Fort Collins is a heating town — 5,797 heating degree days against 624 cooling degree days (NOAA 1991–2020) — which means your ducts carry hot air for most of the year. When those ducts leak, the heat you paid for warms the attic or the crawlspace instead of your living room. The furnace works harder, the rooms at the end of the run stay cold, and the system wears out sooner. Duct leakage is the most common defect we find in older Fort Collins homes, and it is invisible from the living room, which is why it goes unfixed for years.
How to tell your ducts are leaking
The symptoms are specific, and they cluster:
- One or two rooms are always several degrees off from the thermostat.
- Air at the far register feels weak even when the system is running hard.
- Dust returns days after cleaning.
- The system runs long cycles and the house still feels uneven.
- Energy bills climb without any change in how you use the house.
- In summer, the upstairs is hot while the basement is cold — the same imbalance, in the other direction.
The cause is usually at the joints. Flex duct sags, tears at the collar and separates where it meets the plenum; rigid duct joints open up as mastic dries and tape fails; and a run that was cut during a remodel was never properly reconnected. In Old Town’s pre-1930 homes, where the median build year is 1927 and the ductwork was retrofitted into framing never designed for it, crushed and disconnected runs are routine. Our Old Town Fort Collins page covers the neighbourhood’s duct problems in detail.
What you can seal yourself
Start with the ducts you can reach — in the basement, the utility room and any accessible attic run — and only with the system off. Look for disconnected joints, gaps at the plenum, and tears in flex duct. Seal joints with mastic (a thick paste brushed over the joint, then covered with fiberglass mesh tape on gaps wider than a hairline) or with UL-181 foil tape. Do not use ordinary cloth duct tape: it dries out, peels and fails within a year or two, and it is the reason so many “sealed” systems are leaking again.
Replace any flex run that is crushed, kinked or torn rather than trying to patch it — a patched crush is still a crush, and airflow is what you are trying to restore. Insulate ducts that run through unconditioned attics or crawlspaces after sealing them; sealing first, insulating second, because insulation over a leaking joint just hides the leak. And keep every return vent open: closing returns to “balance” the system starves the furnace for air and trips the high-limit switch.
When to call a professional
Call a technician when the leaks are in places you cannot safely reach, when the ductwork is in a tight crawlspace or a steep attic, when runs need replacing rather than sealing, or when the house is still uneven after you have sealed everything accessible. A professional job starts with a duct leakage test, done with the system running, which measures how much air is escaping and where — so you spend money on the leaks that matter instead of guessing. It also includes a static-pressure check, which tells you whether the cold air is a duct problem or a furnace problem.
Nationally, Angi puts duct sealing at $375–$6,000, averaging about $2,250 (2026), and the spread reflects how much of the system is accessible and how bad the leakage is. In Fort Collins, where nearly half the housing stock is pre-1990 and a huge cohort is on its second or third furnace, sealing is often the fix that makes the existing equipment finally work the way it was supposed to. It is also the right first step before replacing equipment: a new furnace on leaky ducts is a new furnace wasting the same heat.
Sealing leaky air ducts vs cleaning vs repair — which job is which
These are three different jobs, and they happen in a specific order. Sealing closes the gaps so conditioned air reaches the rooms. Repair replaces crushed, torn or disconnected runs and fixes collapsed sections. Cleaning removes the dust that has built up inside — and it is worth doing after sealing and repair, not before, so you are not cleaning ducts that are about to be opened up again. In Fort Collins, duct cleaning runs about $343–$398 per 1,200 square feet (HomeYou, 24 October 2026). If a company offers to clean your ducts without mentioning leakage, ask about a leakage test first — it is the test that decides whether sealing, repair or both is the right spend.
Get help with HVAC in Fort Collins
Sealing is one of the best-value comfort fixes in a Fort Collins home, and it starts with a test, not a guess. DNA Refrigeration Heating & Cooling handles duct sealing, duct repair and duct cleaning across Fort Collins — call us at (970) 932-2324. We are open 24 hours, 7 days a week.