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Multiply the original duct component rating by the factor to give the equivalent rating.



Another way of looking at these factors is to compare the length of the original pipe with the length of different sized pipe having the same equivalent duct rating.


1 m length of 60 mm diameter ducting has about the same air resistance as

    0.5 m of 50 mm,   2.4 m of 75 mm,   5 m of 90 mm   and   7.7 m of 100 mm ducting.

1 m length of 75 mm diameter ducting has about the same air resistance as

    0.2 m of 50 mm,    0.5 m of 60 mm,   2 m of 90 mm   and   3 m of 100 mm ducting.

1 m length of 90 mm diameter ducting has about the same air resistance as

    0.1 m of 50 mm,    0.2 m of 60 mm,   0.5 m of 75 mm   and   1.5 m of 100 mm ducting.

1 m length of 100 mm diameter ducting has about the same air resistance as

    .05 m of 50 mm,    0.1 m of 60 mm,   0.3 m of 75 mm   and   0.7 m of 90 mm ducting.


For example replacing 1 m of 60 mm ducting which has a 1 rating by 1 m of 90 mm will give an equivalent rating of 0.2, also meaning you can replace 1 m of 60 mm with 5 m of 90 mm in the installation.

Other components in the setup that have also changed size like bends, splitters etc will change by the same proportions.


*Inlet ducting size changes

Eberspacher say heaters are a lot more sensitive to duct restrictions on the input than the output.

They recommend always increasing the size of the inlet ducting if it exceeds 1 m.

It would therefore be sensible to consider increasing the inlet duct section whenever the outlet ducting size is increased. I have not done so in the examples to keep them simple.


Step Up and Step Down reducers

Increasing and reducing the duct sizes can both be done with Plastic Reducers.

When used to increase the duct size the rating should be 0

When used to reduce the duct size the rating for 100 mm to 75 mm and 75 mm to 50 mm are both 3.2

Ratings for other size reductions are not quoted in the manuals and will have to be guessed.


I have tried to create simpler lists from their manuals in the following pages but I do not know enough to fill the missing values or to choose the correct figures. All I can do is indicate where problems exist.

Major unknown points are marked with blue text.

Dual or larger duct options require a few additional simple calculations as shown below.





Duct rating exceeds the heater guide number

Sometimes simply replacing a high rated part like a right angled hood by a straight hood will be enough to reduce the rating.

A D4 Plus has a larger fan than a D4 giving a considerable increase in guide number.

Where ducting diameter is restricted by a narrow gap, instead of reducing the duct diameter construct a length of rectangular duct of suitable cross sectional area.

If the rating total is only slightly over limits you may decide the small risk of heater deterioration is acceptable.


If the rating total is still too high there are several solutions,



1 Shorten the pipe route

Reroute the pipe to reduce its length or the number of bends.



2 Replace single duct with two ducts

Splitting part or all of the heater outlet duct into two will double the air path which decreases the rating for the doubled up components by a factor of 4 provided both outlets cannot be closed.

eg 1 m hose has a rating of 1,     Two 1 m lengths in parallel reduce the rating to 0.25

This method also has the advantage of giving two separate outlets.

Ducting compliance confirmation test

This is a simple check to see if ducting is within Eberspachers specification.  

It will be more accurate than the calculations.

Make a very small hole in the duct about 30 cm from the outlet and insert a temperature probe into the centre of the duct. Outlets that can be closed must be closed during the test.

Run the heater. The mean outflow temperature measured after the heater has been running about 10 minutes at about 30 cm from the outlet should not exceed 110°C (at an intake temperature of approx. 20°C)

I assume the term ‘mean outflow temperature’ means ignoring short term variations while the heater changes heat settings.

Flexible ducting specification

Eberspacher manuals state   ‘In the event of possible overheating, it is possible for local hot air temperatures of up to maximum 150°C or surface temperatures of up to maximum 90°C to occur, immediately before the defect shutdown. Therefore only temperature-resistant hot air ducting approved by Eberspacher must be used for the heater air system’.

Several suppliers advertise APK ducting for Eberspacher heaters.   

(A - Aluminium heat reflective, P - Paper flexible cardboard, K - Plastic Kunststoff)

Most do not give a temperature but some quote a maximum temperature of only 120°C.


60 mm duct with

50 mm   2.07

75 mm   0.41

90 mm   0.20

100 mm   0.13

75 mm duct with

50 mm   5.06

60 mm   2.44

90 mm   0.48

100 mm   0.32

90 mm duct with

50 mm   10.5

60 mm   5.07

75 mm   2.07

100 mm   0.64

100 mm duct with

50 mm   16.0

60 mm   7.72

75 mm   3.16

90 mm     1.52

Doubling the duct cross sectional area decreases air resistance by a quarter.

Eberspacher manuals quote this rule of thumb for ducting but the duct cross sectional area does not double for each duct size increase.

Without additional calculations this limits its usefulness. I have done those calculations.

From the rule I assume that the air resistance is going to be inversely proportional to the (cross sectional area)2 .  How accurate this assumption is over the range of duct sizes I don’t know but it should be reasonable and is a lot better than having no information at all.


Using the rule of thumb as a guide I estimate:-

The factor for replacing

Eberspacher ducting calculations


Each air heater model has a guide number for the usual duct size(s) used.

Airtronic D2 with outlet hood Ø 60 mm, heater guide number 6

Airtronic D2 with outlet hood Ø 75 mm, heater guide number 12

Airtronic D4 with outlet hood Ø 75 mm, heater guide number 3

Airtronic D4 with outlet hood Ø 90 mm, heater guide number 10

Airtronic D4 Plus with outlet hood Ø 75 mm, heater guide number 10

Airtronic D4 Plus with outlet hood Ø 90 mm, heater guide number 15

Airtronic D5 with outlet hood Ø 90 mm, heater guide number 10

D1LCC with outlet hood Ø 60 mm, heater guide number 6

D1LCC with outlet hood Ø 75 mm, heater guide number 15


Every duct component including those on the air inlet has a rating which uses up some of that guide number.

Add up the rating values which are given on the following pages and if the total is less than the guide number the installation meets the specification.

Inlet ducting is more critical than outlet ducting for heaters.  Eberspachers advice is that if the inlet duct is more than 1m long the duct size should be increased even if the installation is within the limits.

One common method for making this measurement is to use a K type thermocouple connected to a multimeter that has a temperature measuring range.

The majority of multimeters do not have a temperature range but ones with that range are not expensive.

In Jan 2018 ebay DT838 model including a K type temperature probe was under £3.50 post free from China.

Multimeters are very useful multi purpose tools which can also be used for general electrical testing.

After removing the probe the amount of air that escapes through the hole is insignificant.

I don’t know the maximum temperature of Eberspacher duct but found a site advertising “Genuine Eberspacher APK Warm Air Flexible Ducting. This is High grade specification ducting for diesel heaters which has been manufactured for working temperatures between -40°C to +110°C.”

Its maximum temperature will be higher than the working temperature to allow for fault conditions.

APK ducting used here, specification unknown, was not able to handle the temperature.

Ebay multimeter with a temperature measurement range, includes a K type probe, about £3.50

Equivalent rating of the dual duct section

At least one of the branches must be unclosable. This is to prevent accidentally closing both vents which would cause the Eberspacher to overheat and in some cases can damage it.


Single branch unclosable

If one branch can be closed it must not be included in the calculations, the section becomes single duct.

ie  If Branch B is closable the equivalent rating of the 2 duct section is the Branch A total.


Both branches similar and unclosable

Add up the ratings just for one of the two branches and divide that total by 4.


Both branches not similar and unclosable

In many practical installations the two branches will be different.

There are no formulae given in the manuals for calculating the equivalent of two dissimilar paths.

If the branch ratings are nearly the same select the highest rated one and divide that figure by 4.

If the branch ratings have large differences ignore the highest rated branch completely and treat the 2 branch section as a single path.

For the remainder you will have to make a guess at the result, it will be lower than the lowest rated path and higher than ¼ of it.




The total rating for the complete ducting system = Rating for the single duct inlet section +  rating for the single duct outlet section + the equivalent rating for the dual duct section.

See page 5 for examples

3 Increase the size of the duct and its components

Output ducting size increased from 75 to 90 mm *

If Eberspacher had a simple list with every duct component rated like the above example in the Product catalogue it would be simple to calculate the total duct component rating but their lists are confusing and have many values missing.

A further complication is new and older manuals give different values for the same components.

Even their example using the same components has different ratings as shown.

Example calculation in the Product catalogue

Exactly the same example in the Marine catalogue 2008 has different values

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