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 Posted: Fri Feb 10th, 2012 05:51 am
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murphy
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Aside from any factors with sediment or calcium on the elements, is there a basic formula to figure out how fast the water temp will rise in the tank? This would be with no water being used, of course. Like to go from 70* to 120* how long would that take? The water heater in question is a 50 gallon with two 4500 watt elements.

Thanks for any info.

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 Posted: Fri Feb 10th, 2012 11:03 am
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energyexpert
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Murphy,

1 BTU will raise the temperature of 1 lb of water 1 degree F.
1 gallon of water weighs about 8.33 lbs.
1 watt-hour of resistance heat yields 3.413 BTUs.
4500 watts = 15,358.5 BTUs/hour

Approximate 50 gallon electric WH operation:
Volume above top element: 15 gallons
Volume above bottom element: 40 gallons
Initial temperature: 70F

15 x 8.33 x (120-70) = 6248 BTUs needed
6248/15,359 = 0.407 hours to heat top of WH

(40-15) x 8.33 x (120-70) = 10,413 BTUs needed
10,413/15,359 = 0.678 hours needed to heat bottom of WH

0.407 + 0.678 = 1.085 hours to increase 40 gallons by 50F.

David

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 Posted: Sat Feb 11th, 2012 06:29 am
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Geno_3245
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Larger charts

Ordinary tank-type electric water heater: Both elements are not ON at same time: Upper element is ON, or lower element is ON, or both elements are OFF. This is called non-simultaneous operation. Otherwise, two 4500 watt elements would draw 9000 watts and require 50 amp breaker and 8 gauge wire. Water heaters can be rewired many different ways to meet different objectives.
Non-simultaneous sequence: When tank is cold, upper element turns ON until upper part of tank reaches upper thermostat set point.
Next. upper thermostat turns off upper element and sends power to lower thermostat and lower element.
Lower element turns ON until lower thermostat reaches set point. Then lower element turns OFF.
Lower element turns ON-and-OFF to keep water hot during standby hours.
So if upper element ever burns out, the lower element never gets power, and tank has no hot water.
Also it is important to note that math calculations are for water heaters operating under optimal conditions without amp loss in delivery of electricity.
And note that thermostat settings for are always approximate since thermostat reads temperature through wall of tank, and calibration is different for all thermostats.

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 Posted: Sun Feb 12th, 2012 04:04 am
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Geno_3245
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Correction > 9000 watts divided by 240volts = 37.5 amps.
8 gauge wire is rated for 40 amps
Using the 80% rule to figure safe maximum: 40 amps x 80% = 32 amp safe max for 8 gauge wire.
So electrician would use 50 amp breaker and 6 gauge wire for simultaneous water heater with 4500 watt elements.

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 Posted: Thu Feb 23rd, 2012 10:19 am
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RalphVa
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We turn ours off every time we go on a trip. Doesn't take very long to get hot water on return. Used to have a 60 plus 80 gallon tanks. Now have 2 50s.

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 Posted: Fri Apr 27th, 2012 10:21 pm
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ilas
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Hi, I own a Rheem 50gal gas water heater (http://review-home.com/water-heaters/rheem-22v50f1-natural-gas-water-heater-review). When we are away from the house we turn it off to save energy. After returning it lasts usually half a hour for water to heat up for a temperature which is comfortable for a shower. Hope I helped, even if my datas and numbers aren't backed up as well as in energyexpert's comment, they are just plain empirical experiences.

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 Posted: Fri Apr 27th, 2012 11:33 pm
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Geno_3245
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Difference between gas water heater and electric:
Typical gas water heater heats water faster than typical electric water heater.
There are some electric water heaters with 3 element ports that could compete with gas if using largest wattage element.
Typical gas water heater can heat 40 gallons per hour, while typical electric can heat 20 gallons per hour (depending on start temperature of water).

Since gas water heaters expel combustion by-product up the vent, efficiency for typical gas water heater is about 58%. Efficiency for electric is about 99% since elements are immersed in the water, yet it cost 30-40% more per year to operate electric.

Neither efficiency rating includes comprehensive impact of mining, refining, transportation of energy source vrs alternatives. Commonly quoted calculations show that burning coal at power plant wastes far more energy than burning natural gas, but something is missing in the overall calculation since electric costs spike upward when gas-fired generators are used. Maybe energy-expert has figures on this.

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 Posted: Thu May 3rd, 2012 12:29 am
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energyexpert
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There are a lot of variables to consider when deciding the utility mix for generating electricity. A therm of natural gas has 100,000 BTUs. I buy it for about $1/therm. But I'm sure an electric utility pays no more than half of that considering the volume that is used. Burned in an IC turbine (internal combustion turbine or jet aircraft engine) you get about 50% conversion to electricity. 50,000 BTUs = 14.65 kWhs for a cost of $0.50. Coal BTUs will vary from 8000 to 15,000+ BTUs/pound. Let's use 12,000. Since electric utilities buy coal by the train load (10,000 tons) at a time or contract for hundreds of trains of coal, then they may be getting it for $50/ton. That is just $0.025/lb and just $0.21/100,000 BTUs. Coal burning units have efficiencies of 40 to 45%. So you get 11.72 to 13.18 kWhs for $0.21.

As you can see, the direct cost of electricity from coal is much less than from natural gas. However, coal has a lot more indirect costs than natural gas: ash, sulfur, heavy metals in air and water, yes and even uranium with attending acute and chronic health issues. A coal plant releases far more uranium into the air than is permitted by a nuclear plant. There is a small amount of "tramped uranium" contained in coal. But since a coal plant is not a nuclear plant, it is OK for them to release the uranium from the plant.

Anyone interested in doing the research may come up with different numbers. Fuel prices fluctuate. Coal prices may change up or down faster than natural gas or vice versa.

Last edited on Thu May 3rd, 2012 12:34 am by energyexpert

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 Posted: Sun May 6th, 2012 07:45 am
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Geno_3245
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Good analysis, Thanks

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