The amount of debris or mold present in your system could certainly ratchet up the overall price. The cost of mildew removal is around $0.07-0.08 per square foot, on top of your initial cost, according to Blue Book. Removing these elements involves added time, special equipment and even special chemicals. In fact, you may be referred to a specialist if mildew and mold is too severe.
Be careful of dishonest HVAC contractors. There’s no magical formula to see if an HVAC contractor is scamming you, unfortunately, but remember that they typically do this in one of two ways: first, they are charging grossly low prices (far below everyone else), or second, they aren’t including what everyone else is. In the first situation, they will charge you change fees, other hidden fees, or they stole the equipment and are passing themselves off as legitimate contractors (very common in California these days). In the second situation, they are charging you a very fair price, but aren’t including what the other contractors are (i.e. charging $10,500 for the installation which is fair, but not including the ductwork which is a scam unless there is some other, complex stuff going on there).
Although HVAC is executed in individual buildings or other enclosed spaces (like NORAD's underground headquarters), the equipment involved is in some cases an extension of a larger district heating (DH) or district cooling (DC) network, or a combined DHC network. In such cases, the operating and maintenance aspects are simplified and metering becomes necessary to bill for the energy that is consumed, and in some cases energy that is returned to the larger system. For example, at a given time one building may be utilizing chilled water for air conditioning and the warm water it returns may be used in another building for heating, or for the overall heating-portion of the DHC network (likely with energy added to boost the temperature).
The bottom line is: no one knows. There are examples of ducts that have become badly contaminated with a variety of materials that may pose risks to your health. The duct system can serve as a means to distribute these contaminants throughout a home. In these cases, duct cleaning may make sense. However, a light amount of household dust in your air ducts is normal. Duct cleaning is not considered to be a necessary part of yearly maintenance of your heating and cooling system, which consists of regular cleaning of drain pans and heating and cooling coils, regular filter changes and yearly inspections of heating equipment. Research continues in an effort to evaluate the potential benefits of air duct cleaning.
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.
#3 The ONLY time Duct cleaning is needed is when the home had a fire, Bad roof leak on to FIBERGLASS ductwork or house sat vacant for many years with out the air running. And its better to replace than to do “abrasive” duct cleaning. If the duct cleaning has a rotating brush it will remove part of the glue that is holding the Fiberglass in the ducts together, PERIOD no matter how much the say it don’t because the brush is soft.
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