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?
Best Answer
There are two main and opposing risks:
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:
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.
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.
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.
Conflicting Views
From Heated Debate about Hot Water:
Conclusion
The article Residential water heater temperature: 49 or 60 degrees Celsius? published in the Canadian Journal of Infectious Diseases concludes:
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 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.