Over the last few months, when I’ve been telling friends and family that I’ve been ice fishing, those that don’t ice fish themselves are astounded to hear that we have a heater in our shelter. Conventional wisdom would dictate that bringing a heater out on the ice would, as one of my friends put it, be grounds for a Darwinism award. After all ice + heat = water. So it would seem you are putting your life in your own hands when heating the shelter you’re ice fishing in.

So it would seem…

But thousands of ice fisherman go out on the ice each year–with heaters–to enjoy their favorite winter sport. In fact, the hard core ice anglers stay out overnight and leave their heaters on all night long. If they didn’t, they would be icicles by the next morning when overnight temperatures get in the single digits (Fahrenheit) in the middle of January. We’ll ignore the fact that temps get below 0 F consistently in Minnesota’s winters.

So how is that possible? Why aren’t these anglers “swimming with the fishes?” Why is it safe to use a heater out on the ice? I was wondering the same thing, so I did some calculations and found out why it works. The real world is a complicated system with an infinite number of variables. I’m going to make certain assumptions to make this as uncomplicated as possible.

Disclaimer: I am not a physicist, nor do I play one on TV. I took basic formulas I found on the web and did my own calculations to arrive at my own conclusion.

So let’s get started:

In order to melt ice, you need energy, in the form of heat. Let’s forget about the ice for a minute and consider the air within the shelter. When I was out on the ice, the heater warmed the air in the shelter enough to feel warm–almost toasty. For simplicities sake, we’ll say it warmed the air from -5 degrees C to 5 degrees C. This is the equivalent of going from 23 degrees F to 41 degrees F. I need the temperature in Celsius because the formulas I’ll be using below are specified with Kelvin and I can plug Celsius in to them instead.

So the amount the air will be heated up is:

Shelters come in a variety of sizes, but let’s assume a two-man shelter like the one I was in. Just to keep things simple, I estimate the size to be 5 ft long by 5 ft wide by 5 ft tall, or 60 inches by 60 inches by 60 inches. Some simple math gives me the volume of the air I will be heating up:

^{3}

The formula I’ll be using below requires volume to be in cubic centimeters, so I need to convert from cubic inches to cubic centimeters:

^{3}* 16.387 cm

^{3}/in

^{3}= 3,542,400 cm

^{3}

Now I want to find out how much energy it will take to heat up the volume I’ve calculated. To do that, we need a constant called specific heat. It is defined as:

the measure of the heat energy required to increase the temperature of a unit quantity of a substance by a unit of temperature

In our case, we want to warm a volume of air by one degree Kelvin. The specific heat for a volume of air is .001297 Joules per cubic centimeter Kelvin (J/cm^{3}K), so to warm up the air in the shelter one degree Kelvin would be:

^{3}* .001297 J/cm

^{3}*K = 4594.5 J/K

We are warming up the air by 10 degrees C (one degree C = 1 degree K):

So it takes 45945 Joules of energy to warm up the air in my shelter 10 degrees C.

Of course, this warming didn’t happen instantaneously. It’s a small space and didn’t take much time. To keep our numbers conservative, let’s say it took 5 minutes:

So we are producing 9189 Joules of energy per minute to warm up the air.

Are you following me so far? You can go get a cup of coffee if you need a break. I’ll wait.

The original hypothesis is that the heat generated by the heater would melt the ice, causing us to plunge into the icy cold lake. Let’s assume that 100% of the energy calculated above doesn’t warm the air, but instead melts the ice. That’s not true, of course, but that’s a worse case scenario. Now let’s find out how much ice we need to melt for it to be a problem.

The Minnesota DNR sets a guideline that 4 inches of ice is safe to walk on to go ice fishing. That would presume that 3 inches is unsafe. So we will assume we are in the early part of the season and need to melt away just 1 inch of ice to be in danger. Taking the surface area of ice in our ice house (mentioned above at 5 ft by 5 ft), we can find out the volume of ice that we need to melt:

^{3}

In order to find out how much energy it takes to melt this amount of ice, we need to convert the volume to grams because our formula is in grams. Answers.com reports that a pound of ice is about 28.8 cubic inches. We can now find out how much our ice weighs:

^{3}/ 28.8 in

^{3}/lb = 125 lbs

As mentioned above, we need to convert this number to grams:

If you’re still awake, stay with me, we’re almost done.

Now we need to find out how much energy it takes to melt this 56,669 grams of ice. There is a constant called the heat of fusion. Heat of fusion is defined as:

the amount of heat required to convert a unit mass of a solid at its melting point into a liquid without an increase in temperature

Or, stated simply, the amount of energy needed to melt the ice. Ice has a heat of fusion of 333.55 Joules per gram (now you see why we needed our ice volume in grams). That produces the following calculation:

That’s a lot of energy!

Now for the payoff:

We determined above that our heater is generating 9189 Joules per minute of energy. Using that rate of heat distribution, and assuming all the heat generated is going toward melting the ice, we find:

So it would take 34.3 hours to melt the ice enough for it to become a danger to me. And that’s for only one inch of ice! Again, this is assuming that all the heat from the heater is melting the ice and not the air. Let’s also not forget that ice fishing shelters are not air tight. Cold air is coming in all the time, which also needs to be heated up. So now you see that heaters melting the ice isn’t really a problem.

The real danger of using heaters in ice fishing shelters, especially in the castles that have bunk beds to sleep in, is the carbon monoxide produced by burning the propane. Without proper venting, the carbon monoxide builds up and can eventually kill you. Every year, there is at least one news story of someone who left a heater on all night, didn’t vent it properly, and didn’t wake up the next morning.

So if you’re out on the ice, feel free to bring a heater. But make sure to take a break every so often to get some fresh air. Or just open the velcro vents in your shelter. And definitely open a vent if you plan to stay overnight.

Number of hours to melt one inch of ice: 34.3

Number of seconds before I started skimming over your calculations and reading only the witty commentary: 5.6

Knowing somebody geeky/cool enough to have bothered figuring this out: priceless

[...] This post was mentioned on Twitter by DNR Fisheries, MNAngler. MNAngler said: If you've ever wondered why it's safe to use a heater while ice fishing, it's all in the physics: http://bit.ly/d1Ordt [...]

I have yet to ice fish this year. Your post is interesting, and informative, for those who have been wary of ice fishing. the first few times, in Colorado, I had some tense moments, but never any true danger. It has been too many years since I took physics, but I was ok with the heaters. Building a fire on the ice was when I got a little shaky.

[...] a heater on the ice since it tells you ice + heat = water, but it’s actually quite safe. I wrote a post early this winter about why. It’s nice when the outside air temp is in the single digits and [...]

[...] calculations in my last post showed how a heater in an ice shelter is not a problem. However, it would seem to be a bit more [...]

I feel smarter now! But, I’m not going to test this out in the field just going to take your word on the heater being ok.