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Old 13-05-2016, 15:53   #1
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Domestic Air Conditioner High Humidity

I have an 8000 BTU Dometic Turbo air conditioner and March circulating pump. Less than ten years old but little use. For years worked perfectly but then getting HPF shutdown. Cleaned out circulating system with Barnacle Buster. Now no HPF shutdowns and the unit cools adequately, but doesn't take the humidity out of the air the way it used to. Plus the pan is completely dry. Wasn't that way before.

So is there a part broken that controls the humidity? I have reset everything to factory conditions. Cool is great. But the cabin humidity is higher than outside, so bedding feels clammy, etc. Have to run temp down to 70 deg for it to feel comfortable.
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Old 13-05-2016, 16:25   #2
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Re: Domestic Air Conditioner High Humidity

There is no humidity control function.......the humidity is pulled from the air by the evaporator temperature being below the dew point of the air. If your cabin is cooled to 70 and there is no water in the pan......I'd venture to guess there is low humidity already existing.

However, if you're in Corpus I don't see the low humidity being a thing.

When you say you run the temp down to 70, are you achieving 70, or is the unit running continuously and never reaching 70?

If you turn the thermostat to 70, and that temp is not reached, the unit will run until the temp is reached. If this is the case and the there is no condensate water in the pan, then you've got either a Freon problem, or a cooling water problem.

Have you checked your water flow volume? How hot is the cooling water? both under the boat, and in the discharge stream?
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Old 13-05-2016, 16:38   #3
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Re: Domestic Air Conditioner High Humidity

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.
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Old 13-05-2016, 16:51   #4
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Re: Domestic Air Conditioner High Humidity

When it cycles on and off, how long is it on for (like minutes per hour, seconds per minute, etc)? If it only switches on for a very short amount of time spits a lot of cold air, then turns off, there will be no time to pull humidity from the air.

I've seen many installations of huge AC units on smaller boats that resulted in meat locker temps, but were unable to remove humidity at more comfortable temps due to exceedingly short run times to maintain warmer cabin temps.


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Old 13-05-2016, 16:58   #5
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Re: Domestic Air Conditioner High Humidity

That could be it as outside temp has been in low 80's and we are running to 70. Wouldn't use it except feel a bit more secure in marinas when closed up. We are just cooling the sleeping cabin, but last couple of times left the door open and expanded the colony area to include aft head as well. So it runs a while longer. But no noticeable change.

Once it get to temp, it probably cycles cold 3 minutes and then off for ten minutes. We run the fan 100% of time.
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Old 13-05-2016, 17:01   #6
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Re: Domestic Air Conditioner High Humidity

There's your problem. The three minutes of run time won't pull moisture from the air.

Try cooling the rest of the boat, the compressor will run longer, but the longer run times will allow the evaporator to stay cold long enough to pull moisture out.


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Old 13-05-2016, 17:04   #7
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Re: Domestic Air Conditioner High Humidity

No condensate in the drain pan? Is the back side of the evap coil dry?

You cannot wring moisture from the air unless the coil temperature is colder than the wet bulb temperature of the entering air.

Can you take a temperature reading of the coil when the compressor is cycling? First step in solving your problem.

Or short cycling mentioned above might be the issue.
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Old 13-05-2016, 17:06   #8
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Re: Domestic Air Conditioner High Humidity

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Originally Posted by redsky49 View Post
No condensate in the drain pan? Is the back side of the evap coil dry?

You cannot wring moisture from the air unless the coil temperature is colder than the wet bulb temperature of the entering air.

Can you take a temperature reading of the coil when the compressor is cycling? First step in solving your problem.
they can cool the cabin to 70 with compressor cycling with an ambient of low 80's

I'd tend to believe that short cycling is causing this.
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Old 13-05-2016, 17:19   #9
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Re: Domestic Air Conditioner High Humidity

Thanks. I thought as much. I suspect the issue will finally when we get back to 100 deg cabin temps in Texas.

Probably the other issue is showing two 98 deg water vapor emitting bodies in there at night.

THANKS for the replies
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Old 13-05-2016, 17:42   #10
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Re: Domestic Air Conditioner High Humidity

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.
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Old 14-05-2016, 08:40   #11
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Re: Domestic Air Conditioner High Humidity

Possible drain blockage from coil pleumin to drain into pan. Condensate staying inside unit. Blower pushes the condensate into air flow.
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Old 14-05-2016, 08:51   #12
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Re: Domestic Air Conditioner High Humidity

We don't have the humidity and temps you are dealing with, but do have air conditioning on the boat, and a dehumidifier.

You may need to run a dehumidifier for the times when you AC isn't running long enough to dry the air inside the boat. [I'm assuming you are at a dock with shore power...?]

We found a small, quiet, very efficient dehumidifier that works extremely well keeping the entire boat dry. Here is our blog post about it with a bit more info if that is helpful.

Cheers!

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Old 14-05-2016, 09:48   #13
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Re: Domestic Air Conditioner High Humidity

just thought I would throw in my two bits... I agree with the hypothosis that your system is cycling before the evaporator has a chance to ring out the vapor in the air, then once "dew point" is reached it's to late to capture as it is in moisture form on surfaces in your boat. As stated, common problem when boats come here to the Keys from drier climes. Your solution might be as simple as just slowing down the fan speed. Secondly, make every effort to restrict outside air from coming in (not easy on may boats). Lastly, keep interior fans on to move vapor laden air towards evaporator return air. Cheers.
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Old 14-05-2016, 10:09   #14
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Re: Domestic Air Conditioner High Humidity

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Originally Posted by Racer86 View Post
Possible drain blockage from coil pleumin to drain into pan. Condensate staying inside unit. Blower pushes the condensate into air flow.
Hmm, not sure I understand this one. Doesn't the condensate form on the outside of the coils and then drip down into the pan? Dometic unit has a plastic pan that the unit sits in. But the unit is not encased in anything.
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Old 14-05-2016, 10:14   #15
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Re: Domestic Air Conditioner High Humidity

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Originally Posted by bobw100ton View Post
just thought I would throw in my two bits... I agree with the hypothosis that your system is cycling before the evaporator has a chance to ring out the vapor in the air, then once "dew point" is reached it's to late to capture as it is in moisture form on surfaces in your boat. As stated, common problem when boats come here to the Keys from drier climes. Your solution might be as simple as just slowing down the fan speed. Secondly, make every effort to restrict outside air from coming in (not easy on may boats). Lastly, keep interior fans on to move vapor laden air towards evaporator return air. Cheers.
Good points. I have three caframo fans on that side. Will run them to try to get stale air into the mix. Have installed weather stripping on the door, and think I have air leakage under control.

Another issue is that about 10-15% of that space is storage cabinets for clothes, towels and bedding. It takes a lot to get the moisture out of them.
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