How Ducted Take-offs Are Calculated

Why did it order that? This guide explains how Cooledge turns a heatload and a drawn floor plan into a ducted take-off.

• The heatload and the zoning factor pick the system size
• The system size sets the trunk, the zones set the plenum
• Branches step down through BTO, Y-BTO and DBTO fittings
• Flex is ordered in whole 6m lengths, with optional joiners for long runs
• One motorised damper per zone
• What you draw on the plan is what gets ordered

The take-off is not a guess off a spec sheet. It reads the duct layout you drew and turns each part on the plan into a real catalogue item. The same logic that draws the Auto Duct Layout feeds the take-off, so the plan and the order always match. This guide explains the numbers behind it.

From Heatload to System Size

1. Each room's heatload is estimated from its floor area and your watts-per-square-metre figure (125 by default, set in Settings → Floor Plan).
2. The room loads add up to a base load for the system.
3. A zoning factor is applied to get the final load, because you rarely run every zone flat out at the same time. The default is 0.75. You can change it in Settings → Floor Plan → Ducted AC Defaults.
4. The system is sized to the final load. The chosen unit's capacity then sets the duct sizes below.

The Auto Duct Layout status bar shows both numbers, for example Base: 18.2 kW and Final: 13.7 kW (×0.75 zone ratio).

Trunk and Plenum Sizing

The trunk is the main supply size coming off the indoor unit. It is set by the system size:

• 12" at 7 kW and below
• 14" above 7 kW and below 20 kW
• 16" at 20 kW and above

The supply plenum is the box on the indoor unit that the runs feed from. Its number of spigots (1-Way, 2-Way or 3-Way) follows the zones. As a rough guide, small jobs with one or two zones get a 1-Way, large jobs with three or more zones get a 3-Way and most sit on a 2-Way in between. When you draw or auto-route the layout, the plenum matches how the zones group on the plan, so the plan is what counts.

You can change the plenum size and ways on the plan (draw a 1-Way, 2-Way or 3-Way and set its size). The take-off orders what you drew, not the default.

Branches and Step-Down

From the plenum the duct splits down to the outlets through branch takeoffs:

• A BTO (Branch Takeoff): the main run carries on and one branch peels off.
• A Y-BTO (Y-branch): the main run splits into two equal branches.
• A DBTO (Double Branch Takeoff): the main run carries on and two branches peel off.

Each branch is sized to the number of outlets it ends up feeding, then kept within the trunk size above and the outlet size below:

• 1 outlet: outlet size (10" by default)
• 2 outlets: 12"
• 3 to 4 outlets: 14"
• 5 or more outlets: 16"

The outlet size is 10" by default and can be set per area (bedroom and living) in your ducted settings. Fitting labels on the plan show the step, for example 12→10/10" is a 12" inlet splitting to two 10" outlets.

Flex Duct: Ordered in 6m Lengths

Flex duct comes in 6m lengths, so the take-off orders whole lengths, not loose metres. The number on each flex line is a count of 6m lengths, and the name spells it out, for example 10" R1.0 flex duct 6m (supply).

Two rules keep the count realistic:

• Outlet drops and return runs round up on their own. Each run to a diffuser is short and separate, so it takes at least one length. A 4m drop is 1 length. An 8m drop is 2 lengths.
• Trunk and branch runs share offcuts. Their run lengths are added up by size, then rounded up once. So 56m of 10" trunk is 10 lengths, not one per piece.

Note: A run of exactly 6m is one length. Anything over 6m needs more than one, which is where joiners come in.

Flex Joiners for Long Runs

A single flex run longer than 6m is joined from more than one length, and each join needs a joiner. This is off by default. Turn it on in Settings → Floor Plan → Ducted AC with "Include flex joiners for runs over 6m".

When it is on, the take-off adds one joiner for each extra length on every drawn run over 6m, by size:

• A 7m run: 1 joiner
• A 13m run: 2 joiners
• A 19m run: 3 joiners

Joiners are worked out for the order, not drawn on the plan. If your supplier stocks the matching joiner and you have linked it, the take-off uses the real part code. If not, it lists a plain joiner line you can link once.

Dampers and Zones

Zoning happens at the branch, not at each outlet:

• One motorised damper per zone, on the branch that serves that zone.
• The damper matches the size of the branch it sits on.
• A balancing damper (no zone set) is never required.

If a room cannot be reached from the indoor unit through a damper, the plan flags it so you can fix it.

Return Air

The return side is sized alongside the supply:

• A return plenum of 16" under 24 kW or 18" at 24 kW and above. You can change it on the plan.
• A return-air box and grille. When you draw a return grille, the box is ordered to match its size, so a 900×400 grille orders a 900×400 box.
• Return flex is ordered in 6m lengths, one length per run, because each return run is short and separate like an outlet drop.

Multi-Level: Droppers and Risers

On a two-storey plan each level is designed on its own tab and the take-off adds them all into one order:

• A dropper carries a duct down through the ceiling to the floor below. Its riser length is worked out per dropper from the floor-to-floor height in your settings, so a tall space carries more riser.
• For ordering, riser lengths are pooled with the trunk runs, so they do not each force an extra length.

What You Draw Is What Gets Ordered

The take-off reads the drawn layout, so the plan decides the order:

• Every fitting, plenum, damper, flex run and outlet on the plan appears in the take-off.
• Each part is matched to your catalogue, then to your supplier's part code when you have linked one. Anything not linked yet shows as a plain line with the drawn size, so you can link it once and it sticks.
• Parts you did not draw do not appear. Hand edits (a longer flex run, an extra damper, a different outlet) flow straight through with no second data entry.
• On a two-storey plan every level is added into one supplier order.

Tip: If a take-off number looks wrong, check the plan first. The take-off mirrors the drawing, so a surprise quantity almost always traces back to a run, a plenum change or a zone split on the plan.

Common Questions

Why is my flex quantity a count of lengths and not metres? Flex is bought in 6m lengths, so the take-off orders whole lengths. The line name carries the 6m size (for example 10" R1.0 flex duct 6m (supply)) so the unit is clear.

Why did a short outlet run still order a whole length? Outlet runs and return runs round up on their own, because each one is a separate short length you cannot join across rooms. Trunk runs are the ones that share offcuts.

Where do joiners come from? They are added only when you turn on "Include flex joiners for runs over 6m" in Ducted AC settings, then one per extra 6m on each drawn run over 6m. They are worked out for the order, not drawn on the plan.

Can I change the sizing? Yes. Draw the plenum ways and size you want, run flex at the sizes you want and add or move dampers. The take-off follows the plan, not the defaults.

Why are the outdoor unit and pipework in the take-off? Because you place them on the plan. The outdoor unit and the refrigerant pipework run into the same order as the indoor unit and the ducting.

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Related Articles:

• Auto Duct Layout →: the visual designer that produces the layout this take-off reads
• Supplier Ordering →: turn the take-off into a supplier order
• Ducted System Field Reference →: the ducted quote configuration options
• Floor Plan Scanning →: the AI floor-plan flow the designer lives on

Need help with ducted take-offs? Email us at support@cooledge.com.au