Thermal Efficiency Question

[Vaughn McMillan]

OK, this is a completely unimportant question, but I've wondered about it for years, and I figure the collective wisdom of SMC might have some thoughts on it. ;)

Assuming you needed to boil two quarts of water on a gas stove, is it more efficient to start with cold tap water or hot tap water? In other words, is the stove and a good pan more efficient than the water heater (including the heat lost in getting the hot water to the kitchen sink)? I realize there are a number of variables that could skew the results a number of ways, but in general, what's the collective opinion?

- Vaughn

[Steve Ash]

Effiency I am not sure about however we did an expieriement is H.S. Science class and IIRC cold water from the tap boiled faster than the warm water did. Also warm water froze quicker than cold water in another test.

LOYL got you boiling water already Vaughan?:D

[Randy Moore]

Boil the water, it is quicker and in my opinion cheaper.

It didn't take the LOYL long to get you trained. LOL Boiling water already.:eek: :) :D ;)


Randy

[Lee Schierer]

Cold water will boil faster. I believe it is because it has more oxygen entrapped in the water. Like wise water that has been boiled will freeze faster than cold water because it has less oxygen in it.

[Aaron Koehl]

Vaughn,

You're thinking like a systems engineer. ;)
Let's see..

You would like to compare the per unit cost of heating a volume of water to 100° Celsius between two systems, with the goal choosing the most efficient route in terms of dollars (not time).

There are two courses of action:
1. Preheat the water using your home's hot water system, then place the liquid on your stove to complete the boil.
2. Pull cold water from the tap, then place the liquid on the stove to complete the boil.

Phrased differently, the question you pose is whether preheating the water using your hot-water system, then heating the remainder of the liquid on the stove requires less overall energy (and cost) than just heating the cold liquid on the stove.

Things could get complicated if you don't have an electric water heater. We should first model the cost (in Joules) heating of the water on the stove, as that model will be used by both courses of action. Then we should consider the variables in your hot water system-- e.g. is it an on-demand system, is it gas, does it keep a tank at the ready all day long..

Interesting question..

[Vaughn McMillan]

Aaron nailed the crux of the question. I know all about the "freezing boiled water" experiments, and (despite some claims to the contrary) that I can get a pan of 100° water boiling more quickly than a pan of 50° water.

That said, as Aaron summed up so nicely, I'm looking to see what's the most cost effective. As he also noted, the number and scope of the variables is pretty large. (i.e., How much water is wasted just running the hot tap long enough to actually get hot water, or how much gas is wasted keeping a tank of water hot all day?) It's probably one of those questions that can't ever be fully answered.

And you guys don't realize the irony of the "LOML having me trained" statement. :) She can boil water, heat up soup, and microwave a frozen dinner. Beyond that, she doesn't cook. I'm the undisputed cook in the house, and the kitchen is about as much my domain as the woodshop is. (She cleans the dishes though, except for my knives.) I'm not a great cook, but far better than she is. One peek at my avatar and you'll know she didn't marry me for my boyish good looks, for sure. ;)

- Vaughn

[Bill Turpin]

The efficiency of your time is the most valuable. The efficiency of boiling is the true temperature not the apparent temp caused by oxygen content. Pre heated water will truly boil first. Warm water freezes first because the frost on freezer coils is melted and the the better heat transfer makes warm faster. It takes .51 BTU to change temp of ice one degree, 144 BTU to change ice at 32° to water at 32°, one BTU to change water one degree, 980 BTU to take water at 212° to steam at 212°. All these for one pound of water or one pint. I pulled these from memory, books at work

Bill in WNC mountains

[Lee DeRaud]

I'm the undisputed cook in the house, and the kitchen is about as much my domain as the woodshop is.Then you can boil the water any way you bloody well please, can't you? :D :cool:

Leaving that aside, the "infrastructure" issues are irrelevent, given that you're not using the hot water heater just to pre-heat soup base. Bottom line, all you're really comparing is the heat transfer efficiency between (1) a stove burner under a pan and (2) a hot-water-heater burner under a tank. My vote is #2: the guy designing it can optimize it for that one job.

[Ken Garlock]

Hey Biil T. I was wondering if someone would mention the heat of fusion and the heat of vaporization.

Good old physics 101.:)

[Jamie Buxton]

I'd bet that the hot water heater is more efficient at heating water than the open burners on the stove. However, there's another issue. When you pull that first cup of hot water out of the kitchen faucet, there's quarts or even gallons of hot water in the supply pipe from the hot water heater. In my house, and likely in many houses, the heat energy in that water in the pipe is going to be lost. The pipe is insulated, but runs in uninsulated crawl space. That's why if you try to get hot water from the faucet later in the day, you have to run the water a while -- the heat energy which was in that water in the pipe has bled off somewhere else in the house.

So unless you have a very short pipe run from the hot water heater, you'd use less fuel to heat that one cup of water on the stove than in the heater.