Someone please explain why hard water makes water heaters less efficient

[nonameo]

I keep hearing the same explanations for why hard water makes water heaters less efficient. The scale builds up on the elements and keeps it from heating the water as well. This is fine and dandy, but the heat has to go somewhere, and the only place it has to go is in the water. I would think that, as the element gets hotter, the resistance gets higher, and this reduces the energy consumption(and heat output) of the element. That would make the heater work less efficiently, but it wouldn't use any more energy to heat the same water to the same temperature. It would just take longer.

So could someone please explain?

kthx!

 

[Hacp]

Ok i meant to say that it might have a higher boiling point or whatever but then I thought to myself and got confused.

 

[kt]

You're kidding right?

 

[nonameo]

Originally posted by: kt
You're kidding right?

I don't think it's such a bad question.

 

[Brainonska511]

Originally posted by: nonameo
I keep hearing the same explanations for why hard water makes water heaters less efficient. The scale builds up on the elements and keeps it from heating the water as well. This is fine and dandy, but the heat has to go somewhere, and the only place it has to go is in the water. I would think that, as the element gets hotter, the resistance gets higher, and this reduces the energy consumption(and heat output) of the element. That would make the heater work less efficiently, but it wouldn't use any more energy to heat the same water to the same temperature. It would just take longer.

So could someone please explain?

kthx!

You also have to heat the crud that builds up on the heating elements. The crud also doesn't transmit heat as well as the element touching the water directly, thus making it take longer to heat the water.

 

[nonameo]

Originally posted by: Brainonska511

Originally posted by: nonameo
I keep hearing the same explanations for why hard water makes water heaters less efficient. The scale builds up on the elements and keeps it from heating the water as well. This is fine and dandy, but the heat has to go somewhere, and the only place it has to go is in the water. I would think that, as the element gets hotter, the resistance gets higher, and this reduces the energy consumption(and heat output) of the element. That would make the heater work less efficiently, but it wouldn't use any more energy to heat the same water to the same temperature. It would just take longer.

So could someone please explain?

kthx!

You also have to heat the crud that builds up on the heating elements. The crud also doesn't transmit heat as well as the element touching the water directly, thus making it take longer to heat the water.

OK, but I hear a lot of people say that the energy bill is LESS with a water softener. I just find this hard to believe, as the heat is either not used or it goes into the water. There's no where else for it to go.

 

[DrPizza]

I wouldn't think that it makes a difference with electric heat. I believe you are correct. However, with gas heat, the heat transfer wouldn't be as efficient, resulting in more heat lost to the chimney.

 

[SlitheryDee]

Slowing down the heat transfer would cause the same amount of heat to take a longer time to make the trip from the element to the water, from the water to the radiator fins, and from the radiator fins to the air. The house itself is constantly losing heat to the surrounding air, so to make much difference it seems like the radiator must outpace this heat loss to some degree. I honestly don't see how it would become less efficient at turning its power source into heat, but it would still negatively impact its ability to do its job.

 

[Bushwicktrini]

Hard water deposits scale ( mostly calcium) on your coils which acts as an insulator there-by causing the heater to draw more power/gas to produce the same amount of heat.

A good water softener will remove the minerals in your water and also help to remove the scale already in your water heater as the "soft" water will dissolve some of the scale which will make your water heater use less power/gas

 

[Eli]

Electricity is measured in Kilowatt hours.

You answered your own question. It just takes longer.

As far as a gas water heater goes, I honestly don't even know how they work. 😱 I will assume that the scale insulates the water from the flame in the same way that it would insulate it from the electric element.

Edit: Think of it this way. You're right, the heating element stays roughly the same temperature, except now there is a temperature gradient between the element and the water due to the scale. The thicker it gets, the greater the temperature gradient, and the less energy is being transferred into the water. Thus, it takes longer to heat up and uses more energy in the process.

Hmm.. I think Dr. Pizza may be right though. It seems like it would affect gas water heaters much more. I mean, until the scale is like a half an inch thick.. it doesn't seem like the difference in temperature would be that great.

 

[nonameo]

Originally posted by: Eli
Electricity is measured in Kilowatt hours.

You answered your own question. It just takes longer.

As far as a gas water heater goes, I honestly don't even know how they work. 😱 I will assume that the scale insulates the water from the flame in the same way that it would insulate it from the electric element.

Edit: Think of it this way. You're right, the heating element stays roughly the same temperature, except now there is a temperature gradient between the element and the water due to the scale. The thicker it gets, the greater the temperature gradient, and the less energy is being transferred into the water. Thus, it takes longer to heat up and uses more energy in the process.

Hmm.. I think Dr. Pizza may be right though. It seems like it would affect gas water heaters much more. I mean, until the scale is like a half an inch thick.. it doesn't seem like the difference in temperature would be that great.

But you see, even if it does take longer, that means that there is more heat trapped in the element itself. The hotter element is going to have a higher resistance, which would then restrict the amount of energy flowing through the element.

edit: the issue I have is that I see numbers like 20% less energy used with softened water vs. hard water. 11% more energy use for each 1/16 of an inch of scale built up on the element. This just seems way too far out there for a little scale buildup. It just can't be right.

 

[Billb2]

Originally posted by: nonameo

But you see, even if it does take longer, that means that there is more heat trapped in the element itself. The hotter element is going to have a higher resistance, which would then restrict the amount of energy flowing through the element.

The element gets to the same temperature whether it's coated with crud or clean.

 

[Bushwicktrini]

Originally posted by: Billb2

Originally posted by: nonameo

But you see, even if it does take longer, that means that there is more heat trapped in the element itself. The hotter element is going to have a higher resistance, which would then restrict the amount of energy flowing through the element.

The element gets to the same temperature whether it's coated with crud or clean.

Just takes more power to do so

 

[DrPizza]

If it made a difference with electric, it begs the question, "well then, where does the wasted energy go?" Conservation of energy FTMFW.

 

[spidey07]

Originally posted by: DrPizza
If it made a difference with electric, it begs the question, "well then, where does the wasted energy go?" Conservation of energy FTMFW.

Heat loss, they are not perfectly insulated.

 

[nonameo]

Originally posted by: spidey07

Originally posted by: DrPizza
If it made a difference with electric, it begs the question, "well then, where does the wasted energy go?" Conservation of energy FTMFW.

Heat loss, they are not perfectly insulated.

No, but I would imagine that the lime scale would be much more conductive than the air around the back of the element. That's the only other place it can go.

 

[spidey07]

Originally posted by: nonameo

Originally posted by: spidey07

Originally posted by: DrPizza
If it made a difference with electric, it begs the question, "well then, where does the wasted energy go?" Conservation of energy FTMFW.

Heat loss, they are not perfectly insulated.

No, but I would imagine that the lime scale would be much more conductive than the air around the back of the element. That's the only other place it can go.

Think of it this way - the water is in an insulated container. It constantly loses heat (energy). You can heat the water at a rate of 1 degree every 10 minutes with a clean element or 1 degree every 30 minutes with a covered one given the same energy per minute. The heat loss of the water is still the same no matter what. And that is just to MAINTAIN the temperature. Think about when you use hot water and new cold water comes in and it's even worse.

Make sense?

It's not just a function of total energy in you have to factor in the energy out related to time.

 

[nonameo]

Originally posted by: spidey07

Originally posted by: nonameo

Originally posted by: spidey07

Originally posted by: DrPizza
If it made a difference with electric, it begs the question, "well then, where does the wasted energy go?" Conservation of energy FTMFW.

Heat loss, they are not perfectly insulated.

No, but I would imagine that the lime scale would be much more conductive than the air around the back of the element. That's the only other place it can go.

Think of it this way - the water is in an insulated container. It constantly loses heat (energy). You can heat the water at a rate of 1 degree every 10 minutes with a clean element or 1 degree every 30 minutes with a covered one given the same energy per minute. The heat loss of the water is still the same no matter what. And that is just to MAINTAIN the temperature. Think about when you use hot water and new cold water comes in and it's even worse.

Make sense?

It's not just a function of total energy in you have to factor in the energy out related to time.

yes, but keep in mind that usually hot water just sits there waiting to be used. If I take 5 minutes to heat the water to 130, or 20 minutes... it shouldn't matter either way if I wait 30 minutes to actually use said hot water. It still sits in the same tank losing heat for the same amount of time.

 

[spidey07]

Originally posted by: nonameo
yes, but keep in mind that usually hot water just sits there waiting to be used. If I take 5 minutes to heat the water to 130, or 20 minutes... it shouldn't matter either way if I wait 30 minutes to actually use said hot water. It still sits in the same tank losing heat for the same amount of time.

You're missing the point of efficiency - amount of energy transferred over time (not to mention the constant loss I described). Imagine putting an insulator over the heating coils; that's not very efficient.

 

[AlienCraft]

Originally posted by: nonameo
I keep hearing the same explanations for why hard water makes water heaters less efficient. The scale builds up on the elements and keeps it from heating the water as well. This is fine and dandy, but the heat has to go somewhere, and the only place it has to go is in the water. I would think that, as the element gets hotter, the resistance gets higher, and this reduces the energy consumption(and heat output) of the element. That would make the heater work less efficiently, but it wouldn't use any more energy to heat the same water to the same temperature. It would just take longer.

So could someone please explain?

kthx!

You have the right answer, then you go and overthink it. You're co-mingling ampacity with amperage.
You are correct in saying the amount used in the sense that the element never exceeds it's rated ampacity, however since The Crud insulates the element and as such takes longer to transmit the heat that does make it to the surface area.

Thus it takes more time for the rated amperage to produce the expected temperature change over time aka deliver the BTU's.

This can be demonstrated with a new element, an old element, a thermometer and a timer.

That same 'scale- as- insulating- factor' is present on soldering iron tips also.

 

[Jeff7]

Drat, where's my Heat Transfer book?
Ah screw it, it's all the way out in the kitchen, maybe 20 feet away, and I don't feel like getting up just now. So here it is from memory. (Are any of my professors listening? 😀)

A thought exercise which may help:
1) Cut a hole in a box.
2) Put your junk (namely, a 40W light bulb) in the box.
3) Don't open the box.
....
Oh, and the box is perfectly insulated - NO heat can escape.
Turn on the light.

Eventually, you'll be able to melt the glass surrounding the bulb due to heat buildup inside that insulated container.

And "heat" is a bit of a funny term; in my Heat Transfer course, it took on a slightly different meaning than what is normally heard - there, "heat" refered to energy being transferred between two bodies. An object cannot have "heat," as it isn't transferring it to anything. But it does have energy.


So when those elements are turned on, energy flows into them. Resistance in the element converts the electrical energy into thermal energy, measurable as a temperature increase in the element.

Now that energy will want to go somewhere. If the element is in direct contact with water, that energy can be easily conducted away to the molecules of the water, which in turn lowers the temperature of the element.

But if the element is insulated by scale, the energy has to attempt to force its way through it. Less energy is able to do so, due to the low thermal conductivity of the scale. Result: The water doesn't heat as quickly, and the elements get hotter. And that is where the "extra" energy is going - it's making the elements hotter than they should normally be.

Originally posted by: spidey07
You're missing the point of efficiency - amount of energy transferred over time (not to mention the constant loss I described). Imagine putting an insulator over the heating coils; that's not very efficient.

Efficiency is a simple ratio to compare input and output. Efficiency in the case of the water heater would not be concerned with time, but with how much energy it takes to heat a given quantity of water.
Water's specific heat capacity is 4187 J/kg-K.
So ideally, if you put 4187 joules into a kilogram of water, you'll increase it by 1°C.

The question with the water heater's efficiency is, if I put 4187 joules into the elements, how much of that will go into the water? The second question concerns the water heater's insulation - how much of that energy in the water will stay there, and for how long?


If you want to know how quickly it will heat the water, that's not so much a question of efficiency, but of effectiveness. 😛