Join Date: Apr 2014
Location: Bozeman, Montana
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Some thoughts on using a small air to water heat pump to heat the van...
I looked around for something that might work, but could not find anything small enough for what you want -- does not mean there is not one out there.
Daikin has some for houses, but even the smallest one is pretty large for an RV (19000BTU/hr).
If you could find one with a typcal COP (basically efficieny) of 3.5 of the right size, this is an estimate of how much it might pull out of your battery on a cold night. I recorded propane use on a winter trip, and with an outside temp of about 24F, the propane use for my 78% efficient furnace average 0.58 gallons, or 54,000 BTU. At 100% efficieny, this would have been 42,000 BTU or 12,300 watt-hrs.
With the 350% efficiency of your heat pump, it would be 12300/3.5 = 3500 Watt-hrs. This would be about (3500 watt-hr)/(12volts) = 292 amp-hrs from the battery for an overnight of heating, but with a 90% efficient inverter, it would be more like 325 amp-hrs.
So, 325 amp-hrs to provide heat overnight in a pretty well insulated van at 24F outside.
Heating water would add to this. Roughy, heating 5 gallons from 60F to 110F with the same 3.5 COP would be another about 55 amp-hrs.
I think in the end you might need something like 500 amp-hrs of Li batteries, or quite a bit more in lead acid batteries.
This assumes you do a good job of insulating and have thermal window covers -- if you did a really really reallt good job of insulating, you might cut the battery drain in half.
If you use tubing in the floor to heat, then you want to make sure you have lots of insulation under the tubes to prevent a lot of heat loss out the bottom of the van. Maybe a couple inches of polyiso. This and the tubes will make be floor thicker -- maybe about 3 inches thick? This thick floor will effect your headroom.
If you want to use this setup for cooling as well as heating....
The amount of cooling you can get with cold water through floor tubes is limited because if you go too low in temperature with the floor tube water, you will get condensation on the floor. This will limit your cooling capacity unless you add something like a fan coil unit inside for cooling.
And, during summer, the heat pump will have to run one way to heat your water, and the opposite way to provide cooling -- this complicates things. I think that the Daikin units have worked out a way to do this, but I'm guessing it involves several elecrically actuated valves.
So, it seems like a feasible approach (if you can find the heat pump), but pretty challenging, and probably pretty expensive.
Last edited by GaryBIS; 05-24-2019 at 10:59 PM.