Am I out of propane? Or is it just too cold outside?

Anywhere outside the Arctic Circle, propane is under the impression that it’s warm and sunny outside. That’s because propane boils at -44 degrees Fahrenheit at standard atmospheric pressure. We cleverly harness that low boiling point by pressurizing propane in thick steel tanks so it remains a compressed liquid—that is, until we lower the pressure by opening a stove burner, for example. When the pressure drops, some liquid converts to a vapor (gas), that vapor is drawn through the regulator and piping, and dinner is served 15 minutes later.

Most of the year, we don’t give the propane tank a second thought until the regulator indicator light switches from green to red. But in the winter, you may run into a particularly vexing situation where your tank has propane, but it acts like it’s empty! What’s going on here?

The LP tank: Not just for storage!

There’s a catch, you see. The liquid propane inside your tank can’t evaporate on its own. It needs heat. The first law of thermodynamics demands it. When propane evaporates from a liquid to a gas, it must absorb a large amount of heat from its surrounding environment in an endothermic phase change. The amount of heat required is called the latent heat of vaporization, and it’s why ice can form on the outside of your propane tank, even in the dog days of summer.

The propane actually absorbs energy as heat from the ambient air through the tank’s metal walls. As a bonus, the liquid propane wets the inner walls of the tanks, increasing the thermal conductivity. It’s beautiful in its simplicity. The system forms a “heat engine” that keeps the vaporization going.

Cold weather slows the “heat engine”

But thermodynamics isn’t going to let us off this easy. Have you spotted the problem yet?

We all know heat moves to cold, and the greater the temperature gradient, the faster the heat transfer. The inverse is also true. The smaller the temperature difference, the slower the heat will flow. In the dead of winter, there’s usually enough heat in the air to still boil propane (it evaporates at -44 degrees, remember), but it sloooowly transfers through the tank’s walls. As the liquid level falls, there is less wetted surface area to conduct heat. It’s a double whammy. In engineering-speak, there is a lower rate of vaporization, and the propane just can’t keep up with its chores.

Is this a theoretical or practical problem?

All right, let’s wrap up the lecture on thermodynamics. Is this a problem you actually need to worry about?

Practically speaking, this problem usually only manifests with small tanks, like the 20- and 30-lb. DOT cylinders. And if you camp above 20-30 degrees, then you’re unlikely to see a problem. To propane, freezing air is still toasty. But below 10-20 degrees, heat transfer slows drastically. If the delivery pressure drops below 11” of the water column, LP-hungry appliances like furnaces and tankless water heaters may start intermittently failing.

Let’s say you’re running the furnace (22,000 BTUs), the water heater (8,000 BTUs), and two stove burners (5,000 BTUs each). That’s a total of 40,000 BTUs. At 20 degrees Fahrenheit, your 20-lb. DOT propane bottle can’t keep up if you’re not at least ~70% full! Yeesh!

Here’s how to fix slow propane vaporization in winter

The solutions are actually quite simple.

• Keep your tank full. Or keep a spare full tank onsite.
• Use the biggest tank you can. If you’re camping seasonally, ask your local propane company about a seasonal tank rental.
• Heat your tank. Wrap it in a 12V tank heating blanket, for instance. Show your tank some love!

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