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Water Heater Q&A

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Ask Mr. Electricity about saving on water heating costs

This page is just a Q&A.

You'll probably find my general info about water heaters to be more useful.

 

My theory is that you can save energy by turning the water temerature up higher. This way you have to mix it with cold water to cool it down and you wind up using less hot water. When the tank fills back up it has less water to heat. -- Matt, May 2006

Interesting idea, but I'm afraid this won't save energy. Let's say you want 16 gallons at 110°, and your water heater is actually set to 110°. Obviously you'd use 16 gallons of hot water. Now let's say you crank the heater up to 140°. Now to get your 16 gallons of 110°, you use 8 gallons of cold water (80°) with 8 gallons of hot (140°). The question then becomes, which uses more energy, heating 16 gallons by 30 degrees (80° to 110°), or 8 gallons by 60 degrees (80° to 140°)? The answer is that there's no practical difference. You'll notice the product in each case is 480 (16x30, or 8x60), which gives you a clue that similar amounts of energy are required. However, setting the heater to a higher temperature will definitely use a bit more energy, because it will lose heat faster than a tank set to a lower temperature. The greater the difference between the tank temperature and the ambient temperature, the faster the heat from the tank is lost. I double-checked this answer with a physicist to make sure I had this right. In my research I also ran across this page that has a formula for the energy required to heat water, but it's complicated and not for the layperson.

Our electric bill has been unusually high, showing 1500 kWh for one month. We don't have a washing machine or clothes dryer, we go to a laundromat. I had the electric company check it out but they said everything is fine on their end and the problem must be in my apartment. I'm wondering if the problem might have something to do with the 240V electric hot water heater my landlord installed in December 2003, since I think it might have been installed on 120V, and recently it hasn't been keeping the water hot throughout a full shower. -- Moogo, San Diego, CA, Jan. 2005

You left out more than you actually told me. When did your electric bill start going up? Immediately after the new water heater was installed or only recently? How many kWh hours did you typically use before your bill started being unusually high? And since you're writing in the coldest month of the year, what kind of energy are you using for heat? And finally, you didn't tell me whether the new electric water heater replaced an existing electric heater or whether it replaced a gas unit.

Anyway, if your water heater is no longer keeping the water hot then perhaps it's broken (maybe as a result of its being wired in incorrectly). Maybe its thermometer tells it that the water isn't hot enough and it runs continuously without actually heating up the water. A water heater running constantly could certainly up your electric bill to 1500 kWh/mo.

The first thing I would do is to go through my article on finding out Why is my electric bill so high? If you isolate the heater as the cause, then you should have an electrician check out your water heater.

I recently installed a water heater timer (the "little gray box") sold by Home Depot. Timer is set to come on at 5:30 AM go off by 8:00 AM. Come on again at 5:00 PM and go off by 8:00 PM. Based on this, how can you calculate how much we are saving? Also, today I took a shower at 12:00 noon and was able to get hot water despite the fact that it had turned off some hours before I assume that it's water in the tank that was still hot from the morning? -- Ewald Probst, EnergySavings@---.com, 6-03

Good question. Can I calculate how much you're saving? No. To do that I'd need a lot more data: the wattage of your heater, how long it takes to heat X gallons of water from Y to Z degrees, how many gallons you use in the morning, how many gallons you use in the evening, and the standby loss rate of your heater.

But I can offer an estimate: 14-25kWh/mo., for about 40-80 gallons/day. The more water you use, the less your energy savings, because if you drain the tank then the heater has to heat a whole new tank of water anyway. These savings also assume a pre-1998 heater. Modern water heaters are already so efficient that heater timers can't save that much more.

The fact that your tank water was still hot four hours after it turned off suggests that your water heater is pretty good at retaining heat. If so, your savings won't be that great, because even without the timer, the heater wouldn't be constantly heating the water much throughout the day if it's already pretty hot. Another explanation is that your thermostat is set too high, so the water in your tank went from way-hotter-than-necessary down to just sufficiently-hot during those four hours.

By the way, nice email address!

Eemax's website says their EX190T electric tankless unit uses 19kWh. I'm paying $0.16/kWh, so that would be $3.00 for one hour's hot water use, or about $90 dollars a month.  So it seems these devices would be much more expensive for hot water. What am I missing? -- M. Rodber, 10-02

Good question. Remember from our explanation of how much electricity stuff uses that the spec listed is maximum that the device ever might use, and it only uses that maximum when it's working as hard as it can. In this case that would mean multiple faucets on and a very low incoming water temperature. If only one faucet's on, or the incoming water temp is even a little warm, then the heater won't have to work so hard, so it'll use far less than 19kW.

For the model you mentioned, notice that the temperature rise at 1.5 gallons per minute (typical for a shower) is 87 degrees. That doesn't mean heating the water up to 87 degrees, it means heating it to 87 degrees above whatever the original temperature was. If the incoming water was 73 degrees, then 19kWh could heat it to 160 degrees, enough to burn you very badly. But almost all systems like this have a thermostat to limit the maximum water temperature, and so the heater isn't usually working at its full capacity and so it doesn't use as much electricity.

So now that you know that the heater usually uses much less than 19kWh, you're probably wondering how you can do the calculations to compare the cost of a tankless system vs. your existing tank. The short answer is that you can't do so, at least not easily. You pretty much have to trust that the tankless system is more efficient because it's only heating the water when you need it, rather than heating it all the time.

Which is more cost-effective, a gas water-heater (tank) or an electric heat-on-demand unit? I've noticed that our water heater does a good job conserving the heat since it doesn't seem to "light up" very often and if I completely turn off the tank (no pilot, no gas, no nothing) the water is still usefully warm for about 36 hours. So I get the impression it really isn't THAT inefficient and gas is cheaper than electricity. I also live in Monterrey Mexico so it might be that the relatively high temperatures help the tank from not losing its heat as fast as in, say, Alaska. But I'm tempted to buy a heat-on-demand unit. The downside is that it'd be electrical so that while it's only on when we use hot water, it might end up costing more since it's burning electricity instead of gas... -- Craig Steiner, Monterrey, Mexico, 6-01

Wow, you're really going all out. You have warm water for 36 hours after you turn your heater off and you're still trying to eek out even more efficiency. That's commendable, and dare I say, compulsive -- much like myself.

First off, going tankless doesn't mean you have to go electric. Tankless heaters come in both gas and electric models, same as regular tank heaters.

But your question is still interesting -- gas is cheaper than electric, but tankless is cheaper than tank, so which is cheaper: (cheap) gas (expensive) tank, vs (expensive) electric (cheap) tankless? It turns out they're kind of similar, but in this case the gas tank wins (it's a little cheaper). See the water heaters page for the details.

Would I save on my electric bill if I turn off the circuit breaker for my electric water heater when I leave for work in the morning after showering, and then turn it on again when I go to bed so I have hot water in the morning? How long would it take to heat up again? Would this harm the water heater? It's a miniature type (43 gallons) and uses an average of 4881 kWh/year (that's what it says on the label). -- Frank Fragosa, Ventura CA, 12-00

It brings a tear to my eye that my readers are willing to do unusual things to save a lot of energy. Yes, you'll save energy, and it won't harm your water heater. But the easier way to do this is to buy a water heater timer from a home improvement store. It'll save you more money because it'll keep the water off all night until right before you get up, and you won't have to hassle with turning the circuit breaker on and off all the time.

How do I use a water heater timer if there's no plug for the water heater? The power cord goes straight into the wall. -- Frank Fragosa, Ventura CA, 12-00

You can have an electrician install an outlet for about $50. With the money you'll save, it should pay for itself. Or you can install the outlet yourself. I've added instructions for how to install an outlet if you're interested in doing it yourself.

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Last update: July 2006

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