Dear Pablo: Why should I turn down my water heater? Isn't it just as efficient to dilute really hot water with cold water in the faucet than to use barely warm enough water straight out of the hot side?In a strictly theoretical sense you are correct. Using 100-degree water uses the same amount of energy as mixing equal parts of 150-degree and 50-degree water. But your water heater doesn't just make hot water, it stores it as well (unless you have a tankless water heater). That large tank in your basement, garage, or hallway closet holds its water at the desired temperature 24 hours a day, 365 days a year just so that you can have a hot shower at 3 in the morning if you so desire. In real life this storage of hot water creates a difference in efficiency from the theoretical case.
Water heaters are insulated, newer ones much more so than older models. Insulation basically slows the rate of heat loss, so the more insulation you have, the more the heat loss is slowed. Insulation is measured in units of "R-value," which is derived from a formula containing thickness, heat flux (the flow of heat), and the inside/outside temperature. It is the difference between inside and outside temperature that we care about in this case. In any given water heater with consistent insulation the energy lost will be proportional to the difference between the inside and outside temperature. So what this means is that holding 150-degree water at the temperature 24 hours a day, 365 days a year, requires more energy than holding the same amount of water in the same water heater at a much lower 120-degrees.
In my "Water Heater vs. Stove" article I calculated that we need 105 kJ of heat energy to heat one liter of water by 25C (45F). Since a water heater is roughly 67% efficient, I will actually need 156.7kJ/l (105kJ/0.67) per liter just to heat the water, and then an additional amount to keep it at that temperature as some of the heat escapes.
Insulate Your Water HeaterOf course, if you like having scalding hot water available at a moments notice you do have some options. First, you can insulate your older water heater with a specially designed water heater blanket. Your local utility might provide rebates and they might even incentivize replacing the water heater completely. You can also insulate the hot water pipes from the water heater to each faucet. This will reduce the amount of time you have to wait for hot water to make its way through the pipes each time you turn on the faucet because the pipes will stay warmer longer. You can also have your plumber install check valves in the inlet and outlet of the water heater to prevent additional heat loss when the pipes are not in use.
Change Your Shower HeadsIf you're hungry to do even more, you can install a low-flow shower head, or even a shower head with ShowerStart technology that senses when the hot water has arrived and turns off the flow until you are ready. This not only saves water, but it also saves time, letting you do other things while you wait for the hot water to arrive without letting any of it run down the drain.
Consider a Tankless Water HeatersIf some remodeling is in your future, you might even consider a tankless water heater. The advantage of the tankless water heater is that there is no round-the-clock storage of hot water, and therefore no constant heat loss. Since tankless water heaters are typically installed under the sink or near the point of use there is also little to no waiting for hot water so you also save water. The only drawback is that teenage daughters might be inclined to take an endless Sunday morning shower without fear that the hot water tank will run out, because there is no tank!
Just Turn Down the Water HeaterFor those of you who are simply looking for a quick, no-cost energy efficiency and cost-saving solution, just turn your water heater down. In most cases you can get away with 120-degrees or even less (indicated on some water heaters as a triangle or the word "warm"), as long as you don't need scalding hot water for a commercial kitchen, and as long as you don't have a high demand for shower water at any given time. And, as long as you are using soap, don't worry about bacteria left on your plates. Unless you were soaking your dishes in boiling water before, there isn't much difference in turning it down to 120.
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