I have no experience with dual fuel generators, but I have experience using propane for hours in cold weather. Propane is a liquid that evaporates into gas. Evaporation cools the system (that’s why we sweat). When you run a propane tank in cold weather for a long time, the evaporation slows down until the propane freezes entirely, even though the ambient temp is above the freezing point of propane at that pressure.

Easiest way around it is to keep your backup tank in a heated area, or to find a way to keep the tank warm, like submerging it in a tank of warm water.

Hot Air balloon pilot here. I burn up to 60 gallons of propane, twice a day, if weather and passengers permit.

The vapor pressure of propane at 20F is 55PSI, but if you’re drawing it off fast, the temperature in the tank can plummet, and the vapor pressure can drop a lot.

I use waterbed heaters wrapped around my tanks, and preheat to 83F. That gives me 150PSI at the burners. I have redundant thermostats ensuring the temperature does not exceed that set point.

Heat tape, for keeping water lines from freezing, could hold your tanks around 40-50F. Put a couple wraps of tape very low on the tank (you want to warm the liquid, not the vapor) and use Saran wrap to secure. Wrap blankets or fiberglass insulation around everything.

Larger tanks have greater thermal mass; drawing a given amount of vapor from a tiny tank will lower its temperature (and thus pressure) far more than the same amount of vapor from a large tank. Maybe try a 30# or 100# tank?

I’d plumb it into the house lines. The propane supplier won’t, but a plumber will.

So I’m a propane guy (I’m in the biz) but not a generator guy, so I’ll give my knowledge on best guess.

So your 500gallon house tank you never had issue with never ran a higher demand thing like a generator. They suck more gas. Not an issue with a 500g tank though (which when full holds 400 gallons). The liquid doesn’t have to quickly boil off to make more gas because there’s so much more room and the size resists the rapid temp swings from the phase change going from liquid to gas.

A 20 pound cylinder only holds about 4.6 gallons of propane (and that’s if they actually filled it all the way. Many will only fill them to 15 pounds). So your issue is as the number one (at this time, anyhow) poster says. The tank was having to convert so much propane from liquid to gas so quickly that it dropped the tank down to a temp too close to below the boiling point of propane, so there wasn’t the 10 or more psi of pressure moving to keep your generator running. Once the liquid gets down to about -30f you wont have enough gas pressure. So you’d have to keep your tank warmer, which isn’t really an easy thing to do without being dangerous.

What you need to do is get a bigger tank(at least if this is made to be a permanent generator and not a movable/temporary generator).Call around to some propane companies. You’ll probably find someone who will lease you a 330 gallon tank for like $75 a year and hook the system up, but they probably won’t do it if it is a temporary style generator. If that’s the case, get yourself a 100 gallon tank if you think you can load/unload it from your vehicle when you need to go fill it back up. The 100 gallon ones are just tall versions of 20 and 30 pound tanks. Same fittings and stuff.

Now to answer your question about fuel and how long it should last. Like I said before, a 20 pound tank holds around 4.6 gallons of propane. Propane has almost 3/4 the energy as gasoline. So a 20 pound tank (if filled all the way and not just filled to 15 pounds) should last about as long as however long a bit over 3 gallons of gas lasts.

Propane is very cold. If you’ve ever had some squirt out on your fingers when hooking up a tank you’ll know how cold.

At my job we have a propane powered forklift. When it’s anywhere close to freezing out, it’s a bear to cold start. It will behave how you describe, runs for a moment then dies.

As a result we have a plug in heated magnet we throw on the throttle body to preheat the system. Even then once you get it rolling you have to give it throttle until the engine reaches operating temp. Kinda like using a choke but the forklift is fuel injected, so an adhoc choke.

I’ve heard folks with propane car conversions will run the engine on gas to heat everything up and then switch over to the propane. If that is possible with your generator that would be the next thing I’d try.

The reason your cooking gas works is because inside where the ignition is happening is presumably warm and also less complicated than running an engine.

I’m guessing your generator is outside/in an uninsulated space and the engine is having a hard time reaching operating temp with Sub-Zero propane feeding it. Keeping the tanks warm may help, but I think engine temp is a bigger factor.

I have no idea about run times on propane vs gas. I have heard that propane is less efficient, but cleaner burning.

I am far from an expert, but these are things I’ve seen and dealt with. Best of luck!

Your propane is freezing, gotta torch the tank

(Don’t actually torch your propane tank, keep it in a heated area or heat trace it)

I bought a 22KW propane generator. It’s supposed to be plumbed to a large tank, but due to the pandemic I could not get the 500 gallon tank plumbing installed (county permit office was issuing for “emergencies only”). So I tried a 20# cylinder. It iced up after 6 minutes and the generator stalled shortly after. So I bought a 100# tank. That ran for 5 hours until the generator stalled. The tank was still 1/3 full.

What’s happening is that there’s insufficient area for heat transfer. The propane has to vaporize, so it draws heat from the part of the tank in contact with the liquid. The environment provides that heat, but in a small 20# tank there’s not enough surface area to transfer the amount of heat needed to vaporize the quantity of propane required by the generator. The propane stops vaporizing once its temperature drops below the boiling point, and the generator starves. The 100# tank has the same problem when the propane level drops, reducing the surface area in contact with the liquid propane. In the large horizontal tanks, the propane is in contact with a large surface area, and the tank is supported off the ground to provide good air circulation.

Your options are to buy a bigger tank or provide more heat to the small tank. What worked for me was an old washtub filled with water. I put the tank in it, then left the garden hose trickling water into the tub to keep the water from freezing.

If you do opt for a large tank, buy a reconditioned one. They’re half the price of new and last about as long because older tanks were made with thicker steel. Buying will save you money because you’re free to buy from the cheapest supplier. Buy your propane in the summer when the price is low. Get a tank big enough enough to last the whole year. When you lease a tank, the leasing company is the only one that can fill it, so you won’t be able to shop around for the cheapest price. Mine paid for itself in 2 fills (compared my bills with my neighbor who leases their tank).

I’m not sure which generator you have but this one looks to run about 5 to 6 hours on a 20 lb tank. Others I’ve looked at seem to have the about the same runtime.

Propane has a freezing point of -44°F so it should work well at 20°F.

I don’t know why you’re experiencing the issues you’re running into but maybe look into the generator side to see if there’s anything that can be checked like the orifice for clogging.

Good luck!