TinnerFlow™Round Offset & Fitting Takeoff
The triangle gives you centerline travel. The fittings decide the straight pipe cut — and sometimes the fittings eat the whole offset and you are building a dogleg. This is the field method, from the obstruction to the cut.
1. Start at the obstruction, not the offset
Most write-ups begin with "you have a 15" offset." In the field you have a cable tray and a foreman saying get it over that, don't touch it, leave room for the wrap and the hangers. Derive the offset first:
Clearance is a decision, not a formula. The number you leave has to cover insulation thickness, hanger and support hardware, and whatever the other trade needs for future access. On a rated, structural, or shared-pathway run — or where another trade owns the obstruction — this is a Verify First lane item: confirm against the print and the affected trade before metal moves.
2. Then the geometry
For a simple two-elbow offset in one plane, the theoretical centerline travel is:
This locates the two fitting centerlines. It is not automatically the straight pipe cut.
3. Convert centerline travel into a cut
Straight pipe cut = centerline travel − fitting takeoff 1 − fitting takeoff 2
A fitting takeoff must be measured from the fitting’s centerline intersection to its connection end using the same reference on both fittings. Use the actual fitting, approved shop data, or manufacturer dimensions.
Do not subtract insertion twice. When the fitting takeoff already runs to the final connection end and includes the slip/crimp engagement, do not subtract another separate crimp allowance.
4. Crimp direction
On standard round HVAC slip joints, the male/crimped end points with airflow. The female end receives it.
Mark airflow before assembling. Crimp only the cut pipe end that needs a new male end; do not crimp both mating ends.
5. Check the fitting fits before you trust the number
At larger diameters the elbows can eat the entire offset. Two elbows consume takeoff 1 + takeoff 2 of centerline travel no matter what. If that exceeds the travel, there is no straight pipe to cut — the math returns a negative piece.
Below the minimum, the angle is wrong — not the math. Drop to a shallower angle (30°, 22.5°) to stretch the travel, or accept the two fittings butting together. Buying a shorter piece of pipe will not save a fitting that does not fit.
| Symptom | What it means | Move |
|---|---|---|
| Healthy straight pipe between elbows | Normal offset | Cut and install |
| Straight pipe near zero | Fittings meet — a dogleg | Butt the fittings, see below |
| Straight pipe negative | Angle too steep for the offset | Shallower angle |
| Not enough horizontal room | Travel exceeds available run | Steeper angle or reroute |
6. The dogleg — two 45s with nothing between them
When the straight pipe cut comes out near zero, that is not an error. That is a fitting assembly the trade builds on purpose: two 45s brought together to jog the duct over an obstruction and back. Crews call it a dogleg.
Field term → formal term. "Dogleg" is jobsite vocabulary. Published HVAC material calls it an offset using two 45° elbows in sequence. Both mean the same assembly. If a foreman says "just dogleg it," this is the page.
Do not confuse it with dog-legging a duct. Racking joints or throwing a bend in a run to hide a layout miss is the opposite thing, and it is a defect. A dogleg is a deliberate offset around a known obstruction. Dog-legging the duct is covering a mistake. Same word, opposite meaning — know which one is being asked for.
Airflow reality: two 45s in sequence turn air more gently than one hard 90, which is why crews reach for them. But air needs straight duct after a fitting to settle back down — commonly cited as up to roughly 10 duct diameters. Two 45s butted with nothing between them do not deliver the airflow benefit of a spread-out pair. A dogleg is a space solution, not an airflow solution. If a TAB-sensitive run is involved, flag it.
7. Before you order different fittings — check the gores
Most standard round 45s and 90s are built with adjustable gores. Spin the gores and the same fitting produces a range of angles. Odd-angle elbows are custom and expensive, which is why they are rarely stocked.
Practical effect: when 45s eat your whole offset, you may not need to buy 30s. Adjust the elbows already in your hands to a shallower angle, stretch the travel, and get a real, cuttable, hangable piece of pipe between them.
Confirm the fitting is actually adjustable-gore, and that gore count and centerline radius still satisfy the spec for the system velocity. Miter/gore requirements are governed by the standard — verify against SMACNA and the project spec rather than eyeballing it.
8. Do not assume the radius
A common centerline radius is 1.5 × diameter, but 1.0 × diameter elbows exist and change every number on this page. Radius also drives performance — a 1.5D elbow is commonly cited at roughly 30% less loss than a 1D elbow of the same type.
Measure the fitting in your hands. From the connection end back to where the two centerlines intersect. That measurement beats any table, including this one.
9. Worked example — over a cable tray
14" round, duct bottom at 120". A 5" cable tray sits in the path with its bottom level with the duct. Foreman wants it over the tray with room for wrap and hangers — call it 10".
required duct bottom = 125 + 10 = 135"
offset = 135 − 120 = 15"
With 45° elbows on a 1.5D radius (21" centerline radius on 14" round), each takeoff is R × tan(22.5°) ≈ 8-11/16":
| Step | Value |
|---|---|
| Centerline travel (15 × 1.414) | 21-3/16" |
| Less two fitting takeoffs | − 17-3/8" |
| Straight pipe cut | ≈ 3-13/16" |
Under 4" of pipe between two 14" elbows is not a piece you fabricate, hang, or seal well. This is a dogleg. And the minimum-offset check confirms it is close to the edge: below roughly a 12-5/16" rise, two 45° elbows at this radius will not fit at all.
The moves: butt the fittings as a dogleg, or adjust the gores to a shallower angle and gain real pipe — 30° at this radius stretches the travel to about 30" and leaves roughly 18-3/4" of straight pipe, at the cost of needing about 26" of horizontal run instead of 15".
Numbers above assume a 1.5D centerline radius and a takeoff measured to the connection end. Verify both against the actual fittings before cutting.
10. The field sequence
- Derive the offset from the obstruction, not from memory.
- Confirm the fitting angle — and that the fitting fits the offset.
- Calculate theoretical centerline travel.
- Identify the exact fitting at each end.
- Measure or verify each fitting takeoff.
- Subtract both fitting takeoffs from centerline travel.
- Confirm airflow and point both male/crimped ends with airflow.
- Dry-fit, verify rotation and plane, then fasten and seal.
11. What usually causes a short piece
Cutting the straight pipe to the raw triangle length, mixing takeoff references, guessing a universal elbow takeoff, or subtracting crimp insertion twice.
12. Verified-data rule
TinnerFlow does not publish guessed fitting dimensions. Numeric takeoffs belong in the fitting bank only when they include a real source and a verified flag. Blank means check the actual fitting or approved source.
| Safe to publish | Must be verified |
|---|---|
| Offset ÷ sin(angle) | Elbow/fitting takeoff |
| Offset ÷ tan(angle) | Insertion included or separate |
| Male/crimp points with airflow | Manufacturer-specific dimensions |
For rolling offsets, mismatched fittings, specialty systems, or unclear measurement references, stop and verify the layout method before cutting.