AskPablo: Water Heater vs. Stove

brass tap.jpg
This week Steve asks: “Which is better, running the tap water for a minute to get hot water or heating cold water on a gas stove?” In order to answer this question I need to state a few assumptions. Let’s say that tap water is 15C (59F) and our water heater is set to 40C (104F), so that will be our desired water temperature. Let’s also assume that the kitchen is on the second floor and the water heater is located in the garage, with 30 feet of pipe in between. By researching on-line I found that a stove burner (natural gas) is less than 50% efficient (think of all the heat that still escapes from underneath the pot). The water heater may have an efficiency of 67% if it is an older model.

Finding the amount of energy required to heat 1 liter of water by 25C is pretty easy. The specific heat of water is 4.1855 J/gK, or Joules required to raise 1g by 1Kelvin. Since 1 liter is equal to 1000g and a change in 25C is equivalent to a change in 25K we get: 4.1855 x (1000g x 25K) = 104,637.5 J, or 105kJ (99.5 Btu). This means that we need to use 210kJ (105kJ/0.50) of Natural Gas (4.5g) on the stove, or 157kJ (105kJ/0.67) of Natural Gas (3.4g) in the water heater.
Unless you are getting your water directly from the water heater it would be much more efficient to heat cold tap water on the stove. This is because a pipe with a half inch interior diameter and a length of 30 feet holds 1.18l, meaning that you have to waste 1.18l of water before you get your 1l of warm water (and the 1.18l of water that follow will quickly lose its heat in the pipe). Of course this assumes that the hot water loses no heat on its way to the tap, which it does. But I barely passed my undergrad Heat Transfer class so I am not about to dust off my old textbooks and dig out my abacus.
Just in case you are interested in how you might proceed, here is a little taste: It is not enough to know the interior area of the pipe, since the pipe has thickness. This means that, as you go further from the center of the pipe there is more and more material to conduct heat away from the water. Some smart fellow (or lass) figured out to use the log mean radius, whatever that is… So, you get the following equation for heat loss from a pipe: (2 x pi x k x L x dT)/ln(ro/ri), where k is the thermal conductivity of the pipe material, L is the length, dT is the temperature difference between the inside and outside of the pipe, ln is the natural log (consult your scientific calculator), ro and ri are the outside and inside radii of the pipe. All this adds up to a massive headache and that’s why I’m not doing it.
See you next week!
Pablo Päster, MBA
Sustainability Engineer

20 responses

  1. Pablo – nice subject… I’ve been wondering this every day when I make my tea. The thing is, it seems there’s a lot of subjectivity in the matter based on the nature of your tap.
    My tap is blazing hot within seconds so I always fill the pot with tap water, then put it on the stove for the final heating. I’m fairly sure this saves gas.
    Of course, I’ve got nothing to do with paying for the original heating, but as it’s a large building (30+ units) I suspect that it’s fairly efficient.

  2. or even better,
    one of these.

    A “Hot Shot” from Sunbeam. Heats water to hot within 2 minutes. Perfect for a cup of tea, bowl of oatmeal, or to make french-press coffee.

    Much more efficient than the stove or running the tap, as more of the energy goes directly towards heating the water than in the open systems of the hot water heater/pipes, or stove.

    p.s. I don’t work for Sunbeam, but do use this product several times a day.

  3. Since this came up in the context of making tea, it should be noted that it is not advisable to consume water from a standard hot water heater. Anyone interested in doing so should remove a heating element and take a tour of the inside of their tank if they have one. Legionnaires disease is another risk to this practice. I have yet to take a look into tankless water heaters though so if anyone has info there please weigh in.

  4. Yes. Toxic metals, bacteria, and other contaminants are concentrated in a hot water heater tank. The hot water also more readily leeches the lead from the pipe solder if you have copper pipes between your heater and your faucet. Google it ;)

  5. My hot water pipes have insulation around them from part of the trip. Not sure if helps.
    Now here’s one. What takes more energy to heat up(boil?), cold water or hot water? I’ve heard cold water, but I have no idea if that’s true. Suppose I could google…

  6. Ah It’s false

    Cold water boils faster than hot water. If hot water freezes faster, maybe cold water boils faster! Again, this defies common sense—and again, say scientists, it’s simply wrong. Hot water from the tap should in fact boil much faster than cold water. However, using hot water for boiling does not actually save any energy. You may use less gas (or electricity) on the stovetop, but your water heater will have used the same amount of energy to heat the water in the first place. (If you use solar energy to heat your water, of course, that’s a different story.) Some water heaters may introduce additional sediment into the water, giving you another reason to consider starting with cold—at least, if time is not of the essence.


  7. I once saw the sludge in the bottom of a hot water tank that was being changed for a new one. Gross! Best to get your water as fresh as possible from the cold tap.
    Lead-free soldier is a more recent standard. Older pipes would have solder joints containing lead.

  8. Hot water heaters have water forced out of them, not really drawn per se. This is due to the pressure differential being fed on the supply side that points pressurized water straight at the bottom of the tank forcing water out the other pipe due to an opening down stream. The water swirls in the tank quite readily due to the turbulence of water coming in and going out at significant pressure stirring up those nasty contaminants.
    On the topic of heating water, I wonder how a microwave factors in on the efficiency side.

  9. The sludge in the bottom of a water heater is pretty disgusting. It’s recommended that you drain some of it off, using the tap at the bottom, to improve the efficiency of the water heater. But maybe it’s a good way to improve the safety of the water as well.

  10. Where do you think the the “sludge ” in the bottom of the hot water tank came from…. the water of course. So would you rather drink water that had the sludge still in it or the water that has (at least some) of the sludge removed via the hot water heater.
    As for cold water boiling faster than hot…I don’t think so. Conversely, there is speculation that “hot tap” water may freeze faster than “cold tap” water. The reasoning is that “cold tap” water has entrained air and/or dissolved air. Whereas water that has gone through the hot water heater has most of the air knocked out. The air acts as insulation to slow the cooling process. I do not ascribe to this theory, but I do not know if it has been empirically tested. However, if you use hot water in your ice tray, you will have very clear cubes (no air bubbles). Use cold water and have cloudy cubes (air bubbles).

  11. I have a friend who turns on the heater of the HW cylinder (electric) for hot water use once a week. He claims that heating from scratch once a week is more efficient than holding the water at hw temperature for a week. This is counter to my common belief that, for such a short time, turning off and reheating does not break even with holding the heat in the modern well insulated cylinder.
    Any general opinion on this please. Also calculation formulas for proof!

  12. Is it cheaper to heat a kettle of cold water on my kitchenaid range large burner or use an electric kettle. Which unit uses less energy? I do not have an electric kettle and am thinking about purchasing one.

  13. Is it cheaper to heat a kettle of cold water on my kitchenaid range large burner or use an electric kettle. Which unit uses less energy? I do not have an electric kettle and am thinking about purchasing one.

  14. Hi Pablo,
    I agree, but don’t like either way. The Stovetop with a conventional pot waists lots of heat because it doesn’t use a good heat exchanger. My electric hot water kettle uses electricity for heating another waist. Turning on the water to drain and warm the pipes, you have proven to be really inefficient.
    I would like the following:
    1) A very efficient heat extraction kettle for my stove. A folded copper heat exchanger on the bottom of the pot should extract a high percentage of the heat from the burner. I have seen an old one, but I can’t find one now.
    2) A hot water on tap that doesn’t waste any water, and uses my efficient hot water heater to make it hot.
    a) This is an interesting problem. The person who answers it will save billions of gallons of water, and mega watts of power.
    b) I have an idea that I think will work. I am going to prototype it and see how it works.
    c) I have seen the type you can buy at home depot, the flaw is that they cycle often and reduce the efficiency of your hot water heater by making a radiator that is as big as your hot water pipes that is continuously warming and cooling.
    d) Given our energy situation, it is remarkable that our daily efficiency is limited by simple designs of systems in our lives that are far from optimized.
    Thanks again for the informative site.

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