Do Dehumidifiers Reduce Energy Use?
Conventional wisdom will tell you that operating a dehumidifier during months when your air conditioner is running will reduce the strain on and energy consumption of your air conditioner. This is because your air conditioner can only pull a certain amount of energy from the air for a given amount of electricity used. With more moisture in the air, the AC spends more energy reducing the humidity and less on cooling. Then your AC runs longer and uses more energy to maintain a comfortable temperature.
But the dehumidifier doesn't run on wishful thinking. It is not uncommon for an Energy Star approved dehumidifier to use 15 amps ( 1.8 kilowatts ) which, at 13 cents per kilowatt hour, could cost up to $5.62 each day to operate. Dehumidifiers are also quite expensive. So, are they really worth the investment? I recently purchased a dehumidifier to help nail down for all of use exactly how much they can help or hurt our energy efficiency.
This experiment will be ongoing. I will provide the purchase price as well as average monthly energy use prior to installing the dehumidifier and each month, and the energy use with the dehumidifier. Using the electricity price found in my blog Energy Price Tracker, I will determine how much I am saving (or loosing) do to the use of my dehumidifier.
Week 1: It has been a long hot week. Over 100° yesterday, today and tomorrow too. When we plugged in the dehumidifier last week it ran for about two days straight which really concerned me- I could just see the electric meter spinning and spinning in my mind. But, I was emptying the 13 pint bucket two and three times per day so it must be doing some good. Over the weekend I didn't notice much of a difference with the device running. When my wife and I went back to work on Monday, it became evident. My wife turned the thermostat up to 80° when she left the house Monday. When I returned home that afternoon I thought she forgot about it. The house was cool even with the high in the upper nineties. I checked the thermostat and saw there was no mistake, even with the air in the home being eighty, it felt cool. "perhaps it is because of the great temperature difference" I told my wife, "that might be why I felt so cool." The very next day I came home to a sweltering, stiflingly hot home. I had forgotten to empty the bucket that morning and the dehumidifier had shut off during the day. Once I had it emptied and running again the display showed that the humidity in the room was 70%. Indeed, the comfort of a home can be greatly improved by the presence of a dehumidifier. But is it going to save me money?
After the dehumidifier ran for a few days I began listening at night for it to turn on and off. Tracking the machine's duty cycle is part of measuring how much energy it is using. So far it averages cycling every 18 minutes, on for 13, off for 5. So it is operating 72% of the time- about 17 hours, 20 minutes each day. Next, I need to know how much current it is using. With an ammeter and a length of extension cord I measured the current - 0.37 amps. Now that we know the current and amount of time each day (assuming we know the voltage to be 120 volts) we can determine the energy used each day and the money spent on running the humidifier.
120 V /1000 = .12 kV x 0.37 A = 0.044 kW x 17.33 hours per day = 0.77 kWh per day x $0.13 per kWh = $0.10 per day
Ten cents per day! That's three dollars per month. Now I am really looking forward to see what happens to my electric bill. How much will it reduce the strain on my air conditioner?
One important factor to consider is placement- where will you put the dehumidifier? It is heavy, the cord is about 6 feet long, and you will have to carry the bucket to a drain a couple times each day. So put it somewhere that it will be convenient. Also consider that it will work best near sources of moisture- the bathroom, kitchen, bedrooms (snoring puts off a lot of moisture), and house plants. It will also be beneficial if the exhaust is facing a return duct if at all possible so it can deliver dry air back to the air conditioner. I made the mistake of placing it in a bedroom for a couple days facing into the hallway where the thermostat is located. The constant stream of warm air into the hall ran the air conditioner enough to keep the house two degrees cooler than the set temperature. Hopefully, this will not eat away the gains from the rest of the month.
In the second week of use we are becoming more accustomed to the dehumidifier. Our home is sustaining a comfortable feel even though we have the thermostat set at 76°- three degrees warmer than we used to set it during the summer. The dehumidifier is still set to a low fan setting and removing about 13 to 14 pints of water from the air each day.
The second part of this week we shut the dehumidifier off because several people had caught colds and the dryer air seamed to be causing irritation. We also received our electric bill for July. It was a shocker! We used 193 kilowatt hours more than the average July, this cost us an "additional" $24.94. This disappointing news can be tempered however, there may be some mitigating factors.
1.) The first month we only used the dehumidifier for about 2 1/2 weeks of the 4 1/2 that were on the bill, so if this was a very hot July, it may not have had time to have a large impact. This being the first month, the first couple days the dehumidifier had to run more and may have increased energy use significantly for a few days.
2.) If this was a particularly hot July, the average July may not be the measure against which I need to contrast usage. I will need to research the cooling degree days for each of the past Julys which I have information for and divide them into the energy usage during those months to determine how much I should have used given this months weather. Then I can determine how much I saved (or lost) this month by having a dehumidifier. I will report this back soon and will prepare to have this information for August as well. If both months appear to increase usage even considering the weather then I will cut this experiment short and consider the notion that dehumidifiers save energy to be debunked.
3.) The final thing to consider is that my home is on the market. Leaving lights on for showings and open houses may be causing part of the rise from average energy use that I am experiencing.
Week 3 continued
Cooling degree days is a measure of how severe or mild a season is. It is essentially derived by subtracting a standard temperature that we expect to be a comfortable indoor setting from the daily high temperature in a given location. If it is 85° outside and we expect to set the thermostat to 73° then we had 12 cooling degree days that day. when you add these numbers up throughout the month you can compare one year to the next.
To determine if the weather is effecting whether I am saving or loosing energy by having a dehumidifier I am finding the average kilowatt hours used per cooling degree days for months dating back to 2010. I found the monthly total of cooling degree days on wunderground.com. I then multiply the average by the number of cooling degree days we had this year to determine the anticipated energy use. In this case, I expected to use 1,040 kWh, slightly above the average of 1,016 kWh use, but well above what I used last July when we had nearly the same number of cooling degree days. Adjusting the chart we find that little changed with the new math applied.
Given the unsuccessful July, we have begun cutting back our use of the dehumidifier. It heats up whatever room it is in (a result of the condensation process) and seems to be using large amounts of energy. I still believe, however, we need to give it one more chance to prove its value.
We have now revived the dehumidifier with a change of location. It is now in the largest room of the house to maximize the dehumidifying effect but the exhaust is aimed now towards the hallway where the thermostat is located. This is the condition which originally lead to the over-cooling of the house which likely increased July's energy use. This time, however, we have turned up the thermostat an additional 4 degrees to 80°. With the dehumidifier pointed into the hallway and the thermostat turned up, we can keep our home at a comfortable 77-78 degrees. Now when I say "comfortable", I mean it. I'm not comparing my home to what a border guard experiences on the job- in full body armor, in southern Arizona, in August- it really is comfortable. I know that is hard to believe, so I want to know why it feels comfortable even at 77 degrees. The first thing I have tried is to consult the National Oceanic and Atmospheric Association. Their heat index calculator located at http://www.wpc.ncep.noaa.gov/html/heatindex.shtml doesn't provide the answer I was expecting. My assumption was that 73° with 80 percent humidity would have the same heat index (74°) as 77° with 45 percent relative humidity (77°) but at those temperatures, the humidity doesn't make enough of a difference. In fact, the calculator shows that the humidity would have to be at a wood-splitting, nose-bleeding 2% to "feel" like it was only 74°. If you feel you have an answer, tag me on Facebook, Twitter, or send me an email. And thanks for following along.
For a better definition of "comfortable temperature", I have to revert back to how it was described in my energy auditing training. A person is most comfortable when the net energy gain between their body and the environment is near zero. That will differ for everyone, if you release body heat slowly (or produce a lot of heat), you will want to be in a cooler room. On the contrary, if you loose body heat rapidly or don't generate much, you will need a warmer room to maintain your body temperature. This is to say that, while my family is comfortable in our home with a heat index of 77°, you may want it warmer or cooler. If this experiment works, it may be beneficial for everyone to buy an efficient dehumidifier. If you know the temperature and humidity at which you are comfortable, it will be possible to find a higher temperature (and lower humidity) at which you will also be comfortable. Hopefully, this will help you save energy and money.
Happy Labor Day everyone. Hopefully by the end of this week I will have my August energy bill and I will be able to make a determination one way or the other. It has been a very mild August, but I found last month that in Kansas, August is usually (although surprisingly) cooler than July. Following this month's pricing and analysis I will likely only check in from month to month to add data. I will make an announcement if and when I finally break even for the purchase- assuming there are any savings to be had.
I am officially putting an end to this experiment. While an experiment which is conducted more scientifically would need more data to come to a conclusion I think that August's electric bill leaves no need for further study. While this August has been the most mild August dating back to 2010, only once in those 7 years did I use more electricity than I did this month. That year I used 19 more kilowatt hours than I did this year, but we had nearly twice the number of heating degree days. Given the enormous difference between the expected energy use and the actual energy use I see no reason to continue using the dehumidifier. I have to declare this notion debunked- while using a dehumidifier to make a home more comfortable, or to save a basement from water damage is possible- I don't see any way it will save you any money on your electric bill.
Month Expected Energy Use With Dehumidifier Energy Reduction Price/kWh Gain or Loss Running Total
Initial Purchase of the Dehumidifier -$247.24 -$247.24
June 2017 (not used) 865 XXX X X X XXX -$247.24
July 2017 1040 1209 -169 $0.1292 -$21.83 -$269.07
Aug 2017 606 980 -374 $0.1279 -$47.83 -$316.90