How a geeky bachelor gets laundry done

Let’s get this out of the way before I even get started: I do the laundry wrong.

That said, I don’t like wasting energy. Our electric company’s website has a tool that estimates your energy usage by category based on your responses to a questionnaire about what type of house you have, how many occupants, and what types of appliances you have. The questionnaire is sufficiently detailed to make me believe it presents a realistic picture of our energy use. It is an eye-opening look into where our electricity dollars go. After heating, water heating and laundry are the largest energy consumers in our house.

Within the category of water heating and laundry, the dryer is the second-largest consumer.

We already set the programmable thermostat to the lowest temperature we can tolerate, and the water heater is already set to the U.S. Department of Energy’s recommended 120°F. In my view, the next item that would have the most bang for the buck is to not run the dryer any more than necessary, so I take the clothes out before they start to get crispy. (Which brings me to my original acknowledgement: I do the laundry wrong. This post isn’t supposed to be about marital issues, so let’s move along.)

I thought, what if there is a happy medium? I know that there are dryers out there with moisture sensors that stop the dryer when the clothes are dry instead of just running until it hits the time you arbitrarily set at the beginning of the cycle. Since our dryer does not have a moisture sensor, perhaps I could build one.

Moisture sensors in fancier dryers have two metal strips in the drum that the clothes touch as they tumble around. Wet clothes are apparently slightly conductive, whereas dry clothes are not. By monitoring the electrodes, the dryer can determine when it is done. Since I do not have access to terminals like this, and I am unlikely to gain permission to hack up the dryer, I thought that a humidity sensor in the exhaust would be a good means to tell when the clothes are dry. I found an inexpensive humidity sensor at Sparkfun, and waited for a chance to experiment with it. With the rest of my family out of town, I have the perfect opportunity this week.

I placed the humidity and temperature sensors behind the dryer’s lint filter and connected them to the good old Arduino as a simple data logger, and then I captured humidity and temperature readings during the first load of laundry I did earlier this week. This is the humidity and temperature profile of a full 60 minute cycle (PDF, 16kB).

I learned a few things by examining the plot.

  • I thought that the heating element cycled on and off more. In reality, the heating element is on continuously for nearly 45 minutes, shown by the constantly rising temperature line. This makes sense, though, since it obviously takes a while to heat several pounds of clothes and all that extra water by 100°. Think about trying to boil a pot of water by pointing a hair dryer at it.
  • Once the load finally reaches the thermostat’s high trip point, apparently 155°, the temperature drops by 35° in 5 minutes to 120°, the thermostat’s low trip point, at which point, the heater comes on again. The difference between the trip points is called hysteresis. (Bonus word for the day. You come across this word a lot in control systems.)
  • I should listen carefully for the pop when the thermostat cycles the heating element off and take the clothes out immediately. This will maximize cat happiness when she lays down on the clothes as I try to fold them. (Using incorrect folding technique, of course. It’s okay, since the cat is fat enough to press out any wrinkles.)
  • The dryer apparently is designed to keep the heat element off during the last 5 minutes of the cycle in order to bring down the temperature of the clothes, presumably so they wrinkle less. You can see that the thermostat wants to turn the heat on at about 53 minutes, but the heater is turned off shortly afterward because the cycle will be ending soon. (By the way, what are wrinkles?)
  • The humidity of the exhaust air goes down as the cycle continues. Surprise! To me, this shows that merely sampling the humidity of the air coming out of the dryer doesn’t exactly say when the clothes are dry. Once the dryer finally got warmed up all the way, the humidity went to about 10% and stayed there. I know the clothes weren’t dry at 45 minutes, because they were just barely dry at 60 when the cycle ended, and the humidity was basically the same at those two points. (Okay, I admit it: I can tell when clothes aren’t fully dry.)
  • When the temperature line is rising, the heat element is on. For this load, the element was on for around 50 minutes. How much did the load cost in electricity use? 5600 W heater × 50 minutes = 4.67 kWh, or 47¢. I’m not sure exactly how big the motor is, probably 1/4 or 1/3 hp, so that probably brought the total to 50¢ for this load.

So, unfortunately, in this round, the humidity sensor I bought looks like it won’t useful in trying to minimize dryer use. But at least I got to teach the Arduino a new trick and to make a graph!

Taking a second look at the project, I noticed some omissions.

  1. I skipped over a detail of the humidity sensor’s datasheet specifying that I needed to place an 80kΩ load across the sensor output, so this may have affected the humidity readings.
  2. The load was really big, and the full 60 minute cycle did not get it completely dry. Perhaps a fully dry load does produce another significant drop in the humidity of the exhaust air.
  3. I should have weighed the laundry before and after running it through the dryer. How much water did it have to remove? Several pounds, or the better part of a gallon, probably.

I will have to try the experiment again later this week after making a couple changes.

Electric update


In the last 24 hours, where the temperature did not go over 2°F, we racked up another $15 in electricity usage. And our thermostat was set to 60°F for much of that time.


Wicked cold snap

kdk_0226bThis cold snap is crazy. Out of curiosity, I wanted to figure how much it costs to heat our house on days like these, where the temperature hovers around 0°F all day long. The answer turned out to be about $7.

If you’re curious too, I arrived at this number by separating the portions of our electric bills that have to do with heating and cooling from the rest. We have a heat pump with supplemental electrical resistance heat, so it’s not quite as straightforward as looking at a gas bill.

The first step in doing that is to figure out how much of the bill to attribute to heating and cooling. I started with the assumption that the heat pump uses roughly the same amount of energy to heat the house one degree in the winter as it does to cool the house one degree in the summer. This isn’t really true, but we have to start somewhere.

Next, we need a way to correlate each bill with how hard the heat pump had to work that month. The National Weather Service keeps climatology records that make this pretty simple. Among the monthly statistics that they maintain are the number of heating and cooling degree days. (For each day, add or subtract the average temperature from 65°F to get the number of degree days. For example, if the day’s high was 20°F, and the low was 10°F, then the average temperature was 15°F. That’s 50 heating degree days, because the heat pump had to keep the house 50° warmer than the outside air.) The more degree days, the harder the heat pump has to work. The National Weather Service gives us the number of degree days each month, so we can show how hard the heat pump had to work each month. The graph below shows our electric bills plotted against the number of degree days that month.


Another assumption is that aside from heating and cooling, our electricity use is constant throughout the year: above a certain baseline, all of our additional energy use is for heating and cooling. The linear approximation in the graph above shows that in a hypothetical month with zero degree days, meaning that the heat pump did not have to do any cooling or heating all month long, our electric bill would be about $55. Anything above $55 on our electric bill is a result of heat pump use. Specifically, each degree day adds 9¢ to our electric bill. 

Given yesterday’s official high of 29°F and this morning’s low of ‑4°F observed on my car’s thermometer, the past 24 hours are worth 52.5 heating degree days, or $4.73. Tomorrow we might get more official data from the NWS’s observed weather history graph.

I need to revisit one of my assumptions, though. Heat pumps do not work very effectively at very cold temperatures. The heat pump can only maintain a certain maximum differential between the outdoor and indoor temperatures. Below this point, it needs to use the electric resistance backup heat, which is much less efficient than the heat pump operating alone. Since yesterday was extremely cold, and we used resistance heat rather than the heat pump, we can simply subtract the difference between the past two mornings’ electric meter readings, and find 99 kWh used in the last day. At our current average rate of 7.6¢/kWh, that’s $7.52. How much of that was due to heating, as opposed to normal electricity use?


The graph above shows that a hypothetical month with zero heating or cooling degree days would have us using 313 kWh of electricity. However, for the past 24 months, our baseline figure has been 164 kWh, or 5.4 kWh/day. So, if we take out 5 kWh for regular household use, we used 94 kWh to heat the house yesterday, which comes to $7.14.