When it gets cold

Marks RV has an interesting page about the properties of LP gases including both propane and butane. Near the bottom of the page is a table that provides information about the BTU per hour as a function of temperature and tank type. It tells the story of why keeping warm in cold weather gets difficult from many directions.

For the typical RV with 30# propane bottles and a 30 kBTU/hr furnace, the furnace is going to be fuel starved when temperatures get below 10F or your tank gets below 20% full. The factors behind this include the surface area of the liquid in the tank and the fact that it is tougher to get from liquid to vapor when the temperature decreases. A chart on the page puts propane vapor pressure at 0F at only a quarter of its value at 70F. If you have a lot of Butane in the mix, which might happen if the tanks were filled in warm climes, then getting vapor will be even more difficult as butane has to get to 100F to have the same vapor pressure propane has at 0F.

The bottle size is one reason for getting a large, rented, bottle when parked for the winter. From the tables, a 20# tank nearly full could just barely fuel the furnace at 20F and a 65# under mounted tank could be 30% full and keep the furnace going down to 0F.

The website also has a good rundown on troubleshooting your RV furnace that will help you learn how it works.

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