Which is the more energy-efficient way to boil water?

[Feanorcurufinwe]

Assume that your stove and your hot water heater use the same type of fuel. Is it more enery-efficient to run the tap water till it's hot and boil that, or to boil cold water?


on edit: I think the big variable here is the length of piping from the hot water heater to the tap, and how much water you are heating up that you are not actually using...

[Deja Q]

1. Starting with warm water, boil it with the cover on.

:-)

[truhavoc]

2. I am unsure about the energy efficiency, but the water waste...

should be considered as well with respect to the hot water part of the question.

[Feanorcurufinwe]

3. In my situation

a remote homesite, with water supplied by a well, that's not a very big factor, except in consideration of the electricity required to run the well pump.

[Bridget Burke]

4. Alton Brown put hot water into the pot...

When he was showing how to cook pasta. He didn't explain why, but I trust him.

Thinking about it, running one pot full of hot water will probably not cause the water heater to kick in. It just uses already-hot water in the tank. So that method might conserve energy.

[unblock]

7. starting with hot water is most likely TIME-efficient

as it will normally come to a boil faster. this actually depends on the physics of the water, e.g., the bubble content changes the equation, but for the most part, water closer to the boiling point arrives at the boiling point faster.

but that's not necessarily ENERGY-efficient.

as to your point about the water heater kicking in, the pot of cold water will leave the water heater still keeping its constant pool of water hot, whereas the pot of hot water will drain the water heater, causing it to replenish with cold water, lowering the overall temperature of the pool of water, causing it to use more energy to bring it back to the normal temperature.

this last point does have some energy-efficiency implications: the water heater normally wastes energy by maintaining a hot pool of water -- there's a small burner on constantly fighting heat loss of the pool. so if you replace some of the hot water with cold water, the remaining pool does not lose heat as much and the small burner goes more to actually raise the temperature of the pool.

so the "start with a hot pot" solution has an advantage in that it makes the hot water heater temporarily less energy-inefficient.

[unblock]

5. define the bounds of "energy-efficient" please

are we talking about which will the stove use less energy on?
or are we talking about overall home energy use, i.e., is the hot water heater that brings hot water to your tap included?

[Feanorcurufinwe]

6. I'm talking overall energy use.

The electricity required to run the well pump, the propane used to fuel the stove, and the propane used to fuel the water heater. I have a tendency to use hot water because it is quicker, but this is a question I've been wondering about. I'd be willing to take the extra time if it will save energy.

[unblock]

8. i assume you have a normal "tank" water heater, 40 gallons or whatever?

as opposed to a "tankless" water heater?

if you have the tankless variety, i'd say the hot water pot is more energy-efficient because the water-heater is designed more efficiently to heat water than your stove is. you stove loses a lot of heat to the surrounding air, whereas the tankless water-heater traps as much as possible and directs it to the water.

i think the answer is start with hot water either way, but the reasons are different.

[Feanorcurufinwe]

10. Yes 50 nt

[unblock]

9. ultimately, getting the smallest tank is most energy efficient

the bigger the tank, the more energy is wasted maintaining all that heat at the ready.

so if boiling water from a cold water enables you to get a smaller tank, then that would be most energy-efficient.

however, if you're still going to have the big tank, you might as well use the heat you're expending energy to maintain.

[Taverner]

11. Climb to the top of an active volcano

lower the pot, filled with water on a string - just so high that the water starts to boil but not too low as to melt the pot or burn the rope.

[Feanorcurufinwe]

13. The energy involved in getting back and forth to a volcano from my kitchen

probably precludes this solution.

[Salviati]

12. I always use cold water...

Not out of concern for energy efficiancy, but rather the solubility of lead in water and th rate at which it is absorbed is greater in hot water than in cold...