## Sleeping in the van when it’s very cold outside… – page 4 – ram promaster forum gas up yr hearse

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Has anyone considered and/or tried hot water thermal mass as a heating solution for overnighting in the cold? In a passive solar house, water in anything from black 55 gallon drums to pretty transparent columns can be used to absorb heat during the day and radiate it back into the space at night, essentially reducing the maximum high and low temperatures in a space, often drastically.

I could see using a large water tank in an insulated box (perhaps water and non-toxic antifreeze so you don’t have to drain it when not in use) heated with a normal 12v water heating element during the day (either solar or from the engine while driving). At night, when the temperature starts to drop in the van, you open up the box to let the heat radiate back into the space.

You could even make a solar water heat collector for the roof of the van run by a little 12v pump and a dedicated small solar panel–when a thermostat in the collector says "hot up here," it closes a switch to start the pump until another thermostat in the water tank says, "hot enough, thanks." It would be a "drain back" system in that the water is actively pumped up to the collector but flows back down to the tank by gravity, so there is no water in the collector when pump isn’t running.

And no, I don’t have the references and probably not the math skills either to calculate how much water at what temperature you need, but someone here may. Even if it’s not a 100% solution, something like this might reduce the need to run the engine just for heat or allow you to get away without a separate propane, diesel, or electric heating system.

This same 30,000 BTU that is stored in the 40 gallons of water going from 170F to 80F is equivalent to the energy stored in 0.21 gallons of diesel, 0r 0.24 gallons of gasoline, or 0.33 gallons of propane, or 8800 watt-hrs of electricity, which would be about 700 amp-hrs of 12 volt battery!

It seems like you would have to have some way to control the heat release rate from the water tank according to your current need for heat to match the van heat loss, but also to account for the tank is going to lose heat a lot faster when its 170F than when its 80F. Some kind of insulation to keep it from bleeding heat all day long as you are adding heat to it as well as (maybe) some kind of blower with adjustable speed to allow you to blow more air by the tank (and pick up more heat) when its 80F than when its 170F.

For a solar thermal collector to harvest the 30K BTU, full sun is 317 BTU/hr per sqft of collector, and solar thermal collectors are roughly 50% efficient, so you might get about 160 BTU per hour for each sqft of collector. To collect 30,000 BTU over 8 hours of full sun would require (30000)/(8 hrs)(160 BTU/hr-sqft) = 23 sqft of collector area. This assumes the collector is tilted up to catch full sun. Of course if you are driving, waste heat from the engine would easily provide the needed 30K BTU.

The idea you had about having the tank right under the bed might be a lot more practical — 5 gallons of hot water might go a long way toward keeping the bunk warm all night. But, you would still need a way to control the rate of heat flow from tank to bunk — some kind of movable insulation or variable airflow.