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

[256bits]

Generally, nearly all substances expand when heated.

A reference for the knowledge hungry
https://en.wikipedia.org/wiki/Negative_thermal_expansion

 

[snorkack]

Air can be compressed, water is incompressible.

All substances are compressible.
Otherwise, if you knock at one end of an incompressible item, when would its other end move? Answer is, instantly (because the item is not compressible). Therefore, before a light speed signal reaches the other end.
That said, expressing the bulk modulus as "2,2 GPa" is unhelpful. You don´t appreciate how big it is.
2,2 GPa is 22 000 bar. For comparison, the bulk modulus of air, and of any other gas, is equal to their pressure up to normally a few tens of bar.
22 000 bar is unrealistically high pressure for plumbers (and the bulk modulus is nonlinear on the scales of itself, most of time it grows). But it is useful to calculate the fractions.
If you have a 100 l water tank containing 90 l water and 10 l air at 1 bar and you add 100 ml of water into the tank, what happens to the pressure?
Answer: since the free space for air has decreased from 10 l to 9,9 l (you have not let any out), the pressure of air, and therefore water, will increase from 1,00 bar to 1,01 bar. Just 0,01 bar overpressure
If the same tank is completely full with no ullage left (but pressure relaxed to 1 bar) and you tried to add another 100 ml water, what will happen to the pressure?
Answer: since the added water is 1/1000 of the water already in, and the bulk modulus of water is 22 000 bar, you would need to increase the pressure by 22 bar.
That is, if your tank does not burst under these 22 bar, and if your tank is altogether incompressible.
The latter it physically cannot be because of special relativity.
Say you use a more realistic pressure. For example, mains pressure 5,5 bar above ambient, total 6,5 bar.
When you open a tap and let pressurized water from mains to flow into your tank, completely full but under just 1 bar, will any water flow in the tank?
The answer: 5,5 bar is 1/4000 of the water bulk modulus. Therefore, 1/4000 of the tank volume will be added due to compressibility of water - 25 ml will flow in the 100 l tank. However, the elastic expansion of the tank under internal pressure will be an amount varying depending on the geometry and elastic properties of the tank - and may well be much bigger than the elastic compression of water!

Also: it is relevant that it is the weight of water per volume that decreases on heating, not just its mass. Carry your water boiler to a spaceship, where water weighs nothing whether it is denser or less so, and switch on your heating element. Your boiler will not work. The hot water near the heating element will expand, but in absence of gravity, it will have no direction to rise, and it remains near the heating element - the cold water at the other end will not get warm.
Spaceships may not be so common issues, but portable boilers which may be turned altogether wrong way up might be a bit more common.

 

[russ_watters]

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 .

Are you talking about a hot water or steam heating system? This thread has been about domestic (drinking/bathing) water systems. I think in UK both are called "boilers"(and may be combined)? In the US a boiler is for heating your house(whether makes hot water or steam) and the thing that makes hot water for your shower is called a "water heater". Caveat: hot water for heating your house is fairly rare and archaic in the US, but house and domestic water heating may be combined in one device with separate circuits.

Anyway, a heating system needs air bleed (and runs at lower pressure), whereas domestic water doesn't because every sink/shower bleeds air out. Even in a mostly closed heating system air can collect because it is dissolved in and driven out of the water when heated.

 

[sophiecentaur]

Also: it is relevant that it is the weight of water per volume that decreases on heating, not just its mass.

= density?

 

[sophiecentaur]

This thread has been about domestic (drinking/bathing) water systems.

Yes. We don't use steam systems in UK in private dwellings, afaik. Steam has advantages in large /tall buildings with community heating. Is it still in use for large modern blocks?

The term "boiler" is not a good one but it's used as a matter of course in UK. Not the best of terms when the water is seldom anywhere near 100C. Nearly all CH systems use hot water radiators in UK. This is anotherTower of Babel; one of many in the domestic engineering world.

Even in a mostly closed heating system air can collect because it is dissolved in and driven out of the water when heated.

In a closed system, the amount of new water admitted is small (ideally zero). Hot water for washing etc is seldom much above 60C (for safety reasons and legionella). There doesn't seem to be much out-gassing at that temperature.

 

[snorkack]

= density?

Mass per volume has a nice standard term "density". Weight per volume does not have a good standard term I know of. Note that weight per volume goes to zero when gravity goes to zero even when density does not go to zero.
Also note that the derivative of water density with temperature gets very low and goes through zero at low temperature. If your water tank is full of 0 degree water then the density is 999,85 g/l. Turn on the heating element at the bottom of the tank, and warm water will not rise! It will shrink (to 999,97 g/l at 4 C) and therefore sink to the bottom. Only when the bottom water is slightly above 4 C will it start to convect upwards, eroding out the bottom of the icy cold water above.

 

[sophiecentaur]

Mass per volume has a nice standard term "density".

An ideal term for an OP who claims to have little knowledge of Physics. Surely this sort of thread should try to help an OP and not make life more complicated. It's so much more concise and than 'lighter' or 'heavier' yet still familiar.

Perhaps the behaviour of water around 0C would be better for another thread. We could give him Cold Feet?

 

[Ken Fabian]

@sevensages - I think that textbook description is wrong - mixing up what happens when the heater element is on with what happens when it is off. Stratification happens when the heating element is off.

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."

You only get stratification - a hot layer above a less-hot layer - after the heating element is off for a sufficient period of time that the convection from it ceases.

During heating there is convection driven circulation that keeps mixing the contents, preventing stratification. Any stratification is not driven by the heating element making hotter water that rises - a vigorous process - it comes from much slower, less vigorous cooling from pipe inlets and outlets, that are made of metals like copper, brass and stainless steel and cooling of the (imperfectly insulated) tank itself.

The strength of the circulation from that convection when the heater is off is insufficient to mix the contents, allowing the cooled water flowing down to settle. Rather than cause mixing, the slow flow causes the top of that pool of cooler water to rise slowly, as a layer.

I note that our own hot water system has the outlet (and pressure relief valve) lower than the top of the tank, so there is an air space in the top of the tank. Not sure but I expect dissolved gases being released keep it from filling with water or replaced by water vapor. Any corrosion concerns are dealt with by use of non-corrosive materials and a sacrificial anode.

 

[Ken Fabian]

And (duh!) cold water coming in near the bottom will be the main cold water source and will tend to stay there, without much mixing - it won't make the sustained convection cycling that a heating element does, that breaks down any stratification.

 

[russ_watters]

And (duh!) cold water coming in near the bottom will be the main cold water source and will tend to stay there, without much mixing - it won't make the sustained convection cycling that a heating element does, that breaks down any stratification.

There's a major design requirement at work when it comes to putting the cold water in at the bottom and drawing hot water off the top: supplying consistently hot water for as long as possible. If the cold water entered at the top and hot water was drawn off the bottom there'd be more mixing and the temperature would start to drop soon after you start using hot water.

 

[Klystron]

Caveat: hot water for heating your house is fairly rare and archaic in the US, but house and domestic water heating may be combined in one device with separate circuits.

While generally correct for houses in the US, I was somewhat shocked when buying a condominium (condo) unit to find it heated with the same hot water as used by a sink or shower. I have owned 3 condos in a warm climate (CA and NV) that use 'aquatherm' systems to heat the condo. Also a two story house ~1600 square feet with a 75 gallon water heater.

Hot water from the hot water heater outflow runs through a T-connector to a radiator when the thermostat trips activating a taco valve. An electric fan blows through the radiator driving warm air through HVAC air ducts. Sure enough, HVAC techs call a plumber to work on this hybrid heating system.

 

[sevensages]

@sevensages - I think that textbook description is wrong - mixing up what happens when the heater element is on with what happens when it is off. Stratification happens when the heating element is off.



You only get stratification - a hot layer above a less-hot layer - after the heating element is off for a sufficient period of time that the convection from it ceases.

During heating there is convection driven circulation that keeps mixing the contents, preventing stratification. Any stratification is not driven by the heating element making hotter water that rises - a vigorous process - it comes from much slower, less vigorous cooling from pipe inlets and outlets, that are made of metals like copper, brass and stainless steel and cooling of the (imperfectly insulated) tank itself.

The strength of the circulation from that convection when the heater is off is insufficient to mix the contents, allowing the cooled water flowing down to settle. Rather than cause mixing, the slow flow causes the top of that pool of cooler water to rise slowly, as a layer.

I note that our own hot water system has the outlet (and pressure relief valve) lower than the top of the tank, so there is an air space in the top of the tank. Not sure but I expect dissolved gases being released keep it from filling with water or replaced by water vapor. Any corrosion concerns are dealt with by use of non-corrosive materials and a sacrificial anode.

@russ_watters
@jbriggs444
@Lnewqban
@sophiecentaur

What do you all think about what @Ken Fabian wrote here about stratification?

Do you agree with Ken Fabian that my textbook's assertion that hot water in a water heater rises to the top is wrong?

Before Ken Fabian made this post, I was supremely confident that my textbook is right.

 

[Ken Fabian]

I will add that our own HWS has a temperature gauge; the owners' guide informed us (warned us) that when the heater switches on the gauge will initially show cooling, from the cooler water in the bottom and hotter water at the top becoming mixed. The temperature sensor must be near the top of the tank nearer where the outlet is.

When the heating cycle ends and the convection circulation from that ceases I would expect the contents to be well mixed, with little difference in temperature between top and bottom.

It isn't that the heating doesn't cause hot water to rise - clearly it does - just does so too vigorously to allow stratification; stratification arises from less vigorous colder water inflow and from downflow from cooling of the inlet, outlet fittings where there is no insulation, plus some cooling from the tank body and imperfect insulation.

 

[sophiecentaur]

Do you agree with Ken Fabian that my textbook's assertion that hot water in a water heater rises to the top is wrong?

Before Ken Fabian made this post, I was supremely confident that my textbook is right.

Your text book is clearly wrong where it doesn't specify what 'the amount' of water he discusses actually is. You examine the same mass of water as it heats up and cools. Using volume (gallons) is not good for the description od convection and lighter / heavier is also a bad choice because that gallon of water (in a bucket) will always 'weigh' the same, despite expanding and contracting. All you need to say is that hot water, being less dense than cold water, will float up, being displaced by the more dense cold water.

The topic of unwanted stratification is only relevant in a system that's been designed wrong; you would always have at least part of the heat source set low down in the water. In the UK, the hot water tank has a coil of tube (a heat exchanger) which goes pretty near the nottom of the tank. This establishes a gradient of temperature with no stratification - except right at the bottom where there can be a very small quantity of water which the convection doesn't reach. Of course, there's no mixing of water that flows through the 'boiler' (but it never 'boils') and the water that comes out of the taps.. Often, the water that passes through the boiler splits between radiators and water heating. 'Combi' boilers have a heat exchanger in the boiler so, again, the instant hot water from the tap does not flow through the boiler itself.

 

[sophiecentaur]

I was somewhat shocked when buying a condominium (condo) unit to find it heated with the same hot water as used by a sink or shower.

Are you sure it's done that way? In UK there is a heat exchanger to isolate the boiler water from the washing water. It's common to use additive in the sealed heat circuit and you wouldn't want to wash in that stuff.

 

[Klystron]

Are you sure it's done that way? In UK there is a heat exchanger to isolate the boiler water from the washing water. It's common to use additive in the sealed heat circuit and you wouldn't want to wash in that stuff.

After owning and refurbishing several Aquatherm units, I am certain the same water heater tank provides the hot water source for all uses. You are correct and I was mistaken if I implied that used water returns to the basic house tank to be reheated. A much older system in my first condo in Santa Cruz County, California, diverted used water to a seconday "gray water" holding tank IMS.

My previous house featured a large water heater in the first floor garage piped to a 'maniblock' distribution device (also archaic) for household use and a separate distribution device for appliances. Copper pipes then directed hot water to the house Aquatherm heater located in a maintenance attic above a second-floor walkway above a twenty foot drop. I was apprehensive every time I crawled up there for inspection and maintenace on the air filters and fan. The hot water circulated quite a bit then dumped into municipal sewer lines. Mind you, Southern Nevada temperatures do not require much home heating. Apologies if a bit off-topic.

 

[sevensages]

It isn't that the heating doesn't cause hot water to rise - clearly it does - just does so too vigorously to allow stratification; stratification arises from less vigorous colder water inflow and from downflow from cooling of the inlet, outlet fittings where there is no insulation, plus some cooling from the tank body and imperfect insulation.

I don't understand this.

Why would heating the water be too vigorous to allow stratification? Please explain this using simple language.

 

[sophiecentaur]

Why would heating the water be too vigorous to allow stratification?

You are too worried about this, I think. Stratification is a big word but it doesn't mean it's a 'big idea' in your home. It's what you get when the situation is more or less in equilibrium - like an hour after the heat source is turned off. The 'eco' setting for electrical water heating uses an element 2/3 from the bottom of the tank so the bottom 2/3 won't be heated by convection. I wouldn't have used the word "vigorous"; The Power output of the heater will just affect the speed of circulating water

The heat affects the density. If there is cooler (more dense) water above then that will cause convection. Heating a pan on a stove is the most extreme situation because nearly all the pan base is heated and will cause small areas of convection cells.
Stratification is what you get when convection misses a region of water that's the same temperature or lower than the water above and higher than the water below.

Stratification is a phenomenon that can be seen in nature (In water and in the atmosphere) but the heat supply tends to be more complex and of course, the systems were not 'designed' like a domestic heating system. Stratification is one of the features of a Weather event.

 

[russ_watters]

@russ_watters

What do you all think about what @Ken Fabian wrote here about stratification?

Do you agree with Ken Fabian that my textbook's assertion that hot water in a water heater rises to the top is wrong?

Before Ken Fabian made this post, I was supremely confident that my textbook is right.

I don't agree. I think what your textbook says is fine. There's pretty much always more you can dissect out of how things operate, and the textbook doesn't go into excruciating detail, but everything it says is correct/true. Some specifics:

@sevensages - I think that textbook description is wrong - mixing up what happens when the heater element is on with what happens when it is off.

I think it's pretty clear. The textbook is very clear in talking about when the heating element is on:

"As the water is heated, it expands. Since expansion makes it lighter, it rises to the top of the tank"

Heating element on drives convection.

You only get stratification - a hot layer above a less-hot layer - after the heating element is off for a sufficient period of time that the convection from it ceases.

And it says:

"As heated water cools, it becomes denser and sinks to the bottom of the tank, where it is heated again."

That's less clear about whether the element is on or off, but clear enough for me. And probably because the water is always cooling when it's away from the heating element, whether the heating element is on or off. It's just that the convection is much more strongly driven by the heating element. But I see no reason to quibble over how the textbook explained it.

Note: since the water heater is insulated, what happens when the water is not being used is pretty minor/unimportant. Yes, you'll get convection and the heater will kick-on to maintain temperature, but it turns on rarely and the convection and stratification are minor.

I note that our own hot water system has the outlet (and pressure relief valve) lower than the top of the tank, so there is an air space in the top of the tank.

That would be very surprising if true, unless it's some sort of integrated expansion tank. There's no good reason I can think of to have an exposed airspace but there are bad reasons/reasons not to. Can you share the make and model?

 

[russ_watters]

Your text book is clearly wrong where it doesn't specify what 'the amount' of water he discusses actually is. You examine the same mass of water as it heats up and cools. Using volume (gallons) is not good for the description od convection and lighter / heavier is also a bad choice because that gallon of water (in a bucket) will always 'weigh' the same, despite expanding and contracting.

There's some colloquialisms there, but I see no need to come down so hard on the textbook's verbiage for this. The writer clearly understands since s/he points to density in the next sentence:
"Since expansion makes it lighter [than an equal volume of colder water], it rises to the top of the tank, where it can be drawn off. As heated water cools, it becomes denser and sinks..."

 

[russ_watters]

Why would heating the water be too vigorous to allow stratification? Please explain this using simple language.

Stratification just means hot and cold (light and dense) separate. It's usually describing a static situation or the act of creating that static situation (they separate and stay separated, they don't circulate). Convection acts to disrupt stratification.

 

[Ken Fabian]

I am going to stand by what I've said - that sufficiently strong convection disrupts stratification - but admit an exception for a slow rate of heating that is incapable of making a strong overturning circulation. That would be unusual for most tank type hot water systems.

My thinking is based on observation of heating elements in open containers, pots of water on stoves, a basic understanding of convection... and reason.

A heating element (or coil or sleeve) will make an overturning circulation strong enough to break down any stratification and will mix the contents. This won't be true of a heating element near the top - but in practice almost all heating is done either near the bottom, sometimes midway and bottom, or up the sides or around a central fire tube (gas fired) and in ways that make an overturning circulation.

This is an advantage and desirable rather than a problem; the whole tank gets heated - maximising the amount of hot water it holds - and the result is it gets heated uniformly (or very close to it). Convection (strong enough) disrupts stratification. Stratification comes after the active heating and accompanying overturning circulation ceases and is (therefore) not produced by water rising from the heat source.

Call it surmise if people here insist but my reasoning says that subsequent stratification will be primarily from cold water downflows from imperfect insulation pooling in the bottom and - in a not-static situation - cold water inflows.

Stratification - or at least differentiation; a true thermocline will take time sitting still to develop - will happen in a "static" situation but I still surmise it will be due to cooling - around the fittings and tank body - from cooler water sinking and pooling, with little mixing. That will lack the flow rates necessary to make the full overturning convection loop that results in mixing. There is still convection but the upward flow will be in the form of the top of the pooled cool water rising as a layer. (Sort of - can nitpick about thermoclines and whether it is a layer).

I don't expect water of uniform temperature with perfect insulation to stratify - but am not so sure of that to claim it cannot. But I do expect (by reason) that heat conduction within still water will tend to maintain temperature homogeneity.

That differentiation/stratification in a hot water tank is useful, allowing influx of cold water in the lower part of the tank without mixing with and cooling the hotter water nearer the top and that gives longer times between heating cycles. (Ours is set up to run daytime only, to take advantage of rooftop solar).

@russ_watters re the air space - it is definitely there in our water heater tank - the outlet and relief valve are on the side, a few cm below the top - but thinking about it some more I don't know that it will persist over time rather than be displaced with water. I could be wrong that it does persist - which wouldn't be the first time I've got something wrong.

My thinking was it would moderate water hammer type stresses to the tank. I am not sure any of the materials used are especially subject to corrosion - but it does have a sacrificial anode. Ours is a one piece Chromagen heat pump model.

 

[sophiecentaur]

And (duh!) cold water coming in near the bottom will be the main cold water source and will tend to stay there, without much mixing - it won't make the sustained convection cycling that a heating element does, that breaks down any stratification.

You haven't mentioned the Heat Source. At least a part of it should be very near the bottom . The nearer the better; it's part of the design of a good water heating system. You also need to consider how a hot water tank with a heat exchange coil works best. Iirc if the hot input of the coil is at the top the result will be 'very hot' water at the top level and water at the bottom would not be as hot. If hot water goes into the bottom then you would have stronger convection and a more uniform temperature throughout the water column. It's a matter of choice as to what you want; quicker start up with hot water at the top or plenty of warm water, right down to the bottom. This is more complicated than I imagined.

 

[Ken Fabian]

You haven't mentioned the Heat Source.

Actually, yes I did; from my previous comment -

- in practice almost all heating is done either near the bottom, sometimes midway and bottom, or up the sides or around a central fire tube (gas fired) and in ways that make an overturning circulation.

My understanding of those (electric) heaters with midway AND bottom elements is the midway one is a "priority" heating element and will come on first in order to give the user hot water with less waiting; it doesn't make a full overturning circulation and won't mix in the coldest water at the bottom, just heat the upper portion. After the upper part of the tank is hot enough the bottom element switches on to complete the heating. (Not sure if the upper element remains on or switches off when the bottom heating element comes on.)

 

[sophiecentaur]

Actually, yes I did; from my previous comment -

'Indirect ' heating of water in a tank is very common in some places (UK etc). The boiler heats water (same water that goes round the radiators) and passes through a spiral heat exchanger in a tank. That's not mentioned in your comments but is very relevant (in civilised parts of the world Lol ). US and UK argue all the time about domestic electrical wiring and I can see the same hot discussions about hot water, too.
I learned a lot from PF about that 'impossible' steam heating system in many US homes. It's said to be very suitable in some buildings and it's one of those comedic tropes when the noisy heating contributes to situations. It would scare me to death!

 

[Ken Fabian]

'Indirect ' heating of water in a tank is very common in some places (UK etc). The boiler heats water (same water that goes round the radiators) and passes through a spiral heat exchanger in a tank.

Our heat pump hws has the heat exchange pipe wrapped around the tank (welded to the tank I think?) but could have been coiled inside it near the bottom or helically coiled from higher up down to bottom - or be a 'sleeve' - and I think those will be functionally equivalent to piped steam or very hot water from a boiler. Likewise an electric element too. I don't think those specific details will change anything; an overturning convection is desirable and easy to achieve.

The designers may prefer hot coming down a coiled heat exchanger pipe, working with rather than against the (separate) convection within the heating fluid - and some systems may be built to rely on that passive convection to cycle the fluid - which solar water heaters do. But if the fluid is pumped it can go either way and I think the flow direction won't make much difference to the heat exchange; once that overturning convection kicks in the water temperature tends to homogenise from mixing.

 

[sophiecentaur]

Our heat pump hws has the heat exchange pipe wrapped around the tank (welded to the tank I think?) but could have been coiled inside it near the bottom or helically coiled from higher up down to bottom - or be a 'sleeve' - and I think those will be functionally equivalent to piped steam or very hot water from a boiler. Likewise an electric element too. I don't think those specific details will change anything; an overturning convection is desirable and easy to achieve.

Heat exchange and an electrical element are fundamentally different. For heat to pass from the flowing water in the coil to the water in the tank there has to be a temperature gradient, falling from coil input to output. If there were not, there would have been no heating effect. You can calculate the heat transferred by the drop in temperature of the circulating water and the rate of mass flow. If the pump rate is low then the exit water will be significantly cooler. If the flow rate is very high the ΔT will be low. In a domestic system, the flow rate will not be high so the ΔT will be significant.
An electric heater will have a small temperature gradient because of the ΔT across the pipe wall. .

 

[snorkack]

Heat exchange and an electrical element are fundamentally different. For heat to pass from the flowing water in the coil to the water in the tank there has to be a temperature gradient, falling from coil input to output. If there were not, there would have been no heating effect.
An electric heater will have a small temperature gradient because of the ΔT across the pipe wall. .

Or in another fundamental way, the heat in heat exchanger is carried as heat capacity of water inside the pipe and therefore temperature in an heat exchanger must fall along the direction of the flow. It must have lower temperature in the output than input - otherwise it is a cooler. Whereas in the electric heater, the energy is carried as electric field and converted into heat in the wire. Gradient is required between the wire and outside of the wire.

 

[sophiecentaur]

But if the fluid is pumped it can go either way and I think the flow direction won't make much difference to the heat exchange;

But, as @snorkack says, the direction of flow of the input water will affect the power transferred. It's a common technique - even used in the humble Condenser Tumble drier. The exit (hot) air from the drum is passed over the (cold) inlet air from the room in the opposite direction on the other side of the heat exchanger. Condensation occurs all along the heat exchanger path but mostly where hot wet air is close to the coolest dry room air. Amazingly (at least in ours) the emerging air is 'dry' enough not to raise the humidity in the utility room appreciably; almost all the water ends up in the collecting tank.

 

[Ken Fabian]

@sophiecentaur - I am not convinced it matters to whether it will make an overturning convection within the water tank or how well the water heats; yes there will be a difference in temperature between the heating fluid coil as it enters the water tank and as it leaves but it doesn't have to flow downwards to exchange heat. Typically the temperature of water to be heated will be colder than the coolest part (outlet) of the heating coil.

The reasons for making it a downward flow rather than the other direction will be about the convection and flows within the heating fluid side of the system, not the effectiveness of heat exchange within the water tank.

Our previous hot water system was a rooftop solar one, with hot water tank above the solar collectors. Heating fluid was glycol that flowed through a sleeve around the bottom third of the water tank and the flow of that heating fluid was entirely convection driven and that requires the inlet (hot) to be at the highest point of that sleeve. (Not a big height difference with the outlet - more horizontal flow than top to bottom.)

If it had a ground level hot water tank it could not rely on convection and would use a pump to circulate the heating fluid - and it could be made to flow either direction, but typically will be made to go with the convection flows rather than against them. The heating inside the water tank will be the same.