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Water Heater Timers

Posted by gwfl (My Page) on
Thu, Mar 10, 11 at 20:00

Does anyone have experience with the use of a timer on their hotwater heater and wether it saves any money? My wife and I are on the same schedule (no one else at home)and we could shut the heater off for 9 hours or more overnight. Would this really save money or does any savings get used up having to reheat the cooler water?

Gary


Follow-Up Postings:

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RE: Water Heater Timers

I had a timer on a recirculation pump for a gas tank heater, but not on the tank itself.

With an electric water heater, you can put a timer on it. They take a little longer to heat the water in the morning, but not too much since it'll have carryover heat from the night before. Long-term, I'm sure this would save $.
Not sure this is even really an option with gas as the valves don't appear to be designed for the frequent open/closing which a timer would likely utilize. You can set a gas tank heater temperature down, or even to pilot, but again, these controls don't seem designed for that frequent use.

Tankless only heats when there is water demand, but has price and other "fun" issues. :-) Not sure it'll ever pay for itself.


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RE: Water Heater Timers

It will save electricity. You can save even more if you turn it off while you are gone during the day. I don't know how long it will take to pay back the price of having an electrician install one. Have you installed a water heater blanket and good shower heads? Those are good DIY investments if you can't do the electricity yourself.


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RE: Water Heater Timers

You will save, but how much?

Try this: with heater on a fully up to temperature until it clicks off, measure the temperature at the closest hot water tap after it runs long enough to get hot. Let it heat up fully again, shut it off, wait 10 hours, and measure the temperature at the tap again. I'm going to guess about 10 degrees lower.

The energy lost, in kilowatt-hours, is

E = dropT(f) * Gallons * 8 / 3412

where dropT(f) is the difference in water temperature in degrees F, and Gallons is the capacity of the water heater. "8" is the number of pounds per gallon for water. Dividing by 3412 converts from BTU to kwhr.

Multiply by (30 days * 24 hrs /10 hrs) to find kwhr per month, and see how that would affect your bill.

If the drop in temperature is a lot (say 30 degrees), you should repeat the test with less time -- problem being that the tank loses heat slower as it cools, which makes the error in the simple calculation get worse. I can provide an exponential equation that is accurate if you are interested, but you'd need to know the ambient temperature.

Anyway, that gives you the rate of heat loss from the tank when it is close to hot. That is approximately the amount you could save by turning off the heat totally while gone for months vs. just leaving in on, and represents an upper limit on how much you could save.

In your case, you are going to have to heat that water again after only a few hours, and the tank will stay pretty hot, so you will not save as much. Come back with the test results, including the ambient temperature around the tank, and we can estimate that pretty well.


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My guess

And while I'm picking lottery numbers, I'm going to say that your total loss per month is about 30kwhr, but that you will only save about 6kwhr, since the water doesn't cool down that much overnight.


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RE: Water Heater Timers

I tested the temp. drop and found that at peak the temp. was 124 F and after 12 hours of being shut off had dropped to 116 F, an overall drop of only 8 F. This is a new hotwater heater and the garage temp. was approx. 66 F overnight.

Gary


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RE: Water Heater Timers

Your average temperature in the tank over the 12 hours was 119.8 degrees. Had it stayed on all the time, it would have been 124 degrees (maybe less, since it clicks on and off as needed). The rate of temperature loss changes linearly with TEMP(tank) - TEMP (ambient). So the heat loss is proportional to (119.8 - 66) when shutting it off, vs. (124 - 66) if on all the time.

Total heat loss (I'm assuming 50 gallon tank) with the power off was

(124 - 116) * 50 * 8 / 3412 = 0.938 kwhr

It would have been 0.938 * (124-66)/(119.8-66) = 1.011 kwhr

the energy saved is the difference, or 1.011 - 0.938 = 0.073 kwhr. In a year (x 365), that would be about 27 kwhr savings.

You could save the same amount be setting your water heater at 119.8 degrees instead of 124, in the case measured, or by improving the tank insulation by 8%.

I was assuming electric. The same energy savings apply to gas, but the cost of gas energy is usually less than cost of electricity.


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RE: Water Heater Timers

Thank you. Sorry I forgot to explain that, yes, it is an electric unit and is also a 30 gal tank. So at a cost of $0.0395 per kwh it does not seem worth installing a timer costing $40.00 to save just a few dollars a year.

The tank is new and the factory temp. setting is supposed to be 120F. I'll turn it down to 120F.


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RE: Water Heater Timers

Adding more insulation to further reduce the drop would probably have a better rate of return.


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