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Old 07-23-2012, 09:36 PM   #1
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Will a dehumidifier help or hurt?

So, the recent HOT and HUMID weather has me thinking. I have a dehumidifier for cold weather condensation issues, but will a dehumidifier actually make the A/C work better?

I keep going back and forth in my mind, so I'm looking for some expert advice....not necessarily a popularity vote. I am not talking about energy efficiency, but rather, cooling effectiveness. Some of that may be subjective (science) and some of it may be objective (does it feel cooler).

1) So, if I remember my high school science right, condensing water vapor on the evaporator of the A/C transfers heat to the evaporator, making the output air warmer (or less cold, if you will). This is seen as water literally pouring out the drain tube.

2) running a dehumidifier at the same time generates heat from the dehumidifier, but results in less cooling loss (heat gain) over the A/C evaporator due to a lesser amount of water vapor condensation.

Question: does the gain in A/C cooling effectiveness due to drier air inside the AS supersede, equal, or detract from A/C effectiveness?
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Old 07-23-2012, 09:47 PM   #2
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A dehumidifier is nothing more than an air conditioner. While running a dehumidifier inside will bring the humidity down quicker that the AC alone it will be dumping heat inside also.

Since we have not surpassed perpetual motion yet I doubt there is net gain.
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Old 07-23-2012, 09:51 PM   #3
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A dehumidifier is nothing more than an air conditioner. While running a dehumidifier inside will bring the humidity down quicker that the AC alone it will be dumping heat inside also.

Since we have not surpassed perpetual motion yet I doubt there is net gain.
But there in lies the objective part. Will a couple degrees warmer air, but much drier, feel more comfortable. This isn't about perpetual motion.....it's about drier air with available equipment. Many of us, with big trailers, one A/C, and marginally adequate A/C capacity need all the help we can get.

AND, again, will passing drier air over the evaporator result in any significantly cooler air output, due to less heat gain over the evaporator with less humid air?
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Old 07-23-2012, 10:01 PM   #4
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Yes
Warm dry air is more comfortable than cool damp air.
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Old 07-23-2012, 11:43 PM   #5
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You don't see any body using a dehumidifier in their home when the AC is operating do you?

If your AC is marginal, there are lots of things you can do to help the performance effectiveness of it. See the ongoing thread about warm AC.

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Old 07-24-2012, 04:02 AM   #6
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I run a dehumidifier in my 34' when it is parked. Being in south Georgia on the ocean, this is a necessity to keep humidity issues in check.my dehumidifier has a humidistat i set at about 50%...it runs quite a bit. As far a the ac goes, i find that during the day, my ac can't compensate for the heat gain from the dehumidifier. Itypically run one at a time. That being said, warmer less humid air is more comfortable than colder humid air. Many people compensate for higher humidity by turning the tstat lower.
Here in coastal Georgia, i have seen supplemental dehumidifiers in our home ac units on many occasions. In many systems here, the mech engineers will design to 55%rh but it will float up in the 60% range in many applications.
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Old 07-24-2012, 06:26 AM   #7
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You don't see any body using a dehumidifier in their home when the AC is operating do you?

If your AC is marginal, there are lots of things you can do to help the performance effectiveness of it. See the ongoing thread about warm AC.

Dan
Yes, in their basements. Mine included.
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Old 07-24-2012, 06:28 AM   #8
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Originally Posted by c_lewis77 View Post
I run a dehumidifier in my 34' when it is parked. Being in south Georgia on the ocean, this is a necessity to keep humidity issues in check.my dehumidifier has a humidistat i set at about 50%...it runs quite a bit. As far a the ac goes, i find that during the day, my ac can't compensate for the heat gain from the dehumidifier. Itypically run one at a time. That being said, warmer less humid air is more comfortable than colder humid air. Many people compensate for higher humidity by turning the tstat lower.
Here in coastal Georgia, i have seen supplemental dehumidifiers in our home ac units on many occasions. In many systems here, the mech engineers will design to 55%rh but it will float up in the 60% range in many applications.
Thanks, that's the answer I was looking for.
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Old 07-24-2012, 06:54 AM   #9
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Engineering explanation for non-engineers follows. You have been warned…

An air conditioner dehumidifies all on its own, IF the temperature setting on the air conditioner is lower than the dew point of the ambient air. In fact, in wet climates about half the energy used by an air conditioner ends up going into dehumidification rather than cooling, on average.

Here's the kicker; an air conditioner will only dehumidify for the first day or day and a half of circulating the same air; after that, all the water vapor that's going to come out of the air has come out of the air.

If you're cooling air at a temperature of 90F, and the dew point is a sultry 85F (typical for the Gulf Coast, where I live), then the air conditioner will typically lower the dew point of the conditioned air to about five degrees lower than the inside air temperature. If you get the air down to about 70F, the dew point will be about 65F. Relative humidity remains mostly unchanged, and it doesn't feel any drier, but the air is in fact drier as indicated by the lower dew point.

If you use a separate refrigerant-style dehumidifier with the same capacity, then using the same example as above, 90F ambient air with a dew point of 85F, running a dehumidifier provides you with air that is still 90F or a little hotter, but with a dewpoint around 65F as above. The air has the same dew point as with the air conditioner, but the relative humidity is much lower, so the air feels a whole lot drier.

In both cases, you end up with the same dew point. The energy you use to run the dehumidifier, added to the energy to cool the air, gives you a net loss in efficiency. You're better off with the air conditioner alone, in most cases, in terms of energy usage per degree of temperature and dew point reduction.

Home units where people combine a dehumidifier with an air conditioner may make the air feel slightly drier at cooler temperatures if the dehumidifier has a higher capacity than the AC, but energy use goes up, too. Not necessarily something you want for your RV.
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Old 07-24-2012, 07:00 AM   #10
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Engineering explanation for non-engineers follows. You have been warned

An air conditioner dehumidifies all on its own, IF the temperature setting on the air conditioner is lower than the dew point of the ambient air. In fact, in wet climates about half the energy used by an air conditioner ends up going into dehumidification rather than cooling, on average.

Here's the kicker; an air conditioner will only dehumidify for the first day or day and a half of circulating the same air; after that, all the water vapor that's going to come out of the air has come out of the air.

If you're cooling air at a temperature of 90F, and the dew point is a sultry 85F (typical for the Gulf Coast, where I live), then the air conditioner will typically lower the dew point of the conditioned air to about five degrees lower than the inside air temperature. If you get the air down to about 70F, the dew point will be about 65F. Relative humidity remains mostly unchanged, and it doesn't feel any drier, but the air is in fact drier as indicated by the lower dew point.

If you use a separate refrigerant-style dehumidifier with the same capacity, then using the same example as above, 90F ambient air with a dew point of 85F, running a dehumidifier provides you with air that is still 90F or a little hotter, but with a dewpoint around 65F as above. The air has the same dew point as with the air conditioner, but the relative humidity is much lower, so the air feels a whole lot drier.

In both cases, you end up with the same dew point. The energy you use to run the dehumidifier, added to the energy to cool the air, gives you a net loss in efficiency. You're better off with the air conditioner alone, in most cases, in terms of energy usage per degree of temperature and dew point reduction.
Thanks, that make a lot of sense.
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Old 07-24-2012, 07:50 AM   #11
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A dehum will also dry air from showers and cooking without outside air intruding into the trailer. Dry at 82 feels much better than humid a a much lower temp. Everytime a door is opened more hot humid air enters, a trailer is not a sealed environment. Fans are a must to be comfortable. Jim
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Old 07-24-2012, 08:10 AM   #12
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A dehum will also dry air from showers and cooking without outside air intruding into the trailer. Dry at 82 feels much better than humid a a much lower temp. Everytime a door is opened more hot humid air enters, a trailer is not a sealed environment. Fans are a must to be comfortable. Jim
Yeah, that's part of the reason for the question in the first place. I think we all get the need for the DH in the cold months, when our respiration, etc. wets the windows and walls. And the heat of the DH actually helps the interior temp.

I think the above post (Protagonist) answers the high temp and high humidity scenerio.

But I still wonder about those times when you need A/C, but not much (like 80*) and very humid. I have noticed that the A/C doesn't run too much at that temp (in shade....we have a lot of forested campgrounds up here), but the humidity inside is still high. If I turn th A/C lower (temp), it get cold and clammy. I think running the DH and the A/C would dry the air better at a more comfortable higher temp and there would be more than enough A/C capacity to overcome the heat generated by the DH.
I get that it's not very energy efficient, but like you say, our little homes behave differently (small volume, frequent door openings, etc.) than our houses.

Guess I'll have to experiment....just thought some of you may have practical experience.
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Old 07-24-2012, 08:43 AM   #13
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Quote:
Originally Posted by dznf0g View Post
Yeah, that's part of the reason for the question in the first place. I think we all get the need for the DH in the cold months, when our respiration, etc. wets the windows and walls. And the heat of the DH actually helps the interior temp.

I think the above post (Protagonist) answers the high temp and high humidity scenerio.

But I still wonder about those times when you need A/C, but not much (like 80*) and very humid. I have noticed that the A/C doesn't run too much at that temp (in shade....we have a lot of forested campgrounds up here), but the humidity inside is still high. If I turn th A/C lower (temp), it get cold and clammy. I think running the DH and the A/C would dry the air better at a more comfortable higher temp and there would be more than enough A/C capacity to overcome the heat generated by the DH.
I get that it's not very energy efficient, but like you say, our little homes behave differently (small volume, frequent door openings, etc.) than our homes.

Guess I'll have to experiment....just thought some of you may have practical experience.
Have to agree. If the temperature is cool enough that you won't be sweating, running a dehumidifier without the AC is probably better than running the AC in terms of comfort.

Agree with the point about the shower as well. If you're pumping extra water into the air, either a dehumidifier or an air conditioner has to keep working to extract the extra water. However, for RV use, I still think dehumidifiers and air conditioners are an either/or proposition.

UNLESS you're using an absorbent media (zeolite?) for dehumidifying. My comparison was only for refrigerant-type dehumidifiers. An absorbent dehumidifier that uses no electricity except perhaps a fan (no refrigerant), and will have a measurable benefit even when you run your AC, with little to no increase in power consumption. Should have mentioned that before. Sorry about that.
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Old 07-24-2012, 08:57 AM   #14
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I lived in my Overlander for two years while renovating a home and had a permanently installed dehumidifier. I had a cut out through the floor so it would drain to the outside. It was running all day so that when I got home, the AC did not have to work so hard and the trailer cooled off quickly. (Here in the sweltering summers of Tampa FL) It was automatic, so when the AC was on and it would shut off. It worked very well, and I will be installing one in my new project as well.
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