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.
Ok what you said is true but in most cases ducts are in a duct flex format, and knowing that the dust in people's flex duct is just surface dust, any real debris that are in the duct itself will result in very little to no air flow which will keep your unit from performing properly, but if it is true allergy reasons then replacing your duct system is better for overall better air quality, plus if you do pay attention to the warranty on flexible duct is that it only has a 10 year warranty and typically only lasts for 20 years so do your research before having a duct cleaning, and to add one more thing if your duct work is completely metal and you can't see visible insulation then your duct on the inside has insulation inside of it, if you replace that with duct work that is clean metal on the inside and wrapped with insulation on the outside then it will further reduce the air bourne fiberglass and dust.
in response to this reply " should your ducts be cleaned", i have to politely correct the person who answered this. you are not using common sense. first, most of the country uses metal duct work, so for you to say what you did, is wrong. second, the duct system in a house is a circulatory system. that means it draws air into the furnace, it gets filtered, and then it is blown out into the house to heat or cool. So, obviously that air is being brought into the house from somewhere right? Yes, its coming from the outside where there are many allergens and dust particles . For you to says that about whether a duct system is sealed or not, makes no difference. The air is coming from somewhere right? It has to originate from somewhere right? Just look on top of your cieling fan. If there is a bunch of dust up there, then there is obviously a bunch in your ducts. And the duct air is being circulated every day, and every hour through your house. Its simple, dust collects everywhere in your house. Imagine if you never dusted your shelves, or tv? There would be a ton of dust after a while. Well, now think about the air in the duct system. Of course theres gonna be a ton of dust in there. For you to say that it is not benififcial to clean the ducts, is plain ignorant. 90% of the air we breathe is in our household. Wouldnt you want that air to be as dust and allergen free? I could go on for hours about the benifits of duct cleaning, but i just wanted to make a quick point that the person whose says it is not benificial to clean them, is not using common sense. Not to mention, giving the people who clean ducts for a living, a bad name. Use your head .
Because an air conditioner moves heat between the indoor coil and the outdoor coil, both must be kept clean. This means that, in addition to replacing the air filter at the evaporator coil, it is also necessary to regularly clean the condenser coil. Failure to keep the condenser clean will eventually result in harm to the compressor, because the condenser coil is responsible for discharging both the indoor heat (as picked up by the evaporator) and the heat generated by the electric motor driving the compressor.
The most recognized standards for HVAC design are based on ASHRAE data. The most general of four volumes of the ASHRAE Handbook is Fundamentals; it includes heating and cooling calculations. Each volume of the ASHRAE Handbook is updated every four years. The design professional must consult ASHRAE data for the standards of design and care as the typical building codes provide little to no information on HVAC design practices; codes such as the UMC and IMC do include much detail on installation requirements, however. Other useful reference materials include items from SMACNA, ACGIH, and technical trade journals.
Within the construction sector, it is the job of the building services engineer to design and oversee the installation and maintenance of the essential services such as gas, electricity, water, heating and lighting, as well as many others. These all help to make buildings comfortable and healthy places to live and work in. Building Services is part of a sector that has over 51,000 businesses and employs represents 2%-3% of the GDP.
Typically speaking, the lower SEER, more basic models are also more reliable.  Reliability is big with us, so we steer clear of features with reliability issues, and it’s as simple as that. When you factor in the price of repairs and the extra expense of the 21-SEER, you may actually lose money! 16 SEER is the sweet-spot, and it is key to reducing your HVAC installation cost.
Some of the research I have done suggests to have your ducts cleaned when you first purchase a new home to remove drywall dust. Maintain your system well and you shouldn't have to do it again until you want to sell. Those are the two times cleaning was suggested. So for peace of mind maybe it is a good idea to do it even if you buy a used home and then maybe every 5-10 years.
Sounds like your first hvac company was ripping you off. Ask your new company to go to your coil and see if there is a leak. If there isn't the last company was just lining their pockets. If you have had ducts unhooked for a long time I would recommend duct cleaning, trust me I'm not a fan of duct cleaning myself. I have done hvac for 20 yrs. in GA. When I just bought a home built in 1996, we had all the ducts replaced immediately.
I took off stars for two reasons. Minus one star for issues trying to use the $50 for $25 voucher I purchased through Yelp. Although there was no minimum stipulated, the office wouldn't take it for the initial $59 diagnostic fee. If I used the 10% off First service that they offer, then they won't let me use the voucher on the actual repair bill.  I was stuck with paying the full $59 diagnostic fee.
Venice warns homeowners to beware of air duct cleaning scams, especially the sort where unscrupulous cleaners offer a $49 special deal but start piling on extra fees. “It’s a bait and switch scam where they say they’ll offer unlimited cleaning, but then they throw around terms you might not understand, such as extra fees for a ‘main duct line,’” he says. “And many times, these cleaners end up walking out the door with twice the amount of money a reputable duct cleaner would charge. They’ve gotten very sophisticated at upselling.”
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.

There are many interesting comments from several informed individuals. The question that has not been answered is how do duct systems get dusty and dirty? As a RESNET Home Energy Rater, I perform pressure tests on homes and duct systems, and the one common thread is . . . most air delivery sytems have large amounts of air leakage. The solution is to seal the duct system, and pay particular attention to the return duct system and ductwork near the air handler (the 'blower'). As others have said, a captive (sealed) duct system with proper filter maintenance should be more then adequate for most occupants. For those who have allergy, asthma, or other breathing problems, a filter with a higher MERV rating (number should be listed on the package) or a HEPA filter are good choices. These filters will capture smaller particulates and should improve the indoor air quality. Again, make certain that the entire duct system is sealed, preferably with mastic, second choice UL-181 foil tape - never use cloth duct (also known as duck) tape.
As I type this, I am having my ducts cleaned. About 30 including returns and the furnace. $543.00. I am watching the team do it and can see the clear vacuuming tubing and how dirty it has become. I like the gentleman’s comment on “stick your head in your vacuum and breath”….I get it. The team showed me the machine and it’s filtration system…and they will show me it again after the cleaning. However, like I said before, I am actively seeing the tubing that is clear before they start and how dirty it is once they begin and continue cleaning. Reputable company and not a “fly by night” coupon service. People don’t be so cheap, that is your problem.
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In some older homes, there was a grey Mylar flexible duct. After years of use, it started to decompose. A duct cleaning company can come out and find why your air is so dirty and why your building doesn’t heat or cool like it used to. This is a savings on your part. You could also be sucking in Hot, dirty attic air. However, sometimes when a duct cleaning company comes out to clean it, the old duct can be destroyed and have to be replaced. Someone with little experience can replace this ductwork but if it isn’t done right it can ruin a lot of things and cost a lot in repairs.
SEER is useful for comparing one model to another much in the same way that a car's calculate MPG is useful. It's not an accurate prediction of exactly how efficient the system is, but it can tell you which one is more efficient. Also, since SEER is based on a "cooling season", what region you live in will determine how long or short your cooling season is.
Window air conditioners cost less than central units, averaging $300. These systems generally suffice to keep a room cool on warm spring and summer days. They can bring added comfort for a reasonable price but are less powerful than a central air conditioner. If you have a bigger home with multiple rooms, you will probably need to have a larger system installed. Central air can cool several rooms at once, while window AC units usually only cover one or two rooms at a time.
Engineers have pointed out some areas where efficiency of the existing hardware could be improved. For example, the fan blades used to move the air are usually stamped from sheet metal, an economical method of manufacture, but as a result they are not aerodynamically efficient. A well-designed blade could reduce electrical power required to move the air by a third.[28]
You can install the greatest HVAC system ever built, but if your house is not insulated well, it's only going to end up costing you more money. Older homes may have fiberglass insulation that has broken down due to pests, moisture, or improper installation. Spray foam insulation can be damaged by careless repair or modification work or can be improperly installed. If your insulation in compromised, your attempts to save money and be comfortable will be in vain.
BIG THANK YOU for being honest, and advising us that for some reason the fuse/switch was in the reverse position so the AC unit would not properly turn on.   The unit turned back on immediately, and began cooling the house.   And then providing advice that the AC/Heating system needed some TLC, and giving us options to save money by purchasing a maintenance plan that included the filters plus cleaning services since we were already being charged for the service today.
You may consider having your air ducts cleaned simply because it seems logical that air ducts will get dirty over time and should be occasionally cleaned. Provided that the cleaning is done properly, no evidence suggests that such cleaning would be detrimental. EPA does not recommend that the air ducts be cleaned routinely, but only as needed. 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.
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