Webasto fuel fired air heaters are available to match either 12vdc or 24vdc electrical systems with models for Gasoline or Diesel fuel as an energy source. These heaters are highly efficient and reliable devices that can be fit to nearly any type of vehicle in myriad applications.

Their simple design contributes to their reliability and efficiency but also poses some limitations to their operating envelope. The heater has its own electrical and fuel system which will be interconnected to the vehicle systems and therefore the system voltage and fuel type must match between vehicle and heater system.

With their broad acceptance into many vehicle markets and relative ease of installation and operation, many projects have involved Webasto heater upgrades in both Commercial and Consumer vehicle applications. Recreational vehicles specifically geared to the Vanlife movement regularly incorporate a Webasto air heater as a Van Builder provided option or a DIY upgrade due to their small size, high electrical efficiency, low fuel consumption and added attraction of operating multiple appliances from a single on-board fuel source.

Heater Usage & Sizing

These recommendations are guidelines for the rough geography of the contiguous 48 United States, not inflexible rules. There are too many variables involved with exactly how a particular vehicle is built or exactly where a heater is operated or what temperature any person finds to be the most comfortable to claim that these recommendations are perfect for every situation. It is also important to note that these guidelines are referring to everyday regular usage, ie: where the heater lives and works every day.  Occasional short trips outside of the normal working area are of minimal consequence.

Webasto Air Heaters are available in 3 sizes where the sizing is based on maximum heat output capability of the heater. The larger the heat output, the larger the physical heater, the higher the fuel consumption and the higher the cost.

  • AirTop 2000STC - 2kw rated (7000 btu/hr max) controllable 3753 - 7000 btu/hr Gasoline
  • AirTop 2000STC - 2kw rated (7000 btu/hr max) controllable 3070 - 7000 btu/hr Diesel
  • AirTop EVO40 - 4kw rated (13648 btu/hr max boost) controllable 5800 - 11942 btu/hr Gasoline
  • AirTop EVO40 - 4kw rated (13648 btu/hr max boost) controllable 5180 - 11942 btu/hr Diesel
  • AirTop EVO55 - 5.5kw rated (18766 btu/hr max boost) controllable 5800 - 17060 btu/hr Gasoline
  • AirTop EVO55 - 5.5kw rated (18766 btu/hr max boost) controllable 5118 - 17060 btu/hr Diesel

The EVO40/55 heater is physically the same heater with different programming to allow different heating performance. EVO55 models are primarily used in marine applications or very large cargo vehicle applications..

Typically, the appropriate heater for a Transit, Sprinter or Promaster van of any size, will be an AT2000STC model or an EVO40 model, assuming the vehicle is completely insulated, floor, walls and ceiling. The AirTop 2000 will fit anywhere in the rear of the van and also under the front passenger seat in most situations. The EVO40 will fit anywhere in the rear of the van but due to its significantly larger size, might not fit under the passenger seat.

  • If the vehicle is not fully insulated, the default choice will be EVO40 due to the rapid heat loss that will occur on the uninsulated surfaces. Full insulation is always recommended.
  • For more northern areas where temperatures fall farther and there are more days of deep cold, the default choice will be EVO40. More northern areas are roughly defined as everything north of US Interstate 70.
  • For elevations up to 5000' - AT2000 or EVO40
  • For elevations up to 6500' – CO2 adjusted AT2000 or EVO40
  • For elevations exceeding 6500' – EVO40
  • For extreme elevations up to 18000' EVO40 DIESEL

Electrical System

The heater interconnects to the vehicle electrical system directly at the vehicle battery. Connection to the vehicle battery allows operation of the heater when the vehicle is not in use. Electricity is delivered to the heater ECU and distributed by the ECU to allow functionality of the heater while in operation. The main wiring for the heater consists of 2 circuits outside of the heater. One circuit connects the battery positive and negative. The second circuit connects the heater control switch used by the operator. The third circuit connects the fuel pump.

Cabin Hot Air Ducting

As the fire burns within the sealed combustion chamber, a fan within the heater case pulls cabin air into one end of the heater. The air moves through the heater case and across the exterior of the combustion chamber which is hot from the fire within. This causes the cabin air to get hot as it exits the heater case into the operator space. Although it is perfectly acceptable and common to operate the heater with no ducting, flexible ducting can be added to both the intake end and outlet end of the heater case. If the heater is located within a cabinet or a bulkhead it may be optimal to use ducting to get cabin air directly into the heater as well as ducting to route the heated air from the heater and deliver it to the desired area of the cabin.

Combustion Air System

The heater combustion air system is located on the exterior of the vehicle and comprises of dedicated intake tubing, a combustion air fan that moves air through the system, a sealed combustion chamber and dedicated exhaust tubing. The volume of air moving through the system, to feed the demand for heat, is controlled by the Electronic Control Unit (ECU) within the heater, which is reacting to the operator temperature control input setting.

Fuel System

The heater fuel system is located on the exterior of the vehicle and comprises of a standpipe to connect into the vehicle fuel tank supply, fuel lines to transport the fuel, a fuel filter and an independent fuel pump. The fuel pump is powered by supply voltage that is sent from the ECU. The ECU sends 1 pulse of electricity to the pump which results in 1 cycle of the pump. Each cycle (pulsation) of the fuel pump moves a fixed quantity of fuel to the burner in the combustion chamber to keep time with delivery of air via the combustion air system. Fuel is pumped per demand, in a fixed quantity which eliminates the need for injectors, jets or nozzles to meter the fuel. There is no need for return fuel lines since there is no excess fuel because the fuel is pumped per demand. The fuel saturates the burner mat within the burner chamber and as it evaporates from the burner mat, the fumes are ignited into fire.

Air Fuel Ratio

The ECU IS Preprogrammed to time fuel delivery with combustion air delivery to deliver an optimum Air-Fuel (A/F) ratio in the sealed combustion chamber. The A/F ratio is important in that if the balance is off, by having too many or too little parts of fuel to every part of air, the combustion process will be too "Rich" or too "Lean". A "Rich" condition indicates too many parts of fuel per parts of air. A "Lean" condition indicates too little parts of fuel per part of air. Either a rich or lean condition can create excessive pollution, reduce fuel efficiency, reduce BTU output, increase maintenance needs and can cause operational problems with the heater. This operational parameter is benchmarked by measuring the CO2 content of the exhaust gas as a % of the total exhaust gas. When the fuel delivery rate is increased, the % of CO2 content in the exhaust gas will increase. When the fuel delivery rate is decreased, the % of CO2 content in the exhaust gas will decrease. The % of CO2 in the exhaust gas must fall within the specified range to ensure the cleanest combustion process.

  • For the most efficient and clean gasoline burn, the A/F ratio must be 14.7:1. Meaning, 14.7 grams of air to every 1 gram of gasoline. Gasoline has a very unforgiving tolerance outside of this ratio before undesirable effects of being out of balance are encountered.
  • For the most efficient and clean diesel burn, the A/F ratio must be 14-15:1. Meaning between 14 and 15 grams of air to every 1 gram of diesel. Diesel has a more forgiving tolerance than gasoline before undesirable effects of being out of balance are encountered.

The heater has a very basic combustion system which is controlled by a pre-programmed set of parameters meant to deliver optimum A/F ratio which will result in the best overall performance. Unlike the engine in your automobile, the heater does not have the ability to self-test and self-adjust these parameters to take into account the current operating environment with continuous, millisecond timing throughout the course of operation. This simplicity of design helps to keep cost down and helps to maintain a high level of reliability with fewer complex parts to maintain.

Download the Service Manual AirTop 2000STC Spec Sheet


For AirTop 2000STC Models Only

In the event of a major repair which affects combustion chamber related components or in consideration of the heater working regularly outside of its original factory settings (primarily high altitudes), it is possible to adjust the rate of fuel delivery to the combustion chamber to maintain proper burn efficiency. No other parameter is adjustable.

  • After replacement of a major internal component such as a burner assembly or combustion air fan, it is recommended to have the CO2% tested to ensure proper burn has been maintained per Webasto Specification.
  • If adjustment is needed after a major component replacement OR the heater is intended for regular, everyday usage at altitudes above the limits of the factory settings (4900'), it is possible to adjust the fuel delivery to match those conditions per Webasto recommended procedure.  

Download the Service Manual AirTop2000STC_CO2 Adjustment Procedure


When the purpose of this adjustment to the AT2000STC is for higher elevation compensation, this adjustment can only be made at the desired altitude of operation as this is a "Live" adjustment due to the impact of air density on the combustion process. It is not possible to pre-set the heater for operation at any particular altitude.

Webasto Air Heater Correct CO2 Adjustment Procedure

With the Webasto AirTop 2000STC air heater operating every day at higher elevations, and with the A/F ratio properly adjusted for those conditions, the benefits can be noticeable when compared to stock programming.

Webasto Air Heater CO2 Adjustment Comparison Test

The results of our 700+ hour long term run test confirm the effects of proper A/F ratio on carbon accumulation

Download the 2022 Webasto Elevation Testing


**When the heater is tuned specifically for higher elevation operation, there may be a sacrifice of heater performance or reliability at lower elevation.

We do offer a selection of 2kw AT2000STC air heater kits in our store including a pre-adjusted version for operation to 6400' of elevation.

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