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Question DetailsAsked on 2/23/2015

what is the optimal water temperature for water heaters

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There is an ongoing battle about that for the past decade or two - most manufacturers recommend 125 or 130 degrees as a sort of happy medium. True disinfection take 150-160 degrees; effective cleaning in dishwashers (without pre-wash) and cloth diapers (NOT plastic coated) takes about 140 degrees - but both of these are well above the maximum recommended "safe" temperature for sinks and showers and such of 130-135 degrees and risk serious burns in just a few seconds of exposure, so not recommended except for heaters serving ONLY enclosed washing devices like dishwashers or commercial laundry (non-public, non-homeowner use) washing machines.

Also - for a house with small children or elderly people who might expose themselves while alone 130 is the maximum recommnended (about 20-30 seconds exposure need to cause severe burns), except 120 for older/disabled people with cognitive issues, nervous system degradation diminishing their ability to feel heat, or very limited mobility (about 5 minutes exposure to serious burns at 120 degrees). Also, for people with some medical conditions (pulmonary, circulatory mostly) temperatures as low as 90 degrees are the maximum recommended, though that is better handled (say for a person subjectto those conditions living alone) by putting in temperature-limiting mixing valve or mixer faucet on the showers/baths they might use, to prevent excessive rust and possible bacterial growth in the water heater.

However, below about 120-125 degrees the risk of legionnaires bacteria flourishing goes up sharply, as does iron bacteria in the pipes so that is a risk too.

So, all things considered, 125-130 is a commonly accepted range and if the delivery temp needs to be below that for safety, then point-of-use temperature controlling mixing valves or thermal control faucets are recommended, at typically $100-300 at each point of use location, though with nearby bathrooms like back-to-back you can easily rig a mixing valve to feed several points of use.

Answered 5 years ago by LCD


Couldn't agree more with LCD's detailed response above. In addition to the concerns mentioned, here are a few others:

1- how well insulated are your hot water pipes from the heater to point-of-use? If poorly insulated, you'll lose a lot of heat before you get water to the faucet even after letting it run until the hot water appears. You may have to heat your water to, say 140, to get 120 at the faucet.

2- perhaps obvious, but should be mentioned: the higher the set temp, the higher the utility costs for a given heater. For efficiency, go with the lowest temperature you find comfortable to maximize savings.

3- a higher set temp MAY mean more peak demand availability. For example, if 4 people need to take showers within an hour, a higher hot water temp would mean using more cold and less hot water for each shower. Could be useful if you have a smaller hot water heater that tends to run out before it can recover.

4- you may need to adjust the hot water temp over the seasons. My pipes are run in unconditioned crawl space. During winter months the cold water is much, much colder than in warm months. To get the same temperature all year at points-of-use, I kick up the hot water heater a small amount to compensate. And yes, encapsulating the crawl space is probably a better answer but not in the budget yet.

Good luck...and maybe share with the readership what you decided and why!

Answered 5 years ago by SalisburySam

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