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When adjusting my water heater, what temperature should I set it to? Is the answer different if you have:

  • Concerns about diseases
  • A dishwasher
  • Young children or elderly that can be easily burned
  • Electric vs Gas heat
  • Type of plumbing (e.g. PEX or Copper)

Also, is there a preferred technique to check the temperature of the water after adjusting it?

  • @JeffMeden I believe your comment got pulled in from this duplicate question. See gregmac's answer below that describes the risks of setting the temperature too low. – BMitch Feb 25 '16 at 15:42
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There are two main and opposing risks:

  • Too high, and users get scalded
  • Too low, and you risk pathogens, particularly Legionella bacteria, which causes legionellosis (Legionnaires' disease)

Minimum temperature

Legionella risk

According to the paper "Legionella and the prevention of legionellosis," found at the World Health Organization website, temperature affects the survival of Legionella as follows:

  • Above 70 °C (158 °F) - Legionella dies almost instantly
  • At 60 °C (140 °F) - 90% die in 2 minutes
  • At 50 °C (122 °F) - 90% die in 80–124 minutes, depending on strain
  • 48 to 50 °C (118 to 122 °F) - Can survive but do not multiply
  • 32 to 42 °C (90 to 108 °F) - Ideal growth range

Dishwashers

Most current-model dishwashers have a minimum requirement of 49 °C (120 °F). Most have heaters and will heat the interior as needed. If yours is older you may want to check specifications.

Dishwasher detergent varies, but "works best between 50 and 60 °C" seems to be a fairly common statement. There is also cold-water detergent on the market that works at basically any temperature.

Maximum temperature

Burn risk

Setting too high can scald someone using the water. This is particularly easy because when you first open the tap, the water in the pipes has cooled down some, and so its temperature will rise (possibly dramatically) once the water from the tank reaches the point of use.

Young children are at higher risk because their skin is thinner. Some people, especially the elderly, are at higher risk because they may be less sensitive and slower to move away from scalding water.

  • The Consumer Product Safety Commission (CPSC) recommends setting to 49 °C (120 °F)

Piping Concerns

From LegionellaPrevention.o:

Dead legs

Dead legs (branches that are capped off or rarely used) contain stagnant water that can be an ideal breeding ground for bacteria which can contaminate the entire system. This should not be surprising as the water in the dead leg would get warm, but never hot to the temperature of the main flow.

  • If unavoidable, a valve should be installed as close to the main line as possible (no more than 1 pipe diameter away) to minimize risk.

PEX vs Copper

In a study, Legionella seemed to grow a bit faster in PEX than copper (over a period of 500 days), however, over a period of 800 days there was essentially no difference in growth and both pipes had identical biofilms formed inside them.

Copper in theory would cool down faster than PEX (Ed: I can't find any studies on this) but given enough time (likely a couple hours), the temperature of both would cool to the ambient temperature. There's a separate question that discusses if it's worthwhile to insulate pipes, but even with insulation, the water in the pipes will eventually cool to ambient.

  • PEX vs Copper is irrelevant

Conflicting Views

From Heated Debate about Hot Water:

The country’s top experts, represented on the Canadian Commission on Building and Fire Codes (CCBFC), have rejected the lower maximum hot water tank temperature of 49 °C. In spite of this, trusted organizations are telling homeowners to lower their hot water tank temperature to 49 °C as a precaution against scalds from tap water. Some even offer tips on how to find the thermostat so you can adjust it yourself.

The bottom line is that water must be stored at a high temperature as a precaution against bacteria. It can be delivered from the tap at a lower temperature to prevent scalds.

Conclusion

The article Residential water heater temperature: 49 or 60 degrees Celsius? published in the Canadian Journal of Infectious Diseases concludes:

In our opinion, it is important to reduce both the risk of scalds and the risk of legionellosis associated with domestic water supplies.

For water heaters servicing a single housing unit:

  • Electric water heaters should be set at 60 °C to limit the risk of Legionella contamination, and equipped with anti-scald devices to deliver water at 49 °C to the entire household.
  • Gas or oil water heaters should be set at 49 °C, because the risk of scalding is greater with these devices.

For water heaters servicing multiple unit housing complexes:

  • More complex water distribution systems are more likely to be contaminated, and the recommendations from the WHO should apply no matter what type of water heater is used: hot water must be stored at 60 °C inside the water heater by ensuring, at least once a day, the temperature reaches at least 60 °C in the entire tank.
    • Moreover, water should reach the tap at a temperature of at least 50 °C.
    • Taps in these buildings, especially in the bath or shower where most scalds occur, should be equipped with anti-scald devices to decrease the water temperature to 49 °C or less.

Absolute Ideal Scenario

The best scenario seems to be to install anti-scald mixing valves at each human point of use (e.g., sinks, tubs): whether it be integrated in a shower valve, or installed under the sink.

  • The tank gets set to the WHO-recommended 60 °C.
  • Each human point-of-use is limited to the CPSC-recommended 49 °C (120 °F)
    • You may even want it lower (e.g., 110 °F) in kid's bathrooms.
  • Other uses -- dishwasher, washing machine, humidifier -- can get the full 60 °C with no issue.

The downside to this, of course, is cost: you need to install the valves on each hot water tap. For sinks, this is relatively easy to retrofit; for tubs/showers it may be more difficult.

  • "Gas or oil water heaters already installed" = what about Gas or oil water heaters not yet installed? – DA01 Jul 22 '13 at 21:27
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    +1 for the great answer. For measuring the temp, my two thoughts are an IR thermometer on the hot pipe after you've run it for 10 seconds, or filling a pot with hot water and using a meat thermometer. I'd also suggest turning the temp down and periodically measuring until it reaches your min temp, and then turn it up until the heater just turns on, because it maintains the temperature within a range, not an exact value. – BMitch Jul 23 '13 at 1:56
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    @BMitch You should also measure it after running for several minutes (so you're getting hot water from the top of the tank, not just water that was in the pipes). – gregmac Jul 23 '13 at 2:03
  • Excellent answer. I'm told here in NZ, hot water storage tanks must be set to at least 55 degrees. Anti-scald devices are now mandatory on all new installations or alterations. Older houses without the anti-scald devices they say to set them to 55 degrees. In practice by the time the water reaches the tap it drops several degrees. – Matt Jul 25 '13 at 4:21
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    I had no idea there was a minimum temperature recommendation; great info! Can this recommendation be disregarded if you have treated city water (with whatever bacteria-killing agents they use)? – pbarranis Feb 25 '16 at 15:00
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60°C (140°F) is the recommended minimum, as at that temperature Legionella bacteria is killed within 32 minutes. At 66°C (151°F), Legionella bacteria dies instantly.

If you feel this is too hot, yet still want to be safe. You can set the tank heater to 151°F, then install thermostatic mixing valves. These can be in the form of point of use "anti-scald" valves, or a thermostatic mixing valve installed directly on the heater outlet. Thermostatic mixing valves mix cold water in with the hot water, to achieve a more comfortable temperature.

  • Or just be careful when taking a shower by mixing in enough cold water to be comfortable. – iLikeDirt Feb 25 '16 at 15:11
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There are exactly 40 acceptable temperatures when storing domestic hot water in a tank. 140, 141, 142 and so on up to 180 degrees farenheit . With those temperatures being above safe limits a mixing valve is required. Recommended (legal is 118 in most jurisdictions) mixed temperature is 120 but I find happy customers at 130 F. I'm going to assume this is not code everywhere but it should be. Unless of course you are okay with killing the elderly. Legionnaires causes pneumonia and probably kills more than the numbers show. 10% of people who contract it, die. Autopsies often are not done on the demographic affected by legionnaires.

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    "ferinheight" looks funny. Besides, you cannot see temperatures as distinct values: there are more than 40 values between 140 and 180. What about 160.5? Or 170.4? – glglgl Mar 5 at 17:53
  • I normally set them quite high, the point is that the only safe way is store heated water above the scalding temperature and thus requires a mixing valve. You also get more volume of heated water that way. A 40 gallon tank at 180 mixed down to 130 is the rough equivalent of 55 gallons (I literally pulled those numbers out of the air) stored at 130. All figures in "Farenheit" my Autocorrect is messed up. – Joe Fala Mar 5 at 17:59
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After few year but I think it worth to post an opinion.

If tankless: anything you want, boiler won't hold hot water inside long enough to let grow bacteria.

If tanked: continuously 60°C will kill any legionella and also water is not too hot 50° with once-a-week rise to 75° to kill legionella but keeping water colder not to harm your kids.

As plumbing go with PEX-c/AL/PEX-c: it's robust, durable and won't suffer for very hot water as pex-A or pex-B may, also less expensive then copper.

As source of heat go with the cheaper in your area: I'll go for solar-thermal+electric backup if in a sunny area or gas if elsewere reached by gas, electric heat-pump if are with not much sun and no connection to natural gas

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