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We have a tankless water heater.

I want to set the temperature so that it is the most energy efficient. Should I set the temperature of the water so that it is just hot enough for taking a shower?

I figured that if it is set that way then when I take a shower all I have to do is to turn on the hot water and would not require any cold water to bring it down to a comfortable temperature.

Am I overlooking something here?

I guess our laundry washer would also have hot enough water. The dishwasher should be fine too, yes?

Currently we have it set to 120 and it is hit enough. We still have to fix some cold water with it.

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  • What kind of water distribution system so you have? I have the worst setup--un-insulated copper tubing going under a slab. But if you have a better set-up I doubt that setting the temperature lower than 130 F would have a measurable effect on energy consumption. It might lengthen the service life of the heater and plastic parts in the plumbing system.
    – Jim Stewart
    Jan 27, 2017 at 15:58
  • What brand and model of tankless water heater do you have? Is it electric or gas fired? Does it have a remote control for setting the temperature?
    – Jim Stewart
    Jan 28, 2017 at 11:45

3 Answers 3

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Setting the temperature lower would be the most energy efficient setting, but the question is open whether this would be a significant savings. It may be beneficial to the heater--IIRC the instructions for my Bosch Aquastar 125B say that if the heater is set at the lowest flame setting then it may never have to be descaled.

The downside of outputting a lower temperature of water is that this will reduce the ability to supply multiple simultaneous uses.

But there is the question of microbes in the water and specifically Legionella, the cause of Legionnaires' disease. All over the scientifically based world the recommendation is that water stored in tanks should be kept cold or at a temp of 130 to 140 F, not in between. It is not clear how this applies to hot water supply systems that use a tankless heater.

If all the water that ends up abandoned in the hot water pipes after the hot water taps are turned off has been heated to 140 F, then that would have killed "all" the Legionella in the incoming water. As it sits in the hot water line and gets tepid there is not enough surviving bacteria to grow to a dangerous level. But if the heater never heats the water past say 105 F, then Legionella will survive to grow in the hot water supply lines.

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    Useful guidance on Legionella - mainly for UK but probably of interest elsewhere too. Covers many types of hot-water supply arrangements, how to avoid stagnation by siting of low-use outlets etc.
    – RedGrittyBrick
    Jan 27, 2017 at 15:15
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Most guidelines (pdf) (in the US at least) recommend a hot water temperature of 120F, especially if there are children or elderly people in the house. This is to reduce the risk of scalding, but also serves to reduce mineral buildup and corrosion of your pipes.

There are some cases where you would want a higher temperature:

  • You have a weakened or compromised immune system, so a temperature of 140F helps stave off Legionnaire's disease.
  • Your dishwasher does not have a booster heater, so you may want a temperature of 130F to 140F for optimal cleaning and sanitizing.

If you do set your water heater higher than 120F, keep in mind that burns can happen with just 6 (30) seconds of exposure to 140F (130F) water for an adult, so you may wish to install special mixing valves at your fixtures to prevent the water coming out of the fixture from ever being that hot.

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  • I think good quality temperature controlled thermostatic mixing valves are really terrific, but one does hear complaints that some don't work well and cause problems. If the valve is behind a tiled wall, it is expensive and troublesome to replace a poor performing valve, but don't some units fit in a housing (the plumbing equivalent of a standard electrical box) so that the mixing valve itself could be changed without re-tileing? And in Europe the "shower bar mixing valve" is popular. The entire bar valve is accessible for changing, and the connections are standard at 150 mm (6") apart.
    – Jim Stewart
    Jan 28, 2017 at 4:37
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Dishwashers have an electric heating element. If the water going into the dishwasher is not hot enough, the dishwasher heats it. So, see if you can disable this on your dishwasher (and the effect of doing that) before setting your thermostat lower. I may be that the savings on hot water for showers is offset by the electricity to heat the hot water for dishwasher. The washing machine probably will work fine on cold water.

As to the issue of Legionella raised by Jim S., you probably will be setting your hot water heater above 130 just to have hot water that feels reasonably comfortable for a shower or bath.

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    It seems to me that if you lower the temperature of the water supplied by the central tankless heater, you would not want to deselect the heater function in the dishwasher. The dishwasher depends on having very hot water for proper cleaning and microbial control.
    – Jim Stewart
    Jan 27, 2017 at 15:45
  • Yes, there are conflicting priorities: lower elect bill vs more effective and safer dishwasher. Conflicting priorities should be explicitly considered. As I said, "and the effect of doing that".
    – Yehuda_NYC
    Jan 29, 2017 at 14:20

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