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Video · HVAC Installation Craft

Duct Transition: How a Wider New Furnace Meets the Old Ductwork

When a new furnace is wider than the old one, a custom duct transition bridges the gap. Here the old furnace was 14 inches wide and the new one 17 and a half, so the installers built one.

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What you’re seeing in this video

The setting is an unfinished basement with the new furnace in place, its gas shutoff still wearing a green builder's tag. The narrator explains the problem: the old furnace measured 14 inches across and the replacement measures 17 and a half.

The fix is a transition, a piece of sheet metal that tapers from the new furnace's opening to the existing duct. A tech gives a thumbs up holding the fitting he shaped for the job. The seams are taped over and the piece is anchored, so the new furnace and the old ductwork join tight.

Job footageSwiftPro
Topic
HVAC Installation Craft
Length
0:38
Posted
September 30, 2026
Shot by
The SwiftPro crew, Northern Virginia
What they say on camera

All right, something we had to do on this furnace. The old furnace was 14 in wide, the new one is 17 and 1/2. So, when that happens, we have to make an adjustment to the ductwork. And so, you'll see here that this was custom fabricated and installed by our team here.

Oscar, my man over here. He custom made this and made it all join nicely together. Taped over the seams as well. Everything's anchored in place and secured.

So, that way the new furnace can fit with the old ductwork. So, it's called the transition there that he made.

Why it matters in your house

  • Furnace sizes change over the years. If your replacement doesn't match the old cabinet, a fitted transition beats stuffing a gap with tape or leaving an air leak at the plenum.
  • A tapered transition keeps air moving smoothly. Abrupt steps or gaps make the blower louder and push conditioned air into the basement instead of your rooms.
  • Ask whether your new equipment matches the existing duct size. If it doesn't, find out who builds the transition and how the seams get sealed.

What happens if a new furnace doesn't fit the old ductwork?

The installer builds a transition, a custom sheet metal piece that connects the new cabinet's opening to the existing plenum. It should be fastened, sealed at every seam and supported so it doesn't sag. Done right, you'd never know the sizes didn't match. Done poorly, you get air leaks and a noisy system.

Want it done at your place? See HVAC Installation Craft at SwiftPro or call (703) 997-9757.

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See all 234 SwiftPro videos

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SwiftPro Heating, Cooling & Plumbing
4230 Lafayette Center Dr N, Chantilly, VA 20151
44679 Endicott Dr, Suite 211, Ashburn, VA 20147
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