Dehumidifier: running too much OR poor energy efficiency?
jeffwatson
| Posted in General Questions on
I just installed a 30 pint dehumidifier in the basement of my single story 900sqft house.
It uses about 4kWh per day to maintain a set-point of 55%, as measured by a Kill-a-watt.
I’m in Chicago, Zone 5.
My electricity bill without the dehumidifier from March-April was about 200 kWh/month.
Last month, with the dehumidifier turned on midway through saw a usage of 240kWh/month. A/C has not been used yet.
How can I determine if that 4kWh energy usage is due to it just running too much (e.g., too small for conditions?) or if the equipment just takes too much energy to run?
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Replies
Jeff,
Your question is confusingly worded.
Obviously, if you have measured the energy use of your dehumidifier at 4 kWh/day, then the dehumidifier is using 120 kWh per month. (At 15 cents per kWh, that means that your dehumidifier is costing you $18 per month to run.)
Is the dehumidifier "running too much"? I'm not sure what you mean. If you leave it plugged in, it's running exactly as much as you want it to. If you want it to run less, you'll have to unplug it or set it at 65% RH instead of 55% RH.
Dehumidification is energy-intensive. If you want to achieve 55% RH while using less energy, the only way to do that is to purchase a more efficient dehumidifier. Does your dehumidifier have an Energy Star label?
Another way to approach it would be to ask what the moisture sources are and reduce them so that the run time to achieve 55% humidity is reduced. A likely moisture source is air infiltration--humid outside air leaking in and then getting cooler in the cool basement, leading to even higher relative humidity. But it could also be other moisture sources--see https://www.greenbuildingadvisor.com/blogs/dept/musings/fixing-wet-basement
If it's maintaining the setpoint, it's not running too much.
A typical 30 pinter is about a 500 watt load, so to go through 4kwh/day it's running only about 4000 watt-hours/500 watts= 8 hours per day. That's a 33% duty-cycle, so it has plenty of cycle time to spare. The question is whether it's buying you the requisite amount of dehumidification for that power expenditure.
A new Energy Star dehumidifier is rated to reap 1.85 liters/kwh, so for that power use you should be getting a couple gallons per day out of it. Older 30 pinters could be 1.3l/kwh (or less), but you should be getting at least a gallon per day out of it.
If you're burning through 4kwh/day and only picking up a quart or two, it's probably worth buying a new Energy Star dehumidier. If you're getting 1-2 gallons or better for that 4kwh, it's probably worth keeping it and looking for ways to reduce the load instead.
During the summer in Chicago the outdoor dew points are often in the 60sF (or higher), high enough that air leaks into the basement become a significant moisture load, and is likely to be the primary culprit.
Dana - your answer is spot-on for what I was looking for - a way to assess if your dehumidifier is working too hard (for the environment) or just wasting energy.
I understand reducing moisture first off so that there's no need to mechanically remove it is step 1. But once you've done that as best you can, what I was looking for was how do you assess its performance. Because I read other threads all the time about a dehumidifier using a ton of energy where the end-result is that people just accept "that's just the way it is."
So it sounds like the answer is to measure its duty cycle to determine if it's "working too hard" or still has some horses to spare. Then comparing the water output to its expected liters per kWh rating. Sounds obvious in retrospect :-)
In case you're one of those energy nerds who take it to the next step and will be measuring the liters/kwh precisely, it's important to note that the liters/kwh rating test conditions are at 80F/60% RH conditions. At 70F/55% RH it will be pulling out less water, but it will still be substantial if working properly.
Probably more than you ever wanted to know about dehumidifier testing can be found here:
http://energy.gov/sites/prod/files/2013/12/f6/lab_testing_dehumidifiers.pdf
http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/dehumidifiers_cyclic_operation.pdf
http://www.nrel.gov/docs/fy10osti/47215.pdf
Second link is very informative ("Measured Performance of Residential Dehumidifiers Under Cyclic Operation").
Says high cycling rates are very bad, especially if you have a dehumidifier that runs the fan after the compressor turns off (since as much as 42% of the condensing moisture could be returned to the air). They qualify high cycling as run-times from 3-6 minutes.
Makes me feel like testing a theory of setting the fan speed lower to reach longer runtimes & see if my ~4kWh/day number goes up or down.
Question: does a humidifier have to work harder when it drains into its own bucket that's right under the compressor? This is in comparison to if a hose is connected which empties into a floor drain.
And by work harder, I am saying the unit dehumidifies, condenses the vapor into the bucket, but then the liquid water evaporates from sitting & the humidifier has to dehumidify all over again.
J.,
You asked a question about a humidifier -- but I'm guess that's a typo, and you meant to write "dehumidifier."
I've never heard that evaporation from the bucket of a dehumidifier is enough of an issue to affect the efficiency or performance of the dehumidifier. My assumption is that it's not a significant issue.
I'm attaching a graph which shows the average kWh per day of my dehumidifier, at a set-point of 55%.
Ignoring the huge spike on 7/1 (because I had the set-point at 30%), the left of the huge spike is when I was manually emptying the bucket. To the right, is when I had a hose connected.
Bucket based vs hose-based, there's been a steady decline in power usage with hose-based. The dehumidifier used to come on 10 minutes after it would cut off, now it actually stays off for a while before it comes back on.
There's been no difference in anything except that it's hose-based instead of bucket-based. The only hypothesis I have is that the floor drain was constantly re-evaporating when I would manually empty the bucket {not into the floor drain}. Now with the dehumidifier hosed to the floor drain, there's more "movement" in the floor drain & less evaporation taking place.
Have I simply reached some equilibrium state? It just seems pretty wild that the decline in power would happen right after I went from bucket-based to hose-based.
J,
The other possible explanation is that you have just spent 2 months gradually drying out your basement. Over time, the moisture content of the materials in your basement has dropped, because the dehumidifier has been working properly. As these materials dry out, there's less work for the dehumidifier, and your energy use gradually drops.
In other words, the hard work happens in the first few weeks. Then the work gets a little easier.
I concur with Martin. That was my experience dehumidifying my lower level after my house was constructed.
The weather has also changed. The weekly-average outdoor dew point is usually what drives basement humidity loads in the summertime in much of the US, and can vary by quite a bit from week to week, month to month.
Is it normal for a dehumidifier's fan to run constantly when the compressor is not running? Mine does, and I couldn't find any information regarding basic operation in its manual. I only have fan settings of low and high, so I'm always drawing at least 40 W with the fan on low.
Greg, that is unfortunately common. But not all do it, and on some, you can select whether or not it does that. It is not completely illogical, as that helps maintain uniform humidity through the space, an allow the humidistat to get a better reading on the humidity. But with the low efficiency fans used it can use substantial energy. And it can even put moisture that has condensed on the coil back into the air.
If you are brave about tinkering, you might rewire it so that the fan only runs with the compressor. Or it might work to run it on an external humidistat or a timer. Otherwise you have to decide whether it is worth it to shop for one without that "feature".
Dana - I've considered that but the observation doesn't seem to follow.
The humidity & dew points have been pretty high here (high 60s, low 70s). We've still had a decent amount of rain as well.
The dehumidifier seemed to be working its hardest when dew points were not even 60 (months prior).
Well, unfortunately the lower power usage was not a fluke. Went to the basement this morning and it was locked up with an "F0" error code. Even the power-off button didn't work. According to the manual, "F0" indicates voltage or temperature concerns. This unit was hooked up to a surge protector which was plugged into a Kill-a-watt meter which was then plugged into the outlet. Never saw a basement temp less than 70 degrees or greater than 75.
So apparently the sudden decline in power usage just indicated less dehumidification taking place. The other hygrometers I have still read 55% @ 73deg, so I presume this dehumidifier (FFAD3033R1) was probably aggressive in maintaining the set-point. FWIW, this is the second time this exact dehumidifier failed on me. I used this model last year for 2 months in summer & it failed.