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For car camping near the snow line, is there anything that can safely be used as a heater while sleeping?

For example, how large would ordinary hot water bottles need to be, to comfortably emit heat for the entire night?

It seems like electric heaters are unfeasible unless there is mains power available (as even a truck battery would struggle to run a space heater for a quarter of an hour), and gas heaters are obviously unwise inside a sleeping space (due to exhaust).

Has anyone tried using a portable gas hot water system (located outside), to circulate hot water to some kind of radiator inside the sleeping space?

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    Don't forget that the tent doesn't have much insulation value, so heat will be lost almost as fast as you can put it in. – Chris H May 12 '18 at 18:43
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In the outdoors, the way to deal with cold is not to heat the environment, but to insulate yourself. You say "snow line", so it appears you aren't asking about anything particularly cold.

Just get a proper sleeping bag rated for the temperature. Since you are car camping, you can bring some extra supplies like blankets. Get a sleeping bag rated for the normal or a bit above normal temperature, then add a blanket for the unusually cold nights. It's a lot easier to throw a blanket on a sleeping bag to deal with lower temperatures than to open the bag and stick one leg out, or whatever, to deal with higher temperatures.

  • A big factor is how much insulation you have underneath. If you like a thermarest, put a foam mat under it. In a pinch I've even used a couple of towels under my mat – Chris H May 12 '18 at 18:42
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gas heaters are obviously unwise inside a sleeping space (due to exhaust)

There are a lot of different types of gas heaters, many of which would be unsafe to use inside a sleeping space. An indoor rated catalytic propane heater, however, is one of the types that can be used safely indoors. As the heat is produced through a catalytic process there is no flame so the chance of fire is greatly reduced. My understanding is the chemical reaction does not produce harmful exhaust, but it consumes oxygen. If your space is not well ventilated (which is generally not a problem in a tent) you can run into problems due to low oxygen levels because you need oxygen to live and because the chemical reaction will not work with low oxygen leading to a buildup of harmful gases. Indoor catalytic heaters have sensors that prevent this.

  • Various of the catalytic propane heaters have oxygen sensors to turn them off if the O2 levels drop. They work quite well in tents. I don’t use them in my small backpack tents, but in my 6-person dome for car camping they work great... – Jon Custer Feb 13 at 17:44
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It’s decently common to use a nalgene water bottle inside of a sleeping bag to help stay warm during the night.

Before you go to bed you fill the water bottle with water warmed over a stove and place it inside the sleeping bag. It is certainly possible to use boiling water but then you will want to wrap the bottle in something to avoid burning yourself. (There are at least 6 other answers on this site plus other internet places plus my experience of people using boiling water in Nalgene bottles).

I doubt that any larger heated water systems for camping exist, water is just to heavy.

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    Never put boiling water in a Nalgene. – ShemSeger May 12 '18 at 19:42
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    It's not as much of a problem now as when Nalgene first started making bottles, but apply enough heat to plastic and it will leech chemicals. If you want a container for hot liquids, then buy a thermos. – ShemSeger May 12 '18 at 23:43
  • I guess as long as you don't drink the water afterwards you should be fine. Alternatively get a bottle made from steel or aluminium, but then you'll definitely need to wrap it in something to avoid burning yourself. – fgysin reinstate Monica Feb 13 at 12:24
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Hot water bottles work great for keeping you warm at night in your sleeping bag. They shouldn't be relied on for warmth though, if you're consistently cold then you should get a warmer bag, or learn how to set up a proper sleep system. Hot water bottles are great for getting your bag warm for you when you first crawl into it, especially while car camping, because you don't have to pack it on your back. When car camping, you can bring as many creature comforts as you want as long as you have room for them.

I used to attend an annual winter survival training camp when I was in scouts. We definitely kept a couple hot water bottles on the sled that we filled with boiling water right before crawling into our sleeping bags. Your average hot water bottle was good enough to keep you warm in the night, and in the mornings you had liquid water to boil again for your breakfast while everyone else was melting snow and trying to thaw out their water bottles.

There are a plethora of tent heaters available on the market, the gas heaters are safe for indoor use, and pose a greater threat to you tent (melting the nylon) than they do to you.

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    @ShemSeger not all tents do though, and in many cases the vents can be closed anyway (like when the weather is cold, for example). Some car-camping type tents are really quite tightly sealed if the doors are shut. CO is generated in very small quantities so long as there's adequate ventilation but by assuming that all all tents have adequate ventilation by extrapolating from your is dangerous advice. – Chris H May 14 '18 at 15:12
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    @ShemSeger no CO is a big deal. Carbon monoxide (from partial combustion) is lethal at around 0.1% in air, if you don't do something about it (like maybe you're asleep). Carbon dioxide (the stuff we exhale) needs to get to several percent to cause problems. – Chris H May 14 '18 at 20:47
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    Here's an old CDC report as newer stuff I can find doesn't cite individual cases, just things like "several deaths". This is literally people dying from what you're advising them to do – Chris H May 14 '18 at 20:49
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    The gas doesn't care what shape the burner is, and tent heaters weren't common back then. If you're going to give advice that runs counter to widely accepted safety advice the onus is on you to find a safe solution. I'm out of here – Chris H May 15 '18 at 5:53
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    I didn't look for products, I looked for up-to-date studies of the risks. But quoting from your link, provide for quality ventilation and Don’t use the heater when you sleep in the tent which you didn't say, and the latter in particular is critical in the context of the question. I'm actually more willing than most to cook in a well-ventilated tent. – Chris H May 15 '18 at 15:53

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