Originally Posted by Sodafinger
Strange question, but I'm a home brewer...
Can you use an old (stripped down to the tank) water heater as a boiling kettle? My concerns include:
1) Is it safe to cook inside the enamel tank (212 + degrees)?
2) Would using a plasma cutter to cut off the top damage the enamel lining?
3) Would it matter if I use an electric or gas tank?
4) If I use an electric water heater, can I use a propane flame to create a boil?
I've been making my own beer and wine for almost 30 years, so I'm familiar with the process.
Where I can see a problem is that the life of a water heater is determined by the lifespan of the enamel coating on the inside of the tank. As soon as that hard coating cracks, then the steel tank walls are exposed to water, and the tank starts to rust. Normally that enamel coating will take thousands of cycles going up to 130 to 140 degrees or so, which is the temperature that water heaters typically operate at. If you heat the water inside the tank to boiling, the greater temperature variation from hot to cold is going to result in the enamel lining cracking earlier than it otherwise wood. How much earlier, I wouldn't know.
However, I fully expect that it would be safe to use the tank for boiling water (provided you didn't have pressure building up inside the tank) and that it would be safe to drink the beer made in the tank that way.
The enamel lining on the inside of the tank was baked on at about 300 deg. F, so any method of cutting the top off the tank that uses temperatures near or above that will damage the enamel coating. What I'd suggest is that you cut the tank off with an abrasive wheel and a hand grinder. As long as you have a helper misting the area you're cutting with a spray bottle or have water from a garden hose running over the exterior of the tank as you're cutting through it, you'll keep the metal cool and keep the damage to the enamel to a minimum.
You should be aware that the tank for a GAS fired water heater won't be the same as that for an electric water heater. The gas fired heater tank will be in the shape of an elongated donut because it needs a flue channel for the flue gas to rise through the middle of the tank. If you want to avoid that, you need to use the tank from an electric water heater, which will be cylindrical.
I don't see why you couldn't use propane to heat the tank. As long as there was water in the tank the metal tank wall wouldn't get very hot (and wouldn't exceed 212 deg. F), and you'd have good heat transfer from the flue gas into the metal and then into the water. However, you'd have to be very careful around that tank. Since you don;t have a flue channel running up the middle of the tank, you're going to have 400+ degree flue gas rising up the outside of the tank, so that if you get too close to the tank, you'd get burned by the rising flue gas.
It might be smarter to figure a way to provide 220 volt power to the electric heating elements of a water heater than to deal with natural gas piping and the hot flue gas. I expect that it wouldn;t harm the heating element to leave it on until the water boils instead of just gets hot. It'd prolly shorten the element's lifespan, tho.