# Does Turning Down My Water Heater Really Make a Difference?

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.

Of 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.