Here is some info from another board that kinda says why they go out when its cold:
Whether a 100# tank (or multiple smaller tanks ganged together) is needed depends on how cold you want to be out in and how many BTUs the heater is. Give us that info and we can tell for sure. There are certainly situations where some commonly sold vented and unvented heaters will need a 100# tank in MN.
The required cylinder size depends on the combination of:
- BTU draw
- tank temperature
- size of the tank
- how full the tank is
This is due to a physical property of any liquid turning to a vapor, called vapor pressure. The propane in the tank is pressurized into liquid, and we burn the fumes off it.
Propane does not evaporate below -44 deg F, it stays as liquid. As you get closer to -44, less and less liquid evaporates to vapor, thus, less pressure. And that vapor pressure drop off FAST below 30 degrees!:
30 deg = 40% of 80 deg pressure
0 deg = 20% of 80 deg pressure
-20 deg = <10% of 80 deg pressure
Both the vented and unvented heaters, if not supplied with enough pressure (read: BTUs), may not run reliably or safely with too low of a pressure - because if the heater is not getting enough propane for its BTU rating and design, the thermocouple may not get hot enough and the heater can shut down.
Example: If you have a 35,000 BTU heater, and run it off a 20# tank at 0 degrees, you are probably going to have some problems. Like wise for a 75,000 BTU heater and a 100# tank - you would need two 100# ganged together for -20F to push that amount of BTU for more than a couple hours.
And to make things worse for us all, as you draw down the propane tank, you get less and less pressure, too. Here's a BTU chart of temp and fill level for a 100# tank - its proportionally less for smaller tanks. This means that the colder it is, the less usable propane there is, even in a full tank!
As you can see below, 0 deg F and a low 100# tank brings low BTU outputs into play that approach the BTU outputs of common BTU-sizes of vented and unvented heaters. Halve those 0 deg F numbers on the left side (approximately) for -20 deg F.
-----snip!------
VAPORIZATION RATE - 100 lb. Propane Cylinder (Approx)
Pounds of propane in cylinder Maximum continuous draw in BTU/hr
at various temperatures in degrees F.
Code:
--------------------------------------------------------------------------------
pounds
of
propane 0° 20° 40° 60° 70°
100 113,000 167,000 214,000 277,000 300,000
90 104,000 152,000 200,000 247,000 277,000
80 94,000 137,000 180,000 214,000 236,000
70 83,000 122,000 160,000 199,000 214,000
60 75,000 109,000 140,000 176,000 192,000
50 64,000 94,000 125,000 154,000 167,000
40 55,000 79,000 105,000 131,000 141,000
30 45,000 66,000 85,000 107,000 118,000
20 36,000 51,000 68,000 83,000 92,000
10 28,000 38,000 49,000 60,000 66,000
--------------------------------------------------------------------------------
This chart shows the vaporization rate of containers in terms of
the temperature of the liquid and the wet surface area of
the container. When the temperature is lower or if the container
has less liquid in it, the vaporization rate of the container is
a lower value.
-------snip!-------
Also - Smaller tanks' behave differently than you would expect, too. I.e., A full 30# tank DOES NOT put out 45,000 BTU at 0 deg F like a 100# tank with 30 pounds in it does - it is actually less. This is because there is less liquid volume below the liquid-vapor interface.
That's it from Dr. Science - remember, he's not a real doctor, he's just a kook.
http://www.fishingminnesota.com/forum/showflat.php?Cat=&Board=UBB30&Number=1167134&page=4&view=collapsed&sb=5&o=&fpart=all
