Where It Is Going
Twenty to thirty percent losses are ordinary in an unsealed residential duct system, and the causes are dull and mechanical rather than mysterious. Cloth-backed tape dries out and lets go. Flex duct pulls off its collar. A boot gets knocked loose by somebody storing boxes. Insulation compresses where a foot went through it.
None of that is visible from inside the house. What you notice instead is a system that runs and runs, one bedroom that has never been comfortable, and a bill that does not match how carefully you set the thermostat. Sealing the ducts is one of the few pieces of work in this trade where the payback is genuinely quick, which is why people at St Cloud AC Repair will often suggest it before a new system.
- Supply runs separated at the collar, dumping conditioned air into the attic
- Failed cloth tape at joints, boots and the plenum takeoffs
- Flex duct crushed by stored boxes or sagging into airflow-killing curves
- Return leakage pulling hot attic air straight into the air handler
- Boots that have worked loose from the ceiling, drawing air around the register
- Insulation compressed or missing, so the duct sweats and drips
How the Work Is Done
Every accessible joint gets mastic, not tape. Mastic is a brushed-on compound that stays flexible and keeps sealing for the life of the duct, where even good foil tape eventually lets go in attic heat. Where a run has separated, it is reconnected to the collar properly with a strap and then sealed.
Crushed and sagging sections get replaced rather than nursed along, supports are added so long runs are not hanging in curves, and insulation is repaired or upgraded where it has compressed. The return side gets the same attention as the supply, which is often skipped and matters just as much.
Balancing the Rooms Afterwards
Sealing changes the pressures in the system, so balancing belongs at the end of the job rather than as a separate visit. Airflow is measured at each register and compared with what that room needs, then dampers are adjusted until the numbers line up.
This is where the back bedroom that has been hot since the house was built usually gets fixed. Nine times out of ten it is not short of capacity, it is short of air, and the reason is a damper somebody closed a decade ago or a branch that was undersized on the day it went in.
Getting the Return Path Right
Supply gets all the attention, but a room can only take in as much air as it can let out. A bedroom with a single supply register and a door that stays shut pressurises, the incoming air slows to nothing, and the room stays warm no matter what you do at the thermostat.
The fixes are undramatic — a transfer grille, a jumper duct, or an undercut on the door — and they make a startling difference in the rooms that have never worked. It is worth checking before anybody suggests you need a bigger system.
Questions We Get Asked
- How do I know whether my ducts leak?
- Warm registers on a system that is definitely running, an attic that feels oddly cool near the ducts, dust arriving at the vents, and rooms that differ by several degrees are all strong hints. Measuring at the registers turns those hints into numbers, and that is part of the assessment.
- Is it cheaper than replacing the ductwork?
- Considerably, and on most systems it is the right answer. Full replacement makes sense where the ducts are undersized for the equipment, badly routed, or so old that sealing them is putting good money into something near the end of its life.
- Will this actually lower my bill?
- Usually yes, and it is one of the more reliable savings in the trade because you stop paying to cool an attic. The size of the change depends on how bad the leakage was, which is why we measure before quoting rather than promising a percentage.