The EPA is so far behind on useful knowledge because of the lack of funding so don't be mislead. The Hvac old timers were left at the dock so to speak and missed out on the IAQ movement. The ones that are savvy and up to date on the information do Air Duct Cleaning, even tho that will lead to a few less service calls its the right thing to do for their customer. Please take a look into a return duct that has never been cleaned, I dare anyone to say it does not need to be cleaned.
Indoor Coil -- The indoor coil is a heat transfer device. It absorbs the heat from the inside of the house and passes it on to the refrigerant and is pumped outside. Dust that builds up on the coil can hamper its ability to absorb heat. High heat transference coils use very thin metal. Airborne chemicals can cause corrosion which leads to leaks. The constant vibration of the compressor can also cause solder joints to weaken and leak. An indoor coil may operate for weeks with a tiny leak, and you may not notice the loss in performance right away. As soon as a leak is made known, it should be replaced or repaired immediately.
AC units and thermostats have built-in delay features when they're shut down and then repowered. The delay can be as long as 10 minutes. And, if you've subscribed to an energy-saving device from your local power utility, the unit can take even longer to reset. If you've installed the parts shown and reinstalled the disconnect block, repowered the circuit breaker, turned on the switch at the furnace, moved the thermostat to AC mode and lowered the temperature below the indoor temperature, and the unit doesn't fire up after 30 minutes, it's time to call a pro.
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).

He proceeded to clean each heat register by removing the grille and blasting it with an air pressure hose toward the duct opening thinking that the dust would magically be sucked inside. In one room this caused a ceiling smoke detector to go off because the excess dust fooled it into thinking there was smoke. I put my hand in front of a register he wasn’t working on and could hardly feel a suction. Although we discussed how I open and close damper doors, I realized later that he never checked, opened or adjusted them during the cleaning. He could have closed all but the one he was working on in order to increase suction results. His next step was to blast air into the duct and then attach a “squid-like” device that slapped around inside the duct as far as he could reach. This is important! In a newer house, ducts have long straight runs from the furnace. In older houses like mine they can turn and twist. In fact, the duct we have a problem with runs straight up from the basement inside a wall and turns 90 degrees into a soffit for three feet then into two 90 degree turns to get into the floor joist area for another two feet and finally a 90 degree turn up to the second floor register. When he cleaned this one he could only get three feet into the register with the hose or squid because of the turns. That was the case with most. Hardly what could be described as a decent cleaning.

When central air conditioning service fails during a heat spell, you may have to wait days for an HVAC repair technician or an ac contractor to show up, and you'll probably pay at least several hundred for the repair. But if you're comfortable working around electricity and are willing to spend about $50 on parts, you can probably repair your air conditioning service yourself in about two hours and save about $225 on parts markup and labor.
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