Energy Vanguard Blog

5 Questions to Ask When Replacing Your Air Conditioner

Posted by Allison Bailes on Mon, Aug 29, 2011

air conditioner hvac seer efficiency duct system oldAir 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. 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.


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

Tags: HVAC, building enclosure, heating & cooling distribution, comfort