I can't believe the number of people saying a good properly installed central heating system never needs to be cleaned. Maybe they live in hermetically sealed cells? One look into a cold air return and you can see that it definitely needs periodic cleaning. The air in the house "always" has some dust in it and that's where it ends up. We just had ours cleaned and I saw what came out of it, disgusting. But always ask if they use the "air whip" to sweep/beat the inside of the metal duct. If not, don't bother. These folks used a little whip on the end of a pressure hose and swept the entire length of all the ducts. I have lighted bore scope that allowed me to see into the duct after he was done and it was spic and span. But do change your filters often, 2 or 3 times a year.
Dehumidification (air drying) in an air conditioning system is provided by the evaporator. Since the evaporator operates at a temperature below the dew point, moisture in the air condenses on the evaporator coil tubes. This moisture is collected at the bottom of the evaporator in a pan and removed by piping to a central drain or onto the ground outside.

I turned on the AC when he was finished and immediately saw black fuzz balls the size of marbles come out of several of the registers. He said that was expected since everything was shaken up. So much for describing the service as cleaning. I paid him and sent him on his way. I cannot imagine that this type of service is worthwhile in anything but a new house. Apparently in old houses such as mine I can expect that using a whole house humidifier in the winter followed by a great AC can loosen the soot. Maybe this is to be expected for the first few weeks of summer each year. I now have lined the inside of the registers with cheesecloth to catch the fuzz. I’ll see if it stops after a couple of weeks.
All these pro duct cleaners answer with "it's common sense, everything operates better when cleaner" etc. But your explanation is correct. Especially so with flex tube ducting where dust gets "stuck" between the lowest valleys (between wire coil supports) where air flow is drastically reduced -- which is more than 50 pct of the entire interior surface area of the flex ducting. That sounds like a bad thing but also has a good effect of trapping dust from escaping.
Preventing against air leakage is great, but the only way to keep dust and debris out of a duct system would be to completely seal off the return-air side of the system, which would render the system useless. Cold-air returns will always pull dust and other particles into the system. A high-MERV rated filter is definitely a good idea, but it does nothing to keep the return side of the system clean. Definitely agree though to use foil tape to seal seams, etc. Duct tape dries out over time and as it does can actually add more particles to the air.
I called a company name “GREEN RHVAC” with their $29.00 one system air duck cleaning. When the guy came with a truck I asked him to tell me how much he will charge and he said that if he gave me price and I didn’t want the service then he would charge me $89 or he would tell me how much he charge me after he finished the work. I immoderately sense that I made mistake to call him to my house. To cut my lost I asked him to give me the price and it was $1600 for two systems house. I paid $100 to him for my lesson to learn and search online to find this site is very helpfully and I should do it before to call anyone for air duck cleaning.
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The most effective way to clean air ducts and ventilation systems is to employ Source Removal methods of cleaning. This requires a contractor to place the system under negative pressure, through the use of a specialized, powerful vacuum. While the vacuum draws air through the system, devices are inserted into the ducts to dislodge any debris that might be stuck to interior surfaces. The debris can then travel down the ducts to the vacuum, which removes it from the system and the home.
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.
Stick with a maximum of 16 SEER units in most cases.  Like I said above, contractors push higher SEER units because they make more money, but these units can be expensive at this stage of development, and can drive up your HVAC installation cost.  21 SEER units run about twice the price of a 14 SEER unit, so unless you live in a desert and plan on being there for 30 more years... Likewise, a 16 SEER unit typically costs only about $600-$1,000 more than a 14 SEER, and will usually pay for itself over the lifetime of the unit.
The short answer is yes, duct cleaning, done right by a reputable company will help with the smell. It is likely though the smell is coming from another source, ie the crawl space. It could also be that another rodent is dead in the duct work. A visual inspection would be in order to check for any breach in the duct system. A duct pressure test would be able to tell you how much air is leaking from the system over all. A hole large enough for an animal to enter would loses a great deal of air, over 300 CFM. It would be worth looking into having this test done and it should cost around $200. Cleaning the air handler and evaporator coil will greatly improve the performance of the air conditioner. Dirt and dust on the evaporator acts as insulation and reduces the amount of cooling done be the coil. Any quality duct cleaning should include cleaning the air handler and coil, and the tech should allow you to watch him perform the work as it is done.
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Duct cleaning is a band aid, like washing you hands is. Under your description, why wash your hands more than once? If it gets cleaned once, it shouldn't get dirty again. Well that is, unless they touch something dirty. Keeping your hands completely clean is virtually impossible. Same as keeping dust out of your home. You clean your ducts and over a 3-5 year span they get dirty again. Also, duct cleaners are not licensed but should be certified by NADCA(National Air Duct Cleaning Association).
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On the other hand, if family members are experiencing unusual or unexplained symptoms or illnesses that you think might be related to your home environment, you should discuss the situation with your doctor. EPA has published the following publications for guidance on identifying possible indoor air quality problems and ways to prevent or fix them.
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