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Should you use hot or cold water in a kettle or coffee machine?

Written by Phil

A coffee enthusiast and traveler, Phil primarily spends his days thinking and writing about coffee. He also writes for HittingEject.com where he explores ideas of culture, psychology, and travel, and occasionally dabbles in horror.

After finishing that monster post based around the basics of coffee, I started wondering more about water. There are some basic things that people are unsure about, and they have varying reasons for their beliefs. So what is the truth? Should we use hot or cold water in coffee equipment?

Hard water has more minerals, which will result in damage to coffee machines over time. These harmful minerals can be absorbed from water heaters and pipes. Cold water bypasses the water heater and less readily absorbs metals. Use cold water.

Theories

“Cold water will just slow down the boiling process, so why shouldn’t we just use hot to start and speed up the process?” goes the reasoning in some posts I came across, most notably on in the UK subreddit.

Their theories were interesting, ranging from the higher level of dissolved oxygen levels in the cold water, believing hot water was not safe to drink, or that hot water will leech metals from your pipes.

A breakdown of each myth

Water heaters taint your water

If you’re like me before writing this post, you probably haven’t thought too much about how hot water heaters function. As it turns out, they have something called an “anode rod”, which is designed for the sole purpose of corroding and dissolving over time

They last roughly 4-5 years and are there to prevent the more important components in the heater from corroding, namely the steel casing and heating element. 

The exact function and reasoning isn’t important, but there are two ways this can affect the water that comes out of it. In the first case, when they’re functioning as they should, they will be leaching metals (aluminum, magnesium, and/or zinc) into your water. So that’s an addition to the water that otherwise won’t be in the cold water. As well, hard water is determined by higher levels of magnesium and calcium.

In the other case, the rod could be entirely dissolved, and the water will begin to corrode the steel, leeching rust into the water.

Bear in mind, I’m not a chemist, but these additions are extra variables that you’re introducing into your machine and will raise the likelihood of scaling – that hard, white buildup that’ll clog the lines.

Cold water is what’s piped directly into your house. By using hot water, you’re running it through a machine to make it hot, then using that hot water and putting it into a second device to make it even hotter. 

This Vancouver Sun article points out the fact that home hot water heaters can have its contents sitting there for who-knows-how-long. Water has natural sediments in it, which will also build up in hot water tanks.

Water tank manufacturers recommend periodically flushing out the gunk, but who knows how frequently your super has done that.

In the case of apartments or condos, it can be moving through the hot water recirculation system, which means hot water is constantly running through the pipes and back to the sediment-laden heater. Depending on what the pipes are made of, you may get a little extra. Joy.

And if you doubt how gunky a neglected hot water heater can get, ask a plumber.

It likely isn’t dangerous, but it will probably add an aftertaste. The sediment could also negatively affect your kettle or even more sensitive coffee equipment. 

No matter the reason, let’s cut out the middleman machine and jump right for the kettle, shall we?

Status: Valid. Sediment and other metals can be brought along with water that has been sitting in the water heater for extended periods. This can result in negative outcomes for health, flavor, and equipment.

Cold water has more oxygen

To cut this short: Yes, cold water can hold more dissolved oxygen than hot water. Yes, water is H2O, but there can still be dissolved gasses in it.

But why does this matter?

Based on what I can find, water’s flavor is affected by the various gasses that are dissolved in it. The City of Cleveland’s water service is quoted as saying:

“Generally speaking, the more dissolved oxygen in the water, the better it tastes.”

One of the more interesting facts from this research:

Apparently, the dissolved gasses are why water left out overnight has a different flavor in the morning, which is usually less appetizing.

Hot water apparently exudes more oxygen than cold water, and the water sitting in the water heater has more time to let off the gas while sitting, waiting to be used. Thus, there is some merit to the idea that hot water tastes worse.

Status: Valid. More oxygen in the water tastes better. Cold water can contain more dissolved oxygen, and hot water seems to let it off faster.

Cold water boils faster than hot

I don’t know how this is even a thing, but it’s clearly ridiculous. If both hot and cold water are provided with the same heat, the cold’s rate of change will be higher. 

As it gets hotter and reaches the temperature of the warmer water, it will slow down and reach the same rate that already hot water has reached.

Imagine two carriages. One has 6 horses and one has 1 horse. Each adds one extra horse. Which will see the biggest change in speed? But as we keep adding a single horse, the gains will level off.

Anyway, it seems this myth was started to encourage people in the UK to cook with cleaner, safer cold water. Which will continue in the next section…

Status: False. Cold water does not boil faster than hot water.

Hot water is unhealthy for you to drink

I had a good laugh at this one at first because the Chinese believe that cold water is bad for your body (especially for women [source: I lived in China for 4 years]). But in Britain, they think that hot water is bad for you.

Granted, the Brits have a fact-based reason for it: their water heaters were often kept in attics and were not kept in good repair. Dead animals or insects would fall in and make the water… gross. 

But I guess it was ok for showering? According to this video, this is also the reason why they have two separate taps for molten lava and ice water.

In England, some older houses have not yet been updated, and since we can’t be sure which system each particular house has, it’s safer to just use cold water.

Status: Semi-valid, but more so in the UK.

Hot water is more likely to have undesirable elements in it

Since city water is softer, it will be more likely to leach metals, namely copper, from the pipes. If you’re in an older house, your pipes may be made of something different, depending on what country you’re in, and this, too, can be leached into your water. Yes, this includes our favorite heavy metal/neurotoxin: lead.

A basic principle of chemistry is that adding heat to any reaction will make it go faster, and in the case of leaching metals, hot water will leach more than cold. It’s the same principle for why temperature matters for brewing tea or coffee.

Status: Valid. Extra elements and metals can leach into the water, which can then affect flavor, your equipment, and/or your health.

How does it all relate to your coffee machine?

Coffee equipment is doubly cursed as it is both very expensive and finicky. They are finely tuned pieces of equipment that, if you got a good one, will require a vigilant eye on factors that can negatively impact them.

Hot water heaters allow for more sediment to enter the water, and then that same hot water is liable to pick up something extra from the pipes as it runs to your tap. This all could affect your precious brewing equipment by introducing scaling and other impurities that can build up and clog the works. That isn’t good for anyone.

TL;DR – Use Cold Water.

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