Portable air conditioner - one hose or two?

Factual Questions
61

I have a single pipe “10,000-BTU 450-sq ft 115-Volt portable AC unit”.
(Note: the installed location is a 20 x 40 building that is divided into 3 sections each 13 x 19 approximate… the exterior and intervening walls are reasonably insulated with glass wool insulation… as well as the ceiling… 9" of blown cellulose).

Installed the unit in one of the a 13x19x8 foot (approx 250 square feet) room. Note this is a stand-alone building, not connected to a house.

First test it ran for 24 hours. Uses about 1180 watts when the compressor is running… in 24 hours it used a total of 2006 KWh. I had set it on 78 degrees, started at 6 p.m. (outside temp peaked both days around 97 degrees) and at 6 p.m. the next day, the inside temp of the room was 81 degrees. So it ran virtually non-stop through the night and lost ground during the day.

This puzzled me so I began to investigate. First clue… a small opening in the sheathing (used to be a light switch there) … I could feel a very strong influx of hot air, indicating negative pressure inside the room. So then I decided to check the exhaust vent (5 inch diameter) … using an anemometer I measured the exhaust at 2480 feet per minute (28 mph), displacing about 340 cubic feet per minute. Since the entire volume of the room is only 2000 cubic feet, this means the entire volume would be replaced every 6 minutes. The unit does cool the air (66 degrees out of the blower) but it is blowing much of the air it cooled out the exhaust every 6 minutes … and, of course, that air has to be replaced with the hot humid outside air (it is August in East Texas btw).

I did not have a way to monitor temperatures over time… but I suspect that there may have been points when it was actually warmer inside the room than the outside temperature.

Further notes: I DID insulate that exhaust duct and took measures to make sure it was not leaking.

In short … these things are no good. If you stand in front of the vent or use it in a room in a house where the rest of the house is cooled it does not actually increase normal infiltration of outside air … maybe you could conclude that it works but in my situation it does NOT work. It would cost me 100 dollars a month to have the room at 80+degrees in the afternoon.

Taking it back where I bought it. Gonna get a standard window unit.

If you’re determined to try one of these make sure you can return it before you buy!

62

I bought an LG one tube unit that appeared that it was originally intended for two tubes. It actually had a cutout for a second tube that they just covered with a circular plastic grill plug thing (for lack of a better term). I popped out this plug fairly simply and attached a second hose with a few tiny globs of hot glue. It had more vents on the body of the unit that I just covered with packing tape. Pretty simple really, since like I said it appeared the unit was designed for two hoses in the first place. Long story short the difference from running it as it was out of the box compared to running it lightly modified was like night and day. Before the mods it would only cool the room 10 degrees at most, because it was pulling such a strong draft and basically throwing all the cooled air out the window. After modification the unit would easily and quickly cool the room to a point where it could be uncomfortably cold. The unit had a minimum temp setting of 60 and i’m sure it could have easily gotten there but I only tested it down to 65 or so, whereas before the second tube I couldn’t get the room below 75 even after running it for several hours (this is a very hot summer climate)

Long story short, there is absolutely no sense in cooling a condenser by drafting room air to the outside. The only reason to draft air from the outside is if you’re trying to introduce fresh air and remove stagnant indoor air. Of course evaporative cooling requires a complete draft constantly introducing outdoor air, but for a refrigeration unit it just makes no logical sense since it has basically nothing in common with evaporative cooling systems.

I worked as an HVAC tech for over 4 years and really I don’t understand the thought behind the single tube, draft-reliant units other than manufacturing a product as cheaply as possible and giving no thought to efficiency, poor design, and basically wasting large amount of electricity. Basically, I’m calling it a scam until someone convinces me otherwise, which I’ve yet to be…

63

One tube is the exhaust air. The second hose is in no way an “intake” hose…it is a MAKE UP AIR HOSE. The "intake is located on the back of the unit INSIDE under the removable air filter. Absolutely no different than the single hose units. The units that have a second hose have a make up air hose. The single hose units cause a negative pressure to exist in smaller spaces…such as an RV and this negative pressure draws in outside air from widows and cracks the second hose does nothing more than alliviate this negative pressure by equalizing the pressure inside and outside the unit being cooled. Yes it does draw in a small amount of air but it is no more, and no different than the “fresh air” setting on a window mounted unit. It allows a small amount of air to be “made up” or replaced given that the unit is constantly exhausting hot air from the unit…and if you just stop and think…where does all the air that is being exhausted come from?..well silly it comes from inside the unit big cooled …and the unit big cooled has a set amount of air inside and when it is sucked outside…the air to replace it has to come from somewhere. using the second hose keeps the replacement air from being sucked in from cracks and leaks and allows you to Control the make up air. A MUCH more efficient setup than single hose units. So, whatever everybody else blather ed on about here has nothing to do with what the hose is there for. It’s simple physics really.

64

The downside of a two hose system is that they are more expensive, the unit needs two fans so it uses more power and it makes more noise. That somewhat offsets the improved efficiency of using outside air to cool the compressor.

In places where the outside environment is very hot and/or humid (i.e where you need AC efficiency the most) they recommend not using the intake hose because the compressor won’t cool efficiently. In those cases it becomes nothing but a more expensive, louder, single hose system.

65

Does a two hose system really need two separate fans? Since fan motors are one of the most expensive individual parts of an air conditioner (the first is the compressor/housing) it’s in the interest of manufacturers to have one motor drive both the evaporator and condenser fans. This is usually done with a motor that has a spindle that goes through both sides, so the fan sits on or next to the baffle between the hot/outside and the cool/inside, with the spindle penetrating through the baffle. I see no reason why a dual hose system would have any effect on this, because the intake hose just creates a path for outside air to get to the condenser coil.

I don’t know if portable units are actually arranged this way, but imagine looking at the side of one as a cutaway view. You have the evaporator sitting on the bottom at the right, the condenser on the bottom on the left, an insulated panel in the middle, and then the fan motor with squirrel cage blowers in the space behind each coil, then some ducting up to discharges out the top to the right and left. The actual blower for the condenser might be a different size or configuration to better force air through the exhaust hose, or to provide more power and CFMs, and also to sling cold water drained off the evaporator onto the condenser coil, but it’s still tied to the evaporator blower so the speed of each is relative.

So anyway, with a single hose system, there’s an intake grille on the bottom left for the condenser just like the one on the bottom right for the evaporator. Both would have a filter so the coils don’t get clogged with dust and pet hair and such. The only difference is that while the evaporator discharges through a louver into the room the condenser discharges into the hose to the outside. In a two hose system, the second intake hose just goes to a boot around the air intake. So theoretically you could modify any single hose unit into a two hose unit, assuming the intake grille is easy enough to cover with a boot (I’ve seen some where the intakes are through slots on the side, bottom, and the front of the unit, so it would be more difficult to isolate).

It seems that J2Cz has come across two hose units where this intake boot is not isolated from the room’s ambient air, so the unit prefers to draw most of the air from the room and only some from the second hose. Why any company would make a dual hose unit like this is the question, because it defeats the entire purpose, but that doesn’t mean all of them, or even most of them, are configured that way. The only reason I can come up with for why they might do this is because the constricted airflow from two pipes on top of the hotter air intake from outside might mean they can’t adequately cool the compressor and housing, especially since it’s in a sealed box. Another thought is maybe the reduced airflow would cause head pressure would be too high without a little bit of cooler indoor air to help out?

66

Most single hose systems that I’m familiar with are set up something like this. The two vents on the back look identical but they’re not.

The vent on the right is the exhaust. A tube connects to that and runs out the window. It has a pretty powerful and loud fan inside the housing and its only job is to exhaust air. The air that flows through the evaporator coils above them and is blown into the room is driven by another fan inside the top of the unit.

The vent on the left is for air intake (not the air to be cooled and blown into the room, but air to be blown across the compressor and condenser to cool them and then blown immediately outside through the exhaust hose) It does have a filter on it which isn’t shown in the photo.

On a single hose system such as the one in the photo there’s enough pull from the single exhaust fan to take room air in through the intake filter, but you can’t connect a long hose to that intake or it won’t draw enough air. So on a two hose system there is an additional fan inside that vent whose only job is to pull outside air in, where it cools the compressor and condenser coils, and is immediately blown back outside by the exhaust fan.

67

OK so you pundits help me out. I have a rather large house with high ceilings. Despite more than adequate central air conditioning the one place it does not cool efficiently is my bedroom… Having an auxiliary fan put in the duct work to blow more air in here made a barely discernible difference. For last three years I have had a Sharp portable single hose A/C that has saved me. This year after I dragged it out of the closet and reinstalled it, it decided to say good bye. so I am now the proud owner of a Whynter 2 -hose machine that I have not as yet connected.

From your discussion, the air it will likely draw from the “outside” is likely air from other parts of the house, that is much cooler and less humid than the air outside in Arlington TX (we are in the Dallas metro area). Whynter does say this in their FAQs:

"The optional air intake hose pulls air into the portable air conditioner from the outside to help cool off the compressor and condenser coil along with the air pulled from the filter from the room. The hot air generated from the air conditioner is then expelled out the exhaust hose.

Operationally, a dual hose portable air conditioner has two main advantages over a single hose unit. A dual hose portable air conditioner will cool a room faster and will help minimize negative air pressure situation in the room. If the outside temperature is much higher or more humid than the room, the intake hose can be covered with the included cover and it will function like a single hose unit."

Shall I or shall I not connect the intake hose to the outside? I don’t really understand the purpose of “covering” the intake port, why not leave it open to the room?

68

Simple

an AC has to move air across the HOT coil to get rid of the HOT.

1 hose unit sacrifices air from the room you are cooling, blows it through the unit over the HOT coil and out the window.

This of course causes the room to suck air in from the rest of the house

2 house unit pulls air from outside through the HOT side of the ac unit, across the HOT coil and back out the window.

No air from the room is sacrificed, and the room does not suck air from else where, so the AC does not have to work on cooling more air than is in the space it is actually running.

Using the 2 hoses is better.
You dont leave it open to the room because you dont really want it venting your cool air outside to the birds and squirrels?
Why does your central AC not cool your bedroom?
It the duct work run too long?
Does the room not breath? too well sealed?

Central AC needs to pull the air out of the rooms as well as push it in, so if you seal a room it wont cool very well, nor heat.

69

If you read through all the discussion above you’ll see that apparently some (many?) 2-hose units still draw indoor air through the condenser and expel it outside. So the hoses aren’t used in a 1:1 ratio like you’d expect in a sealed 2-hose system. In fact it seems like some are designed as if they’re 1-hose units, with the indoor air drawn over the condenser, and the second hose acts as just a draft balancer.

70

The best answer is to use a window unit if you need a portable A/C. If you’re going to be there 3+ years, install a permanent mini split. Both window units and mini splits are dramatically more efficient than hosed portable A/Cs.

72

So, basically, all air conditioners, regardless of type (even built-in AC), have two heat exchangers (one hot, one cold) separated by a heat pump. The key issue is how air flows.

Ideally, the hot side exchanger’s air flow is always totally separate from the cold side, so that the cold side’s cool space is not contaminated by hot air (e.g. by being sucked into it from outside), one way or the other.

The first catch is that, to keep those air flows separate, you need two fans and (on portable units) two hoses. (Heck, they could even be both driven by the same motor.) That costs money and increases noise.

It’s simpler and cheaper just to arrange a single fan to blow the same intake air (presumably from the cool space) through both exchangers at once, blowing the hot side air outside through a duct, and the cool air inside. But that means makeup air (to replace the air blown outside) has to be drawn into the cool space from outside via a door, window, or leakage. No way around it. Which means that cool air gets warmed up by the very air being blown outside to cool the hot-side exchanger, drastically reducing the effectiveness (never mind “efficiency”) of cooling.

The second catch is that, even if the portable unit you buy has two hoses, you don’t know how the air actually flows inside, and discussion here shows that manufacturers aren’t highly motivated to do it right. In other words, they cheat.

Unless you actually disassemble a portable unit, you don’t know how many fans it has, how the duct hoses are connected to the heat exchangers, or how the air flows. And adding a second hose to a single-hose unit, if it has only a single fan, is useless.

I’m not aware of any objective testing – even by Consumer Reports – that properly addresses these catches, or rates the units in terms of cooling net effectiveness (how large a space a unit of a given cost and power consumption can cool, to what degree).

In my own limited experience, a single-hose portable AC, even if otherwise well designed, is largely a waste of time and money.

It could be really helpful if people could share experiences here with specific units. Please let us know if you are aware of any good portable AC units. I don’t think it’s worth much to report bad ones: those seem to be standard issue.

– Ken