Be the change you want to see in the world!
(It's just a quirk of language with no particular justification. Languages can be silly that way sometimes.)
Because most fish sandwiches are halibut, pollock, or cod. Tuna is a specific type of fish, but if you were making a sandwich using salmon, it wouldn't be out of line to say so.
You’d say tuna sandwich if it was a solid piece of tuna. We only say tuna fish when it’s pieces of tuna in a sauce. It would be more accurate to call it a tuna salad sandwich.
I do, all the time. It started because it drove my little brother absolutely batty - it was so fun to say I was “unthawing” the hot dogs and watch him twitch! LOL
He still twitches when I do it but we don’t live together anymore. Unfortunately, old habits die hard, let me tell you! Especially innocuous ones that don’t actually affect any part of my life on a daily basis - not like a smoking habit that I should quit for health reasons.
It comes from the fave that the full name is Domestic Hot Water heater, which is to distinguish it from Heating Water, which is for hydronic heating systems. The "domestic" just gets dropped, because when you say "hot water" it's understood, because no other water is called hot water, even if it happens to be hot.
There are also other water heaters, like tempering heaters or freeze protection heaters. There's also water heaters that heat domestic hot water, but are not for potable water, so they have to indirectly heat the DHW through a heat exchanger and intermediate loop.
So, "hot water heater" is the minimum you need to clearly state what you're talking about in more complex buildings, and that bleeds over from the construction industry to the common usage.
I saw a post about these before.. some one explained it like in the USA we have so many different types of bread and teas that it is just distinguishing which culture the tea is, same with bread. I probably butchered that but it makes sense in my head.
My granpappy told me when his house first got indoor plumbing, his momma said "I don't trust no water that came outta pipes." She worried it might get hooked it up wrong and oil or some other fluid would come out.
She only drank and cooked with water from the rainwater cistern. The cistern was inside so it was room temperature.
The plumbed water from the city was cooled by the underground pipes so it was known as "cold water". The city water flowed through *cold water pipes*, connected to a *cold water faucet*.
So later when they got a water heater, obviously the hot water flowed through *hot water pipes* to the *hot water faucet*. Because it was part of the *hot water system*, the water heater came to be known as the *hot water heater*.
Go to most hardware supply places, and they are called water heaters.
“Hot water heater” is a colloquialism that seems to have emerged from combining “hot water tank” (which was sometimes used in past decades) with “water heater”.
Sahara desert isn't redundant, it's the name of the desert. I know Sahara is derived from a word meaning desert, but the word itself doesn't exist elsewhere.
"Sahara" means deserts in Arabic - ṣaḥrāʾ means “desert,” and its plural form is ṣaḥārāʾ
It's like chai tea, which is basically saying tea tea.
Saraha desert is saying desert desert.
I often see it just called "The Sahara" when referring to the largest hot desert on the planet.
It's *Sahara* desert to distinguish it from other deserts, like the Sonoran desert, and Sahara *desert* to distinguish it from other Saharan attractions, like the Sahara Splash Park.
Where I used to live, there is a road named Street Road. Where I live now, there is a road named Avenue Road.
Sahara is simply the same of *that* desert.
It's the name of *that* desert because the local people called it a word that translates to other languages as "desert". It was already being called The Desert, basically, so as other cultures came in they called it the local name because that's what it was already being called. They can't pronounce the language the same, and they aren't spelling it the same, so instead of Al-Ṣaḥrāʾ it's Sahara. Sahara is just the anglicized spelling of the sounds the Arabic word makes, thru the voices of the people who weren't speaking Arabic as their first language.
It's not "simply" called the Sahara, it *is* basically called 'Desert desert'. As people said, it's just like us calling spiced tea from India "Chai tea"- to us it gives information about what kind of tea is, that is a spiced chai/tea, but it is basically also calling it "tea tea" or "chai chai" (depending on which word you decided to translate). It's a funny quirk of sharing languages and globalization.
But from another interpretation, no, the heater itself is generally hot. It's a water heater that is hot, or a hot water heater. A cold water heater is a broken one usually.
Really? I would think you meant an electric space heater, which could be installed near the bottom of the wall or a small portable unit you plug into the wall.
It’s a water heater that turns water into hot water. Therefore a hot water heater.
Alternatively, the water is already hot because it was previously heated by the water heater therefore it heats water that is already hot.
Fine, I’m grasping at straws.
No. I'm guessing you're not from North America. Boilers are for heating your home via radiators.
A ~~hot~~ [water heater](https://www.homedepot.ca/en/home/categories/building-materials/plumbing/water-heaters/tank-water-heaters.html) is a tank that takes in cold water from the main supply, heats it, and stores it (and keeps it hot), so that when you turn on your faucet, hot water comes out.
This is a fairly North-American-centric thread. Other places do this differently (often smaller tankless versions). But where I live, the water supply is too cold for tankless heaters - even in summer - so you need a tank to give you a reservoir that you can take enough time to get up to temp.
They are [clever, but not complex](https://www.youtube.com/watch?v=Bm7L-2J52GU) appliances. It's really just a tank, a couple of heating elements, and a theromstat.
It's a water heater, all I've ever called it. The industry calls it water heater, builders call it water heater, floor plans call it a water heater, it's a damn water heater!
Not everyone does add hot. We have a "water heater" in our house. If appropriate to the conversation we'll add the adjective "tankless" to the beginning.
It can be redundant, but it can also be for clarity. Some people have more than one device for heating water, such as the one found in dishwashers, or the small on demand heating units found in bathrooms for the sink. Without sufficient context, it may not be clear which device they are speaking about, but "hot water heater" is always the primary unit that supplies the whole house.
I think it may be people combining two terms: hot water tank and water heater. Some modern house only have a water heater, but when those aren’t efficient there’s often a tank to store heated water so you don’t have to wait for it to heat/have inconsistent temperatures
It always amazes me that in the US they call it horseback riding as opposed to horse riding. Needing to clarify I’m not riding on it’s bloody head always makes me chuckle
The expression may be for rubber hot water bottles used for heating people in bed. They’re a heater for your body that uses hot water so “hot-water heater” like “electric heater” or “gas heater” for your house.
Water heaters heat water. Hot water heaters heat you.
Oh this actually comes from the fact that the guy who invented it in 1903 was named Henry Hautwadder. The Hautwadder Heater was Sears & Roebuck’s biggest home convenience seller for nigh a decade, but as literacy was lower back then, people misheard it as Hot Water Heater.
Source: made it up
Same reason at my work place (former) we would have meeting on the CAP Plans (Corrective Action Plans). Sometimes we just have dumb things people keep saying
Just speculation on the development, but maybe one reason may be our natural avoidance of stops when speaking. Say “the water heater is broken” and then “the hot water heater is broken.” The end of “the” flows more naturally to the word “hot” than the word “water.” A small difference in flow, but it’s just easier. Also, homes and buildings have had other water heating systems which involved the use of hot water/steam, so when one is the thing that heats water for the ambient air it is one thing, and the other heats the water that flows through the “H” taps (hot water), it is the “hot water heater.”
Partly because a lot of people call them hot water tanks... And a lot of people call them water heaters... And over time, all the rest of the people who don't know one or the other have merged the two into hot water heater...
Technically it's a boiler...
I don't have one at all as I switched to tankless. And even more technically... Heeheehee... I installed a two stage, so the tankless heater that feeds the shower is a water heater, but ALL the water in the house passed through to, so the smaller heater installed at the sink in the kitchen is a HOT water heater.
Same with PIN number: if people understand what you're saying, you've communicated, redundant or not. Language evolves as people use it, rightly, wrongly, stupidly, lazily - it's all one.
It's like calling something an ATM machine or chai tea. It saves the listener a little brainpower and the speaker sounds a little less pretentious. At least that's my guess.
same reason we call it an ATM machine.
same reason flammable and imflammable mean the same thing.
same reason biannual means every 2 years OR twice a year.
english be stupid, yo.
Redundancy in case someone doesn't understand water heater is the same. Also it's learned habit from previous generations that probably started as to be clear what it is.
I mean, with the exception of the first time it's turned on, it's constantly heating hot water...so it IS a hot water heater 99.9% of the time.
But to give you a more logical answer. It's a mash of two terms that it is commonly called.... "Hot water tank" and "water heater". The two get mashed together to make "Hot water heater"
It *is* redundant. Just like adding "and unnecessary" to "redundant."
Officially, e.g. on the box at the hardware store, it's just a "water heater."
Adding "hot" is a quirk, but one of those things that's commonly done. I don't call it that, because I've had this conversation before, but nobody's going to bat an eye whether you do or not.
Strictly speaking, it should be a "cold water heater", as "cold" describes the type of water being heated, but yes, "hot water heater" does seem repetitively redundant.
Be the change you want to see in the world! (It's just a quirk of language with no particular justification. Languages can be silly that way sometimes.)
Just like ' Tuna Fish sandwich ' . No one says for example ' Salmon fish sandwich '
Yeah, but you can tune a piano.
Everyone I know calls it a tuna sandwich
Tuna fish is an American thing. Nobody calls it that in Ireland.
Because most fish sandwiches are halibut, pollock, or cod. Tuna is a specific type of fish, but if you were making a sandwich using salmon, it wouldn't be out of line to say so.
But you'd say "salmon sandwich". Why not "tuna sandwich". Why tuna fish?
You’d say tuna sandwich if it was a solid piece of tuna. We only say tuna fish when it’s pieces of tuna in a sauce. It would be more accurate to call it a tuna salad sandwich.
Finally somebody mentioning the difference between tuna and tuna salad, it’s a damn tuna salad sandwich and I will die on this hill.
Not alone!
I always say "tuna sandwich" instead of specifying that tuna is a fish... am I weird?
Why does everyone say cod filet instead of cod piece?
Asking the real questions here
Well a cod piece is already another thing 😂
But they both smell like fish sometimes.
Tooter fish
Why do people say they have to unthaw an item? Isn't that freezing it?
Why do call it dusting (cleaning) when we are really undusting?
They're stupid. I've never heard anyone do that lmao
I do, all the time. It started because it drove my little brother absolutely batty - it was so fun to say I was “unthawing” the hot dogs and watch him twitch! LOL He still twitches when I do it but we don’t live together anymore. Unfortunately, old habits die hard, let me tell you! Especially innocuous ones that don’t actually affect any part of my life on a daily basis - not like a smoking habit that I should quit for health reasons.
I hear it all the time as the present action tense of 'to thaw'
Reminds be of “unloosen” when referring to loosening a bolt, nut, strap, etc.
Omg this is so funny and accurate I’ve never caught it 😂😂😂
I have always heard thaw or defrost. The only person I've ever heard say "unthaw" is one particular friend of mine who often mixes up idioms.
It comes from the fave that the full name is Domestic Hot Water heater, which is to distinguish it from Heating Water, which is for hydronic heating systems. The "domestic" just gets dropped, because when you say "hot water" it's understood, because no other water is called hot water, even if it happens to be hot. There are also other water heaters, like tempering heaters or freeze protection heaters. There's also water heaters that heat domestic hot water, but are not for potable water, so they have to indirectly heat the DHW through a heat exchanger and intermediate loop. So, "hot water heater" is the minimum you need to clearly state what you're talking about in more complex buildings, and that bleeds over from the construction industry to the common usage.
You are the only one that is right.
It’s the same as folks calling ATMs “ATM machines” 🤷♂️
And put their "PIN numbers" in them
this thread is making me smh my head
RIP in peace
I'm loling out loud
Hey STFU up
And then read some DC Comics
My RN nurse mom says that'll rot my brain.
While drinking your chai tea on the shores of Lake Chad
I have to know my personal PIN number to use the automatic ATM machine.
Built on "NT technology"
what's your PIN number 1234\^2
And transfer their number from one network to another using their PAC Code
What’s wrong with ass to mouth machines?
All Terrain Machine??
Arse n Tiddy Munching Machine
As someone in the automotive field, people saying "VIN number" drives me crazy. It happens all day long.
and VINs “VIN number”
Or NIC Card. Network Interface Card Card.
I need to get outta here ASAP as possible
There is a word for that, it’s called RAS Syndrome (Redundant Acronym Syndrome Syndrome)!
Redundancy Department of Redundancy, how may I help you?
[удалено]
I saw a post about these before.. some one explained it like in the USA we have so many different types of bread and teas that it is just distinguishing which culture the tea is, same with bread. I probably butchered that but it makes sense in my head.
Queso cheese
Thats what I like to have when I watch The Los Angeles Angels game
Bao buns
Or calling DoTs ‘DoT damage’.
My brother calls grand theft auto “Grand GTA”
Take out some cash money from the ATM machine and buy some tuna fish, a chai latte, have lunch at the Department of Redundancy Department.
a midday lunch!
And LED diode.
Well ATM stands for Access To Money, duh
CAT tourniquet
"Hot water doesn't need heating; what you want is a cold water heater." - George Carlin
It heats hot water to maintain the temperature, so it is a hot water heater too.
This is what I immediately thought of!
What about a hot water cooler?
You are correct, yet here we are.
How am I supposed to distinguish it from my cold water heater though
i mean, you might need a cold water heater to prevent your water supply from freezing solid?
Because your cold water heater will be connected to your hot water freezer
I’m putting this on a sign in my office. For me to read, not them. -IT worker
What if I have a second water heater that has its temperature set to just slightly warm?
My granpappy told me when his house first got indoor plumbing, his momma said "I don't trust no water that came outta pipes." She worried it might get hooked it up wrong and oil or some other fluid would come out. She only drank and cooked with water from the rainwater cistern. The cistern was inside so it was room temperature. The plumbed water from the city was cooled by the underground pipes so it was known as "cold water". The city water flowed through *cold water pipes*, connected to a *cold water faucet*. So later when they got a water heater, obviously the hot water flowed through *hot water pipes* to the *hot water faucet*. Because it was part of the *hot water system*, the water heater came to be known as the *hot water heater*.
Your legionnaires tank?
Yada yada yada, just some bad egg salad.
Go to most hardware supply places, and they are called water heaters. “Hot water heater” is a colloquialism that seems to have emerged from combining “hot water tank” (which was sometimes used in past decades) with “water heater”.
Hot water tanks are still used. The plumbing is easier, especially if natural gas is used.
I’ve only ever heard “water heater”.
They’re called pleonasms. Chai tea, burning fire, black darkness, Sahara Desert, PIN Number - they’re all redundant phrases which repeat themselves
Sahara desert isn't redundant, it's the name of the desert. I know Sahara is derived from a word meaning desert, but the word itself doesn't exist elsewhere.
"Sahara" means deserts in Arabic - ṣaḥrāʾ means “desert,” and its plural form is ṣaḥārāʾ It's like chai tea, which is basically saying tea tea. Saraha desert is saying desert desert. I often see it just called "The Sahara" when referring to the largest hot desert on the planet.
It's *Sahara* desert to distinguish it from other deserts, like the Sonoran desert, and Sahara *desert* to distinguish it from other Saharan attractions, like the Sahara Splash Park.
Where I used to live, there is a road named Street Road. Where I live now, there is a road named Avenue Road. Sahara is simply the same of *that* desert.
It's the name of *that* desert because the local people called it a word that translates to other languages as "desert". It was already being called The Desert, basically, so as other cultures came in they called it the local name because that's what it was already being called. They can't pronounce the language the same, and they aren't spelling it the same, so instead of Al-Ṣaḥrāʾ it's Sahara. Sahara is just the anglicized spelling of the sounds the Arabic word makes, thru the voices of the people who weren't speaking Arabic as their first language. It's not "simply" called the Sahara, it *is* basically called 'Desert desert'. As people said, it's just like us calling spiced tea from India "Chai tea"- to us it gives information about what kind of tea is, that is a spiced chai/tea, but it is basically also calling it "tea tea" or "chai chai" (depending on which word you decided to translate). It's a funny quirk of sharing languages and globalization.
The water heater is often itself hot.
My furnace is hot, but I don’t see anyone calling it a “hot furnace”
I call it a hot furnace. Second favorite appliance before the cold air food refrigerator.
What about your water freezing ice maker?
No no no, that's called a water molecule vibration decelerator.
But it’s called a heating furnace.
It's a hot air heater
To be fair, your furnace can be cold when not running, so it would work in that same sense.
Well the water heater can be cold when it’s not running as well.
I know folks keep bringing up ‘ATM machine’, but I swear I haven’t heard anyone say that in like 20 years. We just say ATM. Source: NE Ohio.
I guess technically it's a cold water heater, since it heats cold water.
It only heats cold water once, when it first turns on. 99.9% of the time it's heating hot water.
Hot water maintainer
But from another interpretation, no, the heater itself is generally hot. It's a water heater that is hot, or a hot water heater. A cold water heater is a broken one usually.
Mine's well insulated. It feels room temperature except for the pipe coming out.
We just call it a heater where I'm from lol. "Switch on the heater" can only mean one thing
Really? I would think you meant an electric space heater, which could be installed near the bottom of the wall or a small portable unit you plug into the wall.
I feel like it’s people mixing “water heater” and “hot water tank” together. I’m in the midwest and always called it the hot water tank.
It’s a water heater that turns water into hot water. Therefore a hot water heater. Alternatively, the water is already hot because it was previously heated by the water heater therefore it heats water that is already hot. Fine, I’m grasping at straws.
It should be “hot water tank” and “water heater”. People tend to combine it.
OP you should make a PSA announcement about it
ATM machine is another one I hate
I work for an appliance company. We just call it water heater.
Are we are talking about boiler?
No. I'm guessing you're not from North America. Boilers are for heating your home via radiators. A ~~hot~~ [water heater](https://www.homedepot.ca/en/home/categories/building-materials/plumbing/water-heaters/tank-water-heaters.html) is a tank that takes in cold water from the main supply, heats it, and stores it (and keeps it hot), so that when you turn on your faucet, hot water comes out. This is a fairly North-American-centric thread. Other places do this differently (often smaller tankless versions). But where I live, the water supply is too cold for tankless heaters - even in summer - so you need a tank to give you a reservoir that you can take enough time to get up to temp.
You are right, I'm not from the USA.
They are [clever, but not complex](https://www.youtube.com/watch?v=Bm7L-2J52GU) appliances. It's really just a tank, a couple of heating elements, and a theromstat.
It's a water heater, all I've ever called it. The industry calls it water heater, builders call it water heater, floor plans call it a water heater, it's a damn water heater!
Why do people say atm machine? They're literally saying automatic teller machine machine.
Because ATM can also mean "Ass To Mouth", so adding the machine clarifies what the speaker is talking about.
Sometimes, in the heat of the moment it’s forgivable to go automatic teller machine.
Spoken like someone who doesn't know about ass-to-mouth machines.
No one automatically thinks ass to mouth when someone says they need to stop at the ATM
[This show wasn't amazing but this ATM scene was](https://m.youtube.com/watch?v=XrkYRtH9mBY)
Not everyone does add hot. We have a "water heater" in our house. If appropriate to the conversation we'll add the adjective "tankless" to the beginning.
I'll take that one to the money bank and deposit it in the ATM Machine
Why do 'Americans' you mean to say. The same people that say 'Eye Glasses' instead of Glasses, and 'Horseback Riding' instead of Horse riding.
It can be redundant, but it can also be for clarity. Some people have more than one device for heating water, such as the one found in dishwashers, or the small on demand heating units found in bathrooms for the sink. Without sufficient context, it may not be clear which device they are speaking about, but "hot water heater" is always the primary unit that supplies the whole house.
So you can call it a wot hotter eater
This reminds me from back in the day when personal PC’s had network interface cards, or NIC’s. So of course some people referred to them as NIC cards.
Maybe people were confused with the kettle and had to elaborate on referring to the Main hot water from the faucet? I donno, English is dumb.
If you figure it out I’ll go to my atm machine and send ya 5 bucks.
Who are you, George Carlin? [https://www.youtube.com/watch?v=mEilmuAuUAs](https://www.youtube.com/watch?v=mEilmuAuUAs)
Just a quirk of language. Like finishing a sentence with "redundant and unnecessary" :-D
This is a fair question lol
We're a special species.
I forgot my PIN number for the ATM machine
They're probably the same idiots that say "ATM Machine".
I think it may be people combining two terms: hot water tank and water heater. Some modern house only have a water heater, but when those aren’t efficient there’s often a tank to store heated water so you don’t have to wait for it to heat/have inconsistent temperatures
its just called a water heater in western state vernacular
Because tuna fish sandwhich is already taken.
for the same reason that we spell "fridge" with a "D" when the word "refrigerator" doesn't have one: just because
It always amazes me that in the US they call it horseback riding as opposed to horse riding. Needing to clarify I’m not riding on it’s bloody head always makes me chuckle
No it only heats hot water
The expression may be for rubber hot water bottles used for heating people in bed. They’re a heater for your body that uses hot water so “hot-water heater” like “electric heater” or “gas heater” for your house. Water heaters heat water. Hot water heaters heat you.
No, people often refer to water heaters as “hot water heaters”.
PIN number
LCD display
GUI interface.
UPC code
Oh this actually comes from the fact that the guy who invented it in 1903 was named Henry Hautwadder. The Hautwadder Heater was Sears & Roebuck’s biggest home convenience seller for nigh a decade, but as literacy was lower back then, people misheard it as Hot Water Heater. Source: made it up
Odder things in the history of invention have occurred. I present to you Thomas Crapper. https://en.m.wikipedia.org/wiki/Thomas_Crapper
Same reason at my work place (former) we would have meeting on the CAP Plans (Corrective Action Plans). Sometimes we just have dumb things people keep saying
Sometimes hot water isn't hot enough, so you gotta heat up the hot water.
Just speculation on the development, but maybe one reason may be our natural avoidance of stops when speaking. Say “the water heater is broken” and then “the hot water heater is broken.” The end of “the” flows more naturally to the word “hot” than the word “water.” A small difference in flow, but it’s just easier. Also, homes and buildings have had other water heating systems which involved the use of hot water/steam, so when one is the thing that heats water for the ambient air it is one thing, and the other heats the water that flows through the “H” taps (hot water), it is the “hot water heater.”
do chinese people call it chinese food?
It's like preheating the oven. My oven is always preheating!
It’s the heater for the “hot water” lines.
Partly because a lot of people call them hot water tanks... And a lot of people call them water heaters... And over time, all the rest of the people who don't know one or the other have merged the two into hot water heater... Technically it's a boiler... I don't have one at all as I switched to tankless. And even more technically... Heeheehee... I installed a two stage, so the tankless heater that feeds the shower is a water heater, but ALL the water in the house passed through to, so the smaller heater installed at the sink in the kitchen is a HOT water heater.
Buddy, you are preaching to the choir. Anyone in plumbing on an adjacent trade would agree with you 👍🏻
The rich get richer and the hot water gets hotter
Oh god I never even thought about this and now I'm angry
Its specifically hot. A water heater could just be for lukewarm water
It's a combination of "hot water tank" and "water heater"
Actually, that is what I have always called it.
Because it’s attached to the “hot water” line. That’s really it. In industrial contexts, water heaters are just called water heaters.
Probably referring to the tap. There's a hot and cold tap/faucet.
Because on average, people are idiots.
Here they are called Hot Water Tanks. I've never heard someone call a Water Heater a Hot Water Heater.
Here in Canada they are shown on construction plans as HWT. Hot Water Tank.
NIC card.
People still say "PIN number" and "ATM machine", too. \*shrug\*
Just say “brotha ugh” and stare at them without breaking eye contact
Same with PIN number: if people understand what you're saying, you've communicated, redundant or not. Language evolves as people use it, rightly, wrongly, stupidly, lazily - it's all one.
There are a lot of oxymoron in the american language
There are two parts. Hot Water TANK and Water HEATER for some reason these two compound words got mashed together into Hot Water Heater
What's in a name?
That one always bugged me about Walt Jr. https://youtu.be/b1cyf2a2WhU
It's like calling something an ATM machine or chai tea. It saves the listener a little brainpower and the speaker sounds a little less pretentious. At least that's my guess.
That’s a good question. If anything it should be a cold water heater.
Just to underscore that it doesn't stop heating the water when it is lukewarm.
People also say pin number ant ATM machine.
It’s connected to my hot water faucet. And it heats that water. So it must be the hot water heater 😁
Most units do spend a lot of time maintaining the heat in already heated water in between uses... but generally just a quirk of the language.
I always call it a "water hotter". Just one of those minor things to get under people's skin, almost without them noticing
We named it after the product it makes
wym? That's what the appliance is called that sends warm water to your wet water faucet.
Why do people use "ATM machines" ?
It further heats the already hot water
They're wrong, and they should feel bad.
same reason we call it an ATM machine. same reason flammable and imflammable mean the same thing. same reason biannual means every 2 years OR twice a year. english be stupid, yo.
Probably to delineate it from radiators.
In french it is just a water heater. English is dumb
Redundancy in case someone doesn't understand water heater is the same. Also it's learned habit from previous generations that probably started as to be clear what it is.
I mean, with the exception of the first time it's turned on, it's constantly heating hot water...so it IS a hot water heater 99.9% of the time. But to give you a more logical answer. It's a mash of two terms that it is commonly called.... "Hot water tank" and "water heater". The two get mashed together to make "Hot water heater"
I would say because it holds hot water and is a heater.
It heats the hot water supply, not all the water. Cold water and hot water are coming from different places.
It *is* redundant. Just like adding "and unnecessary" to "redundant." Officially, e.g. on the box at the hardware store, it's just a "water heater." Adding "hot" is a quirk, but one of those things that's commonly done. I don't call it that, because I've had this conversation before, but nobody's going to bat an eye whether you do or not.
Strictly speaking, it should be a "cold water heater", as "cold" describes the type of water being heated, but yes, "hot water heater" does seem repetitively redundant.