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#1
Old 07-24-2011, 06:19 AM
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Air Conditioner vs Dehumidifier

I have a window AC that has a dehumidifer. What cost more, to run AC or dehumidifer? When I run the D-H it seems to stay cooler than running the AC. What is the difference?
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#2
Old 07-24-2011, 08:36 AM
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With a dehumidifier, the evaporator coil and condenser coil are both in the room. So while it does remove water from the air, the unit also acts as a heater (i.e. it heats up your room).

With an air conditioner, the evaporator coil is in the room and the condenser coil is outside. Like the dehumidifier, it also removes water from the air, but it also removes heat from the room.

I believe an AC would cost more to run.
#3
Old 07-24-2011, 01:12 PM
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Wait, is it all one unit, with the option to run as a dehumidifier or an air conditioner? Although air conditioners also act as dehumidifiers, there are standalone units as well, but I don't know why one would mount in a window.
#4
Old 07-24-2011, 04:13 PM
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I'm also curious...I haven't seen a window AC unit that has a separate dehumidifier. I recently shopped for (and bought) a window unit; most can be run with the fan separately but I don't recall seeing any that had a dedicated dehumidifier function.

Once, during a horribly humid stretch of weather that made the house sticky despite central AC, I brought my basement dehumidifier upstairs and set it in the kitchen. It put out so much heat that I quickly abandoned that idea and put it back in the basement.

As to cost, I think running AC is most expensive. I live in SE Michigan where it can get seriously cold in winter; but my highest bills have been during very hot, humid summer months when I run the AC a lot. (I leave it on during the day for the dogs even when I'm at work, so it tends to run a lot in July-August.)
#5
Old 07-24-2011, 04:27 PM
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Originally Posted by chiroptera View Post
I'm also curious...I haven't seen a window AC unit that has a separate dehumidifier. I recently shopped for (and bought) a window unit; most can be run with the fan separately but I don't recall seeing any that had a dedicated dehumidifier function.

Once, during a horribly humid stretch of weather that made the house sticky despite central AC, I brought my basement dehumidifier upstairs and set it in the kitchen. It put out so much heat that I quickly abandoned that idea and put it back in the basement.

As to cost, I think running AC is most expensive. I live in SE Michigan where it can get seriously cold in winter; but my highest bills have been during very hot, humid summer months when I run the AC a lot. (I leave it on during the day for the dogs even when I'm at work, so it tends to run a lot in July-August.)
Yeah, kind of weird. An AC is in essence a dehumidifier combined with heat transfer.
#6
Old 07-24-2011, 05:02 PM
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Both the window units I own (one samsung, one LG, both probably manufactured within the past five years) have a dedicated "dry" function. The Samsung's manual says it's for use on humid-but-milder days when running the A/C would result in a chilly, clammy room.

I have no idea on when it's really sensible to use it or whether it works. In Chicago, heat and humidity are usually hand-in-hand.
#7
Old 07-24-2011, 05:12 PM
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Originally Posted by typoink View Post
Both the window units I own (one samsung, one LG, both probably manufactured within the past five years) have a dedicated "dry" function. The Samsung's manual says it's for use on humid-but-milder days when running the A/C would result in a chilly, clammy room.

I have no idea on when it's really sensible to use it or whether it works. In Chicago, heat and humidity are usually hand-in-hand.
Makes no sense. Air conditioning doesn't make things clammy. Chilly, yes, but that's because not only is the temp dropping, but moisture is being removed through recirculation. I could see perhaps wanting to dehumidify without cooling during rainy season, when relative humidity is high and there could be mold/mildew problems.
#8
Old 07-24-2011, 06:49 PM
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Originally Posted by Chefguy View Post
Makes no sense. Air conditioning doesn't make things clammy.
I can think of two reasons AC would have a clammy (humid) quality:

1. A significant source of water vapor is entering the room (water leak, etc.)
2. The AC has too many BTUs and thus "short-cycles".

An example of #2... my father installed central AC in his house that had too many BTUs. (The calculations called for a 2.5 ton unit, and he installed a 4 ton unit.) It cools the house very quickly. Too quickly, in fact; it only comes on for 4 minutes at a time, which is not enough time to remove the moisture. Feels cool and clammy in his house during the summer as a result.
#9
Old 07-24-2011, 08:23 PM
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Originally Posted by Crafter_Man View Post
I can think of two reasons AC would have a clammy (humid) quality:

1. A significant source of water vapor is entering the room (water leak, etc.)
2. The AC has too many BTUs and thus "short-cycles".

An example of #2... my father installed central AC in his house that had too many BTUs. (The calculations called for a 2.5 ton unit, and he installed a 4 ton unit.) It cools the house very quickly. Too quickly, in fact; it only comes on for 4 minutes at a time, which is not enough time to remove the moisture. Feels cool and clammy in his house during the summer as a result.
True enough, although it seems odd that the general instructions with an AC would include the cold & clammy comments. Also, if it's cool outdoors, why would one wish to turn on an air conditioner in the first place?
#10
Old 07-24-2011, 11:35 PM
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I'm having honest problems imagining how the dry function would work, though. Does it bring in outside air that it cools, rather than recirculating? Google only came up with a very unhelpful Yahoo! Answer, and I've got to get to bed.
#11
Old 07-25-2011, 12:21 AM
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Originally Posted by gingerdrass View Post
I have a window AC that has a dehumidifer. What cost more, to run AC or dehumidifer? When I run the D-H it seems to stay cooler than running the AC. What is the difference?
what is the make and model?
#12
Old 07-25-2011, 08:25 PM
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Cold and clammy = A/C unit too big (too many BTU) for the given space it needs to cool.

Cold and clammy with A/C = unit too big. A/C won't run long enough to dehumidify, but cools things off very fast. This is a classic case of oversized A/C.

Calculate room size and get a unit that matches it, and the unit will run long enough to cool and dehumidify.
#13
Old 07-26-2011, 09:00 PM
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An A/C system is a dehumidifier by nature.

An A/C system sees, and is engineered to handle, two "loads"; latent heat (humidity) and sensible heat (temperature).

In both a window system and a central system the latent load is handled by condensation; the humid air passes over a cold A/C coil and the water vapor (humidity) condenses to water. The water you hear draining, or dripping out of the back of your window unit, is water that moments ago was water vapor/ humidity.

The sensible load is carried to the condenser coil where a fan "rejects" the heat out into the environment. Thats why you have a unit in the back yard, or why a window unit needs to sit in the window. In both types of units the A/C needs to have access to the outside to reject the heat from your house. A/C, at it's core, is not about making cool, but removing/ moving heat.

A dehumidifier is designed to handle only latent heat. Humid air passes over the cold coil and condenses to water; just like an A/C system. However it has no "access' to the outside so whatever sensible heat is in the room, remains in the room. The condensing coil blows the sensible heat out the back of the unit----right back into the room.

In fact, there is the "heat of compression" and the heat that the compressor/fan generate so a dehumidifier will actually pit a small of extra heat into the room.

The reason you see dehumidifiers in basements----for example-----is that there is little sensible load, as the basement is underground. However there is often a high latent load so a dehumidifier is used to simply remover humidity/ latent heat/ water vapor.

Clear as mud? I thought so.
#14
Old 07-26-2011, 09:12 PM
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Quote:
Originally Posted by gingerdrass View Post
I have a window AC that has a dehumidifer. What cost more, to run AC or dehumidifer? When I run the D-H it seems to stay cooler than running the AC. What is the difference?
"Cooler" is often subjective. You may feel cooler simply because you are drier. For example, you may feel cooler at 75 @ 50% RH, than 73 at 60% RH.

Both temperature and humidity are forms of heat that you experience/ feel. If you feel cooler using the DH function, use it.

As to the cost, I suspect it is incrementally more expensive to use the DH setting, although I doubt you'd see a big difference in your bill.

In both settings the unit is doing the same thing---the compressor and fan are in service. (and the compressor is by far the biggest energy user)

I'm betting the DH setting slows down the fan to force the compressor to run longer in order to capture more humidity; to "dry out" the room.
#15
Old 07-26-2011, 09:18 PM
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ETA
I can see one scenario where maybe the A/C setting is more expensive that the DH setting:

If the compressor is 2 speed, maybe a DH setting uses a lower compressor speed to simply remove latent heat. I kind of doubt it though, given the cheap prices of window units.

It seems more likely that they're simply slowing down the fan. If that's so, I'd bet the DH setting is incrementally more expensive to run.
#16
Old 07-26-2011, 09:44 PM
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Eta to the Eta

In an A/C system the total load is about 70% sensible and 30% latent.

But the system is controlled by temperature/sensible (via a thermostat) and not by humidity/ latent (via a humidistat). (Thats why it is critical not to oversize a system; an oversized system can't control humidity)

So.......if the questions is "which is more expensive to operate, a dedicated DH or an A/C system?", I'd say the A/C system because it is designed to remover both sensible heat and latent heat, while a dedicated DH system is a smaller system because it is designed to remover latent heat only.

But......if the question is "which is more expensive to operate, an A/C system with a DH setting operating in the DH mode, or an A/C system in the normal A/C mode?", I'd guess the DH setting. (for reasons listed above)

Thats my SWAG and I'm sticking to it.
#17
Old 06-02-2013, 09:51 PM
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AC versus Dehumidifier

Assuming the difference between DH and AC is that hot air from the condenser side coil is ducted back into the room for DH (ie not cooling the room but just removing the humidity) then it depends upon the temperature of the condenser coil. I am assuming that when the AC is on it is hot outside say above room temp, then the unit is having to reject heat to a hotter environment which causes higher pressure in the refrigerant system ie more work on the compressor and more electricity. In this case it would cheaper to run DH.

But, if the unit uses the condensate (water from the air) to cool the condenser coil in AC mode but can't do so in DH mode because it would go back into the room, it may actually be more efficient in AC mode. Kind of depends how the unit works!

Could buy one of those $35 power meters and test it...
#18
Old 06-03-2013, 12:58 AM
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OP may have rotted due to excessive humidity...
#19
Old 05-11-2016, 04:03 PM
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AC vs Dehumidifier

Well in reading all the comments it's no wonder the OP got glazed over. Many didn't notice that the dehumidifier exhausted outside since it's a WNDOW UNIT! Well I have a pirtable AC that has a dehumidifier setting both which vent out the window via a vent hose. So I pose the question again on a one that raindog may have answered.....which is more economical to run? The AC function or dehumidifier function. Ironically, the DH setting makes the room cooler and my unit has no way to control that function. Only on the AC setting can I control the temp. I have an edge star ap8000w if that helps. So in conclusion...DH setting freezing at night....AC setting can adjust temp...if it's cheaper to run DH, I'll just use a blanket. If it's cheaper to use AC, I'll forego the blanket.
#20
Old 05-11-2016, 05:15 PM
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Originally Posted by fernando51454 View Post
Ironically, the DH setting makes the room cooler and my unit has no way to control that function.
The entire room gets colder quicker in dehumidify mode or just the exit air temp is colder? These two are not necessarily the same thing. A lower speed fan setting would result in maximum moisture pulled out of the air, and a cooler exit temp, but will not result in maximum room cooling performance.
#21
Old 05-11-2016, 10:12 PM
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Window units with a "dry" or "dehumidification" mode just run at the slowest fan speed and allow a greater temperature swing than normal to maximize the time to remove moisture without chilling the room too much. That's as sophisticated as they generally get. This mode is meant for damp days that are otherwise too cool for proper dehumidification to happen because the a/c isn't running long enough.

Running a window a/c on its lowest fan setting is actually the least efficient. Because there's only one fan motor for both the indoor evaporator and outdoor condenser coils, the slower fan speed leads to a higher condensing temperature and pressure that makes the compressor use more power. So even though a higher fan speed uses a bit more electricity for the fan motor, it's offset by the reduced compressor load AND it maximizes the cooling capacity as well, thus it doesn't run as long. So really the only time not to run a window a/c at full blast would be if you really do need to dehumidify beyond what the unit normally handles, if it's cold outside (there's edge cases where you might need to do this, but it can damage the compressor, and low fan speed can keep the evaporator from freezing due to lower efficiency), if the unit is oversized, and of course noise.

Last edited by jjakucyk; 05-11-2016 at 10:14 PM.
#22
Old 05-13-2016, 07:49 AM
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Originally Posted by Chefguy View Post
Makes no sense. Air conditioning doesn't make things clammy. Chilly, yes, but that's because not only is the temp dropping, but moisture is being removed through recirculation. I could see perhaps wanting to dehumidify without cooling during rainy season, when relative humidity is high and there could be mold/mildew problems.
I beg to differ. Smaller home A/C units simply cool the air and accept the higher R.H., but commercial A/C units for larger buildings (I think always) cool the air to near freezing, extracting most of the moisture out of the air, then reheat the air to the desired temperature. Not doing this results in unbearably high R.H. and very muggy air. So in large systems simple A/C does in fact make the air clammy.

I believe some window units do in fact have the option of reheating the drier cooler air in some fashion which is exactly what a dehumidifier does. But I have not used any such systems.

I just realized I was replying to a 5 year old post. Darn zombies.

Last edited by rbroome; 05-13-2016 at 07:53 AM. Reason: zombie! darn
#23
Old 06-10-2016, 11:17 AM
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Originally Posted by rbroome View Post
I beg to differ. Smaller home A/C units simply cool the air and accept the higher R.H., but commercial A/C units for larger buildings (I think always) cool the air to near freezing, extracting most of the moisture out of the air, then reheat the air to the desired temperature. Not doing this results in unbearably high R.H. and very muggy air. So in large systems simple A/C does in fact make the air clammy.

I believe some window units do in fact have the option of reheating the drier cooler air in some fashion which is exactly what a dehumidifier does. But I have not used any such systems.

I just realized I was replying to a 5 year old post. Darn zombies.
Actually, Chefguy is more accurate here. Commercial systems do *not* in fact have near "freezing" temperatures. Whether it's a home or business the target temperature is 55 give or take 5. Dehumidifation is a function of sizing the capacity of the unit to the building envelope.

For example if put a 5 ton unit (or 50!) in an environment that needs 2 tons, for example the oversizing causes the unit to "overwhelm" the space, removing sensible heat (the heat you can read with a thermometer) so rapidly it doesn't run long enough to remove "latent" heat. (humidity)

As mentioned up thread an A/C system is fundamentally a dehumidifier. About 70% of the heat removed is sensible and 30% latent (water vapor, or humidity).

That's why sizing is critical. While there are other factors that can produce clammy conditions, sizing is often the culprit.

Reheat systems are uncommon in hospitals and computer rooms. And a few office buildings. But you cant manage humidity by having freezing air temps without causing a host of other problems.
#24
Old 06-10-2016, 06:27 PM
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There's a component to "reheat" in all air conditioners regardless. While the air supply temperature is usually in the 50 degree range, that means the evaporator coil itself is near freezing.* That near freezing evaporator coil ensures the most moisture condenses on it and runs away. This means that the 50 degree air being blown out of the unit is pretty close to saturated too. You can sometimes see this in cars where the a/c will blow out a light fog in certain conditions. That cold but humid air still dries the room as it's reheated by mixing with the room air.

As mentioned above sizing the equipment properly is generally the only component utilized for humidity control. It's only in specific environments where relative humidity needs to be tightly regulated that active reheating would be used. Even then I suspect they'd still do a lot of the control by varying fan speed instead.

*In unsophisticated systems like a window unit, they can easily go below freezing when it's cold outside and ice over the evaporator coil. Central air and commercial systems have expansion valves with sensing bulbs to help ensure that doesn't happen.
#25
Old 06-11-2016, 06:10 PM
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Originally Posted by jjakucyk View Post
There's a component to "reheat" in all air conditioners regardless. While the air supply temperature is usually in the 50 degree range, that means the evaporator coil itself is near freezing.* That near freezing evaporator coil ensures the most moisture condenses on it and runs away. This means that the 50 degree air being blown out of the unit is pretty close to saturated too. You can sometimes see this in cars where the a/c will blow out a light fog in certain conditions. That cold but humid air still dries the room as it's reheated by mixing with the room air.

As mentioned above sizing the equipment properly is generally the only component utilized for humidity control. It's only in specific environments where relative humidity needs to be tightly regulated that active reheating would be used. Even then I suspect they'd still do a lot of the control by varying fan speed instead.

*In unsophisticated systems like a window unit, they can easily go below freezing when it's cold outside and ice over the evaporator coil. Central air and commercial systems have expansion valves with sensing bulbs to help ensure that doesn't happen.
I'm not sure what you mean "There's a component to "reheat" in all air conditioners regardless." There is something called a "hot gas bypass" and that can be used for freezing/ capacity control but it's very rare---I haven't seen one in residential A/C systems in 15 years. They're more common but still uncommon in commercial A/C. I know of no other component that reheats the gas.

Varying fan speed is a good way to manage humidity through slowing the fan speed in a 3 speed motor or through an ECM motor or a VFD in commercial environments.

The primary way to manage humidity in a residential environment is proper sizing. It can be fine tuned-----and most often isn't needed-----though evaporator sizing and fan speed. In commercial environments, a VFD with reheat and even a humidistat

Because of a pressure temperature relationship, different refrigerants will all be at different pressure at any given temperature. It's a given. If you consider 40 degrees "near freezing", that's "near freezing." With the proper sizing of not only the unit but duct work (and other things) an HVAC tech tries to manage the temperature of the indoor coil (by managing pressures) to 40 degrees.

If all things are done correctly, a 40 degree coil will produce a 20 degree drop in temperature of the air flow; so, ideally, the return air is 75 degrees and the supply air is 55 degrees. If all the variables are done right (especially sizing, but that's not all) it will produce humidity of 50-55%--which is ideal for human comfort.
#26
Old 06-11-2016, 06:26 PM
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Originally Posted by jjakucyk View Post
There's a component to "reheat" in all air conditioners regardless. While the air supply temperature is usually in the 50 degree range, that means the evaporator coil itself is near freezing.* That near freezing evaporator coil ensures the most moisture condenses on it and runs away. This means that the 50 degree air being blown out of the unit is pretty close to saturated too. You can sometimes see this in cars where the a/c will blow out a light fog in certain conditions. That cold but humid air still dries the room as it's reheated by mixing with the room air.

As mentioned above sizing the equipment properly is generally the only component utilized for humidity control. It's only in specific environments where relative humidity needs to be tightly regulated that active reheating would be used. Even then I suspect they'd still do a lot of the control by varying fan speed instead.

*In unsophisticated systems like a window unit, they can easily go below freezing when it's cold outside and ice over the evaporator coil. Central air and commercial systems have expansion valves with sensing bulbs to help ensure that doesn't happen.
Expansion valves are like carburetors; they regulate the flow of refrigerant to fine tune the flow of refrigerant based on conditions. For example, on a 80 degree day you need a smaller A/C than if it's 95 degrees. So, a an expansion valve is a "capacity control device", and isn't designed as a freeze protection. If your suction line/ coil temp is approaching 32 degrees, I wouldn't be looking for the TXV to save me. (although a failed expansion valve could "starve" the evaporator and cause freezing)

Whether it's window unit or central system, if it's sized correctly it won't freeze up, because cooler temperatures outside mean lower loads on the house and as a result less of a burden on the house. The result is the thermostat sees less load and "satisfies" and shuts off the unit. So, even on a cool day it shouldn't freeze up---it sees less demands and shuts off.

Maybe........if someone tried to maintain 63 degrees on an 75 day----with an oversized unit------maybe.
#27
Old 08-23-2016, 05:19 PM
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Originally Posted by raindog View Post
Expansion valves are like carburetors; they regulate the flow of refrigerant to fine tune the flow of refrigerant based on conditions. For example, on a 80 degree day you need a smaller A/C than if it's 95 degrees. So, a an expansion valve is a "capacity control device", and isn't designed as a freeze protection. If your suction line/ coil temp is approaching 32 degrees, I wouldn't be looking for the TXV to save me. (although a failed expansion valve could "starve" the evaporator and cause freezing)

Whether it's window unit or central system, if it's sized correctly it won't freeze up, because cooler temperatures outside mean lower loads on the house and as a result less of a burden on the house. The result is the thermostat sees less load and "satisfies" and shuts off the unit. So, even on a cool day it shouldn't freeze up---it sees less demands and shuts off.

Maybe........if someone tried to maintain 63 degrees on an 75 day----with an oversized unit------maybe.
My stand alone ac/ dehumidifier works better to cool my room with the dehumidifier on. As long as I have the hose going out the window for the hot air
#28
Old 08-23-2016, 05:21 PM
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Dehumidifier all the way

My dehumidifier works better than my air conditioner to cool my room. As long as I have the hose going out the window for the hot air
#29
Old 08-23-2016, 05:27 PM
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Another word for a dehumidifier that exhausts the hot air out the window is "air conditioner."
#30
Old 08-23-2016, 08:09 PM
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Large building HVAC cold deck temperatures never get close to freezing.

If the building has a chiller the chilled water temp might be as low as 40 degrees but probably in the 45 degree range.

The cold deck temperature will probably be between float from 50 to 70 degrees depending on the outside air temperature. Either with a chiller or direct expansion system.

Single deck systems with reheat are not as efficient but are easier to control and cheaper to install.
#31
Old 08-24-2016, 11:51 AM
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None of you have explained exactly what a window A/C does on the dry cycle.

I have used several different units over the years, and have 2 Goldstars now. Here are the cycles:

Fan: Room blower only to circulate air in the room.

Cool: Compressor runs to provide cool air, room blower distributes it. When set temperature is reached, the compressor shuts off, but the room blower stays on. This provides for a quick response to warming room temperature, but puts moisture from the evap coil back in the room. Fan speed can be set low or high.

Dry: Low fan speed is selected. When set temperature is reached, everything shuts off. Every 5 minutes, the room blower runs to sample air temperature. If high enough, the compressor starts. If still cool, it shuts off for another 5 minutes. Much less moisture is placed back in the room, but (as mentioned) temperature swing is considerably higher.

Economy: This is a separate button that can be selected only in the Cool cycle. It operates just like the Dry cycle, except the high fan speed can be selected.

Where I live (northern Ohio, in the woods), I can very seldom just let the units run on cool. Way too high humidity in the room (I have sensors all over the house). The Dry or Economy modes are much better.

My main living area is handled by a mini-split system. It has a different dry cycle. The entire unit runs for 8 minutes then shuts off for 3 minutes. Low fan speed only, fixed cycle, the times cannot be adjusted. No wet air from the evap coil is ever put back in the room. Although it might seem to be somewhat limited because of the fixed cycle, it works amazingly well over a wide range of outside temperatures. If you could alter the on/off cycle times it would be perfect.

Dennis

Last edited by mixdenny; 08-24-2016 at 11:56 AM. Reason: Additional information
#32
Old 08-24-2016, 01:43 PM
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Originally Posted by mixdenny View Post
None of you have explained exactly what a window A/C does on the dry cycle.
I did: http://boards.academicpursuits.us/sdmb/...3&postcount=21
#33
Old 08-24-2016, 04:31 PM
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You said:

"Window units with a "dry" or "dehumidification" mode just run at the slowest fan speed and allow a greater temperature swing than normal to maximize the time to remove moisture without chilling the room too much."


But that is not the whole story, the dry cycle completely shuts off all fans, unlike the cool cycle. Crucial difference. You can't leave the circulating fan running in marginal conditions and expect the best dehumidification.

The best system I ever had was when I used a PID controller to cycle a window A/C on and off based on real room temperature, not return air temperature (too much recirculation). Best temperature and humidity control I ever had, but not easily adaptable to modern window A/C units.

Dennis
#34
Old 08-24-2016, 06:10 PM
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I've found this thread several times doing google searches for this exact question. No posts in this thread answer the question, in my case at least.

I have a Honeywell portable AC 16,000 BTU with a single exhaust hose. It has 3 modes: Fan, AC, and Dehumidify.

In AC mode you set the thermostat to the desired temperature and the compressor cycles on and off around that setting. (i.e. when it gets cooler than your setting the compressor stops, and when it gets warmer than your setting it starts again). This is the typical operation of all air conditioners.

Alongside this mode, is DH mode. When set in DH mode the thermostat controls aren't accessible. The fan speed can be set to any speed from lowest to highest, so it has nothing to do with fan speed as mentioned above. However the thermostat can't be adjusted. The compressor cycles on and off on it's own schedule with no user control. It seems to be doing nothing more than what it does in normal AC mode - extracting heat and humidity from the room and dumping it outside. The only difference seems to be that you can't adjust the thermostat setting. The other difference, as noted in the 5 year old OP, is that it actually cools the room better then when in AC mode.

The manual says that DH mode can be used on days that aren't particularly hot but are still humid. Makes sense, except as noted by the OP and several throughout this thread, the DH mode actually makes the room COLDER than can be achieved by setting the thermostat to the lowest possible setting in AC mode.

I'm in a particularly challenging environment for an AC. It is easy for me to see this effect because AC mode is pushed to the very limit of its cooling ability. On a day that is 95 degrees and 90% humidity the unit can be set to the lowest possible setting (I think about 65 degrees) and the compressor will just run full time without ever cycling since it can't get the room cooler than about 80 degress. At this point, however, if I put it in DH mode, it gets the room down to about 75 degrees or even cooler. And the compressor cycles on and off as usual without any connection to the thermostat setting. Something about the DH mode allows it to cool the room more efficiently than AC mode in these conditions.

It has nothing to do with fan speed or exhaust tubes. It seems to use the compressor and fans exactly the same way as AC mode does, but for whatever reason dehumidify mode can cool the room a few more degrees lower than AC mode can at the coolest setting in an environment where the AC is stretched to the limit.

Why?
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