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Short answer
Fabricated ductwork exists because older buildings rarely fit stock fittings. Forcing a catalogue part into a space it does not suit is how duct systems acquire the sharp turns and undersized sections that limit airflow for the rest of the building's life. The reason for fabricating should be visible in the quote. Retrofits in local housing routinely run into joists, chases and finished spaces that no catalogue transition fits, so the choice is between a fabricated piece and a compromise that will be permanent. Ask which sections are being fabricated and why, what the alternative was, and what airflow the finished arrangement is designed to carry, because a duct restriction installed today is a comfort complaint for decades.
Jobs this covers
- Custom supply and return plenums
- Equipment transitions
- Offsets and duct fittings
- Curb and rooftop coordination
- Repair or replacement sections
- Residential and light-commercial fabrication
Common problems and warning signs
- New equipment does not match existing duct dimensions
- A transition leaks or restricts airflow
- A remodel changes duct routing
- An uncommon fitting is damaged
- A rooftop or commercial project needs coordinated metalwork
- Field-built connections are poorly supported or sealed
What service may include
Field measurements come before fabrication. Work may include field measurement, shop drawings or approved dimensions, material selection, fabrication, installation coordination, sealing, insulation boundaries and airflow or startup verification tied to the mechanical project.
What affects the recommendation and price
Custom fittings cost more than stock ones. Dimensions, gauge and material, complexity, quantity, field measurement, shop schedule, access, insulation, finish and installation responsibility affect cost and timing.
What is different around Burlington
Custom fittings are common in older Burlington-area houses, where new ducts have to meet old framing, chimneys and beams that no stock part fits.
What a written quote should tell you
- Why fabrication is required, not stock fittings, in terms of the space
- Material, gauge and whether the work is insulated
- Whether fittings are designed for airflow or only to fit the opening
- How the new work connects to existing ductwork and whether that joint is sealed
- Lead time, since fabricated work is not off the shelf
| Approach | Result |
|---|---|
| Forcing a catalogue part into an odd space | Sharp turns and undersized sections that limit airflow for decades |
| Fabricating for the actual space | A fitting designed for this building's joists, chases and airflow needs |
Common questions
Why is a custom transition needed?
New equipment and existing ducts often have different openings. A properly designed transition connects them without an abrupt restriction or unsupported gap.
Does fabrication include installation?
Only when stated. The proposal should identify who measures, fabricates, delivers, installs, seals and insulates the component.
What dimensions are needed?
The fabricator needs verified field dimensions, connection details, orientation, material and any equipment or drawing requirements.
Is this only for commercial HVAC?
No. Residential replacements and renovations also need custom plenums, transitions and offsets.
What is a plenum?
The plenum is the sheet metal box directly attached to the air handler, furnace or coil, from which the individual duct runs branch: the supply plenum on the outlet side and the return plenum on the inlet. It matters more than it appears to, because it is where air changes direction and velocity immediately after leaving the equipment, and a poorly proportioned plenum creates turbulence and resistance right at the point where the system can least afford it. When equipment is replaced with a unit of different dimensions, the plenum usually has to be refabricated, and that is a common reason a replacement quote includes sheet metal work.
How do I know if I need custom fabrication or stock fittings?
Stock covers standard sizes and standard angles. Custom is needed where the space does not cooperate: a transition between two different sizes at an awkward angle, a plenum for equipment that does not match the previous unit's footprint, a duct that has to fit an unusual joist bay, or an exposed run where appearance matters. Vermont's older housing stock generates a lot of this, because post-and-beam framing, low basements and irregular joist spacing rarely accommodate catalogue parts.
Can I fabricate my own ductwork?
Simple modifications with off-the-shelf fittings, extending a run, adding a takeoff, changing a boot, are within reach of a determined homeowner with tin snips and patience. Custom fabrication is not, because it needs a brake, a shear and the layout knowledge to develop transitions and offsets that actually deliver the intended airflow. The distinction that matters is not difficulty but consequence: badly made transitions and sharp turns create resistance that quietly degrades the whole system's performance in ways nobody connects back to the sheet metal.
What happens if ductwork is improvised rather than fabricated?
Airflow suffers everywhere, not just at the improvised piece. Flex duct crushed around a corner, an abrupt transition, or a takeoff placed badly all add resistance, and static pressure is a system-wide property. One bad fitting can restrict the entire system and produce complaints in rooms nowhere near it. The classic Vermont retrofit version of this is a basement duct system extended by successive owners with whatever was to hand, which explains an astonishing number of persistent cold rooms.
How long does custom sheet metal work take?
It depends almost entirely on whether the shop fabricates from measurements taken on site and returns to install, which is the normal workflow. Measuring, fabricating and installing a modest custom transition or plenum is often a few days end to end instead of continuous work. Complex jobs such as a full plenum replacement or a commercial fit-out run longer. The measuring visit is the part worth being present for, because that is when the constraints of the space get decided.
Who is responsible for making sure new ductwork is sized correctly?
The contractor doing the design, and it should be based on a Manual D duct design following from a Manual J load calculation, not on matching whatever was there. It is worth asking for explicitly, because duct sizing is the most commonly skipped design step in residential HVAC. Equipment gets calculated and ducts get copied. If the equipment is changing capacity or type, the ducts that suited the old system may not suit the new one, and installing efficient equipment onto undersized ducts guarantees disappointment.
Is custom sheet metal fabrication warrantied?
Fabricated components typically carry the fabricator's workmanship warranty rather than a manufacturer's product warranty, since each piece is custom-made. Ask specifically what is covered if a fabricated transition does not deliver the airflow the job was designed around.
What is the most common mistake with a custom fitting order?
Approving a custom fitting based on the shop's stated dimensions without measuring against the actual equipment being installed. A fitting fabricated to the wrong opening size becomes an expensive-to-redo problem discovered only at installation.