I have an electric water boiler in the kitchen. I put fresh tap water into it, boil it up, and use it for my coffee. Then, an hour later, I go back and press the button again and have it re-boil the now room-temperature water, and use it for another cup. Sometimes, several hours pass; sometimes, half a day.

When "too long" has passed, I tend to empty it and put fresh tap water into it, because it feels like it has "gone bad".

Is this silly? Can water really "go bad" like that? Is there any difference whatsoever between freshly poured water and water that has been standing still in the container half a day or even the entire day?

It's still gonna be boiled? Doesn't that "neutralize" basically any kind of water?

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    “Electric water boiler”. First time I’ve heard that phrase being used to mean “kettle” 😉 Mar 5, 2021 at 7:25
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    There are other types of water boiler that are not strictly 'kettles' - When I first moved into my current house it still had a Creda Corvette from the 60s. In the 90s I bought what at the time was a boiler I'd only ever seen in Japan which boiled then kept hot all day example. I see urn types like this frequently in my day job, for locations catering. Google Image search "electric water boiler -kettle" for a whole lot more.
    – Tetsujin
    Mar 5, 2021 at 15:36
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    Just wanted to point out that you can put your boiled water in a thermal carafe or a thermos (depending on how much water you want to keep hot) and it will stay very hot for the better part of a day.
    – levininja
    Mar 5, 2021 at 19:40
  • The literal translation in some languages is that one! And for example when I came back from a year abroad in England almost 15 years ago, brought some kettles as presents with me, and my friends (and their respective families) were quite shocked as it was not really common item. Now it is more common! @ChrisMelville
    – M.K
    Mar 7, 2021 at 8:19

7 Answers 7


It doesn't go bad, but it does change the taste.

When water is just sitting there, water evaporates, but most things dissolved in it don't. Then, each time you boil it, the steam causes additional water to escape leaving the same amount of dissolved stuff in there. So, the concentration of dissolved stuff keeps going up.

Dissolved oxygen also decreases when you heat or boil water, changing the taste.

Finally, even if your kettle is stainless steel, it can eventually rust.

  • Would simply agitating the water re-introduce oxygen?
    – Luciano
    Mar 4, 2021 at 9:38
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    @Luciano somewhat. But water will absorb other substance floating in the air as well, changing the taste in different direction.
    – fraxinus
    Mar 4, 2021 at 10:19
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    if you live in a hard-water area you can get a very strong limescale taste after a while and, eventually, enough limescale will build up in the kettle/boiler to influence the taste of any water left sitting it for a while, even if not boiled. As such it is generally advisable to boil only as much water as needed (which also saves money), and pour out any remainder before the next use
    – Tristan
    Mar 4, 2021 at 10:23
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    My stainless steel kettle is at least a decade old & shows no sign of rust whatsoever. The only time you get rust on SS is if you have something else ferrous in the same 'water bath' like in a dishwasher. One contaminates the other.
    – Tetsujin
    Mar 4, 2021 at 13:52
  • Note if you don’t boil the water ever and leave it out at room temperature, in some places it can develop algae if it doesn’t completely evaporate fast enough. Mar 4, 2021 at 22:40

As far as I know it does not go bad, but there is a good reason not to fill your kettle completely when you fill it from the tap.
Partly filled kettles come to the boil in less time. Also costing less in energy, which can be a concern for some.

So fill the kettle part way, say to the amount of water you actually use and you will spend less time waiting but no more time filling.

I often refill my kettle after using it till empty, having it ready to run and already at room temperature when I want to have an other hot drink.

Here I do not notice much difference in taste between fresh and re-boiled water but that may differ between locations.

  • This is the right answer: you shouldn't be in a position where it comes up in the first place. Mar 6, 2021 at 8:26

Potable water does not "go off" ever*

I wouldn't even consider actually changing the water in a kettle unless I was going to be away from the house for more than a week, for which I'd simply empty it before I left & fill it again when I got back.
Any bacteria acquired through the spout is going to be negligible, and you are flushing the system through with fresh water every time you boil it [or once a day, if you insist on filling it unnecessarily to the top each day, which does nothing but waste energy]. Even if you leave some in the bottom each time, it's not the 'same bit'.

Hard water - high calcium carbonate concentrations leading to limescale & furred-up kettles - will taint the flavour anyway, whether you start with fresh each time or not. The difference in flavour between 'new' & 'mixed with the old' is going to be negligible. Allowing limescale to build up doesn't really hurt anything except your electricity bill & the kettle element which will eventually burn out from overheating.

The solution to both flavour & electricity bill is a water filter.

I live in a very hard water area but grew up in a very soft water area. I consider a filter an essential item, not a luxury.

*assuming it is not later contaminated. This is supported by building regulations, which allow you to cap a pipe without ever needing to allow for the water remaining in that unused section ever being drained or flushed.

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    I'm not the downvoter, but I want to comment on the water filter solution: in the area where I live the government has started advising not to use water filters since they tend to be a breading ground for bacteria and similar. I have seen a filter that was long overdue for a change go very bad at someone else's house.
    – Lavandysh
    Mar 4, 2021 at 14:48
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    @Lavandysh - That's like outlawing baking trays or grease traps because some people don't clean them. It's really tough to legislate for the truly stupid. Water filter has a clock in it. When clock flash, change filter.
    – Tetsujin
    Mar 4, 2021 at 14:52
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    It makes perfect sense. I use the remaining water in the kettle each and every time, I don't see why not!
    – Gigili
    Mar 4, 2021 at 16:28
  • I think it's a good answer but I have a counter-example. We fill the water bin of our coffee maker from an RO filter; all chlorination has been removed by the filter. After a month of the bin not being cleaned, we can feel a biofilm on the inside surfaces of the bin, even though it's only had potable water added to it. But the lack of chlorination allows bacteria to grow, and bacteria can grow with a surprisingly low amount of nutrients. The kettle gets sterilized each time it is boiled, so if boiled often enough, it shouldn't ever get a biofilm like that. Mar 4, 2021 at 19:32
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    'RO' would be reverse osmosis. And I think the issue is that standing water will inevitably be "later contaminated" by things in the air, if the chlorine/etc is removed; odds are that wouldn't happen in a small kettle in a few days time, but given weeks...
    – Joe M
    Mar 5, 2021 at 16:21

If you don't do it too often this might be a good idea for another reason: when you pour out the water (and preferably give your teapot a rinse) you also pour out concentrated chalk residue and any other particles that didn't boil.

Chalk is the white stuff that forms around in your boiling pot after a while. It isn't bad for you at all, but it is bad for your boiling pot. By emptying it completely every once in a while you give your appliance a longer life (independent from how long the water has been in the pot). Chalk doesn't disappear when boiling. So, if you boil more and more water without ever getting rid of the little bit of water at the bottom, it has nowhere to go and the concentration of the chalk will rise and it will stick to the pot after a while. If you empty out the water, the new water will have a lower concentration for a while and you get less residue.

How bad the chalk is depends on the quality of the water where you live. Also, if it really has been a long time since you last used it, I would also clean it because dust still comes in. It's probably not dangerous to drink, but can affect the taste and isn't nice.

  • Chemistry followup: is this "chalk" the same as the mineral calcium carbonate, like in the white cliffs of Dover? Mar 4, 2021 at 13:53
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    Yes, though it's more usually called Limescale in domestic water.
    – Tetsujin
    Mar 4, 2021 at 14:04
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    Depending on what minerals are in your water, you may get other precipitates besides calcium carbonate, though the effect is similar (not dangerous, can impact flavor, over time can cause problems with the appliance). Mar 5, 2021 at 19:06

I work in drinking water quality chemistry at a municipal water authority. The "Potable" water at your tap is treated with chlorine. For the water to be safe from microbial growth, there must be a sufficient "residual" of chlorine. However, the chlorine breaks down over time, for example, water that has sat stagnant in a main for about a week or more will lose its Cl residual, and must be flushed to waste because it is not "potable" anymore.

Heating water breaks down and evaporates the chlorine almost completely. That kettle of warm, non-chlorinated water sitting on your counter is a prime breeding ground for microbes, some of which could be disease-causing. Always dump out the kettle when you're done with it, never drink (or cook with) water from your home’s hot-water heater, and re-boiling water will NOT necessarily make it safe again.

  • Which part of the world are you located?
    – Willeke
    Mar 6, 2021 at 15:48
  • "...never drink (or cook with) water from your home’s hot-water heater, and re-boiling water will NOT necessarily make it safe again...." Seriously? That's like an everyday occurrence...
    – gnicko
    Mar 11, 2021 at 14:20

Ok, so to put it in simpler terms. Lets say you have a bucket of marbles,(Water) and sand (Dissolved solids). Each time you boil the water, two marbles are taken away until eventually there is no water, but dissolved solids instead and this can really mess up the taste, and mess up your pot/kettle in general. So the best thing to do is trust your instincts on this one and just dump out the old water.

My recommendation is to boil it about 4 times and then ditch the batch and refill the kettle.

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    This doesn't work as an illustration. By your theory after a year or so of never dumping out the contents, the kettle would be full entirely of sand. This is demonstrably not true.
    – Tetsujin
    Mar 4, 2021 at 19:15
  • @Tetsujin - the following is not just to you but it's relevant to what you said. || "All models are wrong. Some models are useful." - This IS a useful model / metaphor - but if carried to extreme it gives a wrong impression - as do all metaphors. It should not have been downvoted - other answers which say much the same thing in a different way have been upvoted. . Mar 6, 2021 at 9:50

Your water goes bad because of microbiological growth.

Your tap water is not pure water. There is a lot of stuff in it. And your environment is not sterile and simply cooking it to 100°C for a short amount of time does not sterilize it. You have to cook it for 10 minutes and your container needs to be airtight after.

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    This doesn't occur over the course of days, or even weeks, though. Most sources suggest that tap water is fine to drink when stored for up to six months. It's totally unnecessary to boil tap water for 10 minutes after it's been sitting for a day. Mar 4, 2021 at 13:53
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    … only if you live in an area with non-potable water. Tap water in the EU & presumably the US too will not go "off" at all if you put it in a sealed container. There is nothing in it that can 'grow'. It is sterile.
    – Tetsujin
    Mar 4, 2021 at 14:01
  • @Tetsujin I thought only the police were in the fatally erroneous habit of sealing their kettles. Mar 4, 2021 at 17:25
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    @Tetsujin I don't think I'd go so far as to call it 'sterile'. Sure its perfectly safe outside of very extreme situations but its definitely not 'sterile' in the technical/medical sense of the term, especially when you consider how many miles of pipes the water flows through before it gets to your tap.
    – GHP
    Mar 5, 2021 at 14:41
  • I'm not sure why this is getting downvoted, it's correct. Manufacturers of bottled water add a ton of chlorine specifically to prevent mold from growing while it sits on a shelf. @NuclearHoagie My wife left a pot of coffee out for 3 weeks and a bunch of mold grew in it. It definitely doesn't take 6 months. Mar 7, 2021 at 2:07

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