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Old 14-05-2016, 10:27   #16
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Re: Domestic Air Conditioner High Humidity

I don't have the knowledge to comment on the short cycling issue but I can assure you that here in the Keys my Dometic AC keeps my boat cool with zero condensation. Even with open hatches I don't see condensation but I generally set it at 75 or so. As warm air rises I suppose hot humid air could condense moisture on my cabin roof or refrigerator door but the cabin temp would need to be very low. Then again I don't like ice in my beer either.
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Old 14-05-2016, 10:31   #17
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Re: Domestic Air Conditioner High Humidity

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We are currently in Nassau. Water temp is about 83 degrees. Not sure about circulating water discharge. Strong stream. Humidity outside is 91%. Inside tcabin is 78%. Yes, temp gets to 70 deg within an hour of starting unit, and then does normal cycling on and off to keep it at that.

Had same issue at several Florida marinas on way here. Sometimes humidity in cabin was considerably higher than outside.
Do you have any solar ventilators running? They must be closed with the AC is on or the humidity will never drop.
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Old 14-05-2016, 11:11   #18
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Re: Domestic Air Conditioner High Humidity

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Yes to short cycling. Former hvac tech (granted not marine, but concept is same). If you are reaching temp setpoint (unit cycles off) and not dropping humidity your unit is oversized or your climate controlled space is undersized. Either way your unit is short cycling. Cool the whole boat, with a cabin fan running for greater circulation. Especially in relatively high humidity but moderate temperatures (like the Bahamas, we're here too) you don't have high heat gain from outside so there's less load on the system, exacerbating the problem.
That is the correct answer.
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Old 14-05-2016, 18:32   #19
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Re: Domestic Air Conditioner High Humidity

OT, I know, but aircon to 70F? What's the matter with you people?

We run ours at 26-27C (78-80F) and find that very comfortable.
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Old 14-05-2016, 19:01   #20
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Re: Domestic Air Conditioner High Humidity

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OT, I know, but aircon to 70F? What's the matter with you people?

We run ours at 26-27C (78-80F) and find that very comfortable.

We also keep ours at 78-80, but the op needed to cool to 70 for the compressor to run long enough to pull humidity out of the air


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Old 14-05-2016, 20:40   #21
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Re: Domestic Air Conditioner High Humidity

No solar ventilators. One small dorade type vent. I probably need to cover that.

I have run it as low as 68 deg F and it will get there. But normal is about 74 deg.

My house unit is set at 77 deg. But we have a dehumidifier in the system that keeps humidity below 50%. That combination is comfortable. At higher humidity level on the boat, cooler temp feels better. But to each his own!
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Old 17-05-2016, 18:35   #22
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Re: Domestic Air Conditioner High Humidity

Did opening up more of the boat than just the cabin and head get you longer compressor run times and drier air?


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Old 17-05-2016, 18:49   #23
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Re: Domestic Air Conditioner High Humidity

have worked in the business for many years (not marine but small, med and lg building systems).

1) unit is short cycling or
2) unit too big for boat space that is being cooled or
3) thermostat is too close to unit therefore shuts off too soon to dry air out

if unit is not starting then immediately stopping then not short cycling, I would try opening more of the boat, run more fans to move air throughout, have a remote thermostat added to control unit and make sure its not close to or in direct line of air flow.

rule of thumb, buy unit slightly smaller than needed, unit will eventually shut off as it will get to temp set but it will run longer and cool/dry the air. so if it's less humidity you want, buy a smaller unit, if humidity is not an issue?? then use larger unit sized right and place fans for more air circulation.
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Old 18-05-2016, 07:00   #24
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Re: Domestic Air Conditioner High Humidity

We left Nassau a few days ago and haven't run the AC, since have been at anchor since. The last day in the marina I did run the fans, and opened up the ac'd space back to the head - so the entire port hull and just not the bedroom.. But the issue is there is a door to close off the bedroom, and a hatch board and sliding top to close off the port hull. So each time you want to go into the port hull, you let in lots of humid air. But when it was closed off, it did seem to be less humid. At least the meter said low 75% instead of 90%. But still no water in the pan, so could have also been a less humid day.

The Dometic Turbo unit is 8k btu. I think it is the smallest one they sold. The thermostat has a long cable and is on the wall about six feet higher and two feet forward of the return air vent. So that should not be an issue. When we get back to Texas I suspect the high temp will make this issue go away. And I can work on a solution for port hull entry that detains more ac'd air.
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Old 18-05-2016, 07:48   #25
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Re: Domestic Air Conditioner High Humidity

My two bits, It seems you are keeping the interior too cold. When ever humidity makes it into this air space it goes to dew point before getting to the evaporator. I would acclimate to both the tropics and a drier interior by settling for a comfortably dry 78 - 80 dg interior.
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Old 18-05-2016, 08:09   #26
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Re: Domestic Air Conditioner High Humidity

We only run the AC in marinas, where wind direction may not be optimum for the hatches, or security is an issue. Which has probably been 10 days in the last few months. Otherwise it's hatches open.

When we do run the AC in the Bahamas, we crank it down that low so it will run long enough to have a chance to dry out that cabin. Setting the temp to 80 deg just runs the fan. That does little to dry out the cabin. After running the cabin at low temp, and humidity drops, then can raise the thermostat to comfort level. Or sleep under covers, which is not so bad either!

Again, I don't think we will have this problem when we get back to Texas as AC will run longer when ambient temp starts at 100 instead of 80!
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Old 18-05-2016, 12:46   #27
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Re: Domestic Air Conditioner High Humidity

short cycling?? have it vacuumed and recharged. pressure tested wouldnt hurt. compressor fail comes next/ ask me how i know this.....also have thermosat checked.
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Old 18-05-2016, 13:25   #28
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Re: Domestic Air Conditioner High Humidity

If the copper evaporator coils are cold with condensate on the surface all the way up then it is fully flooded with refrigerant. Your unit was factory tested at 80dg F with 270 CFM of air flow across the coil. Under those conditions the temperature split between the return air and the supply air should be 18 degrees (This is optimal for all Dometic direct expansion units). I realize the airflow is intuitive against what you remembered when it was working normally. Just keep in mind that the ambient water temperature is higher and will be as high as 90 when you get in shallower water in the summer over there and here in the Keys.
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Old 18-05-2016, 15:08   #29
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Re: Domestic Air Conditioner High Humidity

I will check the evaporator coils. I know they get condensate on them. Haven't checked if it was full height. It cools the cabin very quickly though, so I think we just have an issue of unit to large or air space too small. Or temp here in the Bahamas and Florida this time of year not hot enough, but plenty humid!

Zeehag, there is no short cycle issue. AC runs until it gets to set temp, then shuts off until temp raises 2 deg, then restarts and runs to set point, repeat. The only reason I thought I had an issue is because I am not getting condensate in the pan nor is humidity dropping to as low as I would like.
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Old 18-05-2016, 17:34   #30
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Re: Domestic Air Conditioner High Humidity

so thermostat is ok but compressor fail could still be an issue.

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