Air conditioners don't last forever. The good news, though, is that if you have an old air conditioning system, replacing it should save you money. Efficiency has improved a lot since that old hunk of metal and noise was installed in your back yard. Beyond just swapping out the old equipment for new, getting a new air conditioner really needs to involve some thought. Here are five things for you to think about and ask potential contractors. If possible, you should use an HVAC contractor who understands building science and will treat your air conditioning problem using a systems approach. An air conditioner is only one component of many that helps with the heating and cooling of your home. Insulation, air leakage, duct leakage, solar gain through windows, radiant heat problems in bonus rooms, and more all affect the comfort, healthfulness, durability, and energy efficiency of your home. To understand the basics, see our article on Building Science 101. If you ask an HVAC contractor how they're going to decide what size air conditioner to put in, there are some wrong answers that should have you yelling, "Next!"

Here are a few: The first answer is bad because they're assuming the original contractor sized the system properly—and that nothing has changed since. The second is bad because they're assuming that bigger is better. room heater/air conditioner unitsThe third is bad because using a rule of thumb based on square footage doesn't work, and they should measure the house themselves rather than relying on your number.top selling ac units The proper way to size heating and cooling systems is to calculate the actual heating and cooling loads for your home. samsung window ac buy onlineTo do so, the contractor has to measure the house completely, get all the insulation R-values, window types, orientation, infiltration rate, duct leakage, and more.

Then they put all that into their load calculation software and find out how many Btu's per hour your home needs. The most common way of doing this is to use the Manual J load calculation protocol. Proper sizing is important because an air conditioner does two jobs: (i) cools the air, and (ii) dehumidifies the air. In a humid climate, an oversized air conditioner will cool just fine but won't dehumidify well. Also, oversized systems go on and off a lot, and all those start-ups and shut-downs will shorten the life of your air conditioner. What happens inside the cooling equipment is only part of making your home cool and comfortable. The distribution system plays a huge role, and the typical duct system has a lot of leakage in it. If you're paying a lot of money each month to run your air conditioner, you don't want to waste that cooling through a lot of leaks in your ducts, do you? A duct leakage test will determine how bad your ducts are. Duct leakage isn't the only problem with getting cool air into your home.

Many duct systems don't move as much air as they should because of ducts that are too small, kinked, too long, or have other types of constrictions. At a minimum, your HVAC contractor should measure the total external static pressure and make sure it's within the limits specified for the equipment they're installing. Ideally, they'll also measure the air flow to each room to make sure your home will be heated and cooled uniformly. Remember, it's not all about the air conditioner. The overall performance depends a lot on how well the ducts move the conditioned air. If you're serious about getting a new air conditioner installed for peak performance, head over to the ACCA website and download their Quality HVAC Installation Checklist. Use it as a guide to choose the right contractor. In addition to the checklist, they have a nice page on their website on choosing the right HVAC contractor. Getting an air conditioner replaced is a big investment. It's also an opportunity to improve the comfort, healthfulness, durability, and energy efficiency of your home.

It won't happen without your involvement, though, because many HVAC contractors will be happy just to swap out old equipment for new without looking at the bigger picture. Find one who's willing and able to go beyond the box. Even better, get a full home energy assessment. How to Choose a Company to Do a Home Energy Audit Case Closed: Get Those Air Conditioning Ducts out of the Attic A few years ago, a student of mine told a funny story in a home energy rater class. He was an HVAC contractor and said he was installing a new air conditioner for an elderly woman. As he was explaining things to her, he mentioned that they would be installing a 4 ton unit. "Oh, my," she said. "How are you going to get something so big into my backyard?" The confusion here is completely natural. HVAC and home energy pros find this story funny because when you say an air conditioner is 4 tons, we know it's not weight. It's a number that tells how much heat the air conditioner can remove from the house in an hour.

(Let's ignore the issues of nominal vs. actual capacity and AHRI de-rating.) A 4 ton air conditioner is one that can remove 48,000 BTUs of heat per hour from the house. For most people, though, 4 tons means 8000 pounds. (A BTU is a British Thermal Unit, approximately the amount of heat you get from burning one kitchen match all the way down.) Most pros also know how such a common term as 'ton' turned into a bit of HVAC jargon. Before Willis Carrier invented the modern air conditioner, people used to cool buildings in the summertime with ice harvested from rivers and lakes in the wintertime. A Green Homes America article quotes ice production figures from the 19th century Ice and Refrigeration journal, indicating that the 1890 crop from the Hudson River was about 4 million tons. OK, so people used to cool and refrigerate with ice. How does that equate to air conditioning capacity in BTUs per hour, you ask? Well, let's get quantitative and find out. When ice is below freezing and it absorbs heat, the temperature increases.

When ice is at its melting point, 32° F, and it absorbs heat, its temperature doesn't change. If you've had a physics or chemistry class, you may recall that the amount of heat needed to melt ice is called the latent heat of fusion. In Imperial units, that number is 143 BTUs per pound. That's actually a lot of heat to pump into a pound frozen water. Once the ice is melted into liquid water, it takes only 1 BTU per pound to raise the temperature 1 degree. So if you've got a pound of ice at 32° F, you put 143 BTUs into it to melt it completely. Then it takes only 180 more BTUs to raise the temperature of that pound of water from 32° F to 212° F, the boiling point. Anyway, getting back to our main discussion, if you have a ton of ice, it takes (143 BTU/lb) x (2000 lbs) = 286,000 BTUs to melt it completely. You could do that in one hour or 10 hours or a year, depending on how quickly you pump heat into it. Somewhere along the line, though, someone decided to use 1 day—24 hours—as the standard time reference here.

If the ice melts uniformly over the 24 hours, it absorbs heat at the rate of 286,000 / 24 hrs = 11,917 BTU/hr. Rounding that number up makes it a nice, round 12,000 BTU/hr. In air conditioning jargon, then, a ton of AC capacity is equal to 12,000 BTU/hr. If you're wondering how this term got institutionalized, it was probably the usual way. People in the industry start using it, and then the professional organizations make it official. An architecture website has a quote from 1912 that claims the American Society of Mechanical Engineers standardized it. It sounds likely, but their numbers don't work out, so I'm gonna go with Honest Abe on this one and remain skeptical. For the fearless: If you want to read some funny HVAC banter on this topic, check out this thread in the HVAC-Talk forum. And if you figure out what 'heat of zaporization' is, let me know! It's Called an Air Conditioner — Not an Air Cooler! 5 Questions to Ask When Replacing Your Air Conditioner