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Most efficient clothes dryer setting? High/short or low/long?
Old 08-08-2008, 08:24 PM   #1
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Most efficient clothes dryer setting? High/short or low/long?

We have a relatively new electric dryer and I'd like to know what the most energy efficient way to dry a load of clothes is. The goal is to minimze the kWh consumed, regardless of the time it takes. Is it better to run it on low heat for a longer period or high heat for a shorter time? It runs on 220V so I can't plug my kill-a-watt into it to measure. Does anyone know if the heating elements operate less efficiently at high temperatures versus low? If not, it seems that high temp would be the best setting, since it minimizes the time that the unit has to spin. But my fuzzy memory of physics tells me that as the temperature of a conductor increases (e.g. the metal heating elements in the dryer), the resistance also increases, making it less efficient. The dryer has moisture sensors but I find they are not that accurate (the dryer keeps drying after clothes are dry). We're in a mixed hot-humid environment (Dallas).
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Old 08-08-2008, 09:49 PM   #2
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Heating elements are essentially 100% efficient under all conditions. There is nothing else for that electricity to do but turn into heat, so it does. Yes, a conductors resistance increases with temperature, but that only means it draws less current as it gets hotter, it is still 100% efficient at turning that electricity into heat.

I'd need to dig up some numbers, but I'd guess that the motor uses less than 1/10 the energy that the heating elements do. So even if it runs longer at the lower heat, I'd bet it would be less costly to do the long run, low heat setting.

My dryer does not have a variable heat setting. Does the manual state anything about the watts used in each case? I'd assume they are just switching an element in/out, or switching the 220V element over to 110 (it will provide ~ 1/4 the heat energy at half voltage, a bit more if they are on that thermal increased-R curve).

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Old 08-09-2008, 01:23 AM   #3
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High temp could have a negative effect on some clothes (esp. anything with elastic).

If you are truly cheap.. how about hanging the clothes outside for a while and then just bringing them in to fluff/finish (if having them really dry is important and it is humid where you are)?
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Old 08-09-2008, 07:01 AM   #4
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Most efficient would be the least "on" time of the heater. Thus the level of "dryness" and initial wet load determines the energy consumed. If you accept slightly damp end result, less energy used. Lower heat setting will not save you $$ since the heaters would just run longer.
You can't change the amount of heat required to evaporate a given volume of water.
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Old 08-09-2008, 07:28 AM   #5
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Lower heat setting will not save you $$ since the heaters would just run longer.
You can't change the amount of heat required to evaporate a given volume of water.
Not necessarily. For example, with the heaters totally off, the clothes would still eventually dry from the fan and the air currents (just like hanging them outside - no added heat there). So a lower heat setting will result in more air drying.

You are correct that if the heater wattage is cut in half, it will run twice as long to input the same heat energy. And if it were not for the drying effect from the fan and air that is what would happen. In practice, you'll get something in-between. That's why it would be helpful to know the wattage in each mode. If it is 1/4 the wattage, and it does end up taking 4x as long - no savings, a waste of time, and you might just be shooting more A/C or heated inside air to the outside ( a further complication in all this....).

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Old 08-09-2008, 08:47 AM   #6
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Not necessarily. For example, with the heaters totally off, the clothes would still eventually dry from the fan and the air currents (just like hanging them outside - no added heat there). So a lower heat setting will result in more air drying.

You are correct that if the heater wattage is cut in half, it will run twice as long to input the same heat energy. And if it were not for the drying effect from the fan and air that is what would happen. In practice, you'll get something in-between. That's why it would be helpful to know the wattage in each mode. If it is 1/4 the wattage, and it does end up taking 4x as long - no savings, a waste of time, and you might just be shooting more A/C or heated inside air to the outside ( a further complication in all this....).

-ERD50
Right about the air drying bits. In the end we are splitting tenths of pennies. Heaters are usually around 5 KW.
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Old 08-09-2008, 08:53 AM   #7
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Your best drying efficiency is getting a front load washer with a high spin speed and having the clothes come out of the washer barely damp.

It takes our dryer about 8-10 minutes to dry a full load.
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Old 08-09-2008, 09:04 AM   #8
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Your best drying efficiency is getting a front load washer with a high spin speed and having the clothes come out of the washer barely damp.

It takes our dryer about 8-10 minutes to dry a full load.
Hate to dampen the solution, but, a "well spun" load does not change drying efficiency. It does reduce the drying time dramatically. A good spin cycle might improve overall wash/dry cycle efficiency, depending on the washer's motor efficiency. Most household appliance motors are built cheap, not made for high efficiency. They do convert a good amount of electrical input energy to heat.
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Old 08-09-2008, 09:44 AM   #9
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We have a high efficiency washer which gets most of the water out of the clothes. I checked the dryer manual but they don't give any specs for the different dry settings other than cycle x is low temperature and cycle y is high temperature.
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Old 08-09-2008, 09:48 AM   #10
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We have a high efficiency washer which gets most of the water out of the clothes. I checked the dryer manual but they don't give any specs for the different dry settings other than cycle x is low temperature and cycle y is high temperature.
What's the model #? I'm curious - ERD50
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Old 08-09-2008, 02:34 PM   #11
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I've wondered about this, too. This would be a difficult problem to figure out. Factors:
1) As ERD50 mentioned, the whole time the dryer is running you are blowing conditioned air (warmed or cooled) out of your house. Advantage: High heat.
2) The only purpose of raising the temperature of the air is to reduce its relative humidity so the air can accept water from the clothes. If, say, you lived in Phoenix and the air inside your home was at a relative humidity (RH) of 5%, it might not make any sense at all to heat the air. Conversely, if the air in your house is already at 100% RH, you could run the dryer all day without heat and the clothes would never get dry. Advantage: Might depend on RH inside the home.
3) While all electric resistance heating elements are 100% efficient, that only means that al the electric energy goes into making the element hotter. The transfer of energy from the element to the air will almost differ according to the element temperature. I would guess (subject to being shouted down by someone who really knows) that a large surface area at a small temperature above the incoming air will heat the air more efficiently than a small surface area at a much higher temperature. If this effect is important enough, then it would be advantageous to run the elements on "low" setting.

All that said, my Whirlpool dryer has a high heat and a low heat setting, and the high heat setting is marked "Energy Preferred." But what do they know!
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Old 08-09-2008, 03:06 PM   #12
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3) While all electric resistance heating elements are 100% efficient, that only means that al the electric energy goes into making the element hotter. The transfer of energy from the element to the air will almost differ according to the element temperature. I would guess (subject to being shouted down by someone who really knows) that a large surface area at a small temperature above the incoming air will heat the air more efficiently than a small surface area at a much higher temperature. If this effect is important enough, then it would be advantageous to run the elements on "low" setting.

All that said, my Whirlpool dryer has a high heat and a low heat setting, and the high heat setting is marked "Energy Preferred." But what do they know!
I think you are correct. With the lower temperature element, the clothes will absorb a higher % of the heat produced. At the 'high' setting, more of that heat (%) would just get blown out the vent before the clothes have time to absorb it. High heat = quicker drying; low heat = more efficient use of the heat.

Kinda funny, I just had a parallel conversation about this regarding chilling of beer (wort actually, it's not beer at this stage) after brewing it to get it to a good temperature for adding the yeast. It's all the same theories, speed versus efficiency. I actually built a SPICE model to simulate that one, which was 'fun' (for some!). Charged capacitors to represent thermal mass, resistors to simulate thermal resistance, and a 32V battery to simulate an ice bath ata constant 32 degrees F. Worked well. I got fancy graphs and everything, all from about 5 lines of 'code'.

Hmmmm, maybe the 'Energy Preferred' cycle is accounting for removing conditioned air? I've wondered if an outside intake vent would be more efficient - but for the few bucks it might save, it sure doesn't seem worth the time/effort to make a another hole in the wall. And of course there would be some leakage 24/7 from that vent... Which all goes to sow just how 'cheap' our energy still is.

-ERD50
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Old 08-09-2008, 03:14 PM   #13
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We have a relatively new electric dryer and I'd like to know what the most energy efficient way to dry a load of clothes is. The goal is to minimze the kWh consumed, regardless of the time it takes.
No, you really don't want to know the most-efficient answer... it's unplugging the dryer and using a clothesline or a drying rack.

Our kid recently conducted a dryer experiment on a piece of bedding and discovered that efficiency is not correlated to survivability. (She also learned that it sucks to have to unload the dryer with a putty knife.) So regardless of the efficiency, I'd dry the laundry at the permanent-press setting and hope that the fibers aren't hammered too badly.

If you already have a high-efficiency washer/dryer and refrigerator, then the next energy-reduction step is a solar water heater. (Since you're in Dallas I'm going to skip over the part about learning to live without an air conditioner.) Even one 4'x8' collector should work pretty well in your neighborhood despite winter.
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Old 08-09-2008, 04:37 PM   #14
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What's the model #? I'm curious - ERD50
Whirlpool Duet Sport electric dryer, model WED8410SW0. The manual is available here but didn't see any specs in it.

Whirlpool Product Literature Search Results

I hope that there's no way to steal my identity by publishing this information teh interwebs...
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Old 08-09-2008, 07:41 PM   #15
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Old 08-09-2008, 07:42 PM   #16
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My old dryer had a thermistor in the heater tube that would send a signal to the computer. The computer would then cycle the heating element on and off to maintain the selected temp. The element was either fully on or fully off.

My new dryer has both a low and high setting and has an "energy preferred" selection on the dial for both. It just leaves the clothes more wet when shutoff occurs.

Another factor to consider is the heat contribution to the house interior by the dryer. My laundry room gets noticably hotter when I run the dryer on the high heat setting. This is heat that will have to be pumped out of the house by the air conditioner. When I run it on low the temp increase in the room is not noticable.

I read somewhere that the typical dryer fan is 300-350 cfm. If we assume a 2000 sq-ft, 8 ft ceiling house, the dryer would run: 2000 X 8 = 16000 cu. ft. / 350 cfm = 45 min for 1 complete air exchange, or 60/45 = 1.33 air changes per hour. Since ASHRAE recommends 0.35 air changes per hour, we would be way over that, resulting in excess air conditioner operation. So it would seem, at least by this criteria, that it would be best to operate on the high setting when possible to minimize dryer operation time.
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Old 08-09-2008, 07:46 PM   #17
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I've wondered if an outside intake vent would be more efficient - but for the few bucks it might save, it sure doesn't seem worth the time/effort to make a another hole in the wall. And of course there would be some leakage 24/7 from that vent... Which all goes to sow just how 'cheap' our energy still is.
-ERD50
This could really add to the efficiency of the whole setup. If the incoming air (drawn from outside the house) was pre-warmed in by the outgoing air that passed by the clothes, you could save a lot of energy. It would also entirely eliminate the 24/7 loss of conditioned air through the dryer vent.

Why won't we ever see it?
1) No one wants to bang another hole in their wall.
2) Safety. The walls of the outgoing duct would be cooled by the incoming air. They would probably get below the condensation point for the highly humid outgoing air. This would lead to water on the walls, which would collect lint. Within a short time, the duct would be clogged, leading to a fire or at the very least a reduction in drying efficiency. This happens in the real world if the dryer vent duct is too long or if it goes underground or in concrete for any distance.

So--never mind my great intercooling dryer idea.
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Old 08-09-2008, 07:46 PM   #18
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Old 08-09-2008, 07:49 PM   #19
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Old 08-09-2008, 07:50 PM   #20
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It would also entirely eliminate the 24/7 loss of conditioned air through the dryer vent.
24/7 loss? My vent has 2 dampers on it, eliminating both in- and ex-filtration unless the dryer is in operation.
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