Before you choose any duct cleaning service provider, interview as many service providers as you can. Ensure that the service provider is certified by the National Air Duct Cleaners Association or NADCA. Make sure that they hold a good standing in the Better Business Bureau and have all the necessary certifications and license. Be clear about their terms of service. Make sure they have the right equipment to do the job. It is best to go for a company that has been tried and tested successfully by any of your acquaintances or opt for a service provider that has been in the business for quite some time.
Since most states and municipalities don’t license air duct cleaners, you need to check their professional credentials instead, such as NADCA membership. EPA recommends all duct cleaners follow NADCA standards. Member companies must keep at least one technician on staff who has passed a NADCA test. “They have to pass rigorous testing to earn the certificate, and our code of ethics is very important,” Vinick says.
Fuses -- Anyone who has worked with electrical systems knows all about fuses and how they fail. They can burn out over time, may just be loose, or can blow out during an electrical storm or due to overload from another failed component. Of course, that's what they're supposed to do; they stop surges from going through and damaging the rest of the system. When a fuse fails, whatever system it was protecting will stop working.
The performance of vapor compression refrigeration cycles[26] is limited by thermodynamics. These air conditioning and heat pump devices move heat rather than convert it from one form to another, so thermal efficiencies do not appropriately describe the performance of these devices. The Coefficient-of-Performance (COP) measures performance, but this dimensionless measure has not been adopted. Instead, the Energy Efficiency Ratio (EER) has traditionally been used to characterize the performance of many HVAC systems. EER is the Energy Efficiency Ratio based on a 35 °C (95 °F) outdoor temperature. To more accurately describe the performance of air conditioning equipment over a typical cooling season a modified version of the EER, the Seasonal Energy Efficiency Ratio (SEER), or in Europe the ESEER, is used. SEER ratings are based on seasonal temperature averages instead of a constant 35 °C (95 °F) outdoor temperature. The current industry minimum SEER rating is 14 SEER.[27][dead link]
Multiple inventions within this time frame preceded the beginnings of first comfort air conditioning system, which was designed in 1902 by Alfred Wolff (Cooper, 2003) for the New York Stock Exchange, while Willis Carrier equipped the Sacketts-Wilhems Printing Company with the process AC unit the same year. Coyne College was the first school to offer HVAC training in 1899.[12]

I own and live in a mobile home and was told to install a mobile home air condition well I did not want one because the ones for moblie homes are to small,so I install a home air-condidtion and its good for heating and cooling the only thing I do is change is the filter every month or two,What I want to know is how does dirt and dust comes from????
The three major functions of heating, ventilation, and air conditioning are interrelated, especially with the need to provide thermal comfort and acceptable indoor air quality within reasonable installation, operation, and maintenance costs. HVAC systems can be used in both domestic and commercial environments. HVAC systems can provide ventilation, and maintain pressure relationships between spaces. The means of air delivery and removal from spaces is known as room air distribution.[3]

EPA does not recommend that air ducts be cleaned except on an as-needed basis because of the continuing uncertainty about the benefits of duct cleaning under most circumstances. EPA does, however, recommend that if you have a fuel burning furnace, stove, or fireplace, they be inspected for proper functioning and serviced before each heating season to protect against carbon monoxide poisoning. Some research also suggests that cleaning dirty cooling coils, fans and heat exchangers can improve the efficiency of heating and cooling systems. However, little evidence exists to indicate that simply cleaning the duct system will increase your system's efficiency.
My Carrier furnace occasionally (one-two times a day) does not start. It shows error code 15 ("BLOWER MOTOR LOCKOUT -This status code indicates the blower failed to reach 250 RPM or the blower failed to communicate to the variable speed furnace control within 30 seconds after being turned ON in two successive heating cycles."). However, when I turn off and on the furnace power switch it starts and runs fine for a while. Sometimes it runs fine all day, but does not start only once in the evening or in the night. It is 13 years old I was thinking to replace whole thing if repair cost is too high. But I am not ready to pay $10,000. 
As a person with severe mold allergies, I live in a the South where the climate naturally breeds molds.We noticed a difference when we had our ducts cleaned.The company used a vacumn type system and covered all of our vents with plastic that was held up by the suction. We had no damage, no leaking dust/dirt back into the house and it took several hours. I think if you use a reputable company it makes the world of difference. Now we use better air filters and also have a UV light to prevent mold growth on ours system.We 'd do it again when the time comes. From your car's air filter to your dryer...anything works more efficiently when it is not filthy dirty.That's just common sense.
Many homes have a forced-air HVAC system. Both the heating and the central air conditioning units share a ductwork system where they either push in or pull out warm or cooled air. There are also heating and cooling systems that don’t require ductwork — such as ductless mini-splits — but work on the same principles of heat exchange. The national average to hire an HVAC specialist is $2,920-$3,670, with costs varying depending on the work you need done and the equipment you are installing.
Most people are now aware that indoor air pollution is an issue of growing concern and increased visibility. Many companies are marketing products and services intended to improve the quality of your indoor air. You have probably seen an advertisement, received a coupon in the mail, or been approached directly by a company offering to clean your air ducts as a means of improving your home's indoor air quality. These services typically — but not always — range in cost from $450 to $1,000 per heating and cooling system, depending on:
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