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Where I live, cold winters and humid summers rule out line drying for bulky items like blankets and towels for a few months out of the year. When I put them in my electric clothes dryer, which strategy will use less energy overall?

  • A hotter cycle which uses more power for a shorter time
  • A lower heat cycle which uses less power over a longer time
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    If they come out dry either way, then the same amount of liquid water has been converted to vapor and so the same amount of energy has been used. Therefore, in the ideal case, the two configurations are the same. So the question probably comes down to how a particular clothes drier diverges from the ideal case - what are the operational inefficiencies? Does it run past dry in one setting or the other? Does it fail to direct energy as heat at the wet clothes somehow? Does one setting put more physical wear on the device, leading to a shorter lifetime and earlier replacement? – Jean-Paul Calderone Dec 28 '20 at 21:21
  • My best suggestion is to measure it using a kWh meter. The dryer will use a certain amount of power, measured in watts (most likely kilowatts). Multiplying by the time it takes gives the energy used, kWh. – Fred Dec 29 '20 at 8:40
  • @Fred I hadn't thought of that, but unfortunately in this case it will be harder because the dryer uses a special 240V plug, not the standard 120V. – LShaver Dec 29 '20 at 23:54
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The generic and vague wording of the question seems deliberate, but unfortunately a meaningful answer can't be so simple.

The primary factor that determines how long it will take a dryer to dry a load of clothes is not actually temperature, it's airflow. All the heat in the world won't help you if your dryer is overloaded, the lint tray is full/blocked, and your vent is choked. Air needs to move through the dryer for the drying process to work. You can even dry clothes all-year-round by simply putting them into a vented tumble dryer with no heat at all. Might take a while, and they won't come out all nice and warm, but they will be dry.

Now, in order to progress towards an answer of some sort, we need to make a bunch of assumptions...

The first assumption is that the dryer is the older vented variety, not any of the other electric varieties. It draws air from inside the house, uses that air to dry the clothes, and exhausts the air through a wall and outside the house.

Next, let's assume that it's not ancient — it has a sensor on it that stops drying "when the clothes are dry". There are a variety of ways this can be done, but a temperature sensor on the exhaust is common. No need to worry about set duration cycles and over-drying.

The next assumptions will be that the drum motor and fan combined use about 250W and the heating element uses about 5,000W. That's a huge difference, but given that the heating element is cycled on and off, and the motor/fan run pretty-much continuously, it's not as large as it would first appear.

Assume also that the dryer is not overloaded, the load is made of homogeneous items, the lint tray is empty, the vent is clean, internal temp is 21°C and RH is 50%.

Hand-wave a bunch of 'fuzzy' math out of the way and you end up with: Warm and longer dryer cycles consume less energy than hot and shorter cycles.

Towels and blankets can be called 'thick' items. They contain far more water than typical clothing. Since moisture is distributed according to volume, and evaporation is proportional to surface area, you are looking at a dimensional difference.

A short(/hot) cycle will result in rapid initial evaporation over the exposed surfaces, but then — with those surfaces now being dry — the items (especially blankets) become thermal insulators. Insulators resist heat transfer. Since air is continually being vented at the same rate, as the drying cycle proceeds the relative humidity of the vented air drops sharply. What that means is that, as a ratio, more of the vented air is now dry. The 5kW heating element still powers on to heat the incoming air up, but most of the air doesn't have a chance to participate in any evaporation before being vented. In short, hot cycles become very inefficient very quickly.

Warm cycles, on the other hand, don't burst out of the gate with a rapid amount of initial evaporation. Because the incoming air is (relatively) cooler it takes longer for initial evaporation to occur.

Once surfaces are dry, further drying is limited by how fast heat can conduct through the fibres of the items being dried. In the insulation world, this would be a material's U-factor. The thermal properties of polyester, cotton or whatever your blankets and towels are made of, now take over.

If we make a final assumption that you have material homogeneity, what then becomes the primary determinant of evaporation is time. It just takes time to conduct the heat through the insulating surface layer, time for the deep moisture to be evaporated, and time for the tumbling action to evacuate the moisture-laden air from the depths of the material. Whilst temperature does influence the first two, it has nothing to do with the last one.

If a pocket of air deep in a material reaches 100%RH, no further drying at that location will occur until the tumbling action literally forces that air out of the material, and it gets replaced by drier air. At this scale the limit is the same as it was at the very beginning of this answer: Airflow.

So, regardless of how you slice and dice the scenario, airflow is King in dryer operations. Since the impact of airflow is far more tied to cycle length than it is to temperature, the simple fact is that long dryer cycles are what actually dry the items inside a dryer. The 250W motor and fan are doing the heavy lifting. Temperature control is primarily a 'feature' that marketing departments can use to help sell the product to people who 'lead busy lives'. Hot cycles use a lot of power to only modestly decrease drying times.

If you have the time, the most energy-efficient way to use such a dryer is to not have it do any heating at all, and just allow it to use the house's 21°C/50%RH internal air to do the drying.

Given the questions parameters, and all of the assumptions documented above: Long and low is the way to go.

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  • I wonder how the effect of venting heated indoor air might affect this analysis? Long and slow will end up pushing more warm, indoor air outside that my furnace needs to replace. – LShaver Jan 3 at 17:30
  • @LShaver A few years ago the average US house was ~2300ft² which would suggest an air volume of ~20,000ft³. One ASHRAE standard specifies a minimum of 0.35 air exchanges per hour to maintain air quality. That implies ~7200ft³/hr or 120ft³/min. Vented US dryers typically draw 100–125ft³/min. Those numbers would suggest that your dryer is venting the bare minimum amount of air that is considered acceptable. A short cycle might put you below the minimum standard. So, from an air quality/health perspective, short cycles are bad for you (or, at least, aren't doing you any favours). – Tim Jan 4 at 10:29
  • Whether saving money is more important than maintaining health is a personal choice, as always. – Tim Jan 4 at 10:31
  • To be clear: What I meant with the above comment was simply that running a vented dryer can actually help owner/occupiers maintain air quality within their homes. Many modern homes are so tightly wrapped that they do not do this naturally. Whether or not this is an issue for you depends on your house and your climate. It is, at least, something to consider. "More vented air" is not necessarily, or by definition, "bad". "Leaky houses" are not necessarily, or by definition, "bad". – Tim Jan 5 at 0:12
  • Any holes in a house above the NPP (Neutral Pressure Plane) leak air out, and any below the NPP let air in. If your house leaks 200ft³/min naturally, then turning on a 100ft³/min dryer does not result in simple addition of those numbers and a leak rate of 300ft³/min. You just turn the inside of the house into a negative pressure zone, and all holes become inlets — with the exception of the dryer vent which becomes the only outlet. In such a scenario, the actual change in leakage would be small, possibly even zero, as you are primarily changing the direction of leakage, not the rate. – Tim Jan 5 at 0:41
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I tested this with two identical loads of laundry, mostly socks (so, plenty of retained moisture after the spin cycle). I then checked my smart meter's records of how much power was used during that period, and subtracted the "baseline" usage.

  • Tumble dry high: estimated 2.0 kWh, elapsed time 48 minutes
  • Tumble dry low: estimated 1.4 kWh, elapsed time 50 minutes

In both tests, peak power draw was in the first ten minutes of drying, with power draw tapering off sharply during the last ten minutes.

It looks to me like most of the drying time is spent waiting for moisture to migrate to the surface so it can evaporate, and the temperature does not significantly impact this.

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