In my office they have a tap with (boiling) hot and cold filtered water system - its designed for hot drinks, and is fine for black tea. I assume it dispenses at around 100 degrees, though I have not measured it. The cold tap gives chilled water - I'd assume 5-10 degrees.

I brew my coffee using an aeropress, with about 2 spoons worth of coffee and the standard paper filter. Filled up to a mark between the 2 and 3 marking on the aeropress.

What comes out has a slight metallic, bitter taste. I do not get this at home where I use a standard kettle (either leave it to boil and cool for a few minutes, or use my new kettle that can specify the temp - I generally use 85-90 degrees)

I have tried to eliminate the problem thus:

  • Different beans (2 different bags of fresh preground ground coffee, with both "standard for cafetiere grind" and espresso grinds). Grinding at work is not an option and doing at home is too annoying.
  • Different storage (I thought the metal tin I was storing in was imparting bad flavour) - I have now got a glass kilner jar which is air tight and stored in a dark drawer
  • Washing the paper filter before use (I dont do this at home normally)
  • Using the inverted method (with the same, or slightly less grounds)
  • Using another breakout area (with the same system) in case the normal one is broken/dirty/etc

None of this makes a difference so I assume it is down likely down to the temperature of the water being too hot.

What is the best (quickest and least fussy) way of improving my brew?

I have considered (but not yet attempted):

  1. Adding a small amount of cold water before adding the hot (approx 1/4 cold to 3/4 hot) then brewing as usual.
  2. Add the hot then add a splash of cold.
  3. Mixing in another cup and adding that - this is undesirable as I already have to carry the coffee jar, aeropress and cup to the kitchen and back.

What is the best means to get a better brew?

  • Step 1: measure the hot water temperature. My guess: 75°C... way too cold. Prove me wrong! :)
    – hoc_age
    Feb 16, 2016 at 22:40
  • 2
    Could it just be the local water and what you are used to? I find when I make tea at my parents house, it's almost undrinkable to me now as I find the water to metallic. I don't notice it as much when I'm drinking the water straight, but it's amplified when I make a cup of tea. Feb 17, 2016 at 7:20
  • It could be I guess. There is also a water treatment thing attached the system I noticed today
    – NBenatar
    Feb 17, 2016 at 7:21

4 Answers 4


Is it, by any chance, demineralised/distilled water, with no minerals added back in?

Aside from temperature (which you should check - it's OK if it's cooler, but too cool is no good, and knowing the temperature will help you modify other parameters), it's important to have high quality, odour-free water that is free from impurities and that has the correct mineral content. The SCAA actually specifies water standards (see here). In many developed parts of the world, tap water is fine. However, demineralised water is NOT fine. It will underextract.

One solution*: Buy bottled mineral water (like Evian or a cheaper alternative), which usually has too HIGH a mineral content, and blend it with the filtered water from the dispenser. This may seem hard core, but it'd be good as an experiment to see if it cures your woes.

  • Joke only intended after the fact
  • If the water is demineralised, would adding a small amount of salt to the coffee before brewing achieve a similar effect?
    – fredley
    Feb 17, 2016 at 10:34
  • It seems there is no dispenser, but only a hot-water tap. The solution may no apply. I have such a setting in my offices too, although I don't use it for brewing at all (water is treated as you mention). Feb 18, 2016 at 0:15
  • Is there an optimal mineral content for coffee extraction? The water in my area is very hard, but is that actually a good thing when making coffee? Feb 18, 2016 at 10:41
  • @fredley no, because sodium is only required in a small amount. Calcium and magnesium are also important. Look at the specs I linked as a guide.
    – Hoosh
    Feb 18, 2016 at 14:46
  • @WillVousden it's not a good thing for it to be very hard. There's an optimal hardness level. You can either get your water tested, or look at the local council webpages for recent tests in your area or at the reservoir.
    – Hoosh
    Feb 18, 2016 at 14:47

I would highly suggest checking the system which generates the hot water. Regardless of if it is a storage type or on demand type system, it may require cleaning. Storing water at temperature has a tendency to cause the contained minerals to accumulate in the container. If you can take it apart and it is old (more than two years) you may find it has a wealth of accumulated minerals that are imparting "flavor" to your coffee.

If the system is not readily available for inspection, you may considered letting some water from it stand/cool and checking to see if the water has some of the flavor you are describing.

  • Its not really feasible to check the system. The team who look after that sort of thing are usually quite good at maintenance - also the system is pretty new - less than a year old - we used to have an old school water boiler and separate water cooler.
    – NBenatar
    Feb 17, 2016 at 18:24
  • I will however try drinking the hot water (after its cooled) too see.
    – NBenatar
    Feb 17, 2016 at 18:25
  • 1
    It tastes bad. Kind of chemically. I checked under the sink and there is a Cylinder ("Ultra 2000 Water Treatment" from EverPure attached to the heating/cooling/filter unit).
    – NBenatar
    Feb 18, 2016 at 19:06
  • Hopefully that can be swapped for something that won't affect the taste for you. Good luck! Feb 18, 2016 at 19:31
  • Unlikley. Its a big site (2k employees, with probably 20+ breakout areas, each with this system) - I doubt they would change the process because one guy said he didnt like the water. Especially as there is a cafe downstairs that serves pretty good coffee (its cheap, but 2 coffees a day still adds up)
    – NBenatar
    Feb 18, 2016 at 21:25

I think you are attempting to brew with water that is way too cold. To me, this is the elephant in the room in your question.

Though it's only an anecdote (until you measure your water temperature!) I myself measured the "very hot" water taps from a filter machine and from a hot water dispenser in my office: one was 75°C and one was 85°C. The latter is actually the hot water dispenser from a coffee-brewing machine. Both of these are way below what I consider to be adequate brewing temperature (~95°C). I did as fair of a measurement as I could: reasonably calibrated instant-read thermometer, pouring water directly from the tap directly over the thermometer into a pre-heated mug.

See this question about pour-over temperatures and links from (full disclosure) my answer there.

It's also striking how quickly water cools: merely traveling through the air when pouring from a vessel can make the water drop a few degrees C. I think you will probably need a separate heating kettle, and I don't think you should ever expect to have to cool off water before brewing.

Or, like I said before in my original comment: measure the temperature and prove me wrong!


Your hot water is not the problem - you just need to grind coarser! Especially if you're using a darker roast, which is more soluble and will extract faster. Cooling your water would of course help slow down extraction - but I think the grind is an easier variable to control.

  • This week I've gone for standard grind "suitable for all makers", it's not made much difference - it's a lighter roast than I normally go for too. Tastes quite metallic
    – NBenatar
    Feb 25, 2016 at 6:57
  • Ah, if it's still metallic it sounds like it's unrelated to extraction then. Maybe you could try another water source if that's possible?
    – Induction
    Feb 25, 2016 at 7:09
  • Sorry, just read the other comment talking about the water quality... Sounds like that may be your issue :)
    – Induction
    Feb 25, 2016 at 7:09
  • I'm afraid you're right and all sources of hot water go through the same system, unless I go to the cafe and get a cup of hot water
    – NBenatar
    Feb 25, 2016 at 7:13

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