Ducting Example Calculations

The Ducting Conformity Test (page 2) is better than these calculations and should be used as confirmation of the results. It is also the recommended way to check existing installations.

There are serious problems with conflicting rating figures in different manuals so these examples may need revising later. For the calculations I am using those from the latest manuals, not the blue marked ones.

You may decide to try both sets of figures, if it passes both the installation should be ok.

Where a rating value is not known a guess based on similar components will be required.

The examples show that in most installations reasonable errors in the guess will not affect the pass / fail result.

If the result is marginal do the Conformity test or modify the installation to improve the figures.

Dual duct system

Heater Airtronic D2 with 60 mm outlet hood and 60 mm ducting. Heater rating = 6

2a Branch B can be closed

Ignore branch B completely and treat as having a single branch A.

Grille Inlet and Ducting Flange	0.4
0.5 m duct 60 mm	0.5
Heater outlet hood 60 mm straight	0
1.2 m duct 60 mm	1.2
Y splitter 60 60 60 mm	0.25	Value not known, guessed rating
3 m duct 60 mm A	3
45 degree bend, (half 90 degree)	0.1
Outlet screen and flange A	0.4

Total rating	5.85	Rating is less than 6, just within limit but close enough to merit a ducting conformity test to confirm the figures

2b Same as previous example but both Branch A and Branch B permanently open.

The values for the dual duct section have to be adjusted, see Page 2 for details.

Grille Inlet and Ducting Flange	0.4
0.5 m duct 60 mm	0.5
Heater outlet hood 60 mm straight	0
1.2 m duct 60 mm	1.2
Y splitter 60 60 60 mm	0.25	Value not known, guessed rating
Subtotal single duct sections	2.35

3 m duct 60 mm A	3
45 degree bend A, (half 90 degree)	0.1
Outlet screen and flange A	0.3
Subtotal one branch of dual duct section	3.4
Subtotal one branch divided by 4	0.9	Dual duct equivalent rating total

Total rating = Single duct subtotal + Dual duct equivalent total = 2.35 + 0.9	3.25	Total rating is less than 6, well within limit but ideally do a ducting conformity test to confirm the installation is ok.

Heater Airtronic D4 with 75 mm outlet hood and 75 mm ducting. Heater rating = 3

2c As previous layout, 75 mm duct, Airtronic D4, Branch A and Branch B permanently open.

Grille Inlet and Ducting Flange	0.4
0.5 m duct 75 mm	0.5
Heater outlet hood 75 mm straight	0
1.2 m duct 75 mm	1.2
Y splitter 75 75 75 mm	0.25	Value not known, guessed rating
Subtotal single duct sections	2.35

3 m duct 75 mm A	3
45 degree bend A, (half 90 degree)	0.1
Outlet screen and flange A	0.3
Subtotal one branch of dual duct section	3.4
Subtotal one branch divided by 4	0.8	Dual duct equivalent rating total

Total rating = Single duct subtotal + Dual duct equivalent total = 2.35 + 0.8	3.15	Total rating is more than 3, installation does not meet specification. Change to 90 mm or fit D4 Plus.

Increased Ducting Size

Heater Airtronic D2 with 75 mm outlet hood. Heater rating = 12

3b Inlet duct 1.5 m 75 mm, Heater Outlet hood 75mm, Outlet duct 8m 90 mm

Outlet duct would normally be 75 mm, changing to 90 mm improves the installation rating.

Grille Inlet and Ducting Flange 75 mm	0.4
1.5 m duct 75 mm	1.5
Adapter Ring 75 to 60 mm	0.5	Value not known, guessed rating
Heater outlet hood 75 mm straight	0
Subtotal normal 75 mm sections	2.4

Adapter 75 to 90 mm	0.5	Value from D4 tables
8 m 90 mm duct equivalent rating	4	8 x 0.5
Outlet screen and flange	2	4 x 0.5
Subtotal 90 mm sections	6.5	Dual duct equivalent rating total

Total rating = 75 mm duct subtotal + 90 mm duct equivalent total = 2.4 + 6.5	8.9	Total rating is less than 12. Installation is now within limits.*

Heater Airtronic D2 with 75 mm outlet hood. Heater rating = 12

3a Inlet duct 1.5 m 75 mm, Heater Outlet hood 75mm, Outlet duct 8m 75 mm

Firstly doing the calculation using the normal 75 ducting

Grille Inlet and Ducting Flange 75 mm	0.4
1.5 m duct 75 mm	1.5
Adapter Ring 75 to 60 mm	0.5	Value not known, guessed rating
Heater outlet hood 75 mm straight	0
8 m 75 mm duct	8
Outlet screen and flange	4
Total rating for 75 mm duct	14.4	Total rating is more than 12. Installation using 75 mm duct does not meet specification. Example 3b is one solution to this.

Increasing the duct size has brought the previous 75 mm installation into specification

*In practice Eberspacher recommendations are that the size of inlet duct should always be increased if the inlet duct is over 1 m long as the inlet duct is more critical for the heater operation.

This has not been done for this example calculation.

1 Example given in the Product Catalogue 2016

Simple duct system

Examples 2a and 3a are more examples of simple ducting calculations

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