happy2rv wrote:
rhagfo wrote:
Every time I had my empty 20# tank filled, it took 5 gallons to fill, our 30# tanks will take 7 to 7.2 gallons to fill when empty.
It shouldn't take 5 gallons to fill a 20#. Propane should weigh 4.22 pounds per gallon which equates to 4.73 gallons for 20#, so I was low when I said 4.5 gallons. That's assuming it's filled to capacity which can vary by who is doing the filling. Assumption is OPD should kick in at 20# but I don't know how precise those are. 7.1-7.2 gallons should be spot on for a completely empty 30# tank.
Also, to be fair, I did round the usage saying approximately 1/4 gallon per hour. So, assuming 91500 BTU/HR capacity 20000/91500=.2185745 gallons per hour or 21.64 hours for 4.73 gallons. All these calculations are very precise, but reality isn't so precise. I seriously doubt the 20K BTU is 100% efficient and exactly 20K BTU. I also seriously doubt that if you take 10 random 20# propane tanks any of them will hold exactly 4.73 gallons of usable propane. So, I would say approximately 20 hours of continuous run time from a full 20# tank. Again noting if you use exchange tanks they will almost certainly never be full when exchanged.
Happy, I suspect that your first "rounded" 18 hrs is a bit closer to reality, especially when dealing with temps below 32F.
RV propane systems require propane VAPOR, the cylinders are designed to deliver VAPOR but in reality it is filled with LIQUID. So, the tank must convert the liquid propane into VAPOR.
The conversion of liquid propane to vapor propane slows down in extreme cold. This means you will get to a point where there is still liquid propane in the tank but yet you do not get enough vapor.
In other words, less capacity in cold weather.
On a good note, as someone else mentioned, 18 hrs would be if the furnace ran continuously, in reality it should be able to cycle on/off. That can vary from running say 5 minutes per hr in warm weather to full 60 min per hr in extreme cold.
Depends on outside temp, winds, how much insulation the RV has and the inside set temp.
I suspect that if OP has dual switchover with 2 20lb cylinders they would have enough propane for 2 days of extreme weather camping..
But, for comfort level, perhaps bring a third 20 lb cylinder.
If the OP is planning to make a habit of cold weather camping, I would recommend they consider looking into if it is possible to upgrade to 30 lb cylinders.. They are taller and do weigh more, but would reduce the need to haul around a spare 20 lb cylinder..
