I purchased one of those Amazon air duct cleaning coupons for 49.99. When the company showed up, they removed the vent closest to the air intake and immediately told me I needed $1800.00 for all new duct work. They said they could not clean the system until new duct work was installed. I thanked them and said I would give them a call. The only call I made was to Amazon to get a refund. I know I didn’t need new duct work. Be careful to not fall for unscrupulous salesmen and their pitches and scare tactics.
One of the difficulties encountered by many air duct cleaning service companies is that their attention is not really on air duct cleaning. For example, many heating and cooling companies offer duct cleaning as a supplemental service, but of course their primary business is installing and servicing heating and cooling equipment. For these companies, air duct cleaning is not always given the attention it deserves and is often done by less experienced employees whose interest is not in duct cleaning but in repairs and installs. And it is almost always done with inferior air duct cleaning equipment. Still other firms (like carpet cleaners) offer a large variety of services and claim to “do everything.” Because of their lack of focus they may not actually be very good at many, if any, of the services they offer. They cannot simply buy an attachment for their carpet cleaning machine and magically become a great duct cleaner.
There is a lot of good, general information in the article. As with anything, the homeowner should do their due diligence in getting enough information to make an educated decision. Should duct-work be cleaned? YES! Does it need to be cleaned annually or even every second or third year? Not necessarily. It all depends on a bunch of variables. You're right that a well designed and balanced system will deliver the right amount of air flow, but sometimes conditions outside the home/business, make it a necessity to get the ducts cleaned. As for mold? NO filter is going to address a mold issue. If you have mold, you have a moisture issue that needs to be addressed. Also, in my 38 years of experience in the HVAC and sheet metal industries, washable filters are one of the reasons ducts need to be cleaned. They are no where near efficient in cleaning/filtering the air to the level that they should.
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.
Professional cleaning involves getting the dust and debris outside, so they use outdoor ventilation equipment. It is so important that homeowners make sure the professional contractor is using good equipment then, as they could otherwise spread the spores into the home and further agitate sensitive family members. In the case of mold, a cleaner will only be able to tell you it is there, followed by needing an additional mold professional to come out and test or remove the mold. Air duct cleaning industries do not require state licensing, so make sure to check for a company with references.
1. Ducts need cleaning mostly because they were not installed properly. There should be regulatory inpection and approval of clean and proper duct works on new houses. I was extremely unpset when I looked and found significant amounts of construction dust in parts of the ducts in my new house. There is apparently no way to hold the bullder to be responsible for clean ducts.

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).
Heaters exist for various types of fuel, including solid fuels, liquids, and gases. Another type of heat source is electricity, normally heating ribbons composed of high resistance wire (see Nichrome). This principle is also used for baseboard heaters and portable heaters. Electrical heaters are often used as backup or supplemental heat for heat pump systems.
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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