Gasoline or diesel cabin air heaters like the Espar Airtronic and Webasto Air Top ranges are essential for off-grid comfort in vans, RVs, boats, cranes, trucks, specialty vehicles, emergency vehicles and other similar applications. But despite their high performance, overheating issues remain one of the most common problems we see when we talk to our customers.

Gasoline or diesel cabin air heaters like the Espar Airtronic and Webasto Air Top ranges are essential for off-grid comfort in vans, RVs, boats, cranes, trucks, specialty vehicles, emergency vehicles and other similar applications. But despite their high performance, overheating issues remain one of the most common problems we see when we talk to our customers. The good news? Most of these issues trace back to one overlooked area: cab heater ducting design.

In this guide, we’ll walk through the key principles of proper Espar heater ducting layout, the top mistakes to avoid and an overview of diesel heaters ducting accessories. While we focus on premium brand Eberspächer (Espar) heaters here, many of the same principles apply to other premium leading brands like Webasto, as well as budget options like Autoterm or even Chinese diesel heaters.

Here at Heatso, we've supported countless customers who encountered overheating issues due to these common diesel heater ducting mistakes. When we asked an Eberspacher engineer to list the most common cabin heater ducting setup errors, he came up with the below list:

• Too many sharp bends in the ducting: Increases backpressure, reducing airflow.
• Bends placed too close to the heater outlet: Causes immediate hot air buildup close to the heater and restricts proper air distribution which results in heater overheating.
• Incorrect heater duct size used: Incorrect diameter ducts restrict hot air flow, particularly when mismatched with the heater’s rated output.
• Too many T-pieces: If not done correctly - creates uneven airflow distribution and resistance, especially in multi-outlet systems.
• Improper outlet hood setup: Some heater outlet hoods with incorrect ducting setup restrict hot air flow or bend air too sharply, affecting air velocity.
• All outlets closed simultaneously: No escape path leads to overheating and potential heater lockout - even briefly closing all outlets can cause this.
• T-pieces or Y-pieces installed incorrectly: These diesel air heater ducting accessories should only be used as instructed in the manual - all the modifications often lead to trouble.

Every Eberspächer / Epar Airtronic diesel heater or petrol heater comes with a "guide number" - a maximum allowable value that represents how much restriction the air ducting system can have. This is critical to managing Eberspacher ducting airflow effectively.

Each duct component (ducting hose, elbow, connector, outlet hood, etc.) has a line value. Add them up, and they must stay below your heater's guide number to avoid overheating. For example,

• Eberspacher Airtronic S3 (D2L): Guide number ≤ 6 if used with 2.4” (60mm) outlet hood
• Eberspacher Airtronic S3 (D2L): Guide number ≤ 12 if used with 3” (75mm) outlet hood
• Eberspacher Airtronic M3 (D4L): Guide number ≤ 3 if used with 3” (75mm) outlet hood
• Eberspacher Airtronic M3 (D4L): Guide number ≤ 10 if used with 3.5" (90 mm) outlet hood
• Eberspacher Airtronic M3 Recreational (D4R): Guide number ≤ 10 if used with 3” (75mm) outlet hood
• Eberspacher Airtronic M3 Recreational (D4R): Guide number ≤ 15 if used with 3.5" (90 mm) outlet hood

Usually, your Espar original heater installation kit will come with the correct ducting accessories.

Exceeding this rating results in reduced airflow, increased temperature inside the heater, and trigger faults like Error Code 12 or Workshop Code P000115.

Refer to the Eberspächer manual for official duct component line values.

Let’s dive deeper and take the Eberspacher Airtronic S3 (D2L) as an example here. The guide number for the below configuration with 2.4”/60mm outlet hood should be less or equal to 6. Let’s see what is the total guide value for the below setup:

As we can see, the total value is 3.3 which is way below the max allowable value of 6. It means that this heater/ducting/accessories setup is within the allowable heating system limits and also leaves space for future modifications.

From our experience and long discussions with Eberspacher engineers, these are the best practices to stay within the permissible airflow limits and extend the life of your heater:

• Always observe the Line Guide Value for your heater
• Keep ducts straight and short: Reduces air flow resistance and heat loss.
• Use the correct size hot air ducting
• Use genuine, 3-layer, non-toxic ducting: Designed for high-temp operation.
• Always include one permanently open outlet: Prevents pressure buildup. If all outlets are closeable, ensure one remains open at all times during operation.
• Choose Y-pieces over T-pieces: Ensures smoother air distribution.
• Match outlet hood types to duct size: Avoid 90° bends near the heater body.

These cabin heater ducting setup tips apply to many air heater brands - not only Eberspacher / Espar.

We often notice that customers try using different type of ducting hoses which can be dangerous. Make sure to use high-temperature approved ducting to avoid melting, odor release, or deterioration. You can find specialised ducting for Eberspacher, Webasto, Autoterm Air Heaters here:

The heater ducting setup with multiple outlets are often used in boats and they have to be balanced correctly. In multi-outlet configurations, air-blocking elements are used to manage airflow split. These inserts can partially restrict one duct to balance output without exceeding guide number limits. This is not recommended for DYIers. Such system balancing should be performed by a professionally trained Eberspächer technician.

Error 12 is Espar’s way of telling you the heater is running too hot internally due to blocked or insufficient airflow. This is typically a design or installation issue, not a fault with the heater itself. Common causes in general:

• Diesel heater ducting has some kind of restriction (or even more than one) - it could be a bend that is too sharp, improperly used T pieces, incorrect size hot air hoses used, incorrect heater outled hood used.
• All outlets closed during operation

Identifying the issue within the air heater’s ducting setup and resolving it usually solves the problem.

Cabin heater overheating can cause lockouts that require manual resets.

While this guide primarily focuses on cab heater hot air hose setup, diesel heater overheating can also result from improper exhaust hose installation. Similar principles apply - adhere to length limits, keep the routing as straight as possible, and use only genuine heater exhaust accessories.

Yes. It supports both Webasto air heaters (Air Top series) and water/coolant heaters (Thermo Top Evo, Pro series).

Overheating issues in diesel and gasoline air heaters are almost always avoidable. Proper duct planning is your best defense against airflow errors, fault codes, and cold nights.

Before installing, we suggest you do the following:

• Consider sketching your planned ducting layout (include approximate lengths, outlet count, and bend details)
• Verify you're within the heater's guide number
• Identify the correct cabin heater duct size
• Identify the right ducting accessories to avoid overheating or imbalance

This extra step can prevent your Espar heater overheating issues and save hours of troubleshooting and ensure optimal performance from day one.

If you have a more complicated outlet setup (we often see that in boats) - we recommend consulting with the experts - an Eberspacher/Webasto technician or Heatso.