very hard to design a single-zone forced-air distribution system that works as well during the summer as it does in the winter. During the winter, the second floor is usually comfortable. The forced-air system keeps the upstairs warm, but the first floor may feel cold and drafty. During the summer, however, it's the first floor which is comfortable.

While the AC unit may keep the downstairs cool, the upstairs will be uncomfortably hot. You can try to solve the winter problem by oversizing the ducts and registers serving the first floor. But that just makes the summer problem worse. On the other hand, you can solve the summer problem by oversizing the ducts and registers serving the second floor. Now the house won’t be comfortable during the winter. Of course, the right way to address the comfort problems in a leaky house like this is to perform air sealing work and insulation upgrades. Once the building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. is tight and well insulated, there won’t be any temptation to tweak the specs of the forced-air system to address oddities caused by massive air leaks. So now we come to the sad part of the story.

How did my hosts handle the overheating problem? First of all, they turned on the air conditioner to make sure that the first floor rooms were comfortable. The air conditioner appeared to work well, and since the unit was almost certainly oversized — most residential air conditioners are twice as big as necessary — the downstairs rooms stayed cool. Since they knew from experience that their second floor was hot, they kept comfortable upstairs with natural ventilation and fans. They opened the two operable skylights. They also opened both windows in the upstairs bedroom. Then they placed big rectangular fans in the open windows to keep the air moving. (I forget whether the fans were pushing air out or pulling air in, but that detail hardly matters.) There are no villains in this story — just ordinary homeowners trying their best to stay comfortable in hot weather. The result is an energy nerd’s nightmare. The building envelope had several huge holes — the open windows upstairs — with fans accelerating the rate of air leakage through the holes.

And the air conditioner was running around the clock, doing its best to cool the first floor. (Actually, the air conditioner was doing its best to cool the whole neighborhood.) When homeowners like this eventually get fed up with their comfort problem, who are they likely to call? They will probably call an air conditioner repair person. And what are the chances that Bob's HVAC(Heating, ventilation, and air conditioning). will correctly diagnose the ceiling leaks or air-barrier-continuity problems? This 1980s house is typical in many ways. It was slapped together quickly by builders who had no idea what an air barrier is. Its defects were covered up by finish materials that look attractive. And it was probably sold at a fat markup to eager buyers. The problems in this house are fixable, but the fixes aren't obvious to the typical homeowner (or even, for that matter, to the typical handyman or carpenter). Intuitive solutions aren't likely to help, and solutions suggested by a local contractor are just as likely to aggravate the problems as they are to solve them.