Junestan: Water Heater Overheating


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Old 08-06-12, 12:36 PM
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Junestan: Water Heater Overheating

This post was retitled and moved to Water Heaters when it was determined it was not an electric supply voltage problem.

My water heater thermostat doesn't work properly. The water heats up fine but the temp gets down to barely warm before it cycles again - even when I'm not using any hot water. The tech guy (over the phone) said the voltage it's getting is too high (my multimeter reads 134 V on the element) and that makes the thermostat "act wierd". He said the voltage is too high because the breaker is the wrong size. Doesn't make sense to me so thought I'd run it by you folks before I spend a bunch of time and money on a new breaker and Romex.

Here's the info:
20 gal heater, 2000 W, 120V. One thermostat, one element. On a dedicated 20A breaker with 12/2 Romex. The instructions says it should be on a 25A breaker. My breaker switch has not been tripped while the heater has been on it. I live in Susanville, Lassen County, CA.

So could using a 20A breaker instead of a 25A cause higher voltage? If I put in a 25A breaker am I going to have to change my Romex to 10 gauge?

Thanks!
June
 

Last edited by ray2047; 08-06-12 at 02:54 PM.
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Old 08-06-12, 12:43 PM
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The high voltage is a result of the transformer on the pole being bad, not the breaker size. You could have a 15A or a 100A breaker in there, and the voltage would be the same.
 
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Old 08-06-12, 12:46 PM
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Ah, I knew it didn't make sense. Would getting 134V instead of 120V make the thermostat function that way? Should I call my electricity provider or is this really a non-problem?
 
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Old 08-06-12, 12:51 PM
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Unless it's a digital thermostat, voltage shouldn't have an effect. I'd call the poco and get the voltage issue checked out if I was you.
 
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Old 08-06-12, 12:58 PM
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POCO is power company right?
Thanks
 
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Old 08-06-12, 01:01 PM
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PoCo is a generic abbreviation for Power Company; it means your electric supplier.
 
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Old 08-06-12, 01:03 PM
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Thanks again - you just saved me about $100. Will call my power co.
 
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Old 08-06-12, 01:30 PM
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Take your meter to a friend,s house a few blocks at least from you and check what it reads in a receptacle at their house. If it is a digital change the batteries or buy an analog multimeter.
 
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Old 08-06-12, 01:56 PM
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Thanks for that idea. I checked several receptacles and got readings of about 138 on all. Changed the battery in my multimeter and got readings close to 120 on all. Now I get a reading of about 113 on the element of my water heater. So it's not the voltage. But my water heater still lets the water temp get too low before it kicks in to heat it up.

The heater is almost brand new. When I first had it installed (I did the electrical; a plumber did the plumbing) it would overheat then shut down. I had to press the reset button every morning. The tech people had me replace the element then the thermostat. Neither helped. Someone suggested I check to see that the thermostat was snug up against the heater. It wasn't and after fixing that the heater stayed on. Now I just have this problem. Do you suppose the thermostat was damaged by the overheating?
 
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Old 08-06-12, 02:26 PM
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Does anyone know how much elec a water heater like this should consume in an average month?
 

Last edited by junestan; 08-06-12 at 04:14 PM.
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Old 08-06-12, 06:29 PM
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FWIW: Just installed a 2000 W, 120V 6-gallon water heater. The instructions not only called for a min. 25 amp breaker, but 10g cable.
 
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Old 08-06-12, 07:15 PM
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Mine also called for a 25A breaker. You'd have to use a 10 gauge wire with that. But I've talked to at least 3 different electricians that say a 20A should be fine. 2000W/120V = 16.7 Amps needed. 20A should be w/in OK range. And 12 gauge wire is OK w/a 20A breaker. Plus my breaker switch has never tripped.
 
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Old 08-06-12, 07:25 PM
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Electrical supply to a water heater must be designed to 125% of the electrical load. For a 2000watt heater operating at 120 volts that works out to 20.85 amperes so by a very strict interpretation of code a 25 ampere circuit is required at minimum.

To calculate the electrical consumption of the water heater you need to know the temperature rise of the water, that is, the difference between the water supply temperature and the water outlet temperature; the amount of heated water used and the kilowatt hour rate of the electricity. It requires one BTU (British Thermal Unit) to raise the temperature of one pound of water one degree Fahrenheit. A gallon of water weighs approximately 8.34 pounds.

If your incoming water temperature is 60 degrees F. and you use 30 gallons of 100 degree water it takes just a bit over 10,000 BTUs to supply that heated water. Since a kilowatt of electricity equals 3,413 BTUs it would take 2.93 kilowatt hours of electricity to heat. If your kWh rate is ten cents per kWh then this amount of hot water would cost twenty-nine cents. ($0.29) Understand that 30 gallons of 100 degree water is a ten-minute shower if your shower head flows 3 gallons per minute.

Once you calculate your actual hot water usage the rest is simply plugging in your local numbers and doing the multiplication.
 
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Old 08-06-12, 07:38 PM
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What great info - thanks!
Do you think the fact that I'm using a 20A breaker would prevent the thermostat from working properly? It heats fine but allows the water to get too cool before heating it back up, even when I'm not using it.
 
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Old 08-06-12, 07:53 PM
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No. Your problem could be simply that the differential between element on and element off is too wide. It could be the placement of the thermostat on the heater itself. A 20 gallon water heater is mighty small and the normal 20 degree differential could easily manifest itself into a VERY noticeable differential in outlet water temperatures.

Most larger water heaters have two elements, each with their own thermostat and since the heated water rises to the top of the tank it is the lower element that does the lion's share of the work. Since the incoming water is at the bottom of the tank it will energize the lower element long before the water in the upper section has cooled significantly. I suspect that you either need a larger storage tank (larger heater) or you will just need to get used to the shortcomings of that tiny water heater.
 
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Old 08-06-12, 08:05 PM
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Both the thermostat and element are at the bottom of the heater. When I haven't been using any hot water for a while (ie - just got up in the morning) I put my hand on the outgoing hot water pipe at the top of the heater. Sometimes it's nice and hot; sometimes it's barely warm. If it's barely warm and I wait long enough the water gets hot again. It doesn't make sense and is driving me crazy.
 
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Old 08-06-12, 08:10 PM
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Have you replaced the thermostat? If not it could be defective and if you have replaced it then I suspect it is just the nature of the beast. I could give you a design for a better thermostat but the cost would be prohibitive.
 
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Old 08-06-12, 08:25 PM
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When I first got the heater it kept overheating. The tech support people had me replace first the element then the thermostat. Neither helped. Then someone told me the thermostat had to be snug against the tank. I found that it wasn't. Once I fixed that it no longer overheated. So it has a new thermostat and element. If thermostats are cheap I might try one more just in case. How much would the thermostat you're suggesting cost?
 
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Old 08-06-12, 10:30 PM
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Water heater thermostats are fairly inexpensive, around $10 as I recall. IF/when you replace it go to a Radio Shack and ask for some heat conductive compound, just a small tube. Smear some of the compound against the back of the thermostat where it contacts the tank wall and it will work slightly better.

Forget what I said about designing a new thermostat. I used to work in the instrumentation and controls field and I had to have very precise control of temperatures. The unit I would often use had a base price of $800 some ten years ago.

You might also try running some hot water about 15 or 20 minutes before you are going to really use it. This will bring in cold water, actuate the thermostat and then the wait time before use allows the element to bring the tank back to the desired temperature.
 
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Old 08-07-12, 08:25 AM
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Yep, $800 is a bit over my limit. I'm already running the hot water in the morning to prompt it to heat up. I think I'll try another thermostat and the heat conductive compound. I have one last question for you: the thermostat is a Rheem. Would it be OK to try another brand, like a Camco, as long as it's a single-element 120V and looks the same? Many many thanks for all your help.
 
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Old 08-07-12, 10:41 AM
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If your water heater uses a strap-on (surface mount) thermostat then yes, they are pretty much universal. Certainly the internal mechanism is universal.

[ATTACH=CONFIG]2476[/ATTACH]
(Image courtesy of waterheatertimer.org)

Notice that you will have a red button on your thermostat. This is the overheat safety thermostat. Do not use a "bottom element" thermostat unless your safety thermostat is separate from the element thermostat. Thermostats are not voltage specific.
 
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Old 08-07-12, 11:14 AM
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I was going to suggest a different brand of thermostat because over the years I have learned you can buy brand new items that are defective. If it was a whole lot that was defective and the next one is from the same lot then it too will be defective Even worse is if you go to BigBox because of their easy return policy. it is easy for someone to buy a new item, put the old one in the new box, and return it as new. Then the old defective one ends up on the shelf.
 
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Old 08-07-12, 02:20 PM
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Here is my thermostat. I found a Camco (thermo disc single element) on Amazon that looks identical. I couldn't find any "heat conductive compound" but got some "heat sink compound" at Radio Shack. It says it "increases thermal conductivity between CPUs and heat sinks or water-cooling solutions". Is that comparable?

I have also bought "new" but used-and-broken items at Home Depot. I never buy an opened package anymore.
 
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Old 08-07-12, 06:35 PM
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I couldn't find any "heat conductive compound" but got some "heat sink compound" at Radio Shack. It says it "increases thermal conductivity between CPUs and heat sinks or water-cooling solutions". Is that comparable?
Yep, that's the stuff! askljdfwepjmnadpfgv[
 
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Old 08-14-12, 09:13 AM
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resigned

Installed the new thermostat 4 days ago. Used the heat conductive compound which made it stick nicely to the tank. I don't see any difference in the performance of the heater though. It still lets the water get too cool before heating it back up. So I am resigned. I installed this heater as a replacement/backup for the hot water provided by my expensive and inefficient oil-fired boiler. It is still a great improvement over that. Thank you all very much for your help. And Furd - a big thanks to you for your help and specific info.
 
 

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