When adjusting my water heater, what temperature should I set it to? Is the answer different if you have:
Also, is there a preferred technique to check the temperature of the water after adjusting it?
There are two main and opposing risks:
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:
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.
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.
From LegionellaPrevention.o:
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.
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.
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.
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.
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 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.
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.
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
There are exactly 40 acceptable temperatures when storing domestic hot water in a tank. 140°F (60°C), 141°F, 142°F (61°C) and so on up to 180 degrees Fahrenheit (82°C). With those temperatures being above safe limits a mixing valve is required. Recommended (legal is 118°F (48°C) in most jurisdictions) mixed temperature is 120°F (49°C) but I find happy customers at 130 °F (54°C). 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.
