What would be the most efficient way to heat water? If you put it on the stove, you have to heat the pan too; not very conservative. If you use the microwave, you probably use less power, but for longer (I don’t know what the total power usage would be though), plus you would only lose the energy that the water would transfer to the container (hardly any considering the material the container is made out of).
It depends on what you mean by “efficient”.
If you’re looking for the method that will waste the least energy, I’d guess that an immersion heater would be your best bet. Because the heating element is completely submerged, the energy has nowhere to go but into the water.
But perhaps you’re asking which way raises your utility bills the least. If that’s your concern, get yourself a big magnifying glass, and wait 'til noon. =B^)
Laugh hard; it’s a long way to the bank.
We have a setup in the milkhouse called a Freeheater. Basically it preheats the water going into the hot water heater by recovering heat from the process of cooling down the milk. When you refridgerate the milk you’re taking heat out of the milk. Instead of just wasting that heat this pre-heater uses it to heat up the water going into the hot water heater. My guess is you couldn’t get much out of the system on a homeowner scale. However the milk coming out of a cow at body temperature and then taking all the heat out to drop it to just above freezing, well that’s a useful amout of energy.
The most energy efficient way to boil water is to boil it in a vacuum, because it’ll boil at room temperature. Not very good for cooking, but that’s how you phrased your question.
If you want to speed up the process of boiling, make sure you have the lid on the pot. This traps the heat of the steam which is ordinarily lost when heating an open pot.
Guy Propski:
Would a vacuum really be most efficient? At some point, energy must have been expended to create that vacuum (unless, of course, we’re talking about the vacuum of outer space, in which case the water would freeze, rendering the point entirely moot). How much energy? How does it compare to the energy expended to boil the water on a stovetop?
I have absolutely no idea as to the answer - I’m just pointing out that the vacuum is not the “free” energy source you seem to suggest.
~ Complacency is far more dangerous than outrage ~
Two points:
1: In Arizona, they had water pre-heaters available that would run the just-compressed freon in the air-conditioner through a heat exchanger that pre-warmed the water going into the water heater. I never had one, so I do not know how much it saved on energy costs.
2: I had an idea for an on-demand water heater that would use a little microwave at every point you would want hot water. The magnetron, which generates the microwaves, would be cooled at the front and since the microwaves would not be able to escape the pipe/waveguide, voila! 100% efficiency.
Seems like you could also put your water into a well insulated vessel. Then measure out, to precise amounts, several chemicals that when combined with the water will produce energy and another product(s) that could easily be seperated from the water. Thus all the energy intended for the water heating is produced right in the water and you don’t need all that extra equipment cluttering up your basement.
Years ago a friend of mine built a solar water heater using cleaned, burned-out flourescent tubes and PVC pipe fittings. He got the plans from a Mechanics Illustrated mag, IIRC. It actually worked pretty good.
Do any of you older twiddlers out there remember seeing this article? It was maybe 20 years ago, in the early days of interest in solar energy.
Yes, I was stoned. No, I wasn’t hallucinating. 
Peace,
mangeorge
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STARK–you can create an area of very low pressure (if not vacuum) in this manner:
(1) Obtain two pots; one fits inside the other. The smaller is the pot with the lid, and also has holes in the bottom (sort of like a double boiler).
(2) With the smaller pot inside the larger and in the lowest position, fill the pots with water.
(3) Place the lid TIGHTLY on the pot, then pull the smaller pot upward (like a piston). Presto! Low pressure, and therefore lower boiling point.
I wonder if they sell this at Williams-Sonoma?
In California, we just put some black plastics tubes on our roof, run the water thru them, heats the water, costs nothing.
There are solar water heaters in Arizona, too. The reason for the air-conditioner/water-heater heat exchanger is to help save electricity, since most Arizona homes are completely electric.
Since the question seems pretty open ended let my throw in my five cents (hey, the rates have gone up!).
On the river I like to take a warm shower. I take a thick, black, plastic bag and fill it with water and then let it sit in the sun for about an hour. It’s very efficient (doesn’t cost a cent) and works very well. I use this to also heat water for dishes. I guess it would depend on how hot you needed the water to be and what you were going to use it for.
As for the home, that would depend on how much you paid for electricity (microwave) or how much you paid for gas (as if you had a gas stove). In Utah, natural gas is far cheaper than electricity so it would be cheaper for me to heat water on the stove. Again, it depends on what you mean by “efficient”. It costs less on the stove therefore, IMHO that is the most efficient way for me to heat water unless I have time to use the sun.
Best!
Byz
I just run water through a coffee maker without coffee or a filter. Seems to be the quickest way.
How about applying this to a pool?
7,000 gallons…
I feel the need to point out that boiling water in a low pressure situation has no practical application. The purpose of boiling water has to do with the temperature - unless you are for some reason trying to run a steam engine out of your home. Killing bacteria, making hot drinks and cooking vegetables or pasta all have to do with the temperature and not the physical act of boiling water.
As for pure energy efficiency, you are right, it takes less joules to heat the objects in amicrowave and less heat energy is lost in tha setting.
“C’mon, it’s not even tomorrow yet…” - Rupert
If you need a graphic solution, http:\ alk.to\Piglet
A quandrary.
Why doesn’t the friction of water splashing around make it hotter?
Actually, it does. But the increase is so small that it can’t be measured.
Virtually yours,
J Matrix
NOAA weather station radio gives the temp of our water supply (lake) and it’s the same as the water coming out of the tap. Should be enough friction in the trip from lake to tap to increase the temp? Thermometer just too crude?
But isn’t splashing different? (Someone mentioned splashing.) Then you’d be exposing more water surface and get more evaporation and evaporation is cooling rather heat producing.