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.
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.
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.
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.
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.
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.
“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.
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.