Wall split dehum facility ?

[fridge doctor]

Hi all,

I have tried a search for this but nothing has come up.
Basically how does a typical small wall split AC unit operate in De-humidifier mode? Obviously a unit designed for the purpose has two coils in the air handler but a wall split only has the one which as you know can be either hot or cold.... but not both. So how can it claim to be a De-hum unit, anyone any ideas ?

[paul_h]

They run at a very low fan speed and high TD, cycling the compressor and fan off to maximise drip time for water to drain away.
I haven't looked into them too much, it's more for humid coutries that are 24C but really humid like in asia. Here in australia it's the heat we worry about, not the humidity so much, so I've never needed to know much about the dry cycle. Panasonic service manuals are pretty in depth if you can get you hands on any one of them, you can see how they work.
But basically it drys by a set program of fan and compressor cycles to minmise cooling but maximise condensate collecting and drain times.
Eg on normal cooling mode, the indoor fan will still run at set speed when the compressor cycles off on temp setting. On dry mode the fan cycles off too, to avoid blowing all the condensate back into the room, and small differences in operation mode like that.

[Thermatech]

You can sometimes find a full description of the Dry mode control stratagy in service manuals & sometimes a good description in manufacturer training material.

This is a cooling mode dehumidification fuction to maximise moisture removal & cool room to set temp.
Generally system cools to set point using a combination of indoor fan off & extra low speed & low speed. This makes maximum condensation or even frost on the indoor coil.
Then the compressor stops for a few mins to allow the condensation or frost buildup to drain away.

Cheap split systems offen have very crude Dry mode control stratagy based on room return air temp & set temp only.
The premium brands have more complicated control stratagy which also controls indoor fan speed & outdoor compressor based on indoor coil temp in addition to return air temp & set point.

The exact control stratagy will depend on the manufacturer & unit model.

Most of the large split system manufacturers design systems for a global market but this type of cooling dehumidification has limits.
So for extreem humidity conditions more specialised solutions may be required as I am sure others will soon point out

[brunstar]

yeah with the dry function on both replies above by slowing the fan speed down we reduce the air flow over the coil allowing the moisture to stick to the coil and not change the room temperature that much, by running in cooling anyway you are removing moisture so that is dehumidifying.
With the daikin ururu sarara, it has a split indoor coil which heats up half of the coil and traps the hot gas in half of the coil and then the other half runs liquid through the second half of the coil and this is a more accurate dehumidification process, with this strtegy they can also achieve humidification.

if you want to see how this one works here is a link.
http://www.youtube.com/watch?v=eHoKyB33h0U

[Abby Normal]

many air conditioners can run in enchanced dehuimidification mode, the problem is that some times from a psyhcrometric point of veiw, you can only control humidity by using reheat or a desiccant.

You can get some situations where a cooling coil, alone, can either supply air at the correct temperature, but the dewpoint is too high, or at the correct dewpoint but the dry bulb temperature is too low.

This will occur when required SHR are low and the process line as drawn on a chart is steep.

This leaves you with three choices.

Endure 75% RH and do not act surprised if you develop a mold problem, use a dehumifiier with reheat, or use a desiccant

[Gary]

Hi all,

I have tried a search for this but nothing has come up.
Basically how does a typical small wall split AC unit operate in De-humidifier mode? Obviously a unit designed for the purpose has two coils in the air handler but a wall split only has the one which as you know can be either hot or cold.... but not both. So how can it claim to be a De-hum unit, anyone any ideas ?

You seem to have the impression that it takes a second coil (presumably a reheat coil) to dehumidify. Not true.

The evaporator coil removes the moisture and the colder the evaporator the more moisture it removes. By slowing the blower speed we make the coil colder.

Reheat does not remove moisture, it simply extends the running time so that the evaporator has more time to remove the moisture.

[Abby Normal]

reheat extends running time because the supply air temperature is too cold

without reheat it is simply an air conditioning unit that is doing the best it can to help control humidity. Adding reheat to it makes it a dehumidifer as it can respond directly to the space humidity without over cooling the space.

When we do a good design job and physics are not against us, a simple air conditioner can properly control temperature and humidity, be none-the-less an air conditioner is something that responds to space temperature that gives us crude humidity control as a side benefit. Some times crude control is fine, other times it is not.

Other times it is not the case of physics going against us, but rather a design problem either mechanical or architectural that gives too much humidity. In those situations a dehumidifier makes for a good band-aid

[Abby Normal]

a nice section on this concept in chapter 4, including "the impossible process" that you deny exists.

http://www.hcbcentral.com/default.htm

click on software, there is a downloadable program that simulates a refrigeration cycle, does some pyschrometrics and other items

[fridge doctor]

Yes, I was referring to a re-heat coil. I accept that of course AC alone is in fact de-humidification as in moisture removal, but depending upon where you live in the world your immediate reaction to the term de-humidify can be different. To some this will mean the removal of unpleasantly high (hot) humid air from a room to make the conditions more comfortable. To others it may mean the removal of dampness (typically from a wall) in what is sure to be a an unpleasantly cold room, and for this purpose a source of warmth is an essential element.

So, it would seem that a wall split, allowing for some variance within manufacturers design in terms of actual operation procedure, can only really tackle humidity of the hot unpleasant type described first, and really isn't much use for the second category of moisture removal ?

However, this has raised another point which had not occurred to me. It has always been my belief, that the unpleasantness felt by a person on a hot day, is as much (if not more) to do with high humidity as the temperature itself? I was always told that a person can be comfortable in a temperature much warmer than he/she is used to PROVIDED the humidity is low. High humidity = sweat rolling off your forehead = unpleasant - even at just 20 degrees or less.

So, if that theory is correct, is there not possibly a case for running domestic AC on Dehum instead during periods of high humidity. If the dehum facility works more efficiently and faster(at moisture removal) than it would in AC mode, then there is a substantial energy saving factor to be considered as the unit is clearly not running for so long ?

[Gary]

There is a price for moisture removal. The system will in fact run less efficiently and for a longer period of time in dehum mode.

[paul_h]

Yes, I was referring to a re-heat coil. I accept that of course AC alone is in fact de-humidification as in moisture removal, but depending upon where you live in the world your immediate reaction to the term de-humidify can be different. To some this will mean the removal of unpleasantly high (hot) humid air from a room to make the conditions more comfortable. To others it may mean the removal of dampness (typically from a wall) in what is sure to be a an unpleasantly cold room, and for this purpose a source of warmth is an essential element.

So, it would seem that a wall split, allowing for some variance within manufacturers design in terms of actual operation procedure, can only really tackle humidity of the hot unpleasant type described first, and really isn't much use for the second category of moisture removal ?

However, this has raised another point which had not occurred to me. It has always been my belief, that the unpleasantness felt by a person on a hot day, is as much (if not more) to do with high humidity as the temperature itself? I was always told that a person can be comfortable in a temperature much warmer than he/she is used to PROVIDED the humidity is low. High humidity = sweat rolling off your forehead = unpleasant - even at just 20 degrees or less.

So, if that theory is correct, is there not possibly a case for running domestic AC on Dehum instead during periods of high humidity. If the dehum facility works more efficiently and faster(at moisture removal) than it would in AC mode, then there is a substantial energy saving factor to be considered as the unit is clearly not running for so long ?

Domestic split a/cs are comfort coolers, nothing more. They do not belong in server rooms, industrial processes or taclking specific problems like moisture removal.
I'm not having a go at you, after all you are just asking about how the drying feature works.
I'm just saying that these cheap splits are cheap for a reason, and just because they are easy to buy and install, doesn't mean engineers should be looking to use splits for anything other than to cool (edit: or heat) a room with a few people in it.

The dry mode in splits is made to remove humidity when the temperature isn't so high. So yes there's energy savings in running in that mode if the indoor temp is 24C but the RH is 90%. But due to the nature of the mode with a lot of off cycling, it would take a while to have much effect IMHO, it would need to be correctly sized for the room for one, which isn't often the case.

[fridge doctor]

Thanks Gary, and yes excellent points Paul. Unfortunately I am now having to deal almost exclusively with cheap units so your observations are particularly relevant to my circumstances. Duly noted and taken on board. Thanks guys.