Dear Jim: Last time my furnace was serviced, the technician asked if I had ever had the ducts cleaned, and I see ads on television about it. How can I tell if they need to be cleaned and how do I select a company? – Sean N.

Dear Sean: A variety of contaminants can accumulate in the ductwork of your HVAC (heating, ventilation, air-conditioning) system. Some cause no problems and others cause just dust. Still others, though, such as mold, bacteria and very fine particles, can cause health and allergy problems.

The reasons most people have the HVAC system cleaned are readily apparent problems with allergies, excessive dust and odors in their homes. People also often have the system cleaned when they move into an existing home or a newly constructed home where fine drywall dust has settled everywhere. A clean HVAC system does operate more efficiently and uses less energy.

Although it is commonly referred to as “duct cleaning,” it is important to have the entire system cleaned. This includes the furnace and air conditioner blower, heat exchanger-coils and drain pans in addition to the ductwork. Air conditioner coils can collect a lot of dirt because they get damp when the unit is running.

If your HVAC system has never been cleaned and your family is experiencing some problems, you might decide to have it cleaned. The typical cost is from about $500 to $1,000 depending upon the size of your home and the complexity of the duct system.

You might first want to have it inspected. Look for a company that will deduct the inspection cost from the total project cost should you choose to have it cleaned. The inspection can be conducted by looking into ducts and the furnace blower unit using mirrors at corners. A mini-television camera can also be pulled through the ducts.

When selecting a duct-cleaning company, there are several factors to consider. As with any project, the reputation of the company is highly important. Although the ducts can be inspected afterward to see how well they have been cleaned, your best assurance is using a reliable contractor.

The NADCA (National Air Duct Cleaning Association) is the trade group that certifies HVAC system cleaning contractors. The certification is legitimate and requires extensive training. For a company to be certified, it must have a certified specialist on staff. A national listing of certified companies, along with its number specialists, can be found at www.nadca.com.

When selecting a contractor, ask about the type of cleaning equipment each uses. If an indoor vacuum unit is used, it must have a HEPA (high efficiency particulate arrestance) filter on the exhaust to trap all the particles.

Another option is an outdoor truck with a built-in vacuum unit and a long hose. Since it is located outdoors, the exhaust filter is not critical. These large vacuums are very powerful. The only problem is that during winter they suck the heated air out of your house during the cleaning process.

Dear Jim: I am going to install hardwood flooring that has foam on the back. It will rest tightly on the subflooring over a crawl space. I wonder if I should install two-sided foil over the subflooring first. – John F.

Dear John: Installing the foil sandwiched tightly between the foam and subflooring should not cause any problems, but it will not provide much benefit either. For foil to be an effective radiant heat insulator, there should be an air space on one or preferably both sides.

If you have a problem with water vapor or humidity entering the house through the floor, staple the foil up under the floor joists. This also provides the air space.

Send inquiries to James Dulley, Bangor Daily News, 6906 Royalgreen Drive, Cincinnati, Ohio 45244 or visit www.dulley.com.