Heating, ventilation, and air conditioning (HVAC) systems have been shown to act as a collection source for a variety of contaminants that have the potential to affect health, such as mold, fungi, bacteria, and very small particles of dust. The removal of such contaminants from the HVAC system and home should be considered as one component in an overall plan to improve indoor air quality.

One of the best ways to know if your ducts or vents need to be cleaned is to just check them. Your eyes will be able to check for indications of mold, dust or pollen buildup. You might also be able to smell the presence of mold, which would be a clear indicator. Ductwork will have some buildup of dust, since the return registrars pull air back in. However, this doesn't mean your ducts have too much debris, and you can easily clean them with a duster or vacuum.
A little simple math can help determine the size system you need. A rule of thumb is 20 BTUs per square foot. So, a 500 square foot room would need 10,000 BTUs to cool or warm it efficiently. This assumes that you live in a temperate region and have adequate insulation with no energy loss. In the real world, all units have some degree of energy loss. This is reflected in an HVAC system's SEER rating for cooling and AFUE rating for heating.
Natural ventilation is the ventilation of a building with outside air without using fans or other mechanical systems. It can be via operable windows, louvers, or trickle vents when spaces are small and the architecture permits. In more complex schemes, warm air is allowed to rise and flow out high building openings to the outside (stack effect), causing cool outside air to be drawn into low building openings. Natural ventilation schemes can use very little energy, but care must be taken to ensure comfort. In warm or humid climates, maintaining thermal comfort solely via natural ventilation might not be possible. Air conditioning systems are used, either as backups or supplements. Air-side economizers also use outside air to condition spaces, but do so using fans, ducts, dampers, and control systems to introduce and distribute cool outdoor air when appropriate.

If the one noticeable spot is where the wall [or ceiling or floor] is white.......first.....take a pair of needle nose pliers and bend each louver of the register to a straight outward position [as they all come brand new with a slant to each side]....so the register is shooting all supply air straight outward. If its a round register [in ceiling].....no help there. You also should always burn soy or beeswax candles.
I guess I should present my credentials first, I am a 30 year veteran of the heating and cooling industry. I started at the bottom and have participated in every area of the trade. Installer, service, maint. Ownership, rep, territory manager and so on. A forced air duct system is a large vacume. Like your vacume it has an air intake (return air) and an exhaust (supply registers). A filter just like a vacume (located next to furnace or air handler). Now I want you to visualize something. Put your head inside your vacume cleaner and breath. Get the picture? This is what your duct work looks like. Do you clean your vacume when it’s dirty? If it’s full and the filter is dirty you lose performance and it starts poluting the air you breathe. It’s common sense. I recommend hiring a duct cleaning outfit that you have researched to be honest, background checked, drug tested, insured, and highly recommended by your piers in the area. The service is a necessity, but like everything, it should be done by a qualified company.
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.
I to had an odd layer of white'ish dust building up especially during seasonal periods when the HVAC ran a a lot. I carefully swept up the fine dust into a small pile and had it analyzed. Turns out it was very fine fiberglass powder dust. It turns out it is the insulation form the attic! (My attic has blown in insulation). Turns out if you (I) had small gaps around my HVAC registers in the ceiling whee the airs blows into the room. What occurred in my case was that the exiting air at that seam creates a vacuum effect at the grate vent cover and draws and then blows fine insulation dust from the attic into the house. Removing each register grate and sealing each tin outlet gap with calking between the dry wall ceiling and tin fixed the issue. A couple were so bad with bowed gaps I had to screw down the bowed tin to the attaching stud better. Checking the very same glass top desk that catches and shows any dust accumulation now shows only very small amounts of normal house dust build up over several weeks. A big, big improvement. You might have a look by simply removing a couple vent covers yourself.
I originally took off one star because of the SNAFU with getting the part ordered.  When Alberto called me on the phone he admitted the person had "gotten busy with other things" and apologized.  THAT was taking the high road and I was impressed by that.  Sadly, for reasons I won't guess at, Alberto has decided to take the low road in his comments and imply that they got the part as quickly as possible.  Well, we called at 2:30 so did someone spend six and a half hours finding the part??  That seems very doubtful.

In modern buildings, the design, installation, and control systems of these functions are integrated into one or more HVAC systems. For very small buildings, contractors normally estimate the capacity and type of system needed and then design the system, selecting the appropriate refrigerant and various components needed. For larger buildings, building service designers, mechanical engineers, or building services engineers analyze, design, and specify the HVAC systems. Specialty mechanical contractors then fabricate and commission the systems. Building permits and code-compliance inspections of the installations are normally required for all sizes of building.
So, naturally I respond with, “you don’t tell your doctor which medication to prescribe you, do you?” The point is, on the residential level, who you get to install your HVAC unit is far more important than which unit you choose.  Keep your options open and let your HVAC contractor make a few recommendations – that’s why you spent time finding a reputable contractor!
I am a service technician [since '92].....and as to double filtering.....I have never recommended it. Granted.....it would be noticeably helpful if the fiberglass filters are the main ones used [which I also never recommend.....as long as system can deliver appropriate air flow with a pleated cotton filter...and any decent service tech can check the air flow for proper cfm]. Any particles small enough to pass through a pleated cotton filter are going to pass through a second filter as well....unless the second filters are so tight that the system is starving for air flow. Has anybody ever accidentally put two filters in their automatic drip coffee maker? What if cars used two fuel filters or air filters? I have told some homeowners to try using second filters at each register [the black thin filter material cut-to-fit....same type used in many window air conditioners].....but I only suggest this when I know they will still get at least 400 cfm per ton of air flow [350 absolute minimum]......as some homeowners know others who say it has helped and it gives them a peace of mind knowing they are doing something to help contribute to a solution, so it HAS to help [even though I think it falls in the "one-born-every-minute" category.....similar to duct cleaning].
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.
Natural ventilation is a key factor in reducing the spread of airborne illnesses such as tuberculosis, the common cold, influenza and meningitis. Opening doors, windows, and using ceiling fans are all ways to maximize natural ventilation and reduce the risk of airborne contagion. Natural ventilation requires little maintenance and is inexpensive.[19]
An example of a geothermal heat pump that uses a body of water as the heat sink, is the system used by the Trump International Hotel and Tower in Chicago, Illinois. This building is situated on the Chicago River, and uses cold river water by pumping it into a recirculating cooling system, where heat exchangers transfer heat from the building into the water, and then the now-warmed water is pumped back into the Chicago River.[24]
Air duct cleaning service providers may tell you that they need to apply chemical biocide to the inside of your ducts as a means to kill bacteria (germs) and fungi (mold) and prevent future biological growth. They may also propose the application of a "sealant" to prevent dust and dirt particles from being released into the air or to seal air leaks. You should fully understand the pros and cons of permitting application of chemical biocides or sealants. While the targeted use of chemical biocides and sealants may be appropriate under specific circumstances, research has not demonstrated their effectiveness in duct cleaning or their potential adverse health effects. No chemical biocides are currently registered by EPA for use in internally-insulated air duct systems (see Should chemical biocides be applied to the inside of air ducts?).