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Heating an RV Off-Grid: Vented and Unvented Heaters

There are two ways to heat an RV off-grid and they are not the same kind of thing. A diesel air heater is vented: it burns outside air and exhausts outside. A portable propane heater is unvented, and Mr. Heater's own manual says it consumes oxygen from the room. That difference decides everything else, including where each one is not allowed.

By OriginRV. Last reviewed on Nov 2, 2026.

The short version: there are two ways to heat an RV off-grid and they are not the same kind of thing. A diesel air heater is vented: it pulls combustion air from outside, burns it in a sealed chamber, and sends the exhaust outside. A portable propane heater is unvented: it burns the air in the room and puts everything it produces back into the room. Mr. Heater says so in its own manual: "This heater consumes oxygen from the room in which it is installed." That one difference decides the rest of this page.

Where these heaters are not allowed

This comes first because it is the part a product page will not tell you, and it is stated by the maker rather than inferred.

Mr. Heater's own product information for the Buddy series reads: "Restricted from all indoor use in Massachusetts and Canada." It also reads: "Restricted from use in dwellings in California." California permits the Buddy series in a garage or a tent but prohibits vent-free heaters in a dwelling entirely.

Mr. Heater separates a residential standard from a commercial one. The residential products are the Buddy family and the vent-free heaters. The commercial products are the unit, garage and forced-air heaters, and their stated requirement is "1 square inch of ventilation for every 1000 BTUs." The company's own guidance is to check local, city and state codes before using any gas heater indoors.

The difference is where the exhaust goes

A vented heater has two air paths that never meet. Combustion air comes in from outside through one pipe, the burner burns it in a sealed chamber, and the exhaust leaves through another pipe. The heat reaches the room through a metal heat exchanger. Nothing that was burned enters the living space.

An unvented heater has one air path. It takes the room's air, burns it, and releases the products of combustion into the same room. That is why the manual asks for a ventilation opening, why it sets clearances to anything that can burn, and why it names rooms it may not be used in.

Both can be the right choice. They are not interchangeable, and the rest of this page is the difference written out as numbers.

Diesel air heaters: what the makers publish

These are the vented option, and the makers publish real figures for them.

Webasto's Air Top Evo 40 produces 5,100 to 12,000 BTU per hour across its control range, with a boost figure of 13,650. It burns 0.18 to 0.43 liters of diesel per hour across its control range, or 0.49 in boost, and draws 15 to 40 watts at 12 volts, or 55 watts in boost. Webasto rates it to UL 307A, CSA B140.10 and RVIA standards, with TEI listing number 23D01. The Air Top Evo 55 runs from 4.0 to 5.5 kW, which is 13,650 to 18,800 BTU per hour.

The smaller Air Top 2000 STC produces 3,100 to 7,000 BTU per hour. It is CARB approved and EPA SmartWay verified. Webasto makes a broader claim for the air-heater range rather than for this model alone: its field guide reads "use as little as 1 Gallon (3.7 liters) of fuel in 22 hours", and its catalogue gives the same claim as "Uses as little as 1 Gallon of fuel in a 22 hour period" with no liter conversion.

Eberspaecher's Airtronic 3 series publishes a fuel consumption of 0.1 to 0.55 liters per hour, an electrical draw of 26 to 55 watts, and a service life of up to 5,000 operating hours.

For estimating, Webasto's own Australian technical literature gives the rule of thumb directly: "On average a heater needs 100 ml diesel per hour, per kW."

The installation figures the makers state are worth having before you plan a location. The standard exhaust pipe length is 0.8 meters. The heater surface near the warm air outlet reaches about 80 degrees Celsius. The fuel pump should sit within 1 meter of the tank.

The altitude ceiling, and the curve nobody publishes

Altitude matters to a combustion heater, and the makers handle it differently.

Webasto's Air Top Evo 40 has automatic altitude adjustment as standard up to 2,200 meters, and a high-altitude variant that reaches 5,500 meters. Eberspaecher's Airtronic 3 states automatic altitude adjustment up to 5,500 meters. Mr. Heater rates the Portable Buddy to a maximum elevation of 7,000 feet.

Eberspaecher publishes a rate. Its heater catalogue, in the altitude-kit section, states: "It begins reducing fuel feed at around 1,400 m, which immediately starts to reduce heating performance by around 9% for every 1,000 m in altitude." That is a figure a reader can apply to any altitude above 1,400 meters.

Webasto publishes a chart rather than a rate. Its altitude documentation gives heating power against altitude per model: "AT Evo 40 - 100% heating power up to 2,200 m; High Altitude version: Heating power max. 2,8 kW up to 5,000 m." Webasto also states the limit and a warning: "All Webasto Fuel Operated Heaters are designed for use at elevations not to exceed the maximum recommended operational elevation, which varies by product and model. Please consult your operation manual to determine the maximum operational elevation for your specific product and model. Use of any Webasto Fuel Operated Heater at elevations higher than those recommended may lead to reduced performance and may require increased service and maintenance."

The two makers answer the question in two different shapes. Eberspaecher gives a rate that applies above 1,400 meters. Webasto gives a ceiling per model, plus a power figure for the high-altitude variant. Neither is a single number for your coach, and this page will not estimate one.

Portable propane: the manual is the safety document

For the unvented heaters, the specification sheet matters less than the clearance and ventilation numbers in the manual. For the Mr. Heater Portable Buddy MH9BX they are:

Clearance to combustibles: 30 inches above, 24 inches in front, 6 inches at the sides, 0 inches behind.

Ventilation: a minimum opening of 9 square inches, which the manual illustrates as a 3 inch by 3 inch opening. The larger MH12HB requires 12 square inches at ceiling level and at floor level. The manual adds: "Do not use other fuel burning appliances inside."

Rooms: bedroom use is on the "LO" setting only. Bathroom use is not permitted at all.

The MH9BX produces 4,000 or 9,000 BTU per hour, heats up to 225 square feet, and burns 0.044 gallons per hour at the low setting and 0.099 at the high one. A 1 lb cylinder gives about 3 hours at maximum and 6 hours at minimum.

What the ODS is, and what it is not

Every heater in the Buddy family carries an Oxygen Depletion Sensor. Mr. Heater describes it this way: "This device senses the amount of oxygen in the air surrounding the device. When the oxygen level drops below a specific level, the device will shut off automatically."

It is a backstop. It is not ventilation, and it does not make the opening optional. The manual asks for the 9 square inches whether or not the sensor is fitted, and the sensor's job is to catch the case where the room has already lost oxygen despite the opening. The tip-over switch works the same way: it shuts the heater off when it is tipped past a set angle, and it does not make an unstable surface acceptable.

For the carbon monoxide question itself, there is a separate guide on this site.

What this does not cover

The built-in furnace has its own guide, and it is a different system with a different fault path: RV furnace not working. The carbon monoxide question has its own page: RV furnace and carbon monoxide.

Two parts in this system are not covered here. An electric fireplace and heated floors are both electrical appliances rather than combustion heaters, and their faults are element, thermostat and breaker faults. They are recorded as gaps rather than folded into a page about exhaust.

Sources

Every figure on this page comes from a document the maker publishes. These are the documents, so you can check any of them.

Related guides

RV furnace not working for the built-in system. RV furnace and carbon monoxide for the alarm question. Winterize your plumbing for the other half of getting a coach through a cold night.

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