I'm trying to design an all terrain/all weather RV for some extreme camping and I'm wondering if it's possible to have a multiple fuel source heating/cooling/refrigeration system. I want to be able to heat and cool with wood/pellets, coal, #2 oil/K1/diesel, LPG/LNG. I'm thinking I can build a multiple fuel firebox (with only one fuel running at a time) and use an ammonia based system for the cooling (like my propane fridge upta camp has). Thoughts, ideas, things I should research/consider? I'm expecting the living space of the vehicle to be roughly 30'×8'×8' = ≈2K ft³.

Additional information:

  • I'm planning a hull design cabin such as big orange or the Globecruiser.
  • Here is a cutout image of a firebox, for reference. Other cutout images I've seen call it a combustion chamber.
  • I'm planning on having the furnace part on the exterior of the vehicle with no direct access to the inside. I was thinking of building a heat exchanger that will pump water through the firebox/combustion chamber into some ½" PEX tubing in the floor and walls of the vehicle that will be sandwiched between two layers of metal and fill poured with a castable refractory cement.
  • Once I get my camper built, I'll be on a very limited retiree budget - so I don't want to buy propane or heating fuel any more than I have to. I prefer to run mostly wood (chunk or pellet) or coal whenever possible.
  • Air conditioning is not a matter of living fancy, it's a matter of breathing with my lung issues. I'm expecting to go all kinds of places across north and south America and I might even make my way across the ice bridge in northern Alaska to hit Asia, Europe and Africa. So, I will need both heat for the cold locations and air conditioning for the 120°F day in the shade places.
  • I do plan on having a couple different electric generation sources including solar, but I didn't want to have to build it to a size that would be required for air conditioning.
  • An absorption refrigerator is a refrigerator that uses a heat source (e.g., solar energy, a fossil-fueled flame, waste heat from factories, or district heating systems) to provide the energy needed to drive the cooling process. The system uses two coolants, the first of which performs evaporative cooling and is then absorbed into the second coolant; heat is needed to reset the two coolants to their initial states. The principle can also be used to air-condition buildings using the waste heat from a gas turbine or water heater. Using waste heat from a gas turbine makes the turbine very efficient because it first produces electricity, then hot water, and finally, air-conditioning—trigeneration. Absorption refrigerators are commonly used in recreational vehicles (RVs), campers, and caravans because they can be powered with propane fuel, rather than electricity. Unlike more common vapor-compression refrigeration systems, an absorption refrigerator can be produced with no moving parts other than the coolants. [1]
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    Sure using a multi fuel as a base you can figure out the heat load required and make a heat exchanger or direct exchange conversion chamber. The problem and the reason these are not common is the gas is toxic and the amount you would need for a system this large would present a real hazard if a leak. I would be more concerned up front with the legal issues if those can be overcome even solar can be used to heat the chamber.
    – Ed Beal
    Commented Jan 1, 2021 at 1:11
  • 3
    If you are going to burn diesel, burn it in a diesel motor connected to an electrical generator and recover heat from the cooling system and exhaust. If you don't need the electricity for charging batteries or refrigeration, run an electric heater with it to keep a load on when running. See sailboat generator systems for the well-done version, or 18-wheeler APUs for "often less well-done, but could be improved."
    – Ecnerwal
    Commented Jan 1, 2021 at 15:54
  • Never thought about boats - although I did consider getting a small box truck sized reefer unit at one time.
    – ShoeMaker
    Commented Jan 1, 2021 at 16:00

2 Answers 2


Don't make perfect the enemy of the good, or the project never finishes.

Space dimensions do not decide HVAC requirements. Insulation value x surface area does x air leakage.

So it's all about the insulation. Walls with 6" of aerogel and good door/window sealing will be a completely different energy picture than J random converted step van with #16 aluminum on 3/8" plywood and a 1/4" crack under the door.

Where you lost me, however, was "multiple fuel firebox". Firebox???? It sounds like you want to rocket off into space and design your own stove from scratch that will take everyfuel. Forget it, that's a Manhattan Project to do with any level of safety or sophistication. I assume you want a heater that you can refuel without getting your pajamas wet, yet that will also not kill you from carbon monoxide? Generally you have 2 options:

  • Build a hillbilly nightmare that will burn the place down or gas you
  • Buy a quality, efficient product off the shelf that Just Works and you can run it while you sleep so you don't have to sleep in long johns and spend the first hour of the morning trying to restore heat while your hands go numb.

Since I gather you don't want three totally separate furnaces in that compact space, you better select one that's a safety and functionality winner and stick with it. There's not going to be any shortage of propane or diesel anytime soon.

I gather you'll probably want to cook. Think about that as part of the heat source decision, pretty dumb to need 2 different fuels for the two tasks.

When you're RVing, you don't need air conditioning. If you really want it, cover the roof with solar panels and you'll have plenty of power to run a compressor based A/C system. You'll want a sensibly sized battery pack anyway, because if you're too fancy to live without A/C I'm sure you're too fancy to live without electric conveniences. Running an off-grid electrical system, sizing panels, batteries, loads etc. is a topic all its own.

Solar plays well with A/C because the only reason you need air conditioning in the first place is solar gain. Thus the need and the power supply arrive in same proportion.

I seriously doubt an ammonia based absorption refrigerator would even be capable of scaling up to air conditioning size.

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    There are absorption chillers (LiBr/water) used for air conditioning, but they're mostly an industrial thing -- I doubt anyone's made such a thing suited for cooling a RV, because most of them are oh, the size of the RV :P Commented Jan 1, 2021 at 1:23
  • I generally agree, off the shelf will be safer than homebrew all-in-one. A franklin stove will burn most any solid fuel safely and efficiently. A multi-fuel heater can burn most any liquid fuel. LPG heaters are cheap and small. LNG isn't practical on this scale. Pick one, two, or all three. Combo LPG/electric refrigerators exist but are not all that efficient on electricity. It may be wise to double up here, just use the unpowered refrigerator as an insulated box. Solar + batteries is easy but not necessarily cheap. Batteries are expensive, insulation is cheap.
    – MacGuffin
    Commented Jan 1, 2021 at 9:41

I would pick a single fuel or 2 that is easy to transport and purchase when you get to your favorite spot. What you are trying to do has already been done in the commercial setting. The company I worked for sold and serviced "3" fuel boilers, nat gas, #2 fuel oil and coal/wood. A boiler or furnace that is designed to burn anthracite (hard coal) can with design changes also burn bituminous (soft coal) or wood chips or anything that can be chipped or pellitized. If I were designing this item I would stick with a wood, propane, or #2 fuel oil burning device. Coal burning can be added but why would you do it. Burning coal well is an "ART" and if you use soft coal is a dirty head ache waiting to happen. Also coal is heavy and hard to transport. There were also 4 fuel boilers; coal (soft), fuel oil, nat gas, and electricity, but that is another story. What you are trying to do can be done but why try to re-invent the wheel. Pick something that is easier to do but remember anything that is not designed and built properly can also be very dangerous to your health and life. Be careful and stay safe. My 2 cents.

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    Do you have links to your example 3 or 4 fuel boilers? That would be very helpful. I don't care if I build or buy the furnace - if what I'm looking for already exists. :)
    – ShoeMaker
    Commented Jan 1, 2021 at 15:10
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    You were speaking about the possibility of making or producing this unit on your own. I was just referencing that this concept has already been produced and that there may still be some of these units in use today.. The units that I saw and mentioned were used in schools and hospitals and those would definitely be too big to be portable. You could try Keystoker at 570-385-3873 or Yukon-Eagle at 800-358-0060 to view there products or get ideas.
    – d.george
    Commented Jan 1, 2021 at 18:17

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