Keeping warm in sub-freezing weather, using only a tiny amount of fuel for heat.

[islanderxx]

So glad your able to stay warm when needed. I have a small uninsulated 1500 Promaster cargo van. Keeping warm is no problem without using anything then the vehicle heater when l first get up. I put an electric blanket over my top sheet then 2 thin fleece blankets to keep the heat from escaping. I don't turn the electric blanket on until abt 2am when l notice the cold starting to get uncomfortable.Just need to turn it to 1-2 on the dial. Everything is plugged into my Vtoman power station. Didn't like my power station when l first got it, but as l expected it was due to user error...that would be me ! After small learning curve (l'd never lived in a vehicle - always had sticks.n' bricks), l'm warm and happy. Except quick potty break in the middle of the night. My electric blanket isn't expensive or fancy and doesn't use much power. Granted only have had a few nights where temp around low 30's.

You can't more efficient than that.

 

[islanderxx]

Any thermal mass will do; don't have to be expensive. My grandma would heat her iron on the wood burning kitchen stove for us kids at night, wrapped in a couple of towels. I have known guys that use plain, old rocks.
I use a modern version of an old fashioned hot water bottle. Water is easily heated on a camp stove.

I'm assuming this is to help keep you warm while in bed. There is a big difference between heating an iron or rocks and water. Heat up 2 pounds of iron or water at 160° (100°F 𝚫) and go to bed. The amount of heat in BTU for Iron will be 25 BTU, and for water, it's 200 BTU. As you can see, with water, you'll be taking about 10 times more BTU than iron. Use water because of its heat capacity.

 

[CosmickGold]

It's that nighttime potty break(s) that keeps me from bundling up. Them young whippersnappers wouldn't understand!

I understand.

At age 77, I understand too. But I don't let midnight potty breaks interfere with staying warm! That little electric blanket under my jacket is all above the waist, so it goes with me to the potty. I just unplug the mini-blanket first and sit on the potty with it still keeping me quite warm. Then plug it back in as I crawl into bed again.

 

[Happy Camper]

Another option that might work for some are heated vests/jackets, gloves, socks, etc.

They are powered by power banks and generally do a very good job.

If you have multiple power banks and enough juice to recharge them, it's a viable option.

If you have enough solar to make it work, that's about as close to sipping fuel and you'll get.

 

[LargeMarge]

... but as l expected it was due to user error...

.
Any time I am involved, I automatically presume 'operator-error'.
If anything goes right during the first couple-three dozen attempts, I am pleasantly [[strike-through] 'surprised'] amused.

 

[quilting22]

I just had the good fortune of two nights of 19F cold outside as I slept in my van. I say "good fortune" because it's rare to have a winter where temps ever drop into the teens even once here in Arlington, Texas. These two nights of cold were just what I needed to test my van for winter warmth, and the test was a marvelous success, requiring only a tiny amount of heating fuel.

No one uses electric heat from house batteries to heat their entire van, as the electrical drain would be tremendous, killing the batteries. But I don't need to heat the whole van, I only need heat myself as I sleep. That's the trick, focus on heating yourself, not the whole van. It only takes a tiny fraction of the electricity to heat one person, as it would take to keep an entire van warm.

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So yesterday I installed this 12-volt jack on the wall right next to my bed.

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I also bought this 12V electric car blanket to plug into it while I sleep. At bedtime, I set this blanket to medium and wrapped it around my chest over my T-shirt, then put on a comfortable thin-but-insulated jacket and zipped it up over the electric blanket. Then I went to bed under two regular blankets. (It's amazing how much warmer two blankets are than one, regardless of what two blankets you use.)

I also had thick socks on my feet to ensure my toes stayed warm, and insulated gloves on my hands (required by my too-easy-to-chill fingers). Then I slept in total comfort, toasty warm all over! The cold was not uncomfortable to breathe, because my body was so warm. (I cut a small slit in two of the glove fingers so I can still use the touch screen of my cell phone even with the gloves on.)

I'd been concerned I might be wrong about the power draw, and might be about to kill my house batteries before dawn. But checking the gauge in the morning, I found the pull on my batteries had only dropped them 0.2 volts. (What a relief.) Now during the day, the solar panels can bring them right back up again.

Concerned water in the van might freeze, expand, and break containers and pipes, I keep the van about 10 degrees above freezing, ensuring all water stays in the liquid state. So I have a propane furnace with a thermostat on the wall, which I set to 41 degrees F. Set so close to the outside temperature, it requires very little propane to accomplish my goal even when temperatures are in the teens outside.

Planning well ahead for this, I have my van fully insulated on walls and ceiling with HaveLock wool. The metal van floor is covered with a layer of inch-think foam insulation, and that is covered with a layer of white linoleum. This combination makes my floor very soft on my knees. I cringe when I see videos of people putting hardwood floors in their vans, having seen pictures of the damage and callouses done to knees of van dwellers who must crawl over a hard floor every day. My floor actually feels good on my knees when I need to get down there for something.

I have lots of windows for light, but to keep the cold out, I cut a section of that inch-thick foam for each window. These covers are held in place by a bungee cord over each window running from top to bottom over the center of each. Then I place a two-inch-square block of wood under each bungee cord at the window's center to hold that foam insulation firmly against the glass. (Works like a charm.)

Having made all these insulation preparations -- and by keeping the temperature only a little above freezing -- the propane needed to heat my van is as insignificant as my mini electric blanket's drain on my batteries. Warmth in my van is almost free!
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But be sure to have backup ways of staying warm! I have two propane tanks so if one goes empty, the regulator automatically switches over to the second tank and shows me a red warning that the first tank is empty. (After I refil the first tank, the red warning will show when the second tank goes empty)

But what if batteries go dead and I can't run the furnace? As my furnace backup, I have a little cook stove burner connected to the same propane line, normally shut off both at the stove and at a valve on the wall.

But what if both tanks run out of propane? I doubt I'll ever let that happen; yet to be safe, I have plenty of blankets and warm clothing to put on (plus my little electric blanket to use if I still have electricity).

Cold winters should be fun, not brutal!

If a cold emergency should arise, l keep insulated thermal long-johns and a snow suit in my van. Both fold/roll down to very manageable size.