Fall is the time of year many people winterize their RV and put it away until the spring camping season. For those of us who live in our RV full time or like to go camping in the winter we should be making preparations for the cold winter temperatures ahead. Making sure your propane tanks are full is one way to combat those cold winter temperatures. Even though you have made preparations, have you ever woken up in the middle of the night to find the inside of your RV freezing and your furnace not working? If you are like me your first thought was probably I was out of propane. Especially, when it seems like you just bought propane. How could the tanks be empty already? Well, it might surprise you the tanks most likely were not empty, but the propane can’t evaporate quickly enough to meet the demand of your furnace. Let me explain.
When you buy propane you are buying a liquid that has been compressed approximately two hundred and seventy times. So, the propane molecules are really cold and tightly packed into the propane cylinder. In other words, those propane molecules want to get out as quickly as they can. Once the liquid is in our tank it starts to boil or evaporate because propane boils at negative forty four degrees Fahrenheit. Remember I said it was was really cold. It is actually this vapor that is formed from the liquid propane we use in our furnace and other appliances.
Appliances are usually rated in terms of BTU’s or British Thermal Units. My furnace is rated at 40,000 BTU’s per hour. A BTU is a measure of the amount of energy required to raise or lower one pound of water one degree. One pound of propane is equal to 21, 960 BTU’s. My thirty pound propane tank holds 7.2 gallons of propane weighing approximately 30.24 lbs. This potentially gives me 664, 070 BTUs from my thirty pound propane tank. Remember though, our propane tanks should only be filled to eighty percent of their capacity as room has to be left in the tank for the propane to vaporize and expand. It is the rate of vaporization that can sometimes make it seem like we are out of propane during the winter time.
In winter the cold temperatures will effect the rate of vaporization by causing it to slow down. As the temperature gets colder the amount of BTU’s available from our propane will decrease. Continuing with my thirty pound propane tank let’s assume my tank is sixty percent full. At twenty degrees Fahrenheit the propane will vaporize at a rate that will produce approximately 50, 400 BTU’s in one hour. When the temperature drops to zero degrees Fahrenheit the rate of vaporization will slow down producing approximately 25,200 BTU’s in one hour. Now let’s assume my same propane tank is twenty percent full. At twenty degrees Fahrenheit the rate of vaporization will produced approximately 30,240 BTU’s in one hour, while at zero degrees Fahrenheit the rate of vaporization will only produce approximately 15,120 BTU’s in one hour.
Earlier I said the furnace in my RV is rated at 40,000 BTU’s. That’s 40,000 BTU’s per hour if my furnace were to operate continually for one hour. Most RV furnaces are designed to cycle several times per hour and not run continually. However, looking at our example above if my tank is twenty percent full and it is zero degrees Fahrenheit outside the propane is vaporizing at a rate that only produces approximately 15, 120 BTU’s per hour. If my furnace runs for five minutes six times times per hour it would have ran thirty minutes and used approximately 20,000 BTU’s of propane. But the rate of vaporization would not have produced that many BTU’s, so I could have potentially woken up in the middle of the night to a freezing RV and thought I was out of propane. So, is there anything we can do about this, or are we doomed to waking up in the middle of night to freezing temperatures inside our RV?
One trick when it gets cold is to place an electric blanket around your propane tank. The electric blanket will effectively warm the tank. The warm tank will increase the rate of vaporization which will in turn increase the amount of BTU’s available in an hour. So, the next time you wake up in the middle of the night and your furnace is not keeping you warm it might be time to bring out the electric blankey. Your toasty warm propane tank will thank you for it by increasing the rate of vaporization of the propane producing more BTU’s than your sad cold propane tank.