Dehumidifier - will it make a difference?

[MartinF]

I agree with Thinwater. 37% is already at the lower limit for comfortable living. Humidity will be low in the winter anyway since you are heating cold dry air for your living space. Insulation and double-glazing is the answer. A lot can be accomplished with cling-film, expanded polystyrene and clear sticky tape.
Good Luck!
M

[derfy]

Originally Posted by EmeraldCoastSailor

I don't have a winter situation to deal with but my dehumidifier works wonders to make the boat more comfortable. When I sail I unplug it and lay it down in the head in the shower area out of the way. At the slip it sits on the counter and drains to galley sink. It comes out when I cruise so as to be able to use the shower portion of the head (and I don't have a generator so running it isn't in the cards unless I am in a slip).

With the unit running I have found I don't need to run the AC to keep the boat comfortable in the hottest most humid days on the panhandle. Come November the unit comes out till the heat/humidity returns in the spring. I bought it at Home Depot for less than $200.

The only drawback is the size but for me it works well.

I have been looking for one that will sit on the galley counter and drain into the sink. Could you share which unit (mfr/model) you bought from Home Depot? They have many types - none indicate an external drain.

Thanks

[Sailmonkey]

Originally Posted by derfy

I have been looking for one that will sit on the galley counter and drain into the sink. Could you share which unit (mfr/model) you bought from Home Depot? They have many types - none indicate an external drain.



Thanks

Almost all of the dehumidifiers Home Depot carries (or that are made for that matter) have a provision for an external drain. Such as this one. https://www.homedepot.com/p/Whynter-...-204619877-_-N

The external drain attachment is on the back.

[alctel]

As already mentioned, get one of the new(ish) Desiccant type ones. Small, pretty quiet and pull an insanely large amount out.

[CarinaPDX]

I use a compressor-type dehumidifier in the spring and fall; it draws about 400W. In the winter it isn't needed as the heaters keep the interior warm and dry. The main thing is to ventilate, and use more heat. Too many folks close up their boats to keep the heat in, thus trapping the moisture. The ends of the boat and the hull below the waterline are more difficult; again circulating the air will help but insulation is the ultimate answer. I put thin, frosted sheets of Lexan under my hatches to insulate and keep moisture from the cold bronze/glass. With the portlights I use the same material and put it just inside the screens and outside the glass. It is good enough for the milder winters in Portland but in colder temps a better insulation would be desirable. BTW I use frosted Lexan for privacy, with good light; otherwise use clear.

Greg

[derfy]

Originally Posted by Sailmonkey

Almost all of the dehumidifiers Home Depot carries (or that are made for that matter) have a provision for an external drain. Such as this one. https://www.homedepot.com/p/Whynter-...-204619877-_-N

The external drain attachment is on the back.

OK, Thank you.

[CarinaPDX]

I just looked at the dessicant dehumidifier linked. It is an interesting option, being lightweight and quiet. But there ain't no such thing as a free lunch - this consumes 615W while removing 15 pints/day while my Arctic Aire consumes 315W while removing 30 pints/day. In the winter it wouldn't matter (you want the heat) as long as it is able to keep up with the humidity, but in the spring and fall (when I use it) I wouldn't want the heat, just the dry. For me the weight is a small issue, but of course the quiet would be nice. YMMV

Greg

[MarkusK]

Originally Posted by SVRocinante

Good day folks

Our first winter living aboard and the only real issue we're having is condensation! Warm, moist air hits the cold metal flanges of the portlights & hatches and we get rain!

I'm about to plunk down ~$200 on a Dehumidifier and wondering if it will actually make a difference. As I sit here writing this post it is 22 (degrees F) outside & 72 inside with 37% humidity.

If I'm understanding the relationship between Temperature, Relative Humidity & Dew Point correctly, I would have to get the humidity down to around 14% to stop the condensation at this moment.

Does anyone have personal experience with using a quality dehumidifier? Am I correct in my assumptions? If so, will one of the typical home units be able to do that?

Thanks for any and all help,

Dehumidifiers are used to control humidity, not condensation. As a rough rule-of-thumb in the HVAC trade, Mechanical (compressorized) dehumidifiers are used in warmer applications, say above 40°F Dew Point. Desiccant below 40°F DP.

37% RH at 72°F is already dry ~45°F DP…at the low end for most mechanical dehumidifiers.

Your real problem, condensation, happens when an object is at or below room DP. Your metal flanges have a surface temperature that is well below 45°F room DP temperature, so they will sweat. Two ways to prevent this:
1- insulate them and incorporate a vapor barrier on the warm side to keep insulation dry.
2- raise the surface temp above DP temp by blowing air on them.

A properly sized dehumidifier will reduce the number of days/season where condensation will occur, but it won’t eliminate condensation.

[senormechanico]

If it's practical, why not insulate those portlights on the outside as well?
Who cares what it looks like if it's keeping you warm and DRY ?

[Symphony]

What they all said. Good advice on the difference between condensation and relative humidity.

Below 40% RH is unhealthy for your mucous membranes and skin. Below 30% and you are in the Sahara.

[DITB]

Agree on the desiccant dehumidifier, for when the temperature is lower.
Understanding of dew point is essential - as MarkusK writes, any surface inside your boat which is lower than dew point will condensate.

1) heat up those surfaces (like using insulation, or ventilate air inside more. Insulation is way better though, especially if you can insulate from the outside)

2) remove moisture the best you can. To cover the entire cabin, consider connecting the intake of the dehumidifier to external air (put a hose on, connect via porthole or similar). This “pressurizes” you cabin.
3) avoid any flame Inside. Any combustion adds moisture. Same for cooking, showering, sweating, breathing - they all add moisture. Minimize. Well, don’t stop breathing

Insulation is an important part of your solution. It will lower condensation levels, but has to cover any area affected. Below deck is an issue in cold water unless your boat is out of the water (obviously). Arrange your insulation such that where dew point meets temperature, moisture isn’t an issue. Like insulating on the OUTSIDE of your hull, if at all possible.

If you could keep the dew point inside your cabin lower than the outside temperature, you wouldn’t have condensation BUT that isn’t always possible. The lower the temperature outside, the greater the challenge.

[CaptTom]

Originally Posted by robert sailor

Some of your problems may be helped by the type of heat you are using. We find electric heat to be the worst for humidity and diesel one of the best.

OK, I'm lost here. If you're creating a hot metal surface, and blowing or circulating air over it to warm the cabin, what does it matter how that metal got hot? No moisture is being introduced.

Obviously we're not discussing venting the flue gasses into the cabin. That would make a huge difference.

[mikedefieslife]

Originally Posted by CaptTom

OK, I'm lost here. If you're creating a hot metal surface, and blowing or circulating air over it to warm the cabin, what does it matter how that metal got hot? No moisture is being introduced.

Obviously we're not discussing venting the flue gasses into the cabin. That would make a huge difference.

Electricity, radiators etc.. all dry heat. So impossible for one to create more moisture/humidity than another. Now gas burning directly in the cabin is another matter e.g. cooker hobs, ovens etc..

Ventilation is the key to removing/preventing condensation (as much as can be). Hot air can carry more moisture than cold air. By venting the hot air out of the boat, you are also venting out the moisture with it. Say it's 25c in the boat with 70% humidity, and 10c outside the boat with 80% humidity, even though the humidity is greater outside, you will still dry the boat better by expelling the warm air and replacing it with cold, since the cold air can't carry as much water vapour.

Most will realise how daft this is, as it costs good money to heat the air. On my boat my Webasto Diesel heater that serves three radiators can use up to 0.6l per hour or €0.79 an hour at full load.

At that price, a dehumidifier begins to look economical. It will keep the boat drier, and keep heating bills down by a) not wasting warm air b) it creates heat itself c) drier air is easier to heat than moist air.

[derfy]

Originally Posted by Sailmonkey

Almost all of the dehumidifiers Home Depot carries (or that are made for that matter) have a provision for an external drain. Such as this one. https://www.homedepot.com/p/Whynter-...-204619877-_-N

The external drain attachment is on the back.

I see this unit has electronic controls and digital readout. This usually means a device will not recover from a glitch on the dock power. Gadgets with good ole analog knobs with hard switches will usually keep going after a power failure. It is getting hard to find them.

I don't leave my inverter on when leaving the boat for extended periods, so that is not an option.

Do you know if this unit will recover from a power outage? I would be very surprised if it did.

[Sailmonkey]

I don’t have that exact model, one VERY similar. Complete with electronic push buttons. Mine does resume operation after power failure. I think most manufacturers now have gotten wise to the fact that not resuming after a power failure isn’t acceptable.