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5 Questions to Ask When Replacing Your Air Conditioner


Air conditioners don’t last forever. Shocking, I know. 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.

1. Will the contractor address your air conditioner replacement using a house-as-a-system approach?

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.

2. How will the new air conditioner be sized?

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:

  • Well, you’ve got a 3 ton AC in here now, so we’re just going to put in a new 3 ton unit.
  • Well, you’ve got a 3 ton AC in here now, so we’re going to add another ton to that and give you a 4 ton system. You’ll be nice and cool!
  • You told me your house was 2400 square feet, so that means you need a 4 ton air conditioner.

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. The 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.

The proper way to size heating and cooling systems is to calculate the actual heating and cooling loads for your home. To 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.

3. Does the contractor test for duct leakage?

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.

4. Does the contractor assess the air flow in the duct system and make recommendations for repairs?

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.

5. Are you using the Quality HVAC Installation Checklist from the Air Conditioning Contractors Association (ACCA)?

If you’re serious about getting aacca quality installation hvac checklist new air conditioner installed for peak performance, head over to the ACCA website and download their Quality HVAC Installation Checklist. They also created a nice contractor scorecard (pdf) to help you choose the right contractor. Finally, ACCA has a great page on their website on just for homeowners trying to find a good HVAC contractor. Check out these great resources.


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.


Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and writes the Energy Vanguard Blog. He also has written a book on building science. You can follow him on Twitter at @EnergyVanguard.


Related Articles

The Magic of Cold, Part 1 – How Your Air Conditioner Works

How to Choose a Company to Do a Home Energy Audit

Building Science 101

Case Closed: Get Those Air Conditioning Ducts out of the Attic


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This Post Has 18 Comments

  1. I would like to send this to
    I would like to send this to several HVAC contractors that are struggling with these issues, this would be a great segway to getting them to understand the importance of proper planning prevents you know what

  2. Buddy:
    Buddy: Feel free. My intent here is to educate homeowners because I think one of the reasons contractors don’t do it is that they think homeowners don’t care. If their customers start asking for this stuff, they’ll either have to do it or lose customers. But the contractors definitely need to get on board, too.

  3. Great principles. But… I
    Great principles. But… I think the contractor is ill suited to carry these responsibilities. Until we completely revolutionize the industry, the residential HVAC contractor is just not the kind of person who likes to work with numbers (if he were, he would be in a different job). And he is in a business where lowest bid gets him a job much more reliably than a sophisticated approach. The customer all too often does not have the budget to do things right. It may be the best we can achieve is to fight the worst of the wrong practices. 
    As you describe this is actually a small job of engineering and not well matched to the training and aptitude of the HVAC person. To analyze a house as a system this way, is actually a consulting function… and consulting engineering is at a whole different cost and price level than prevails in the HVAC business. 
    I am pessimistic about finding the brain talent in the HVAC industry as it stands. We need to simplify their job if possible, so there is a better chance of overall success. For one thing the equipment companies could and should build manometer probe points into their systems, to facilitate and encourage measurement of “ESP” (External Static Pressure). As much as possible the equipment should take on some duties of measurement, as Carrier’s Infinity control system does now. And possibly a more corporate way of doing HVAC service, can enforce good practices and inject more engineer-type services where they are so badly needed. 
    This industry is broken. If people worked as you imagine here, there would be far fewer problems. But there is a long and twisted road to get to that goal. 
    Best of luck — M. Johnson

  4. M. Johnson
    M. Johnson: I share your frustration. But…I know a lot of really smart people working in the HVAC industry who are more than up to the task of doing things right. Yes, a lot of the guys in the field have minimal training and skills, but there are HVAC companies out there that not only can do it right, but are doing it right. 
    Yes, customer budgets often don’t allow for quality installation when you look only at the upfront costs, but if you can show them the full benefits—monetary and otherwise—then I think they’ll see that higher upfront costs come back to them.

  5. Chris: The
    Chris: The ENERGY STAR checklist is a simpler version of what ACCA offers. Both are good, but maybe the ENERGY STAR checklist helps more with customers because their brand is more recognizable.

  6. Yes. That is exactly it.
    Yes. That is exactly it. Piggyback on a trusted brand and this give instant credibility. 
    The link I offered was a bid comparison checklist homeowners can use in comparing contractors. This is a very useful tool.

  7. In addition to everything you
    In addition to everything you wrote, I also give friends and family members what I believe is the most important advice of all — do their homework BEFORE the AC dies. Few people are going to take time to interview potential contractors when it’s 90 outside and their AC is broke.  
    Another tip is to pay attention to your system’s duty cycle during hot weather (how many minutes on and off). This can help assess how much the replacement can be downsized. 
    And finally, when in doubt, hire a 3rd party designer. Allison and Chris provide this service, as do I. I’ve yet to have a project where I wasn’t able to save clients more than my fee, while ensuring they get the right system for their needs (pardon the blatant self promotion).

  8. @David Butler: So you endorse
    @David Butler: So you endorse measuring AC duty cycles as a way to evaluate sizing? Good to hear so far. Ideas on how to get this short of the homeowner sitting with a notepad and clock? My own is an energy monitor, but that is a $300 niche product. Can such a number be estimated reasonably from smart meter data?

  9. M. Johnson – Take a look at
    M. Johnson – Take a look at the HOBO dataloggers by Onset ( They offer some low-cost loggers than can monitor temperature, relative humidity, voltage, current, and a number of other measurements. If you have access to the device you wish to monitor, a hour meter suitable for the device voltage will be less expensive. If you want a really cheap method to measure run time of a 115Vac device, head down to your local thrift store and pick up an electric desk clock with an analog dial. Set your “new” clock to midnight and tap the cord into the load side of the device controller. It will only run when energized – just like an hour meter, but cheaper.

  10. @Mark, if homeowner is
    @Mark, if homeowner is motivated, cycle timing is easy and doesn’t take long. It only needs to be done once (preferably for two complete cycles), can can be carried out while watching TV, reading or surfing the net. If blower starts and stops aren’t obvious to occupants (noise and/or air movement), I recommend taping a strip of confetti or toilet paper to the grille. 
    Here are some general rules: Cycle timing should be done in late afternoon after a daily high at or above the local design temp. The customer should note the outdoor high and thermostat setpoint. Keep in mind that peak cooling usually lags daily high by an hour or so. Fan delays should be disabled or accounted for. And finally, don’t change the thermostat setpoint in the hour or so preceding a test. 
    For an existing home, cycle timing is actually more accurate than a Manual J, provided you have the equipment’s expanded performance data. If other efficiency improvements will be completed along with a HVAC change-out, cycle timing will be less useful, but still provides a great point of reference to calibrate the load calc model. 
    As Tim pointed out, a data logger would be a refinement. It removes the homeowner from the loop and enables you to evaluate cycle times under a variety of conditions. Just be sure to log both indoor and outdoor temperatures. You don’t have to monitor HVAC system power directly since cycles will be obvious on the indoor temperature trace.

  11. Another easy way to better
    Another easy way to better size a system. Get the total airflow measured by flow hood, and divide by 400 or 350 for humid climates. 
    I routinely find broken down 5 ton systems and measure 1250 airflow and install 3-4 ton condensing unit replacements. Works like a charm. However it should be noted that it is not proper science.

  12. The problem is how HVAC
    The problem is how HVAC contractors are “trained”. Most start out as young guys “paying their dues” as “tin knockers”. This a a lot of hard physical labor that doesn’t pay very well. Not exactly the type of job “build science” people want to do for 2-3 years while they “pay their dues”. The “tin knockers/installers” get promoted as they learn enough to run service/repairs. 
    Nowhere along the line are proper building science concepts ever handed down from one tech to another. 
    Add in the homeowner that thinks that HVAC should last forever and gets sticker shock when it comes time to replace it in the middle of the summer/winter. We want cool/warm NOW, and don’t want to spend anymore than we absolutely have to.

  13. Hey Bob, 

    Hey Bob, 
    You hit the head on the nail about the “training” thing with reference to AC installers. This is a project I have been working on for a long time and almost have it figured out.  
    In the near horizon I will be releasing training tutorials in a “no brainer” formats, and eventually hope to cover the entire array of HVAC tech in building science. AND THEY WILL BE FREE TO ANYONE WHO WANTS TO TAKE ADVANTAGE. 
    Big words, so you will just have to wait a bit and I will start releasing the content, as it is developed.

  14. @Chris: Do you really believe
    @Chris: Do you really believe you can cover what’s necessary in Building Science, in a “no brainer” tutorial format? I am interested in seeing how they turn out, but am curious how well that format can fit the science.

  15. Hey Johnson. 
    Hey Johnson. 
    I will just have to show you. I dont want to prop up my genius only to be laughed out of Dodge. Although I dont mind a good laugh even if it is at my expense.

  16. As always Allison…great
    As always Allison…great stuff. I think you are correct in that most homeowner’s want this information, but don’t know they should ask it. Buddy….please get this out to all of SC!! And if people listen to David and prepare for that day before that day happens, things will begin moving on the right track. As someone else above commented…it is a long twisty road, but there is no way to get to the end without beginning to travel!

  17. Why ac throw is LAMINAR in
    Why ac throw is LAMINAR in operation theater?

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