B Why does hot water rise to the top of a water heater?

[sevensages]

TL;DR

Why does hot water rise to the top of a water heater?

I am enrolled in a plumbing program with Stratford Career Institute. In the chapter in my plumbing textbook about Water Heaters, the textbook authors assert that hot water in a water heater rises to the top of a water heater. The authors assert that cold water sinks to the bottom of a water heater.

Here is an exact quote from the textbook:

"The design of the storage tank takes advantage of water's natural properties. Figure 18-2 describes how a water heater functions. The cold water supply pipe typically connects to the water heater at the top of the tank. A pipe on the inside of the tank directs the cold water to the bottom of the tank. As the water is heated, it expands. Since expansion makes it lighter, it rises to the top of the tank, where it can be drawn off. As heated water cools, it becomes denser and sinks to the bottom of the tank, where it is heated again. This circulation of water within the tank ensures that a reservoir of hot water will be available at all times."

Before I read this textbook, I never would have thought that if a given quantity of water was heated, that it would expand. I don't understand this. Does water expand when it is heated? Let's say that I have an empty 5-gallon size bucket. I pour exact one gallon of 50 degree fahrenheit water into this 5-gallon bucket. Then I put the 5-gallon bucket with one gallon of 50 degree fahrenheit water on a stove, and I heat the water in the 5-gallon bucket up to 100 degrees fahrenheit. Would the water in the bucket start occupying more than one gallon of space in the bucket when the water reached 100 degrees?

 

[Lnewqban]

Water expands when it is heated.
That is the reason behind the pressure relief safety valve that you see at the upper section of each water heater.

Please, see:
https://www.engineeringtoolbox.com/water-density-specific-weight-d_595.html

https://www.watts.com/resources/references-tools/t-and-p-relief-valves

 

[sevensages]

Water expands when it is heated.
That is the reason behind the pressure relief safety valve that you see at the top end of each water heater.

Please, see:
https://www.engineeringtoolbox.com/water-density-specific-weight-d_595.html

Well, my textbook says that water expands when it is heated.

So if a gallon of 50 degree fahrenheit water is heated to 100 degrees fahrenheit, the 100 degree fahrenheit water will be more than one gallon?

 

[jbriggs444]

Well, my textbook says that water expands when it is heated.

Your textbook is correct.

So if a gallon of 50 degree fahrenheit water is heated to 100 degrees fahrenheit, the 100 degree fahrenheit water will be more than one gallon?

Let us see what Google can dig up about the density of water as a function of temperature...

Search phrase: density of water versus temperature

First hit: https://www.engineeringtoolbox.com/water-density-specific-weight-d_595.html

You care about the blue curve on the bottom that cuts off at 212 degrees Fahrenheit. The pressure in the water heater will be a bit higher. So maybe you care about a curve in the gap between the blue and yellow.

 

[Lnewqban]

So if a gallon of 50 degree fahrenheit water is heated to 100 degrees fahrenheit, the 100 degree fahrenheit water will be more than one gallon?

Yes.
The occupied volume at 100° will be more than one gallon.

Note that the gallon is a unit of volume in British imperial units and United States customary units.
In both cases, at 50° and 100°, your mass of water will still be approximately 8.34 pounds.

 

[sevensages]

Both my textbook and Lnewqban have told me that water expands when it is heated. But I am still confused.

The idea that if I heat one gallon of 50 degrees fahrenheit water to 100 degrees fahrenheit, the water will expand to more than one gallon is VERY counter-intuitive to me. In fact, it is so counter-intuitive that I need someone to spell this out for me because I am unsure if it is true. I think that the statement that "water expands when it is heated" might mean someone OTHER than if I heat one gallon of 50 degrees fahrenheit water to 100 degrees fahrenheit, the water will expand to more than one gallon.

In other words, I think it might be true that water expands when it is heated, but I think that it somehow might also be true that if one gallon of 50 degree fahrenheit water is heated to 100 degrees fahrenheit, the 100 degree fahrenheit water will still be one gallon (not more than a gallon). Can someone help me understand this?

Just telling me "water expands when it is heated" ad nauseum is not going to make me understand this.

 

[jbriggs444]

Both my textbook and Lnewqban have told me that water expands when it is heated. But I am still confused.

The idea that if I heat one gallon of 50 degrees fahrenheit water to 100 degrees fahrenheit, the water will expand to more than one gallon is VERY counter-intuitive to me. In fact, it is so counter-intuitive that I need someone to spell this out for me because I am unsure if it is true. I think that the statement that "water expands when it is heated" might mean someone OTHER than if I heat one gallon of 50 degrees fahrenheit water to 100 degrees fahrenheit, the water will expand to more than one gallon.

In other words, I think it might be true that water expands when it is heated, but I think that it somehow might also be true that if one gallon of 50 degree fahrenheit water is heated to 100 degrees fahrenheit, the 100 degree fahrenheit water will still be one gallon (not more than a gallon).

You are wrong about this. A "gallon" is a unit of volume. It is not a unit for a fixed "quantity of water".

If you heat one gallon of water from 50 degrees to 100 degrees, you will then have more than one gallon of water. If you have it in a graduated cylinder, the cylinder will be more full. The water you have will be hotter and less dense. It will still weigh the same as before you heated it.

This assumes that the water is free to expand.

If you lock the water in a tight box with no air space anywhere and do not allow the box to expand, the volume of the water will be unchanged, of course.

 

[sevensages]

You are wrong about this. A "gallon" is a unit of volume. It is not a unit for a fixed "quantity of water".

If you heat one gallon of water from 50 degrees to 100 degrees, you will then have more than one gallon of water. If you have it in a graduated cylinder, the cylinder will be more full. The water you have will be hotter and less dense. It will still weigh the same as before you heated it.

Ok I believe you. You and Lnewqban have made me understand this. Thank you very much.

 

[sevensages]

Yes.
The occupied volume at 100° will be more than one gallon.

Note that the gallon is a unit of volume in British imperial units and United States customary units.
In both cases, at 50° and 100°, your mass of water will still be approximately 8.34 pounds.

I believe you. Now you have made me understand this. Thank you.

 

[sevensages]

I thought that I was through with this thread, and then jbriggs edited his post to write this:

This assumes that the water is free to expand. If you lock the water in a tight box with no air space anywhere and do not allow the box to expand, the volume of the water will be unchanged, of course.

What if the water is in a water heater? Wouldn't there be a little bit of air space in a typical water heater? If the water is in a water heater, would there typically be enough air space for the water to expand when the water was heated?

 

[vela]

In other words, I think it might be true that water expands when it is heated, but I think that it somehow might also be true that if one gallon of 50 degree fahrenheit water is heated to 100 degrees fahrenheit, the 100 degree fahrenheit water will still be one gallon (not more than a gallon). Can someone help me understand this?

When you say "water expands when it is heated," you're saying the volume the water occupies increases. But when you say "if one gallon of 50 degree water...the 100 degree fahrenheit water will still be one gallon (not more than a gallon)," you're saying the volume of water doesn't change. Both can't be true.

 

[sevensages]

When you say "water expands when it is heated," you're saying the volume the water occupies increases. But when you say "if one gallon of 50 degree water...the 100 degree fahrenheit water will still be one gallon (not more than a gallon)," you're saying the volume of water doesn't change. Both can't be true.

That is why I had to create this thread.

 

[jbriggs444]

What if the water is in a water heater? Wouldn't there be a little bit of air space in a typical water heater? If the water is in a water heater, would there typically be enough air space for the water to expand when the water was heated?

I am not a plumber, so I do not know for sure whether there is air space or just a pressure relief valve. In my area, code requires a check valve so that expanding water cannot force its way back "upstream" toward the water supply.

We can look up the bulk modulus for water. It is about 2.2 gigaPascals (GPa). That is about 22,000 atmospheres. You can think of that as how much pressure it would take to change the volume of some water by 100 percent. [More properly, it is 100 times the pressure it takes to change the volume by one percent]

If we look up the density of water at 50 degrees Fahrenheit, that's 0.99965 in grams per cc. At 100 degrees, it is .99308. That is a change of about 0.69 percent.

So we would need to apply $22000 \times \frac{0.69}{100} \approx 150$ atmospheres to prevent the water from expanding and keep it in the heater. This matches what I can read off the graph I'd posted up-thread. Without a pressure relief valve or some air space, your water heater may have problems.

 

[sevensages]

The bulk modulus for water is about 2.2 GPa

I am not a plumber, so I do not know for sure whether there is air space or just a pressure relief valve. In my area, code requires a check valve so that expanding water cannot force its way back "upstream" toward the water supply.

We can look up the bulk modulus for water. It is about 2.2 gigaPascals (GPa). That is about 22,000 atmospheres. You can think of that as how much pressure it would take to change the volume of some water by 100 percent. [More properly, it is 100 times the pressure it takes to change the volume by one percent]

If we look up the density of water at 50 degrees Fahrenheit, that's 0.99965 in grams per cc. At 100 degrees, it is .999308. That is a change of about 0.69 percent.

So we would need to apply $22000 \times \frac{0.69}{100} \approx 150$ atmospheres to prevent the water from expanding and keep it in the heater. This matches what I can read off the graph I'd posted up-thread. Without a pressure relief valve or some air space, your water heater may have problems.

I don't know what bulk modulus means. I've never seen or heard the word bulk modulus in my life.

I just looked at the picture in figure 18-2 in my plumbing textbook, and it does show an air gap at the top of the water heater. So I suppose that most water heaters do have an air gap at the top of them, which would allow the hot water to expand.

 

[jbriggs444]

I don't know what bulk modulus means. I've never seen or heard the word bulk modulus in my life.

Which is why I tried to explain how a bulk modulus works.

I just looked at the picture in figure 18-2 in my plumbing textbook, and it does show an air gap at the top of the water heater. So I suppose that most water heaters do have an air gap at the top of them, which would allow the hot water to expand.

Thanks. That makes sense since water will release dissolved gasses when heated. Though I am not certain how one prevents the gasses from building up in the water heater over time.

 

[sevensages]

Which is why I tried to explain how a bulk modulus works.


Thanks. That makes sense since water will release dissolved gasses when heated. Though I am not certain how one prevents the gasses from building up in the water heater over time.

I don't know anything about water releasing dissolved gasses when heated.

I think it makes sense that the water heater would be designed to have space at the top for the water to expand because that if the heated water expands and rises to the top, and if the cold water sinks to the bottom, that would allow a circulation of water in the water heater, which is desirable.

 

[Drakkith]

Per the 2nd link in the 2nd post:

When water is heated it expands. In a 40-gallon water heater, water being heated to the thermostat setting expands by approximately 1/2 gallon. The extra volume created by this expansion usually pushes back into the water tower of public water supplies, or into the well tank in homes with a private well, resulting in negligible pressure increase. However, on public or shared water supplies were a backflow device is installed on the water main feeding the house, the pressure can no longer push back into the tower or well tank. With no place for the expansion to go, pressure will dramatically increase.

So in my house, where there is no check valve or backflow device installed, the water is free to expand into the house pipes and public water supply and there is little to no pressure increase inside my water heater tank. If the water expanded into an air space I would expect to see a slight 'burst' of high pressure hot water whenever I first turn on the hot water in my sink or bathtub as the air expands and pushes on the water, but I do not see this so I guess the pressure in the tank is the same as in the supply pipes. Perhaps others have this issue if they have check valves or backflow devices, but I do not know.

 

[sevensages]

Some water heaters have two heating elements, one at the top and one at the bottom. Some water heaters only have one heating element. If the water heater only has one heating element, the heating element is at the bottom of the water heater.

If a water heater only has a heating element at the bottom of the water heater, an air gap at the top of the water heater would be beneficial because it would allow the circulation of the water in the water heater as water is heated in the water heater. If the water heater only has a heating element at the bottom of the water heater, and if hot water rises to the top and cold water sinks to the bottom, the circulation of the water in the water heater would cause all the water in the water heater to get heated. If the water heater only has a heating element at the bottom of the water heater, and if the water did not circulate in the water heater, you might have hot water at the bottom of the water heater and relatively cold water at the top of the water heater instead of the entire tank being hot water.

 

[Lnewqban]

If the water is in a water heater, would there typically be enough air space for the water to expand when the water was heated?

Air can be compressed, water is incompressible.

Air in the system is desirable because it acts as a spring against which expanding hot water can move, avoiding excessive increase of internal pressure that could rupture the walls of the tank, which are much weaker than the pipes and fittings (due to greater diameter).

Nevertheless, as air is compressed by the expanding water, the internal pressure of the whole water system also increases.
Most codes require either a bladder expansion tank (located above the water heater) or a relief valve (located upstream of the main shutoff valve of the building) to protect the entire plumbing system (besides the built-in pressure-temperature relief valve of the water heater).

Please, see:
https://plumbingtoday.biz/blog/3-things-that-can-cause-your-water-heater-to-burst

https://www.freshwatersystems.com/blogs/blog/how-to-fix-common-water-heater-expansion-tank-problems

 

[sophiecentaur]

TL;DR Summary: Why does hot water rise to the top of a water heater?

Before I read this textbook, I never would have thought that if a given quantity of water was heated, that it would expand.

OMG this thread has really got out of hand. Why bring in how water behaves under vast and unrealistic (for plumbers) pressures? Let's walk before we run.

Your "given quantity" is too loose a term and is confusing. You need to say "given mass". Then you can heat it and cool it and the only thing to change is its volume. Your book says:

TL;DR Summary: Why does hot water rise to the top of a water heater?

Since expansion makes it lighter

and that is wrong because the word is not 'lighter' but 'less dense'. Many text books attempt to use too many friendly words and that can totally misdirect the reader.

Start with 10 lb of water, heat it and it expands, cool it and it contracts but you still have the same mass of water. There's no way to restrict the volume of the water you've got in your tank . (Not measurably).

What happens in water heater tank is convection.
Start with a tank, full of cold water. Put a heating coil near the bottom and the water around the coil will warm up and expand. It's density will be lower than the surrounding water; it will move upwards and the cooler water will fall towards the bottom. That's Convection and the water will carry on circulating and mixing until it's nearly all at nearly the same temperature.

You may have come across 'Immersion Heaters' that have a long heating element, which reaches right down to the bottom and also a short element that only reaches 1/3 of the way down. Convection doesn't work downwards so the short element only heats the top 1/3

 

[russ_watters]

I am not a plumber, so I do not know for sure whether there is air space or just a pressure relief valve.

I think it makes sense that the water heater would be designed to have space at the top for the water to expand because that if the heated water expands and rises to the top, and if the cold water sinks to the bottom, that would allow a circulation of water in the water heater, which is desirable.

There should be no air in the system. Oxygen + metal = corrosion. Plus, if there is air at the top, it's just going to go out the outlet pipe. There should only be an expansion tank, which has pressurized air on one side, water on the other, and a flexible membrane in between.

 

[sophiecentaur]

There should be no air in the system. Oxygen + metal = corrosion.

If the system doesn't 'breathe' then the amount of air is limited.
Closed systems have an expansion vessel (as above) at a high point which is pressurised when installed. There's an overflow which will let water out if the pressure get too high but we're only dealing with around a Bar.
We're getting confused with several different factors here. I've lost where we are going with this.The actual expansion of water is important in that it causes convection but only in an indirect system with a 'hot tank' and a heat exchanger. Combi Boilers predominate (in UK at least) which heat water on demand and the circulation is by pump and not by convection.

 

[Lnewqban]

We're getting confused with several different factors here. I've lost where we are going with this.

From post #1:

Before I read this textbook, I never would have thought that if a given quantity of water was heated, that it would expand. I don't understand this. Does water expand when it is heated?

 

[russ_watters]

Closed systems have an expansion vessel (as above) at a high point which is pressurised when installed.

The expansion tank could be anywhere but it is typically installed by the water heater, which can be anywhere - high or low. Mine's in my basement.

There's an overflow which will let water out if the pressure get too high but we're only dealing with around a Bar.

3-6 barg, typically. We're talking about household domestic water here; that's the pressure you get from the city service.

 

[Tom.G]

Hi @sevensages, lets see if this helps explain the water expansion with temperature.

You have probably heard of molecules, they are two or more atoms bound together. For instance water is two atoms of Hydrogen bound to one atom of Oxygen, this leads to the chemical formula for water, H₂O... and they are not in a straight line, The H atoms are bound the the O about 105° from each other; that results in a water molecule taking up a bit more volume than expected

When the temperature of something rises the, molecules jostle around and bump into each other more often that at a lower temperature. This, on the average, keeps the molecules slightly further away from each other.

You can see that same effect in a crowd of people. If they are standing still they can/will be rather close to each other, rather tightly packed. On the other hand, if they are dancing to some wild music they wil be further apart.

With both water and people, their may be the same number of molecules/individuals, but when they are randomly moving around they will take up slightly more space.

Hope this helps!

Cheers,
Tom

 

[Baluncore]

In this discussion, the only "solid" parameter that does not change, is the mass of water involved in the experiment. A fixed number of molecules, has a fixed mass, and so weighs an amount determined by the acceleration due to gravity, wherever the experiment is being conducted.

The state variables, volume in gal(imp/US), temperature(°C/°F), or the confining pressure, (Pa/psi), are all interdependent.

 

[256bits]

Note that the gallon is a unit of volume in British imperial units and United States customary units.
In both cases, at 50° and 100°, your mass of water will still be approximately 8.34 pounds.

I have to point out a slight correction, and some unit trash.

The two gallons comprise different volumes in the two systems.
As do the two gils, which is 5 fluid ounce in Imperial, but only 4 fluid ounce in the American system.
Four gils make a pint for both systems.
This makes the cup size different for both = 10 oz Imperial vs 8 oz American.

Imperial gallon = 4.546 litres = 4 quart = 8 pint = 16 cup = 160 fluid ounce == weight 10 pound water.
American gal = 3.78 litres = 4 quart = 8 pint = 16 cup = 128 fluid ounce == 8.34 weight pounds water.

The American gallon is based on the wine gallon, based on 8.34 pounds of water at 39 F.
The British adopted the imperial gallon, based on 10 pounds of water by weight at 62 degree F, in the early 1800's.

Watch out in recipees. Whether they are using Imperial or American units, your cake may not quite come out expected under an incorrect assumption.

 

[sophiecentaur]

Watch out in recipees.

Whatever the recipe, volume is not conserved. It may be a convenient unit for measuring everyday fluids but it's hardly a good basis for talking Physics.

The expansion tank could be anywhere

True. I seem to remember a bleed screw on mine. Mine is actually at the highest position and the boiler is on the ground floor wall. But, of course, the boiler is closed and uses its own pressure; I'd forgotten that. The hot tank is upstairs .

 

[snorkack]

Generally, nearly all substances expand when heated.
Nearly all, because there are some very significant exceptions. Water is one of these few exceptions.
As stated, most substances expand on heating - but different substances expand to a different amount.
Substances can also expand differently in different directions. This happens to some solids and some composites. However, many solids and all liquids (and gases) expand equally in all directions.

I mentioned that water is one of the few and significant exceptions that do not always expand when heated. Water sometimes does expand when heated.
When you warm ice Ih from absolute zero, then it first shrinks slightly. Reaching about 934 g/l at -180 C.
On further warming, ice expands, significantly, from the said 934 g/l at -180 C to 917 g/l at 0 C.
And then on melting, shrinks drastically - to almost 1000 g/l of water at 0 C.

Interlude: how can you even measure expansion of a liquid?
Most liquids expand on warming - but so does the solid vessel which the liquid is in!
Well, you can make a careful comparison of vessels with rulers which you try to not let heat up with the vessel. It´s possible, but it took a lot of work, and a lot of mistakes.

Water shrinks to almost but not exactly 1000 g/l on melting:
https://www.engineeringtoolbox.com/water-density-specific-weight-d_595.html
1 l of water weighs 999,85 g at 0 C. And water shrinks further on warming - weighs 999,975 g at 4 C.
For the measurements: the British and USians use their (different!) gallons. (Which gallons are in Canada?). Much of the world uses metric... which has the advantage of some more rounded measurements!
Not all of them exact.
When the French revolutionaries back in 1790s enacted the metric system, they intended to define metre so that the circumference of Earth around poles - they knew that Earth is flattened like an orange and the equator is longer - should be exactly 40 000 km. They also intended to define kilogram so that 1 cubic metre of water at 4 C should weigh exactly 1000 kg.
They made measuring errors with both. When the etalons of metre and kilogram have been measured again, it turned out that the circumference of Earth was 40 008 km, not 40 000, as it should have been, and that 1 cubic metre of water at 4 C and 1 bar weighed just 999 kg 975 g, not exactly 1000 kg. The metre ruler had been made 0,2 mm - 0,02% - too short, and the kilogram weight, given the metre ruler, 25 mg - 0,0025% - too heavy.
The French decided against correcting their measures (they would have had to tinker with measures all the time as they got increasingly improved measurements!). So they settled at Earth being approximately but not exactly 40 000 km around and water weighing approximately but not exactly 1000 g/l.
In any case, water above 4 C starts expanding. It is 999,1 g/l at 15 C, 997 g/l at 25 C, and 958,4 g/l at 100 C (all at 1 bar).

 

[256bits]

(Which gallons are in Canada?).

We are on metric. Before it was the Imperial; as the commonwealth countries followed Britain.
It would be interesting to have a synopsis what the French colonies used as a system of measurement prior the Louisanna purchase and formation of Upper and Lower Canada, but never talked about.
I found this difficult site
https://www.bing.com/ck/a?!&&p=025e...zc2V0cy9kb2N1bWVudHMvTWV0cm9sb2d5LnBkZg&ntb=1