Take a look at this label on this 40 gallon electric water heater.
It lists three wattages:
Upper is 4500W.
Lower is also 4500W.
And of course that totals…
4500 wa - whatt?
Who wrote this?
Actually, that’s not a mistake.
You’ll find similar labels on most conventional electric water heaters.
Oh and, by the way, that second listed wattage is what it would run at if hooked up to a 208V electrical supply rather than 240,
which you’ll usually find in office and large apartment buildings.
Using Ohm’s law we can determine that this thing’s heating element has a resistance of 12.8 ohms,
which at 240V would draw 18.75 amps and thus 4500 watts
but at 208v it would only draw 16.25 amps which works out to 3380W.
Math is fun!
Anyway, how can those two numbers add up to themselves?
This label would suggest 2+2 is 2.
And I’m pretty confident that’s wrong.
Well, you might have guessed by the labels “upper” and “lower” that there are two separate heating elements in the water heater -
indeed there are, and they’re both the same power rating.
The reason for this silly math has to do with the thermostats.
Yeah, there’s two of those, too.
But they’re cleverly interlocked in a way that not only prevents them from running at the same time which would overload the electrical supply
but also makes the operation of the water heater smarter and more effective.
A water heater like this is an extremely simple device.
It’s really just an exceptionally well-insulated cylindrical tank of water with a few holes in it.
Up top there are holes to let water in and out.
Down below is another hole with a spigot to let you flush out the tank periodically,
and then there are two holes in the sides of the tank under these covers.
The heating elements stick into the tank and thus into the water through those holes and when power is run through them
they get real hot.
This is really just an overgrown kettle.
Now, there are actually two more holes in the tank, one of which leads to the temperature/pressure safety valve up top.
Heating water makes it expand slightly which increases the pressure inside the tank.
Ordinarily this isn’t a problem.
The tank may seem like a sealed pressure vessel but it’s still connected to the rest of your plumbing.
As the water inside expands, some water will simply move out of the tank by pushing the water in the supply line backwards.
However, if you have an entirely closed plumbing system with backflow prevention,
then that won’t be possible so you probably have an expansion tank installed with your water heater.
At least I hope so.
But anyway if for whatever reason pressure starts building up too high in the tank and/or the water inside is getting too hot
then this valve will pop open, releasing that pressure and spraying hot water onto the floor,
and you will have a very bad day but at least the water heater didn’t explode.
That’s nice.
And the final hole in the tank is one many people overlook.
Because the tank is made of steel which, I don’t know if you’ve heard but doesn’t really like to be in prolonged contact with water,
a sacrificial anode rod often made of aluminum or magnesium pokes down into the water to take the corrosion bullet for the rest of the tank
and keep it from rusting.
The top of that rod lives buried underneath the foam insulation that's underneath this plug,
and it can actually be replaced!
If you do that regularly, it can extend the life of your water heater significantly.
But almost nobody does that, including me.
Yay laziness!
But this isn’t a home improvement channel…
So what's so special about this water heater that’s making me make a video about it?
Well, earlier I said the thermostats make it smarter and more effective.
Allow me to explain.
Notice that even though this is a 40 gallon water heater,
its rated capacity is somehow 53 gallons.
Where did it magic 13 extra gallons from?
Well, I suppose we can let water out of the tank while also heating incoming water, can’t we?
Yes, but there’s more to it than just that.
First, how exactly does the water flow into and out of this tank?
Both pipes are at the top,
but the incoming water supply pipe attaches to a dip tube that travels all the way down to the bottom of the tank -
meaning cold water enters the tank from below and pushes hot water out the top.
We can actually observe this with the thermal camera.
Right now the tank has a fairly uniform temperature, just a few degrees above ambient.
But when I open a tap and start using its hot water,
we see that it’s only getting colder at the bottom of the tank.
And here’s something pretty fascinating about water: so long as you reduce turbulence and keep things relatively still,
the hot and cold water inside the tank doesn’t mix.
Like, at all.
You can see that as I continue using hot water, the region that’s cold is getting bigger,
but the hot parts up top are still just as hot as they were when I started.
This happens because hot water is less dense than cold water,
so in effect it “floats” to the top of the tank, and as a result a surprisingly sharp boundary between hot and cold is maintained.
Now you might think that over time that boundary layer will disappear and the water will mix together but
for the most part it doesn’t.
Any water that cools down inside the tank (like for instance the water clinging to the slightly cooler tank walls)
becomes more dense and simply sinks to the bottom, piling in with the rest of the cold water.
After a very long time without adding heat energy to the tank the remaining hot water does start to cool down,
but it takes a lot longer than you’d think.
Like, the better part of a day.
It’s pretty wild.
This thermal stratification combined with how we fill the tank from the bottom-up
turns out to be really useful.
Remember that we have two heating elements sticking into the tank at two different heights.
As you continue using hot water, eventually the cold/hot water boundary reaches the bottom heating element and its thermostat,
kicking that element on so it can start working to heat the water as it flows into the tank.
[sound of water trickling in pipes]
[hissing noise from the element starts]
Now, 4500 watts may seem like a lot of power, but in the context of water heating it really isn’t.
It takes gobs of energy to heat water.
At most the water heater can put 4500 joules into the water per second, or 1075.5 gram calories per second.
On a cold winter day the water entering the tank might be 10 degrees Celsius if we’re lucky,
and our target temperature is generally around 50 degrees Celsius.
So we need to attain a 40 degree temperature rise.
If we divide the gram calories we have every second by the 40 degrees we need to increase in temperature,
we find that we can only heat 26.9 milliliters of water per second, or about 1.6 liters per minute.
That’s not too bad, really, but the lowest flow shower heads you’ll generally find here are 1.5 gallons per minute,
and they may go as high as 2.5.
Sticking with the smaller number, that’s nearly 5.7 liters per minute of flow.
Of course most people temper hot water with a bit of cold water
so we might only have 4 liters leaving the hot water tank every minute
but that’s still more than double what we can heat with 4500 watts.
So we’re going to run out of hot water eventually.
There’s just no getting around that.
Even if we try and heat the water as it enters the tank,
the water is simply moving out too quickly and it won’t reach the target temperature by the time we’ve used up the hot water already in it.
But, remember there’s a second heating element above the one at the bottom.
It isn’t any more powerful
but with a bit of strategy we can use it to chase the water as it leaves the tank.
And that’s exactly what the water heater does.
With our theoretical 4 liters per minute flow rate, the lower heating element will only achieve about a 16 degree temperature rise.
That's nowhere enough, but also that’s not nothing.
So long as the lower element is turned on,
in effect the bottom of the tank is filling up with 26 degree water
rather than 10 degree water.
And once that tepid water reaches the top element,
well now its thermostat kicks in and switches the power output to itself.
We have the same 4500 watts that we did before, but now it’s up here -
working to heat the water we’ve already started heating.
[hissy water heater sounds]
[CLACK, hissing stops]
[a new and louder hissing begins]
We can still only achieve a 16 degree temperature rise with 4 liters per minute of flow,
but that’ll get us up to 42 degrees.
Not exactly where we want to be, but still plenty hot.
We will of course still run out of hot water - once the top element kicks on the bottom switches off,
so the water entering the tank is now truly cold and once it’s up near the top...
well, game over.
At best we’re getting 26 degree water out of the tank, now.
But this strategy bought us some extra time with a limited power source simply by changing where we applied that power, and heating the same water twice.
This is where we magicked 13 extra gallons from.
4500 watts is 4500 joules per second, and that's 16.2 million joules per hour or 3.87 million gram calories.
And lift 96.8 liters of water by 40 degrees celsius.
So over an hour, this water heater can bring 25.5 gallons of cold water to our target temperature,
and that is where this “1st hour rating” comes from.
I reckon it’s only listed as 13 gallons over the true capacity
since it was calculated using a 208V supply and assuming we start with freezing cold water.
Quite conservative.
In any case, this means that over an hour the energy it can add to cold water plus the energy it has stored in 40 gallons of already hot water
is the same as if you had 53 gallons in the first place.
And on 240V like I have here it may be closer to, like 65 gallons.
The strategy employed here is frankly genius, especially considering how simple this device is.
All we have are two bog-standard thermostats.
There are no electronics here, there’s no control scheme, it’s just a thermostat for each heating element.
The only complication to the circuit design is that the top thermostat always takes priority.
If that heating element is switched on, the thermostat breaks the circuit to the lower element to prevent it from running.
Now, you might think this would cause some sort of operational conflict.
What if we want the bottom thermostat to run but the top one happens to have kicked on?
Well, think about this for a sec:
because hot water stays at the top of the tank naturally, and we fill it with cold water comes in at the bottom,
for the most part that just doesn't happen.
In fact, top element rarely gets used at all.
It basically only comes on after you have used up more than half of the water in the tank.
If you don’t use at least that much, the bottom element will take care of things.
Same goes for keeping the tank warm between uses.
And with a low-flow shower head, 15 minutes in the shower is unlikely to use much more than 20 gallons of water.
So, yeah, the top one just doesn't need to run.
But if you do happen to use up all the hot water, or even just come close to that,
well now the strategy of giving the top element priority helps us again.
While we can of course use the bottom element alone to heat the water,
after all the hot water it generates down there will float up to the top,
doing that means we have to heat the entire volume in one go.
Whatever hot water it generates at the bottom has to travel through all the cold water on its way up to the top which will of course cool it down.
Oh, by the way, I should mention that whenever the elements are actively heating,
there is some mixing and churning of the water going on inside the tank thanks to the convection currents that generates.
You can see that happening here with this kettle.
That’s how we can heat the entire tank of water using what amounts to two hot sticks.
Anyway, if we want to heat 40 gallons of cold water by 40 degrees celsius,
it’s gonna take over 6 million calories or about 7039 watt-hours.
With 4500 watts to play with, that’s gonna take about an hour and a half.
However, if we only send power to the top element, then in effect we only have to heat half of the water.
We’ll still get some convection currents around the element, but we won’t be mixing all of the water together -
the bottom of the tank will stay cold.
So we’ll get the water in the top of the tank (which remember is what leaves first) up to our desired temperature in half the time.
So even after completely running out of hot water, you’ll get truly hot water again after only 45 minutes or so.
Only once that’s warm and “ready” will we send power back down to the bottom element to heat up the rest.
I know that I’m weird, but I think this is amazing!
I only recently learned that there are two thermostats on the water heater.
In hindsight it probably should have been a little more obvious since, y’know, there’s two identical covers on the front but
I just assumed that was for cost cutting or something and thought there was one thermostat controlling both elements.
Well, OK I guess there still is, technically, but… y’know what I mean.
The simple physics of how hot water behaves in a tank allowed us to implement a very strategic method of heating it
using nothing but a dip tube and two interlocked thermostats.
Just doing that covers pretty much all possible concerns:
how best to heat a cold tank for the fastest-possible hot water, how to chase it on its way out for some extra reserve capacity,
and how to do both those things with a more reasonable amount of input power.
And that’s what I call … neat!
You might have noticed that the water heater is hooked up to some sort of mystery box.
[CLACK] What exactly is inside that box is not important for you to know
but it allows me to disable the water heater at-will.
Or I guess... selectively enable it.
The water heater only has power if the red light is on.
See, I’m in the middle of some experiments.
Water heaters, as I said, are exceptionally well insulated.
In fact you would have no idea there’s 40 gallons of piping hot water in there by touching the tank as it feels stone cold.
The only heat you can really feel is what leaks out of the supply pipes - I really oughta get some sleeves for those.
Anyway, because this thing is so well-insulated that means it’s effectively a battery.
Remember that math I did earlier?
It takes over 7 kilowatt-hours to heat up 40 gallons of water by 40 degrees C.
And that’s… a lot!
Electric water heaters are among the most power-hungry things in your life,
and the cost to operate them is a big reason many people chose to use gas instead for water heating.
I mean, just look at this! Yikes! And those are old numbers!
Oh and by the way, the reason the cost range is so narrow is because
energy is energy and water is water.
Now, I have a time-of-use rate plan with my utility and power in the middle of the night is quite cheap.
So for the past couple of months, I have only been letting this thing operate between 1 and 5 AM.
And believe it or not, I pretty much can’t tell.
Now, I can feel the comments already -
aren’t you supposed to keep your water heater hot at all times? Aren’t you the least bit worried about diseases like Legionaries?
Well, sort of and as a precaution I did set the temperature a fair bit higher before I embarked on this journey...
but on the other hand for years I have been shutting off the water heater whenever I leave for more than a day
and I’m not dead yet!
But in fairness, that could be a risk and more research is definitely needed there,
so I am not endorsing this practice. Do not do this at home.
I can however tell you that when that thing is “charged up”
it will stay hot for way way wayyy longer than I thought was remotely possible.
I've taken a shower at 9:00 at night - meaning the water heater had not operated for sixteen hours! - and the water was still scaldingly hot.
And remember the whole “hot and cold water won’t mix” thing?
Yeah, you’d think that after a while it would but no!
It just doesn’t!
I’ve taken a shower in the morning, probably using up a good half of the tank’s volume,
and in the evening - over ten hours later - the water from the kitchen tap was still too hot to touch.
More quantitatively,
I measured the temperature after running approximately 10 gallons of water through the kitchen tap at 132.4 Fahrenheit.
Five hours later - five hours of the water heater being entirely off -
the water measured 124.3 degrees.
Just swapped the digits around.
Somehow, even with at least 10 gallons of cold water sitting in the bottom of the tank for five hours,
we lost only 8 degrees of temperature.
It’s wild but the cold and hot water really just don’t mix.
Now obviously this strategy won't work for families. Good luck spreading 40 gallons a day around.
But I think it’s worth pointing out that this thing hardly loses any energy when it’s not getting used.
And that makes it a really effective energy storage device - and a large one at that!
This 40 gallon tank stores 70% of the energy that the original Chevy Volt’s battery pack could!'I hope you can see that there’s a lot of potential for energy management applications, here. You shut this thing off for an
I hope you can see that there’s a lot of potential for energy management applications, here.
If you need to shut this off for an hour or even two, you you won’t even notice.
And, what if you put a thermostatic mixing valve on its output?
Then, you could charge up a more sophisticated water heater well beyond your target temperature when energy is cheap and abundant,
not only storing that energy for later, but also extending the tank's capacity.
As a matter of fact that idea is already in use with some heat pump water heaters today.
I’ll be keeping track of energy costs here and will report on how this experiment continues to go.
I’ll probably do that on my second channel so you might want to subscribe if you haven’t already.
But I can already say with complete confidence that a conventional, tanked water heater can be an important part of an energy-management strategy.
People keep asking for my thoughts on tankless water heaters
and frankly this experiment is reinforcing my previously-held belief that they’re not the be-all-end-all.
They need tremendous amounts of power and can’t store energy for later.
Plus they have unique maintenance considerations.
I really think they only make sense when you have severe space constraints,
and personally... I gotta say that if you feel you need the “endless” hot water they can provide,
it might be worth considering adjusting your routines.
But that’s just, like, my opinion.
Anyway, that whole section was basically a teaser for my next video which will be on home electrification.
That’s coming soon. At least I hope.
For now, I hope what I’ve said here will get some gears turning.
Many of the obstacles we think are insurmountable are in fact a piece of cake with just a bit of strategy and a dash of management.
Thanks for watching.
♫ scaldingly smooth jazz ♫
You’ll find similar labels on most conventional electric water heaters.
Oh and, by the way, that second listed wattage is what would blet deh buh dih be deh
And by the way, that second listed wattage is what it would run at -
what the heck was that noise?
I heard a weird noise.
Right now, the tank has a prettyform out… prettyform?
It's pretty uniform.
Same goes for keeping the tank warm between uses.
And a low-flow shower head fit… oh.
I skipped a word!
...combined with how we fill the tank from the bottom up turns out to be really useful.
Remember that we have two heating elemen…
[belches] I GOTTA BURP
Explaining how your water heater works can be a tankless job.
OK, that's not mine, plenty of plumbing companies the world over use that one.
Besides, like I said I don't exactly have an affinity for tankless water heaters.
Especially since electric ones need, like 10 kW just to provide one shower.
Then you might as well get one of those electric shower thingies.
Anyway, toodles.