I've heard 'Hot water heater' and 'Water heater' being used interchangeably to refer to an appliance which generates a supply of heated water.

The wording of 'Hot water heater' feels redundant, as heaters usually make things hot.

Is this phrase grammatically correct?

  • I agree with you that hot water heater is redundant (and strange to me. Never heard that. I do see that though on googling.) Maybe it's a mix-up of hot water tank and water heater? I call the device either a hot water tank (when there's a problem, like a leak) or a water heater (when shopping for one). Mar 12, 2014 at 6:51
  • It is grammatically correct, but (assuming you're referring to the standard household device that heats water) stupid. While a lot of people use the term you will get roundly ridiculed if you use the term on, say, a bulletin board frequented by plumbers or construction people.
    – Hot Licks
    Feb 10, 2015 at 23:14
  • It's not per se redundant. I can heat up water, but that doesn't necessarily make it hot. Jun 4, 2015 at 1:21
  • "Hot water heater" is a fairly common term in Australia, and is used even by some major heater manufacturers and gas companies. "Hot water system" and "hot water unit" may also be used here.
    – nnnnnn
    Jul 9, 2020 at 7:37

7 Answers 7



Hot water heater

The phrase is definitely grammatically correct (it has no objective construction or structural issues). Whether it's appropriate usage or not is a different a question.


I can think of two ways to parse the phrase:

Hot [water heater]: a water heater that is hot.

You know, because... it gets quite hot... as it heats the water.

[Hot water] heater: a water heater which further heats water that is already hot.

If you live in a desert, this might somewhat apply.1 I do not know if there are special appliances for heating water that's already hot.

Short, nice answer

The phrasing is, indeed, somewhat redundant, unless there is a very special case that holds. A heater of object X does conventionally make object X hotter.

[1] Getting water when in a desert is a separate problem.

  • There is another parsing that depends on cultural norms. About 80 years ago, a constant source of hot water was a rarity in most households. "Hot water" was a compound noun expressing a concept 'of itself' and quite different from "water" (which was always cold). A heater could heat anything, but a {hot water} heater was the thing that produced this distinct substance.
    – Greybeard
    Jul 9, 2020 at 9:40

There is actually a relevant article discussing terminology (ie usage) at Wikipedia [emphasis mine]:

Domestically, water is traditionally heated in vessels known as water heaters, kettles, cauldrons, pots, or coppers. These metal vessels that heat a batch of water do not produce a continual supply of heated water at a preset temperature. Rarely, hot water occurs naturally, usually from natural hot springs. The temperature varies based on the consumption rate, becoming cooler as flow increases.

Appliances that provide a continual supply of hot water are called water heaters, hot water heaters, hot water tanks, boilers, heat exchangers, geysers, or calorifiers. These names depend on region, and whether they heat potable or non-potable water, are in domestic or industrial use, and their energy source.

So, the terms, whether appealing to logic or otherwise, are not used consistently. I suspect that this terminology may be different again here in the UK.

  • Wow! Calorifier! Never heard of it. But I like it. Not sure I would have picked up on what it meant without context though. But seeing it here and thinking about it for a sec it makes sense.
    – Jim
    Jul 9, 2020 at 6:47

The appliance used to heat water is a "water heater" Some cars use electrical elements to warm the seats. There are "electric heaters". "Electric" is an adjective used to describe the type of heater My car uses hot water to warm the interior. This is a "hot water heater". "Hot water" is an adjective describing the type of heater.

  • Hello Dwight. In looking at 'answers to older questions', I've come across this one – for the first time. Though your answer seems to possess a nice logic, you have to remember it's English we're dealing with here, not maths. The way 'hot water heater' is used by some (and that's what counts) is as a synonym for 'water heater'. The researchers at Wikipedia give us this information. Feb 10, 2015 at 20:42

In linguistics such a phrase is called a pleonasm overlap (or redundancy). Like "tuna fish", or "free gift". English has lots of examples.


This one is a pet peeve of mine. I never remember hearing the term "hot water heater" until I was stationed on an American submarine. (I grew up in Arizona and Wisconsin.) On the sub, I had to deal with it because there were numerous water heaters, all bearing labels such as "Hot Water Heater #7" and "Hot Water Heater #12". Few, if any, other sailors on the sub were bothered by the terminology at all, and I thought I might have slid into another dimension or something. The term just sounds so stupid to my ears.

The funny thing was, I was a reactor operator on the sub, and it actually did have four hot water heaters:

  1. The nuclear reactor itself. 500°F+ water went in and 500°F++ water came out.
  2. The pressurizer. This was a huge upside down sealed steel pot that had electric heaters in the bottom. A pipe at the very bottom connected it to the primary coolant system. Superheated boiling (500°F+++) water was kept within the pressurizer. It was maintained within a certain liquid water level band with superheated steam at the top. It acted both to pressurize the primary system so 500°F+ water would remain liquid, and to act as a surge volume as the primary coolant would expand and contract in volume due to changes in its temperature.
  3. & 4. The port and starboard steam generators. These used the pressurized 500°F+ water of the primary system to create steam on the secondary (non-nuclear) part of the system. The superheated, high pressure steam was then used to run generators and the main engines.

Personally, I think those should have been labeled "Hot Water Heater #0" through "Hot Water Heater #3" respectively. All the other ones used for cleaning, cooking, etc., should have just been water heaters.

  • What do F+, F++, and F+++ mean? Are the plus signs an indication of how much pressure the water was under? It seems weird to talk about having heated the water from 500 to 500 - isn't that the same temperature?
    – nnnnnn
    Jul 9, 2020 at 7:22
  • @nnnnnn The specific temperatures and pressures the reactor plant ran at were classified. So I was using a shorthand obfuscation, which obviously were a little too unclear. So, 500`F+ is something more than 500 degrees Fahrenheit. Each additional plus meant even hotter. Yes, the minimum pressure to keep the water liquid at ascending temperatures would go up with temperature, but I was only focused on conveying the heat.
    – RichF
    Jul 9, 2020 at 8:01

"Hot water heater" --> This would suite more a heater for the house that heats up the stuff, the hot water that comes out of your hot water tap.

"Water heater" --> This is generally just a thing that heats water without any specification to how hot or warm the water is going to get.

Also neither of them are boilers or water boilers so neither of them are supposed to produce boiling water. One of them however hot water.


The term hot water heater specifically refers to a device or equipment that produces 'hot water' (of course?).

The generic term water heater refers to that which produces either hot water or steam (i.e., a 'boiler', which term may also be used for that which produces hot water).

The adjective hot modifies water, not water-heater, (of course!).

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