Our 25 y/o, 3000 sq.ft.home has two H/AC systems. Over the last ten years we have replaced the furnace at one end of the house and the air conditioner at the other. The remaining furnace/ac are now failing, so it's time to finally make the systems compatible, but I have questions about the ductwork. My house is unusually dusty (noted by people other than my mother-in-law) and I have two allergy-ridden children. When I have the new systems installed, would I be better off just replacing the ductwork, or having it cleaned and checked for gaps.

As a certified ASCS by NADCA I would have to say it is a waste of money to replace the ductwork unless it is in very poor condition. Most contaminants can be removed by a NADCA certified duct cleaner. Replacing the ductwork is like putting a tourniquet on a paper cut. Gaps are going to exist in all residential ductwork. That is not the problem. Most of the time the problem results from poor filtration and/or poor filter maintenance. DO NOT ust the cheap fiberglass filters. They filter nothing. I would suggest an extended surface filter such as Spacegard. At least use a good pleated filter and inspect and change it on a regular basis. This will keep tou system and house cleaner as well as prolong the life of your system.

P.S. You can find a NADCA certified cleaner by going to NADCA.com and searching for one in your area.

I agree with Duct1, A spaceguard filter will do wonders for the air quality. Do not except the 1" thick plastic filters many duct cleaners are attempting to sell. They are simply to restrictive and will cause overheating and poor cooling. You should consider however caulking all joints that are accessible on the cold air side of the duct system. this will aid in prevention of pulling dust from basements and attic areas. Proper cleaning of the ducts will help also, But beware of rip off scams. We call them blow and go guys in the industry. Duct cleaning cannot be complete if they do not clean the blower and AC coil as well. The coil is the area in which most fungal growth is found. Many folks feel that mold is growning in the ducts themselves. The zink in the steel will kill most fungal growth. add the fact that no moisture is found in the ducts. No mold can develop. This does not mean that the dust is not causing your ill health it however is typically not the cause of your concerns. If you have carpets in the home, you should consider getting the carpets replaced or removed. Typically carpet that is ten years old weighs almost twice as much as it did from new. The cleaning chemicals, dust mites, and other things get trapped in the carpet that cannot be cleaned or vacummed out. A good book to get on indoor air quality and general healthy homes is My House Is Killing Me by Jeffrey May. This book may shed some light on your health concerns.

The Home Care Club Is correct. We see many"blow and go" guys in the industry. That's why they should be NADCA certified. I'm not saying all members are on the up and up but you stand the best chance of a good job with a NADCA member. That being said, if they show up with a machine called a "Rotobrush" send them packing. This machine is popular with the unknowledgable and does little but stir up dust.
Als the coils should definatly be cleaned. The only real places mold can grow is in the returns where they are in floor joists and in ductwork that is made of ductboard. This is a ridgid fiberglass coated with a foil covering. When this is contaminated with microbials the best thig to do is demo it and start over. The only "extra" that would be worthwhile is a UV light in the return. This kills microbials. Putting it in the supply side has adverse effects of plastic parts.

Duct leakage is very common with homes that are very dusty. Assuming the duct work in your house is 25 years old it is very possible that not only does it leak the insulation may have also degraded. Both of these conditions will result in high energy consumption.
There are a couple of simple tests you can do to check the duct system integrity. First, with the unit fan only running cover all the return grills with plastic to totally block the air to the unit. Then check the supply grills for air flow. If you are getting air from the vents then the return system to the unit is leaking. Second, turn on the unit in emergency heat or the furnace, and set the thermostat to allow to run for several minutes i.e. 85F. (make sure the return grills and all supply vents are full open no blockage). With a reliable thermometer measure the return air temperature and the supply air temperatures. If minimal leakage, adequate insulation, and proper equipment sizing you should get a greater than 30F rise in the supply vents in several minutes. A slow rise or if 30F is not acheivable, then ther is leakage.This will help you in determining the type of maintenance required.
Duct cleaning is a good maintenance tool to maintain a duct system that is functioning properly. Poorly insulated or leaking duct will reduce efficiency of a new unit and increase operating expence.

Duct work dust is often generated by house cleaning. The more you vacuum, the more dust you stir up in the air. The more in the air the more movement around the house.

The solution is NOT to stop dusting. Don't say the tech. said not to dust any more! The solution is in keeping a good filteration system working. The right filters and the right replacement/cleaning schedule. Changing a filter to often can be just as bad as not at all. The more the filter "loads" the better it works. To a point this is benifical. If it overlaods is restricts the flow of air. The air starved blower will them start sucking the dirt through the filter, blowing it into the furnace/duct system, back into the air and eventually on to the table and rugs. Where you vacuum it back up again and blow it into the air.

You mentioned your were going to change your failing system. As an allergy prone person (molds, grasses and cats) I would recommend an electronic air filter or a hepa type air filter. The electronic will even remove bacteria and some of the larger size viruses in the air. Two of my family menbers have had them and I plan to install one in my home.

To almost all points I agree fully. Most people use the cheap fiberglass which filter NOTHING. However on the electronic air filters I have my doubts. I have remediated many systems that use them and I'm not a big fan. While they do catch most of the larger contaminants the smaller ones below 10 microns, are a concern. And if not maintained punctually will not work at all. These filters MUST be serviced (cleaned) on and absolute ragular basis. If not the ion field will not function and everything will pass through since there is nothing really to prevent airflow. I have seen too many that were left uncleaned for months at a time. And it is questionable if they actually remove microbials. A UV light system will actually kill most of the microbials passing through the air stream in the ductwork.

Electonic filters will only work properly if several conditons prevail. The first being the duct system is properly designed. The second is the air flow thru the filter does not exceed the size of the filter.
Air flow thru a duct varies in speed from faster in the middle to slower around the edges. In round duct the air flow is the same thru the entire opening, Until you bend it. Such as the return duct where the air is bent to face the furnace or air handler for the fan. Many folks install turning vanes in the duct to correct this. Although this helps it does not cure the issues. As the turning vanes turn the air around the corner of the return duct the air picks up speed. Although the volumn of air stays the same. The air speed exceeds what the manufacture of the air cleaner requires for their unit to work properly. Sopme areas the air is hardly moving other it is moving to fast to properly clean. EAC units are tested in a factory with ideal air flow systems. Not what is found or can be duplicated in the field by most HVAC people. The Benefit of HEPA type filters such as Space Guard, Honywell etc. it that when they load up in one area because of poor duct design the air is forced to areas that are not as dirty. You will always get clean air because of this. I am not saying that EAC units do not work, its just that unless the contractor doing the install understands how they work, and you have the ability to have an extended level duct with no turns for at least four feet fefore the EAC unit, it simply will not properly operate. And in most cases this cannot be done.

That says it better than I ever could.