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Pipe down those rumbling heating ducts

Ideas on what might be causing the racket

11Thank you for your time.

-- Christine McBride, e-mail

ANSWER -- As the heating season is now upon us, I thought this would be an excellent time to answer your question. Noisy forced-air heating systems are a common complaint and I will offer some advice for relief of this issue, which is usually nothing more than a nuisance.

There are two main causes of your noisy ducts, with one being within the scope of normal repairs and the other more difficult.

The first possibility is poorly secured or installation of the heating ducts and plenum on the heating system. This may have been an original defect when the home was built and did not become a problem until the newer furnace was installed. Since you have upgraded your furnace a few years ago I can safely assume that your home is more than 20 years old and it is unusual, but not unseen, to have poor-quality ducts originally installed. Most HVAC contractors employ good sheet metal technicians or licensed gas fitters who have the right tools and expertise to properly install ducts for a forced-air system. What is more likely, is that the ducts may have become loose or were poorly secured or installed at the time the new furnace was put in.

The way to check for this possibility is to follow the furnace warm-air ducting from the main plenum to the elbows or junctions where the branch ducting attaches. Push on the ducting gently to see if any movement occurs.

If there are loose or damaged supports between the floor joists, open joints in the ducts, or easily moving sections, then that is likely the cause.

This defect can often be rectified by adding additional support brackets, or tightening up the existing ones, to prevent movement of the ducts when the furnace fan engages.

In this same light, a problem with the ducting may have occurred when your new furnace was installed. To accommodate this upgrade, the old plenum from the original furnace would have to be disconnected and a new connection made between the new furnace and the main duct. The housing for the new furnace is usually much smaller than for the old one, often requiring substantial retrofitting of the metal ducts. To accomplish this with minimal disruption to the old ducts requires substantial skill. Sometimes, the old ducting may have been partially unsecured from the floor joists to ensure a proper fit with the new sections. If this is not properly re-fastened, there could be significant looseness, which may cause the ducts to rattle when the furnace comes on. Also, vibration from the exhaust fan or blower fan of the new unit may cause more movement in the ducting than the previous furnace.

If the ducting in the basement is obviously loose or has significant movement when tested, simple repairs may solve your problem. If the ducts are mostly hidden behind a finished ceiling in the basement or in walls in the home, these repairs may be tougher to diagnose and difficult to rectify. In those situations, little may be done without major demolition or removal of wall and ceiling coverings, which is likely not warranted.

The other possibility for the new noise is simply due to a higher-quality air-movement system in your new furnace. Most new furnaces have much more efficient and powerful blowers than old ones and the noise you hear may be caused simply by an increased volume of air moving through the ducts. Also, the faster air may be heating the cool ducts more rapidly than the old blower, causing the ducting to expand rapidly. This thermal expansion and contraction is often the cause of banging noises and can be quite noticeable.

The problem of noisy air movement from your new furnace is likely emanating from the old ducting, so visual inspection for obvious defects may allow you to fix the problem with minimal effort. If the ducting is hidden, or the problem is not apparent after your personal evaluation, calling a licensed heating, ventilation, and air-conditioning (HVAC) contractor to look for solutions is the way to go. This may be done as part of annual servicing on the heating system or as a separate service call.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the president of the Canadian Association of Home & Property Inspectors -- Manitoba ( Questions can be e-mailed or sent to: Ask The Inspector, P.O. Box 69021, #110-2025 Corydon Ave., Winnipeg, MB. R3P 2G9. Ari can be reached at (204) 291-5358 or check out his website at