Without proper ventilation, carbon monoxide can be lethal at concentrations of 1000 ppm (0.1%). However, at several hundred ppm, carbon monoxide exposure induces headaches, fatigue, nausea, and vomiting. Carbon monoxide binds with hemoglobin in the blood, forming carboxyhemoglobin, reducing the blood's ability to transport oxygen. The primary health concerns associated with carbon monoxide exposure are its cardiovascular and neurobehavioral effects. Carbon monoxide can cause atherosclerosis (the hardening of arteries) and can also trigger heart attacks. Neurologically, carbon monoxide exposure reduces hand to eye coordination, vigilance, and continuous performance. It can also affect time discrimination.[15]
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 next step was cleaning the cold air returns. He found a good spot to drill an 8” hole for his vacuum hose. The procedure matched the other air ducts except that when he finished at the two cold return registers he drilled several holes in the cold ducts in the basement and blasted air into them. Because of our houses age and the upgraded return system, I noticed that the main sheet metal return duct had no top but was just butted up against the ceiling in the basement. When he blasted air in, dust and crap shot out the edges along the ceiling. Once he finished that he removed the filter and “swept” out anything at the bottom of the return with his hand into the 8” hose.

I took off stars for two reasons. Minus one star for issues trying to use the $50 for $25 voucher I purchased through Yelp. Although there was no minimum stipulated, the office wouldn't take it for the initial $59 diagnostic fee. If I used the 10% off First service that they offer, then they won't let me use the voucher on the actual repair bill.  I was stuck with paying the full $59 diagnostic fee.
I noticed a lot of negative feedback, though unfortunately there are many dishonest companies out there. I currently work with a company that cleans duct work and we DO NOT use any scare tactics or up sell services or equipment. Once every 10years of course there are exceptions. We clean supply, returns, air handler, housing, motor, and fan. Though be discerning when qualifying anyone to work in your home. I would ask for REAL REFERENCES to contact.
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.
HRV/ERV Ventilators: These whole-house ventilators exchange air. They are ideal for homes with what is called a “tight envelope,” homes where house wrap has been applied and airgaps in the attic, around windows, doors and outlets have been sealed. Air in these homes becomes stale and polluted, so exchanging the air is important to IAQ. Heat recovery ventilators (HRV) are designed for cold climates. Energy recovery ventilators (ERV) are best for warm climates.  Your cost for an installed HRV or ERV will range from about $2,000 to more than $5,000 based on its capacity and features.
A do-it-yourself approach will only scratch the surface because it is hard to reach some of the ventilation running underneath the floor or into the wall. Although it may help in clearing some of the dust and debris, you really need a professional to guarantee your system is truly spotless. The proper procedure involves the use of a powerful vacuum system with multi-brush attachments designed to loosen debris and feed into the suction. Particles are then blown outside of the house or passed through a HEPA filter inside.

EPA does not recommend that air ducts be cleaned except on an as-needed basis because of the continuing uncertainty about the benefits of duct cleaning under most circumstances. EPA does, however, recommend that if you have a fuel burning furnace, stove, or fireplace, they be inspected for proper functioning and serviced before each heating season to protect against carbon monoxide poisoning. Some research also suggests that cleaning dirty cooling coils, fans and heat exchangers can improve the efficiency of heating and cooling systems. However, little evidence exists to indicate that simply cleaning the duct system will increase your system's efficiency.

Many disconnect blocks contain two cartridge fuses. Check them before you proceed with repairs (Photo 3). A blown fuse is a sign of a failing part inside the condensing unit. So don't just replace it and think you've solved the problem. Instead, replace the parts we show here. Then install new fuses and fire up the unit. If it blows again, call a pro—you've got more serious issues.
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