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Old 09-11-2009, 09:22 AM   #1
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Reducing 15kw Heat Strip


My home has a 3 ton electric air handler installed with a 15kw heat strip. I live in Texas and the winters do not get that cold. Is there any way I can reduce the heat strip so it only uses 10kw's of power? The 15kw heat strip just kills me on my power bill. I have a degree in mechanical engineering so I know may way around electrical things, I was just wondering if it was possible.

Thanks,

Adam

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Old 09-11-2009, 10:35 AM   #2
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Reducing 15kw Heat Strip


Yes, they make smaller heat strips. You'd have to research what sizes are available that fit in your air handler.

But I'm not sure how that will lower your electric bill. A smaller heat strip may simply run longer.

If this is coupled with a Heat Pump perhaps you have the option to install primary, secondary and tertiary heat strips, depending on your heat pump and air handler. Or perhaps your system is not optimally configured.

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Old 09-11-2009, 10:49 AM   #3
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Reducing 15kw Heat Strip


I actually was trying to modify my 15kw heat strip to only run at 10kw's. I was assuming it would save money to only run the 10kw because it would be losing less power, and everywhere I have read 10kw would be more than enough to heat my size home. My air handler is 20 years old so I don't think I could find a heat strip that would fit.
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Old 09-11-2009, 11:09 AM   #4
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Reducing 15kw Heat Strip


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Originally Posted by tx_reeg View Post
I actually was trying to modify my 15kw heat strip to only run at 10kw's. I was assuming it would save money to only run the 10kw because it would be losing less power, and everywhere I have read 10kw would be more than enough to heat my size home. My air handler is 20 years old so I don't think I could find a heat strip that would fit.
It will make no difference in your power bill. The heat strip does not "lose" 15kW in the sense of wasting it - it pushes 15kW of heat power into your home while it is on. When the temperature drops below a certain level, it turns on and runs until enough heat energy has been pushed into your home to raise the temperature enough to turn off the thermostat. How long it takes to push that much heat energy into the house depends on how much power the heat strip provides. A 15kW heat strip will run 1/3 less time than a 10kW. The total heat energy produced is the same, by definition, since that's what your thermostat actually responds to. It's also what you pay for on your electric bill.
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Old 09-11-2009, 12:23 PM   #5
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Reducing 15kw Heat Strip


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Originally Posted by mpoulton View Post
It will make no difference in your power bill. The heat strip does not "lose" 15kW in the sense of wasting it - it pushes 15kW of heat power into your home while it is on. When the temperature drops below a certain level, it turns on and runs until enough heat energy has been pushed into your home to raise the temperature enough to turn off the thermostat. How long it takes to push that much heat energy into the house depends on how much power the heat strip provides. A 15kW heat strip will run 1/3 less time than a 10kW. The total heat energy produced is the same, by definition, since that's what your thermostat actually responds to. It's also what you pay for on your electric bill.
Thanks for the info. I did not know that.
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Old 09-11-2009, 02:52 PM   #6
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Reducing 15kw Heat Strip


Supposedly you get the max efficiency by having the heater on all the time, so if you have a 10 kw or 11 kw or 12 kw or . . .heater running at 100% duty cycle on the 97.5% percentile day at your location then you are doing the best you can.
If you occasionally wear a sweater you might get by with less than 10 kw.
http://www.degreedays.net/
http://web2.airmail.net/danb1/climate.htm

Depending on how many heaters of what size you have, you can get several intermediate levels of power dissipation.
Half the voltage = 1/4th the power, and since you'll be running these particular heaters at less than full power I can't imagine any safety concerns with this type of cross-strapping.

Or get a heat pump and divide your heating elec. bill by 3.
1 Therm of energy can be had from approx. 0.71 gal of fuel oil, 0.77 gal of gasoline, 100 cubic feet of natural gas, 1.1 gal of propane, 29 kwh of elec heat, 8.4 kwh of heat pump heat (COP = 3.5), 4.2 kwh of heat pump heat (COP = 6.9), 14 pounds of wood.
Or go solar. Texas must have high insolation.

3 tons = 10.5 kw[?]

Last edited by Yoyizit; 09-11-2009 at 05:21 PM.
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Old 09-11-2009, 05:12 PM   #7
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Reducing 15kw Heat Strip


You may be able to disable 5KW of that heat package.
If your air handler is in the attic. It may save you some money.
Since the hotter the air is in the duct work. The more heat the duct work loses in an unconditioned space.
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Old 09-11-2009, 05:55 PM   #8
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Reducing 15kw Heat Strip


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Originally Posted by Yoyizit View Post
If you occasionally wear a sweater you might get by with less than 10 kw.
[?]
Or.... how bout a snuggie Does the same thing apply in the summer if you wear less you don't need as much cooling?

Last edited by newtech; 09-11-2009 at 06:39 PM.
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Old 09-11-2009, 07:17 PM   #9
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Reducing 15kw Heat Strip


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Does the same thing apply in the summer if you wear less you don't need as much cooling?
Never tried it, but wearing wet clothes in moving air might work; kind of like your own personal evap. cooler.

I knew of people in Austin who talked about sleeping under damp sheets back when almost nobody had A/C. She didn't mention fans but it would have worked much better. Maybe they didn't have elec., either. . .
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Old 09-13-2009, 02:14 AM   #10
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Reducing 15kw Heat Strip


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Originally Posted by tx_reeg View Post
I actually was trying to modify my 15kw heat strip to only run at 10kw's. I was assuming it would save money to only run the 10kw because it would be losing less power, and everywhere I have read 10kw would be more than enough to heat my size home. My air handler is 20 years old so I don't think I could find a heat strip that would fit.

A heat strip is nothing but a big resistor, basically you need to make your heat strip have more resistance. If the construction is that of parallel coils of wire, removing a few of the coils could reduce it to less power. Since I don't know what kind of heat strip you have I can't tell you how possible this is.

Basically if you have 15kw and your running it at 240 volts, then your heatstrip is about 62 amps and 3.8. ohms. If you for instance were able to make the coil of wire longer, or remove some parallel portions of the element, you could increase the resistance and reduce the power. 10kW would be 41 amps and 5.8 ohms.

Something else to consider, if you wired two heat strips in series, you would double the resistance and reduce the power by half. You'd have 7.5kW heater using two 15kW heaters. If you wired them in parallel, you'd have 30kW...
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Old 09-13-2009, 03:47 AM   #11
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Reducing 15kw Heat Strip


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Originally Posted by Yoyizit View Post
Never tried it, but wearing wet clothes in moving air might work; kind of like your own personal evap. cooler.

I knew of people in Austin who talked about sleeping under damp sheets back when almost nobody had A/C. She didn't mention fans but it would have worked much better. Maybe they didn't have elec., either. . .
It worked better without running a fan. A fan would cause the sheets to dry out faster. And then they would wake up sweating in the middle of the night.

Some people in AZ also used that same method.
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Old 09-13-2009, 03:47 AM   #12
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Reducing 15kw Heat Strip


Jeff.

Your just making it sound more complicated then it is.
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Old 09-13-2009, 11:08 AM   #13
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Reducing 15kw Heat Strip


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Jeff.

Your just making it sound more complicated then it is.
I'm just explaining the theory. If you understand ohms law, it helps understanding the physics of what is going on with resistive heating.
Power = Volts * Amps, Resistance = Volts / Amps.
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Old 09-13-2009, 12:01 PM   #14
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Reducing 15kw Heat Strip


Understanding ohm's law means nothing in this case. If you really don't know how strip heaters are configured.

And your post shows you don't know.
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Old 09-13-2009, 01:03 PM   #15
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Reducing 15kw Heat Strip


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Understanding ohm's law means nothing in this case. If you really don't know how strip heaters are configured.

And your post shows you don't know.

OK, you were right, I was wrong. You win. Now lets see some helpful guidance on re-config, rather then just insulting me.

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