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I'll typically bring hot water from the tap to the boil instead of waiting longer for cold water. This hot water comes from water heater with a large storage tank. Is this considered safe?

For example, are heater storage tanks known for festering nasties not killed by boiling? Is different piping used for hot water or different soldering on pipe fittings? Do hot pipes cooling down go through a temperature more conducive to bacteria growth?

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If your goal is fast boiling water, a good electric kettle might be better. –  Jefromi Feb 18 '12 at 6:16
    
Which country do you live in? –  Mien Feb 18 '12 at 8:18
    
Australia. Just spent 5 days in hospital from E-coli, returned to hear local water supply had a E-coli outbreak, so I'm a bit paranoid about all nasties now. –  jontyc Feb 18 '12 at 9:03
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Not related to safety, but one reason that freshly-drawn cold water is recommended for tea and coffee is that it has higher dissolved oxygen content (cold liquids dissolve gasses easier than hot liquids), and this extra oxygen that remains in the water when boiled (it takes time to drive off) aids in the extraction of flavors. –  Sam Ley Feb 18 '12 at 18:23
    
Very interesting. I wonder if boiling hot tap water would be then ideal to cook foods, minimizing removal of flavors? –  jontyc Feb 18 '12 at 22:27
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2 Answers

up vote 8 down vote accepted

Unless your hot water tank is very close to your hot water tap, this is a very energy inefficient. As Jefromi notes it would be faster to boil water in a electric kettle first, and then pour it into the pan. Put the pan on the heat at the same time if you are really in a hurry

Hot water systems are normally hot enough (above 55°C, 130°F) to keep water borne nasties at bay, plus if you are on town supply water it will be chlorinated etc

Normally on the first few meters of hot water pipe are copper, then it switched to normal crimped plastic plumbing. This will vary depending on your local building codes

The rate at which pipes lose their heat would ensure it never sits in the danger area for long, not that I think this is a big issue for clean plain water

In general, modern copper pipes are not soldered, they are crimped using special hand tools

Old or non-renovated houses may still be 100% copper pipes that have been soldered. This poses no extra safety risks with just clean water in the pipes

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Worth adding that older pipes can have lead solder, hence the old advice about not using hot water for cooking. But if the plies aren't old, it's perfectly safe. –  yossarian Feb 18 '12 at 2:22
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Whether copper pipes are soldered or crimped depends on location, age and type of construction, and probably the plumber doing the work. For example, in my area there's very little new construction, and there's probably not a crimped pipe to be found for miles. New work in old houses, at least in my area, is usually soldered. –  Caleb Feb 18 '12 at 6:01
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Good answer - just wanted to add a detail. Legionella is the dude normally associated with hot water heater disease. They are fully killed above 140F, and have trouble living above 120F. If you have people in the home who would be particularly susceptible to disease, you should set your tank closer to 140F (though that increases the danger of scalding). Legionella spreads when people turn their heaters way down (under 115F) for efficiency (a better way is to get a more efficient unit and insulate piping). –  Sam Ley Feb 18 '12 at 18:30
    
Nice addition Sam, could have stood as an answer. You've got me know thinking to put the hot water tank up at 140F and remove piping insulation so I don't get scalded :) –  jontyc Feb 18 '12 at 23:32
    
@SamLey, if you put a thermostatic valve after the boiler, it can be at 60ºC (?) without scalding anybody and without legionella risk. –  BaffledCook Feb 23 '12 at 19:24
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May not be a direct answer, but too long to be a comment. In short, there is really no need to use hot tap water for cooking.

Unless the temperature of your hot tap water is near boiling point high, it does not necessarily boil significantly faster than room temperature or cold water. I am not saying hot water does not boil faster than cold water, just not to a meaningful extend.

See the following excerpt from Scientific America

"..cold water will be absorbing heat faster while it is still cold; once it gets up to the temperature of hot water, the heating rate slows down and from there it takes just as long to bring it to a boil as the water that was hot to begin with.."

The reverse is even more interesting: hot water may freeze faster than warm water:

..It all depends on how fast the cooling occurs, and it turns out that hot water will not freeze before cold water but will freeze before lukewarm water. Water at 100 degrees C, for example, will freeze before water warmer than 60 degrees C but not before water cooler than 60 degrees C..

In addition to heath and energy/money concern, I do not see any reason to use hot tap water for cooking.

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