Is one large boil more fuel efficient than two small?

Although I don't have a answer I also would like to know my guess is with all variables the same that it would be slightly more efficient to use a single boil due to the pot having to heat up as well as the stove Thats just a guess though

In general the larger diameter pot is more fuel efficient.
Provided you have a lid on the pot, how much water is inside the pot does not make much difference at all to the efficiency.

Cheers

Roger,

I am skeptical of that. I have been experimenting with trying to boil 6 cups or so in a variety of stoves over a varitey of fuels. I have a sidewinder with a 2qt pot and have used esbit and alcohol. I can not boil that amount no matter what I do. I have tried the penny stove and the included stove with all different levels of alcohol.

By contrast I can take my keg and boil 3 cups very easily. I have come to the conclusion that perhaps its easier to boil twice.

Of course what I really think is that the new jetboil ti is going to be the answer but I dont have one yet and the early test seem to be a bit biased. they are not taking into account the number of boils you can get from a cartridge as compared to alcohol.

"I am skeptical of that. I have been experimenting with trying to boil 6 cups or so in a variety of stoves over a varitey of fuels."

We just spent 5 days doing just that with 2 1.3L Cones at 6000' and 50° temps.
We used less than an ounce of alcohol for each boil.

Something is amiss.

I treat all my water using Aqua Mira, and can heat 24oz of cold water hot enough(close to a boil) for a meal and coffee etc, using one Esbit tab,with a Evernew .9 un-coated short Ti. pot,TD Sidewinder Cone,with the TD Gram Cracker stove. In my book, its not only more efficient, but saves time too. I have also used the BPL Ti wing stove, with the BPL 550 mug, and a tightly wrapped alum. windscreen and was able to do a double "heat" from one Esbit tab, but you've gotta be quick about it, with regards to refilling the mug.

I too have been treating the water first so that I don't need to bring it to a "rolling boil", just to the point of boiling. Much more fuel efficient and time saving.

Hi Mark

Sorry – I don't understand. I was talking about fuel used per litre boiled.

What are you talking about? It sounds like you were trying to boil 6 cups of water over a stove with inadequate fuel capacity, but I may be mistaken. Details?

Jetboil? OK for car camping, but hardly light-weight.

Cheers

Roger,
They just shifted from boiling 6 cups to heating 6 cups of treated water.

Yeah, I agree that there is no difference. As you say, the wider pots usually used will provide a bit of a boost, but, the few secods needed for refueling a too small stove lets the water cool a bit before finishing the boil…probably about even.

I just spent many days boiling away thousands of gallons of maple sap on the family farm. Efficiency is important with that much boiling. Most people run the depths of the sap in the pan as shallow as they can without burning the syrup because you can boil more with the same amount of fuel that way. I was keeping all that boiling sap in a 2 foot by 8 foot pan only 1 inch deep.

It seems logical that the same principle would apply to backpacking to some degree. For example, if using an alcohol stove I'd expect to be able to boil 1 liter, twice, with less fuel than I could boil 2 liters once. (1/2 liter or about 2 cups is what I usually boil.) There's a reason that alcohol stoves aren't recommended for larger amounts of water. If you try to heat too much water at once efficiency suffers.

You'd have to test your set-up to know for sure, but I think you're likely to find there is a point where two boils will be more efficient for you but below which one boil is better. As has been pointed out, all things being equal a larger bottom on your pot (within reason) will tend to be more efficient in transferring heat.

Buck, I think I agree. Though the maple pans are also optimized for evaporation.

I am curious about this. I need to make 40+ ounces of hot water to feed 3 people. With alcohol I went with 2 boils simultaneously with this system.

http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=46146

It uses about 2 ounces or less of alcohol and usually I get a full boil and always plenty hot to FBC.

What systems make 40+ ounces boil in one pot with alcohol?

"We just spent 5 days doing just that with 2 1.3L Cones at 6000' and 50° temps.
We used less than an ounce of alcohol for each boil."

My experience nearly exactly (40 ml) at 9000-10,000 feet at even lower temperatures with a Evernew 1.3 pot in a Caldera cone.

If you want to break it down to pure science then it doesn't matter whether you heat up 1 liter of water twice or 2 liters once. The amount of thermal energy required to boil 2 liters of water is the same no matter how it's done.

To figure out something more real world though I'd tally up all the constants along with real world heat loss to see what matters and what doesn't. Power from your stove is constant, heat capacity of the water is constant along with the change in temp to get the water up to boiling no matter if its one or two liters your heating. The mass and time are directly related so that only means one liter will heat twice as fast as two. With that in mind you can factor in heat loss as an unknown rate for each setup and determine which loses more heat based on the time it takes to boil 1 liter vs. 2 liters. Whichever takes longer to boil all 2 liters overall is then using more fuel and less efficient.

If it takes exactly twice as long to boil two liters then your heat loss is the same and it doesn't matter which method you use. If boiling 1 liter isn't half the time as boiling 2 liters then you are losing more heat somewhere in the setup. In this case to boil the water you have to heat up both the pot and the water along with anything else touching the setup in addition to the heat loss through the pot. Since the pots are probably fairly similar in size the total transfer through the pot will probably be pretty close to one another. So I'd expect that boiling 1 liter twice would be less efficient since you'll be dumping energy into heating the pot up twice instead of only once.

There are lots of tiny heat loss factors that make it difficult to analyze perfectly. Your best bet is to minimize most of the heat loss factors for either case. Use the most practical windscreen, use a pot of optimal size for the stove/burner, and use some kind of lid on the pot.

–B.G.–

I used a Brasslite alcohol stove. Don't know the model offhand, but I believe it's the Turbo something-or-other.
I used a two quart aluminum pot with cover. The base of the pot is about 6" across.
I used a windscreen for all the tests.
The temperature was about 50 degrees with winds about 0-3 mph.
The water was about 65 degrees.
I let the setup cool between tests.
The type of alcohol fuel was unknown, but appeared to be either HEET or denatured or a combination thereof.

In the first test I attempted to boil two quarts. When the fuel had burned out the water was hot but it hadn't boiled.

I then boiled 1 quart. It boiled in 12:40.
I then boiled 1 pint. 7:00
I then boiled 2 consecutive pints, the first boiled in 4:30, that was quickly dumped out and replaced with cool water, and the second pint reached a boil for a total time of 10:40 for both pints (1 quart.)
I boiled another quart, 10:55.
I then boiled another pint: 5:58.
(In another aborted test a pint of water started boiling when I wasn't paying attention, but must have been around the 5 minute mark.)

I also did a couple of tests with 99% rubbing alcohol. It boiled 1 quart in 5:40! (Another quart had caught me by surprise when it took about the same time to boil on the first rubbing alcohol test.) Judging from the flame length there was a faster burn rate than denatured, so I don't know how much water rubbing alcohol boiled per oz relative to denatured/HEET. I tried to boil 2 quarts but it ran out of fuel before it boiled. Rubbing alcohol produced a lot of soot. I have often used it in thru-hiking when necessary, mixed with whatever remaining HEET I had left.

Some of the results don't quite make sense. It seems clear that there were seemingly small variables that made a significant difference. Perhaps this model of stove burned hotter when nearly full. Another variable was the exact windscreen placement, primarily how much air was getting underneath, although I tried to make sure there was a good gap on each test.

The test wasn't conclusive for this setup comparing 1 quart boiled at once vs 1 quart boiled as two separate pints. I wasn't surprised to see that 2 quarts at once was too much for this stove. With my Caldera Keg set-up I just boil the amount of water it was designed for: about 2 cups at a time. This test was conclusive that rubbing alcohol boiled much faster with this setup.

I asked almost the same question about a year ago, and wound up doing a slew of tests: 2 X 8 oz. vs. 1 X 16 oz.

http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=34089

My interest was about using a smaller pot (the cup from a Snow Peak Solo cookset) to reduce pack volume; I imagine boiling half the water in the same size pot would take a hair longer because of the air space between the top of water and the lid.

The short of it is there's no increase in efficiency boiling 8 oz. twice (actually takes a bit longer, especially if you need 16 oz. or more for meal and drink), and this is compounded with the headache of juggling the hot pot twice, maybe running out of fuel or having the flame blow out when the pot is lifted (and waiting for the stove to cool before refilling), the interruption of the mid-way pot switch (typically, you can judge your fuel to boil 16 oz. and simply do other things (or nothing!) while you wait for the flame to go out), and many other things that simply add up to a PITA.

Anyway, I've pretty-much settled in on using my Snow Peak Bowl (now with a Zelph Starlyte stove), which has been praised here by a few. It only weighs 1.8 oz. (same as the SP Solo Cup, but with almost twice the capacity) and have sought other ways to increase alcohol fuel efficiency, mainly with heat reflectors, windscreens, and otoshibutas (drop-lids).