my question - our propane bottles ran out. Exchanged one bottle for a new one. (trailer has access  for 2 propane bottles) . The other night we needed heat. I had the switch straight up and not turned to any one bottle. When we were ready to use the furnance, I ran out and flipped the switch to the new bottle.  Propane indicator showed red. Furnance ran for two hours, never got warm. Ran outside, propane indicator still was shwoing red. Turned the heat off, slept cold.  Next morning - hubby turned the switch back to the propane and gradually through the morning the propane indicator became green. 

 I guess my question, is do you have to wait for these to prime themselves, or what?  What exactly did I do wrong?  Thanks.

 add: the valve switch - do you need to keep that on to keep propane in the lines?

When both tanks run out and they are replaced, air is introduced into the gas line. Some appliances will not initially start until the air is bled out with new propane. Sometimes this may merely involve a couple of start cycles, or holding an override/restart button. A worse case scenario would involve physically bleeding the line.

Obviously the best way to handle the situation is to try to keep at least one of the tanks full for a back up. You can select that one while the empty is being refilled, and more importantly, the line is constantly charged with propane.

If the gas unit is off and the bottle valve is off, gas is trapped in the line and a restart is usually immediate. The newer appliances usually have electronic ignitions and an automatic shut off valve. No gas will flow when the appliance is not being used.

Hope you're warmer the next time.

Bol  Gard