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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

3 Answers 3

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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. –  William Shakespeare 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

To answer this question in general, it's important to note that hot tap water systems are not always considered potable in many parts of the world. In some places they don't attain or maintain a high enough temperature, and older systems (even in places like the UK) can occasionally use hot water reservoirs which are more open to contamination than cold water from the tap. So, in general, be sure that your hot tap water is actually intended to be potable and has the necessary safeguards.

TFD's answer discussed one obvious safety concern raised in the question regarding temperature. Given environmental concerns about wasting heat and energy, as well as warnings not to have scalding hot water from taps, many people tend to lower water heater temperatures as low as possible. But it's important to keep temperatures always at least above 120F (50C) to avoid conditions which can allow bad bacteria like Legionella to propagate. (Sam Ley mentioned this in comments, but to be clear -- any temperatures above 120F will cause Legionella to die off, but the question is how much time it will take if you have a contaminated water source: at 125F it could take hours; at 140F it only takes a few seconds.)

However, assuming a well-functioning water heater that is not set to an inappropriate low temperature, the major safety issue with using hot tap water for cooking or drinking is not bacteria, but other dissolved substances. Hot water will absorb any contaminants in pipelines much faster than cold water.

The main concern here is lead. Government agencies are generally in very strong agreement that one should NOT use hot tap water for cooking or drinking for this reason.

  • From the CDC: "In all situations, drink or cook only with water that comes out of the tap cold. Water that comes out of the tap warm or hot can contain much higher levels of lead. Boiling this water will NOT reduce the amount of lead in your water."
  • From the EPA: "Only Use Cold Water for Consumption: Use only water from the cold-water tap for drinking, cooking, and especially for making baby formula. Hot water is likely to contain higher levels of lead. The two actions recommended above [i.e., "flushing" water lines with fresh water and using only cold water] are very important to the health of your family. They will probably be effective in reducing lead levels because most of the lead in household water usually comes from the plumbing in your house, not from the local water supply."
  • From a New York Times article on the subject: "Lead is rarely found in source water, but can enter it through corroded plumbing. The Environmental Protection Agency says that older homes are more likely to have lead pipes and fixtures, but that even newer plumbing advertised as “lead-free” can still contain as much as 8 percent lead. A study published in The Journal of Environmental Health in 2002 found that tap water represented 14 to 20 percent of total lead exposure."

The links have more information, but in general be aware that one does NOT need to have an old house with lead pipes for this to be a concern. Soldering in newer pipes can also contain lead which will leach into hot water much faster than cold. I think the CDC and EPA are probably being a little overly cautious here, but unless you've actually tested the water from your faucets for contaminant levels, it may be best to err on the side of caution and let the water run cold first from the tap before getting water for drinking or cooking (particularly when small children or pregnant women are involved).

For myself, I've always followed this practice and was taught it when I was very young. I also remember being told to do this for flavor reasons, which would also be very relevant for drinking and cooking. A few years ago when I had a discussion with a person who had never heard of this practice, I said we should both get glasses of hot water from the tap, allow it to cool, and compare drinking it to water drawn from tap cold. We both agreed that the hot water had more "off" tastes to it when it had cooled.

I can't say that this would be true everywhere. (I've since moved myself, and I haven't tried it again.) But if your hot tap water actually tastes different, it's clear that something is changing in it, which could involve more rapid absorption of some contaminants somewhere in your plumbing. Is this dangerous for most healthy adults? Probably not (unless you still have actual lead pipes), though again you'd need to do actual testing to know. But if my water tastes better from the tap cold, why would I use hot top water for cooking? As others have noted, it's likely not going to save you energy, and in some cases could actually be harmful.

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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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