We just got back from a trip into Canada, we rented a motorhome and the kids and one of the couples we went with stayed in their tents.
Two of the night got really cold, we survived because we flipped on the motorhomes furnace but they other got a little frosty :-)
This may sound stupid but is there any options for connecting into a motorhome's existing furnace to provide supplemental heat to our fellow tenters? I would think a low watt electric blanket would likely be the easiest but...?
Be careful if using the motor homes furnace to not pump CO2 into the tent. Guess you could use an electric heater in the tent...assuming you have sufficient electricity available. Then need to be careful heater doesn’t get knocked over.
So why not just get better sleeping bags? No safety issues and a little cool air on the face in the morning seldom hurts
I think SteveAE is going in the right direction - besides deer camp we (kids and I) periodically encounter sub-freezing temps in both the Sierras and our local mountains. A Mr. Buddy heater, an electric heater, even a lantern are great for warming the inside air in the evening before, and morning after a night's sleep, but proper gear will ensure a good rest with no safety risk. I would think an electric blanket would come closest to being safe. The right stuff includes a down or synthetic bag rated for the expected weather, and most importantly, a self-inflating sleeping pad, closed cell, or thick fiber mat to insulate the sleeper from the ground. Too many campers are lured into thinking that cots or air mattresses are camping equipment, they're not, they're "heat sinks", and can be downright hazardous as a hypothermia risk. You can get a ThermaRest, REI, EMS, Kelty, or MegaMat self-inflating sleeping pad from discounters or Ebay for nearly the same cost as a quality air mattress or cot. They are all but leak-proof, will last a lifetime, and are a lot easier to store.
One thing overlooked is the volume of space within the tent. Bigger isn't always better. Five people in a "cabin" style tent can keep it warm from their body heat, but a solo person in the same tent will be very cold. A full rain fly on the tent that goes all the way down to the ground will also help with keeping the tent warm, but at the expense of some air flow for cooling during the day.
If these temps are expected to be a regular occurance for them, I'd look into better sleeping bags (get ones rated for at least 10-degrees F LOWER than expected temps) and possibly a smaller tent. The bigger tent can be used for gear.
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Having spent probably 700-800 nights in a tent over the years I wouldn't recommend trying to use a heater through the night. Most tents now are synthetic materials as are many sleeping bags etc and they can quickly go up in flames should they come in contact with your heater. You want to be sure you're awake when a heater is in use. You also need to worry about gases or using up all the O2 with a flame or catalytic heater.
Good sleeping bags are the only way to go. I've winter camped quite comfortably in temps around -30F at night with the right gear. You will want something like a little buddy heater to take the chill off a little prior to getting out of the sleeping bag in the morning though.
Back in my tent camping days I came up with a old car radiator, set it inside the tent and ran rubber hoses to the outside under the tent flap door. Then I converter to pipes and ran through the fire pit and looped back to the radiator hoses.
Then I filled the radiator up with water and played with where to keep the height of the metal pipes over the fire pit to keep from boiling the water.
Made a great tent heater back in the day. Would sit a battery operated fan behind the radiator fins and it would run you plum out of a three room tent.
Of course the downside is you have to keep the water hot all night.
Nothing is free....
We first used the white gas Coleman catalytic heaters and finally switched to the Mr Buddy heaters when they hit the market. For tents it is hard to beat a well placed Mr Buddy heater. They will run about 7 hours on one propane canister.
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The people in the tents also survived, didn't they, just didn't like the cold? That was not clear from the way you said it.
I think the key is the sleeping bag and what is underneath it. I've set up a tent on frozen ground, put down a space blanket and closed-cell foam, and two people have been fine together inside a winter-weight double bag with temperatures dropping into the teens.
I've also gotten so cold in a tent at 40 degrees that I got up and moved around all night to keep warm. That was Daytona, February, summer-weight bag on top of a cot, which most of the time worked OK in Florida, but not when it got that cold.
I've never heated a tent, but I have slept under an open shelter facing a small fire, in near freezing temperatures. That was short sleeps, someone has to get up to keep the fire going. A more modern equivalent might be a lean-to shelter with a couple of Mr Buddy heaters aimed at you.