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View Full Version : Cost savings after water heater is replaced?

Uncommon Sense
06-30-2003, 03:19 PM
A 130 gallon - 480 volt - 3 phase - 30KW. water heater was replaced with a
65 gallon - 208 volt - single phase - 4.5KW. water heater.

Both have dual elements.
The cost per KW hour is 3.0 cents.
Assume they both ran an average of 8 hours per day.
The temp on both was set at 130 degrees F.

Is the smaller one more cost effective?

cmosdes
06-30-2003, 03:51 PM
If they both run for 8 hours, how in the world can the one which uses less KW not be cheaper?

H1 : 30KW * 8 hours = 240KWhours
240 KWH * .03 = \$7.20

H2: 4.5KW * 8 hours = 36KWhours
36KWhours * .03 = \$1.08

However, H1 has heated twice as much water, so really we'd need to say it cost \$3.60/65 gallons.

Since this is GQ, is it okay for me to say I *think* the difference between water heaters will be in how effeciently they heat the water and how well they retain heat?

A water heater which is more efficient at heating the water will run for a shorter period of time (assuming identical insulation). A water heater which has better insulation (assuming identical heating efficiency) will not have to re-heat the water as often.

Also note that often power costs are variable according to the time of day. A 130 gallon heater might be able to heat all the water for the next day at night, when electricity is cheaper. The 65 gallon tank might have to refill and heat water during high cost times.

sailor
06-30-2003, 04:06 PM
All electric water heaters are 100% efficient in converting electricity into heat. The only difference is going to be in heat lost through the insulation. Other than that, nothing.

Uncommon Sense
07-01-2003, 07:25 AM
I was thinking that the 3 phase heater would get the job done more efficiently, more watts per gallon. I was also concerned about the length of time that it would take to heat the water in each tank if they have to re-heat the water each morning. Since they would only run for 8 hours, the water would certainly cool down overnight. I think the smaller one would run longer to get the water up to 130 degrees - again there are more watts per gallon for the larger heater.
The last concern would be maintaining 130 degrees during daily useage. The larger tank may not have to run as much to maintain the 130 degrees whereas the smaller one will certainly be working more to maintain the 130 degrees. We can assume
a 5/10 percent water exchange rate per hour for the larger/smaller heaters.

Are these things to consider?

[possible hypo. situation]
If the larger one is really only at 30KW for two hours (out of the eight) and the smaller one is running at 4.5KW for four hours (out of the eight) then the difference becomes \$1.80 for H1 and \$.54 for H2. Closer. Is there a practical situation where the difference would be negated? Or am I reaching here?

cmosdes
07-01-2003, 09:22 AM
If sailor's assertion that electric water heaters are 100% efficient (this can't be true, but is probably pretty damn close), then the only difference between the heaters will be in their insulation and heat retention properties.

In other words, if the two heaters are identical in both energy conversion and heat rentention, it shouldn't matter how much power they draw. All power will be converted to heat energy of the water. The smaller one will take longer to heat the water, but it won't use any more energy. (Technically it will use half the energy, but it will have heated half as much water).

The water exchange rate won't matter because what we are talking about is having to raise a certain amount of cubic centimeters of water a certain number of degrees over the course of a day. That requires a specific amount of energy.

The size of the tank makes no difference in maintaining the temperature of the water. This is strictly a function of insulation of the heater. However, given identical insulation material, the one with the smallest surface area/volume ratio will be more efficient.

Again assuming 100% conversion of electrical engergy to water heat energy, if the smaller tank runs 15/4.5 = 3.33 times longer, you break even. I used 15 instead of 30 since the larger tank heats twice as much water. So if H1 runs for 1 hour that would cost \$0.90/130 gallons. If H2 runs for 3.33 hours, that would cost \$0.45/65 gallons, or \$0.90/130 gallons. Bear in mind that the only reason the tanks should be different in cost should be due to insulation differences.

In conclusion:

The larger tank will heat the water faster without a doubt. The only way to determine cost difference is to look at the insulating properties of the heaters.

Blown & Injected
07-01-2003, 09:50 AM
The heating element in an electric water heater is emmersed in the water so 100% of the heat in the element goes into the water. Unlike a gas WH that must be vented - some of the heat goes up the vent in those WHs.

I thought there was a difference in effectiveness in different voltages and phases.