Some systems include an "economizer mode", which is sometimes called a "free-cooling mode". When economizing, the control system will open (fully or partially) the outside air damper and close (fully or partially) the return air damper. This will cause fresh, outside air to be supplied to the system. When the outside air is cooler than the demanded cool air, this will allow the demand to be met without using the mechanical supply of cooling (typically chilled water or a direct expansion "DX" unit), thus saving energy. The control system can compare the temperature of the outside air vs. return air, or it can compare the enthalpy of the air, as is frequently done in climates where humidity is more of an issue. In both cases, the outside air must be less energetic than the return air for the system to enter the economizer mode.
If you are using existing ducting, it will have to be inspected. Proper ducting loses around 2% to 5% of your energy. Old, leaking ducts can lose 50% or more. A contractor will need to have the ducts inspected and replace any parts ahead of time. If you are changing the size of your HVAC system because of significant changes to your home, you might need to replace the ductwork regardless.

The AC contactor (relay) and start/run capacitor(s) (see illustration below) fail most often and are inexpensive. So it's a safe bet to buy and install those parts right away, especially if your air conditioning service unit is older than five years. The condenser fan motor can also fail, but it runs about $150 — hold off buying that unless you're sure that's the culprit.
Our unit is 16 years old and we've had it checked out recently since it seems to be running excessively. Over all the tech says it is putting out well; it's the correct size, we have no shade and we've been having extreme heat with high humidity. The info you provided is very helpful and will help us make an informed decision when the time comes. Thank you so much.

Had duct cleaned (apartment) however, dust is still settling throughout the apartment. Contractor said no other cleaning necessary until two (2) years. Is there a time period before dust no longer settles prior to that time? Should the vendor had worn a mask? He did not cover his face the vents and dust was blown throughout the apartment (had family help with the cleanup). I had to leave when they left because I have asthma and I couldn’t breathe without coughing. Please help!

I live in a 1950's ranch house in the South. The prior owners were smokers. Based on recommendations from one of the local conservation organizations, I decided to have a home energy audit done. As part of their findings, they recommended additional insulation in the attic and duct cleaning and sealing. This turned out to be a very problematic process. In terms of the ducts, it was necessary to remove the metal grates. However, these grates had been painted over many times and shoe molding covered the bottom of the grates. The vendor was not able to remove the grates without damaging them in the process. Because these grates dated back to the 1950's, it was difficult to find appropriate replacements. The ones I finally discovered were correct except they were 1/4" wider. This meant that the shoe molding had to be carved out. The duct cleaner could not provide any assistance with this problem. It took me a month of long evenings replacing the vent covers myself, carefully chiseling out the shoe molding and repainting. In terms of the extra installation in the attic, the same vendor used blow in insulation. Unfortunately, they did not recognize that the 13 floodlights whose mounting extended into the attic were not designed to be buried under insulation. The problem manifested itself in terms of expensive flood light bulbs failing continually. So, I had to have all of the light housings replaced. Yet another unexpected expense. Despite the expense, pain and suffering, I cannot report any improvement in my heating costs or the "quality" of the air. To be fair, I do not have any allergies and did not undertake this for air quality improvements but rather heating efficiency improvements.
When these aren't properly maintained and sanitized, they can build up dust, pollen, and even mold. This means when the air conditioner or heater turns on, the spores are released into the air for you to inhale. While some are less harmful, people who have allergies or sensitivity to certain spores will react negatively. Plus, mold is dangerous for anyone to ingest and should be avoided by regular maintenance.
Professional cleaning involves getting the dust and debris outside, so they use outdoor ventilation equipment. It is so important that homeowners make sure the professional contractor is using good equipment then, as they could otherwise spread the spores into the home and further agitate sensitive family members. In the case of mold, a cleaner will only be able to tell you it is there, followed by needing an additional mold professional to come out and test or remove the mold. Air duct cleaning industries do not require state licensing, so make sure to check for a company with references.

Just had my air ducts cleaned. They used a high pressure air hose in all the heat ducts and cold air returns. He showed me the stuff that came out, the reason for the duct cleaning was the fact that I had a problem. With rodents. There was quite a few mummified rats and mice, and he said I wouldn’t have to have them done again unless I incur another problem or have renovations done. Cost me 345.00 for a 2 storey large old home. Not a bad price as I had them done when I moved in 28 years ago and it cost me 500.00. Lots of dog hair, toys, balls and assorted goodies came out as well. Took them 2 hours. I did have sanitizer put in the ducts because of the dead rodents.
Whether or not central air adds value to your home depends on your location. You probably won't recoup the entire cost for the installation, as it is an expensive project that isn't as flashy as something like a new deck. But this invisible upgrade can help you sell your home if you live in a hot climate. And many buyers are willing to pay a bit more for a working AC.
2. If air quality is so important and duct cleaniliness is so uncertain, why not adopt doubling filtering; that is, in addition to the existing main furnace filter, add a filter at each air outlet. I understand replacing so many filters would be a burden, but air outlets can be designed for easy filter replacement. The builders and home design engineers should bear responsibility to provide good and low maintenance homes, rather than prioritizing on fancy or costly featuers for homes.
20 yrs ago I bought the house I am in and a Central H/A was installed at that time. At the beginning of ea season I have the unit serviced. Every yr since I have owned the house I have had to have freon added and been told that I must have a freon leak. Fast forward to this yr when I hired a new company to do the service. They went up into the attic as part of their overall servicing. Nobody else has ever done that. He came down showing me pics on his camera phone of ducts that were not even connected to anything and in my estimation have probably been laying there for 20 yrs. Of course he fixed this. I have not needed freon this year either. I have been complaining to friends about the huge amount of dust and how I can't keep up with the cleaning away of it for a long time. Now it makes so much sense.To me it is like that duct work was just up there laying around and every time my unit was blowing, all the insulation, debris, dust and whatever else over the years was just randomly filling up those ducts. What now that they are re-connected? Will it be better? Or am I someone that needs to have my ducts cleaned? I am so confused after reading all these comments. Can one of you experts tell me what you would do before I spend a bunch of money that I don't really have? Thank you for any input you can provide. Also, feel free to respond to email address denise@digitalflak.com
You and your family spend hours sitting on your favorite chairs, couches and sectionals. Over time, dander and body oil become trapped in the upholstery, but with regular cleanings you can keep these delicate fabrics looking, feeling and smelling great. We examine your furniture and select the best process to safely clean and protect any kind of upholstery, including leather. 
An air conditioning system's SEER is especially important if you live in a climate that changes temperature dramatically. The SEER is determined by the cooling output during the winter divided by its electric input during the winter. The higher the rating, the more efficient it will be. In January of 2006, the U.S. put standards in place for cooling units which are still in effect today. They must have a minimum SEER of 13. So, if you live in a home with a system installed before the new standards went into effect, consider having it replaced. SEER 13 units increase home efficiency by 30 percent.
Did you know that cleaning your ducts and vents can help airflow and increase energy efficiency? According to the Environmental Protection Agency (EPA), keeping your air ducts and vents clear can increase energy efficiency and indoor air quality. While the cost to clean ducts and vents might sound high, the benefits to homeowners–especially those sensitive to allergens–is worth considering.
In the United States, HVAC engineers generally are members of the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE), EPA Universal CFC certified (for installation and service of CFC HVAC devices), or locally engineer certified such as a Special to Chief Boilers License issued by the state or, in some jurisdictions, the city. ASHRAE is an international technical society for all individuals and organizations interested in HVAC. The Society, organized into regions, chapters, and student branches, allows exchange of HVAC knowledge and experiences for the benefit of the field's practitioners and the public. ASHRAE provides many opportunities to participate in the development of new knowledge via, for example, research and its many technical committees. These committees typically meet twice per year at the ASHRAE Annual and Winter Meetings. A popular product show, the AHR Expo, is held in conjunction with each winter meeting. The Society has approximately 50,000 members and has headquarters in Atlanta, Georgia.
One of the difficulties encountered by many air duct cleaning service companies is that their attention is not really on air duct cleaning. For example, many heating and cooling companies offer duct cleaning as a supplemental service, but of course their primary business is installing and servicing heating and cooling equipment. For these companies, air duct cleaning is not always given the attention it deserves and is often done by less experienced employees whose interest is not in duct cleaning but in repairs and installs. And it is almost always done with inferior air duct cleaning equipment. Still other firms (like carpet cleaners) offer a large variety of services and claim to “do everything.” Because of their lack of focus they may not actually be very good at many, if any, of the services they offer. They cannot simply buy an attachment for their carpet cleaning machine and magically become a great duct cleaner.

Just had my air ducts cleaned. They used a high pressure air hose in all the heat ducts and cold air returns. He showed me the stuff that came out, the reason for the duct cleaning was the fact that I had a problem. With rodents. There was quite a few mummified rats and mice, and he said I wouldn’t have to have them done again unless I incur another problem or have renovations done. Cost me 345.00 for a 2 storey large old home. Not a bad price as I had them done when I moved in 28 years ago and it cost me 500.00. Lots of dog hair, toys, balls and assorted goodies came out as well. Took them 2 hours. I did have sanitizer put in the ducts because of the dead rodents.
I hope that those customers you have with breathing problems don't follow your advice too closely. You are doing them a disservice. Dirt in your ducts does not always stay there, depending on what the contaminants are. I bet you tell your customers not to worry about mold either- just to kill it with bleach. Educate yourself before opening your mouth- You may kill someone someday with your opinion.
Central home air conditioner service systems consist of two major components: a condensing unit that sits outside your house, and the evaporator coil (often referred to as an A-coil) that sits in the plenum of your furnace or air handler. The refrigerant in the A-coil picks up the heat from your home and moves it to the outdoor condensing unit. The condensing unit fan blows outside air through the condensing coil to remove the heat. The condensing unit houses the three parts replaceable by a DIYer: the AC contactor, the start/run capacitor(s) and the condenser fan motor. The condensing unit also houses the compressor, but only a pro can replace that. The A-coil has no parts that can be serviced by a DIYer.
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