100 lb propane tank freezing

[walleyehunter80]

I have a 100 lb propane tank outside of my 24'X24' garage. Its hooked to my gas line that runs strait to my furnace. during the day my furnace seems to run fine, but when the sun goes down and the temps dip bellow zero my furnace seems fuel deprived and wont ignite.

what can I do to correct this, what has worked for you? The tank will last me on average a month when my furnace is set a 41 deg. and I just filled my tank 3 days ago.

[BobT]

The boiling point of propane is somewhere near -44 degrees so it shouldn't be freezing at zero degrees. I wonder if there's something wrong with your regulator.

[walleyemaniac]

I'd say check your regulator as well shouldn't have got cold enough to affect it. If it was out in that wind yesterday, maybe. Try moving the tank somewhere that the wind won't hit it or wrap it with some kind of insulated blanket (moving blankets work good). When it gets really cold a little bit of wind will affect them and just getting the tank out of the wind some how will make the differance.

[Surface Tension]

Could be your tank is undersized for the furnace. Undersized meaning not enough surface area.

[clownshack]

I agree, too small of a tank for very many btu's. I have had problems with pulling only 40,000 btu off of a 100lb tank in extreme cold. Call a local propane dealer and have them size one for your application. I use a 150 gal tank and I am pulling 100,000btu right now at -30f. If I pull any more the pressure starts to drop.

Forgot to add: having a dealer fill your tank is way cheaper then bringing a 100lb to the gas station. Most dealers fill by the gallon on site. It cost almost 2x more if I go to the gas station.

[boatfixer]

Could be your tank is undersized for the furnace. Undersized meaning not enough surface area.

Frank and cs are correct. Too much draw, not enough surface area for the liquid in the tank to draw heat and produce vapor pressure fast enough to keep up with demand.

[BobT]

The tank size explanation makes sense. Didn't think of that.

[walleyehunter80]

i wrapped the tank with 25' of heat tape and duct taped it on real good, got er pluged in now and everything working great and its -14 out right now. thanks for the info

[Twins Guy]

no propane expert but as the tank loses gas it will actually cool (for example if you've used a 1# propane tank you will notice that it frosts up with use).

this is explained by the ideal gas law PV=nRT where

P = pressure

V = volume

n is the amount of substance (propane molecules)

R is the Regnault (universal gas) constant

and T is the absolute temperature (in Kelvin degrees I believe)

so as the tank "burns off" it may actually cool beyond ambient temps. the larger the tank (assuming constant/same use of propane) the less noticeable this effect should be.

your solution with the heat tape should help to keep pressure up in the tank as well so your furnace shouldn't be choking.

[boatfixer]

Absolutly right. Not enough ambient outside temp to "re-supply" the heat lost from the tank due to the boiling process results in vapor pressure loss. a smaller surface area will lose heat much faster than a larger surface area. Your example of the 1# tank is spot on.

You will also notice the difference between a full tank and a tank that is half full. As "n" is increased, its contact with surface area is increased. Very good explanation TG!

[Twins Guy]

awe thanks!

[Tom7227]

Very interesting LP threads recently. The answers at the end here make sense. It was interesting to see the 'wrong' answers come up pretty consistently on the different threads.