Duct cleaningand sealing

When is duct cleaning worth it?

When there’s a reason you can see: substantial visible mold inside the ducts, mice or insects in them, or heavy dust and debris blowing out of the registers. The EPA does not recommend duct cleaning as routine, and it has never been shown to prevent health problems.

A renovation that filled the system with drywall dust is another fair reason. If you do clean:

• Fix the moisture first. Mold comes back if the cause, usually a sweating duct or a wet crawlspace, isn’t fixed.
• Clean the whole system. A proper job puts the ducts under suction, loosens the dirt with brushes or air whips, and also cleans the blower, coil and drain pan. Vacuuming the registers isn’t duct cleaning.
• Ask about sprays. Ask what any coating or chemical applied inside the ducts is, and why it’s needed.
• Watch the price. A whole-house offer far below $300–$700 usually means a quick pass and a long sales pitch.

What’s the bigger duct problem in Upstate homes?

Leaks. Ducts in crawlspaces and attics lose cooled air through loose joints, torn flex duct and crushed runs, and the return side pulls in damp crawlspace air or hot attic air.

Signs to look for:

• Rooms at the end of long runs, like bonus rooms, that never get comfortable
• Bills that climb with no change in how you use the system
• A dusty house even with clean filters
• A musty smell when the system starts
• Flex duct lying on the crawlspace floor, kinked, crushed or disconnected
• In manufactured homes, torn belly wrap or a loose crossover duct under the floor

How does humidity damage ductwork?

In a humid Seneca summer, cold supply ducts in a damp crawlspace sweat. The water soaks the insulation, rusts metal duct and grows mold, and a leaky return pulls that damp air into the house.

Sealing and reinsulating stops the sweating only if the crawlspace moisture is dealt with too, often with a vapor barrier on the ground. Indoors, a system that short-cycles leaves the house damp even with tight ducts; our guide on why a house stays humid with the AC running covers that side. Closed-up lake houses and second homes often have both problems at once.

What does sealing or replacing ducts involve?

Sealing means mastic or UL 181 foil tape on every joint and seam the technician can reach, with sagging runs strapped up and torn insulation replaced. Replacement is for runs that are crushed, wet or falling apart.

• Sealing and repairs usually run $500–$2,500. Crawlspace height and attic headroom set the price more than the size of the house.
• Replacing some runs costs $1,500–$5,000, with the rest sealed.
• A new duct system for a house of about 1,600 square feet runs $4,000–$10,000.

Cloth duct tape fails on ducts within a few years, so it shouldn’t appear on any quote. Ask for a duct leakage test before and after sealing, so the result shows up as a number.

If the system is due for replacement anyway, fix the ducts in the same job: a new heat pump can’t make up for air lost in the crawlspace. The AC and heat pump replacement page covers the rest of that quote, and our HVAC replacement cost guide has full system prices.

What should you ask before paying for duct work?

Ask what the technician saw and why the work is needed, and ask for photos from inside the ducts, crawlspace or attic.

• For cleaning: is the price for the whole system or per vent, and does it include the blower and coil?
• For sealing: which runs, with what material, and is there a leakage test?
• For any work: the contractor’s South Carolina license number, which you can check at verify.llronline.com.

A tune-up is a good time to have someone look under the house; see HVAC tune-ups. When you’re ready, get a ballpark price for duct work.