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When I am going to boil a pot of water for cooking, will the total energy consumption be less if I start with hot water from the tap?

A few considerations:

  • Both the water heater and the stove burn natural gas (methane)
  • The water heater is less than 10 years old (so relatively efficient and well-insulated)
  • The pipes are not insulated, so when I start filling the pot the water will be cold (assume 8 deg C for Wisconsin, USA)
  • I need 5L of water in the pot
  • By the time a pot is full the water from the tap is hot enough to steam (let's say 49 deg C)
  • I'd estimate there are about 5m of 2cm diameter pipe from the tap to the water heater
  • 2
    One thing you may want to consider when using water from a hot water heater for cooking or consumption purposes is that such heaters can accumulate metal within the water via leaching of the tank walls & water evaporation concentrating metal within the water over time. – Fred Mar 3 '17 at 8:13
  • @Fred for a (relatively) new water heater, is this cause for concern? Wouldn't the metal be iron, aluminum, or copper, which is harmless in small concentrations? – LShaver Mar 6 '17 at 17:45
  • It's just something to consider. – Fred Mar 7 '17 at 12:58
  • Related: sustainability.stackexchange.com/q/815/3379 – LShaver Jun 6 '18 at 14:47
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Stovetop vs water heater efficiency

A gas stove-top is about 44% efficient (source), and a modern natural gas water heater with a tank is up to 67% efficient (source (pdf)).

Basic formula

From this answer on Chemistry.SE, the energy needed to heat a liquid from one temperature to another is:

Q = (mass)*(specific heat)*(change in temperature)

For water, the specific heat is simply 1 kcal/(kg °C), so we can leave this out. The mass of water is 1 kg per L.

Total energy needed

Applying the formula:

Q = (5kg)*(100°C - 8°C) = 460kcal

Energy needed from each source

Starting with cold water in the pipes, volume is length * π * radius² = approximately 1.5L:

  • In the pot we'll need to heat this 1.5L from 8°C to 100°C = 138kcal
  • The remaining 3.5L is heated from 8°C to 49°C in the water heater = 143.5kcal...
  • ...and then the rest of the way to 100°C in the pot: 178.5kcal

If the water at the tap is already hot, the breakdown is:

  • Heat from 8°C to 49°C in the water heater = 205kcal
  • And from 49°C to 100°C in the pot = 255kcal

Final comparison

  • Stovetop only: 460kcal / 44% = 1045kcal
  • Using the water heater, starting with cold water at the tap:
    • (138+178.5)/44% + 143.5/67% = 934kcal
  • Using the water heater, starting with hot water at the tap:
    • 255/44% + 205/67% = 886kcal

So using hot water from the tap will save anywhere between 11% and 15% compared to using just cold water.

  • Typically, by "getting to boiling", one doesn't mean boiling it all away, which is, I think, what you've calculated. It more commonly means raising it to 95-100 deg C, with a few grammes of steam produced. – EnergyNumbers Mar 18 '17 at 16:43
  • @EnergyNumbers... oops. That's a good point. I corrected the answer. – LShaver Mar 19 '17 at 21:29
  • You might want to add the case: "Stovetop with pot skirt: 460 kcal / 60% = 767 kcal". It increases the thermal efficiency by a factor up to ~1.3, according to "An Investigation of Skirts". And in the final comparison, you might add the heat loss from the 1.5 l hot water cooling down in the pipes after using the tap. Stovetop does not look so bad after all! – tanius Apr 23 at 1:25
  • @tanius interesting, but looks like that applies specifically to outdoor stoves? – LShaver Apr 23 at 3:06
  • In the paper there is a list of effects of a pot skirt – most also apply to indoor stoves. Some don't, so the effect would be weaker indoors. – tanius Apr 23 at 8:32
1

Any water heater built in the last 10 years will be significantly more energy efficient than your stove. At the same time you might want to consider a number of other variables if you are really into crunching the numbers on this.

1) you mentioned that your pipes are not insulated but didn't say how much pipe the water travels through before reaching your kitchen sink, or how close that pipping may or may not be much colder areas such as exterior walls, unfinished basements etc. Without knowing that kind of information no one can really say how much energy you are loosing in between your water heater and your sink.

2) The time of year, and whether you are trying to heat or cool your home, matters a great deal if you are thinking about how either choice plays out in terms of CO2 emissions. If you are boiling water in the winter, a lot of the energy that your stove "wastes" while heating up a pot of water goes into the kitchen's air and is helping to heat your home. That might matter a lot if you have a pretty wasteful forced air system attached to an older furnace, or might not really help if you have an efficient radiant floor set up or a geothermal heat pump etc. Conversely, turning your stove top on in the summer will make your AC to run, which in turn compels the nearest coal or natural gas power plant to burn more fuel. Make no mistake, even if you have solar panels, everyone who is tied to the grid is consuming at least some dirty energy at night, cloudy days, and any time appliances are drawing more than panels provide at that precise second.

If you are really concerned about this you also might want to look at a solar water heater on your roof which is way cheaper than PV solar panels, eligible for a 30% tax rebate, and will drop your water heater bill by anywhere from 50%-80%

  • The pipe length is mentioned in the last bullet point - 5m of 2cm pipe. I also doubt things like unheated areas/external walls make much difference over the few seconds it takes to fill a pot. But, that's a good point on home heating, especially since the water heater is in a locked utility closet! As for a solar water heater, I'd love to -- but it's not a possibility since I rent! – LShaver Apr 16 '17 at 4:36

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