Dehumidifier - when to turn it on?

[jakeroyd]

Most marina / boatyard supplies are subject to interruption over winter so I just set the timer I use to say 4 periods of a couple of hours per 24 hrs.
Make sure the machine you use has auto restart and auto defrost.

 

[alanch]

Very interesting thread. Many years ago I was involved in the layup of very large Tankers in the far east. The fundamentals were, remove as much water as possible from the vessel (drain lines etc.), seal up the accommodation and engine room, and run huge dehumidifiers continuously for some weeks to get the humidity right down. Once this was achieved they only needed to be run for a few hours once a week. Some of these ships stayed there for years like that and when reactivated sailed merrily away. As for dehumidifiers on small boats I use mine in the winter, when the boat is still afloat and unused. It has a humidity setting and cuts in and out 24/7. I also use several tube heaters on thermostats and the internal temperature never drops below 5 degrees. It has worked for me like this for 10 years without problems. Luckily power cuts in my marina are rare, although the recent flooding on the river knocked the power off for some days, but the outside temperature was quite high, fortunately.

 

[davidej]

Lovely put down Mr Pmagowan but I fear it may be lost on him.

No not lost on me but I recognise that my response was over-agressive - more lounge than PBO style. For this, I apologise.

I know enough physics to realise that a hot body cools faster than a cool one but the quote seemed to suggest that I should warm up my yoghurts before I put them in the fridge or it would't work properly.

Please explain to me where I have gone wrong as I did not claim to be an expert in this field - only that the advice seemed to me to be counter-intuitive. My thoughts went as follows:

As temperature rises, the amount of water vapour the air will hold rises and the fraction of the maximum that is actually in the air is the relative humdity.

Thus as humid air cools, the RH will (at some point) reach 100% - the dew point- and "excess" (ie the amount over 100%) water vapour will condense out.

I assumed (perhaps wrongly) that a de-humidified worked by cooling the air to its dew point. From that I deduced that the warmer the air, the more cooling is required to reach dew point.

Obviously if manufacturers advise that they work better in warm conditions, I am sure they know what they are talking about but perhaps someone would be kind enough to explain where my elementary physics has gone wrong.

 

[Rivers & creeks]

As the system we use is working very well I will not change it.

When I fitted the Dorade blanking caps one previous winter black mould appeared on the salon ceiling in a couple of places.

You do yours your way, I'll look after mine, thank you!

Then I think it's quite clear that the excellent ventilation is what is keeping the boat mould free; great ventilation is way better than a dehumidifier.

 

[Cantata]

Then I think it's quite clear that the excellent ventilation is what is keeping the boat mould free; great ventilation is way better than a dehumidifier.

I have to say I agree. I only have power at my berth on a token-in-the-slot basis anyway, so have never bothered with machinery. I keep a hatch cracked open, and the boat has dorades so there is a bit of air circulation. Never had any damp problems at all in 10 years, all year round. I thought dorade blanking caps were only intended to be used in serious voyaging with green seas over the deck.

 

[pmagowan]

No not lost on me but I recognise that my response was over-agressive - more lounge than PBO style. For this, I apologise.

I know enough physics to realise that a hot body cools faster than a cool one but the quote seemed to suggest that I should warm up my yoghurts before I put them in the fridge or it would't work properly.

Please explain to me where I have gone wrong as I did not claim to be an expert in this field - only that the advice seemed to me to be counter-intuitive. My thoughts went as follows:

As temperature rises, the amount of water vapour the air will hold rises and the fraction of the maximum that is actually in the air is the relative humdity.

Thus as humid air cools, the RH will (at some point) reach 100% - the dew point- and "excess" (ie the amount over 100%) water vapour will condense out.

I assumed (perhaps wrongly) that a de-humidified worked by cooling the air to its dew point. From that I deduced that the warmer the air, the more cooling is required to reach dew point.

Obviously if manufacturers advise that they work better in warm conditions, I am sure they know what they are talking about but perhaps someone would be kind enough to explain where my elementary physics has gone wrong.

It is explained in the formula and explaination I included in my post. In non-scientific language:

The dehumidfier cools air. Cold air can hold less water. The amount air is able to be cooled depends upon its original temperature and the temperature of the cooling element. So, if the element is 5C and the air is 5C then there will be no cooling at all. Of course, this is a simplification as the cooling element of the dehumidifier would achieve lower temperatures if the coolant is originaly cooler prior to compression but the formula for this would be complex. If the air is at 30C then if the dehumidifier is 100% effective it will cool the air from 30 to almost 5C thus changing the temperature by 25C. Then you must take into account the ability of the warmer air to absorb the moisture so that the warm air is more efficient at drying the boat also. As the 5C dry air rewarms it will absorb significant amounts of water to reach an equilibrium. There are also pressure effects which are in the formula and temperature and pressure have a relationship.

Your statement presumes that the dew point of air is a set temperature and warm air must be cooled more to reach it. It is not fixed and thus the dew point of warm air will be much higher than the dew point of cold air (given generally equal conditions). For example, take a box with an inch of water on the bottom and fill the top with 5C dry air. The air will 'absorb' water (as per the formula) to reach an equilibrium. Due to being cold it will not absorb much water. You would then have to cool the air to get it to release that small amount. If the same was done with hot air the air would absorb much more water and you would release lots more when you cool it. The ease of cooling would also be less as per the description above.

Of course this is all a gross simplification as is the complicated answer with the formula. To undertand how to make an apple pie from scratch you would first need to understand how the universe came to be.

 

[davidej]

I expect you will think I am being argumentative but there are still some statements that I take issue with..

Lets put aside the question of pressure. We agree that the amount of water vapour air can absorb (the saturation point) rises with temperature and that the fraction the the air actually contains is the RH.

Thus with a given (fixed) amount of water vapour, the RH will rise as temperature falls. The dew point will be dependent on that amount - the greater the amount of vapour in the air, the higher the dew point. Hence misty moisty autumn days

Maybe a de-humidified doesn't need to cool air to dew point to get some moisture to condense out, but I stand by the rest of my physics

 

[pmagowan]

I expect you will think I am being argumentative but there are still some statements that I take issue with..

Lets put aside the question of pressure. We agree that the amount of water vapour air can absorb (the saturation point) rises with temperature and that the fraction the the air actually contains is the RH.

Thus with a given (fixed) amount of water vapour, the RH will rise as temperature falls. The dew point will be dependent on that amount - the greater the amount of vapour in the air, the higher the dew point. Hence misty moisty autumn days

Maybe a de-humidified doesn't need to cool air to dew point to get some moisture to condense out, but I stand by the rest of my physics

No. Misty moist Autumn days occur when the ground becomes the dehumidifier element due to radiative cooling during the night when the air remains relatively warm and therefore contains lots of water. The air is then cooled by the ground and the water condenses into a fog or mist.

One of the main things you seem to be missing in your physics is that the amount of water vapour is not fixed (previously you seemed to fix the dew point now the amount of water vapour). Warm air will absorb mose moisture from its surroundings. this is the whole point of a dehumidifier. You take moisture out of the air so that it can absorb more moisture from the thing you are trying to dehumidify. Yes, if you have a wierd controlled experiment where you have a small fixed amount of water that is totally absorbed into an amount of cold air then heat the air up you would need to cool it down a lot more to reach the dew point. However this is a completely ficticious scenario that does not happen in any environment where a dehumidifier would be used.

 

[geem]

As the system we use is working very well I will not change it.

When I fitted the Dorade blanking caps one previous winter black mould appeared on the salon ceiling in a couple of places.

You do yours your way, I'll look after mine, thank you!

Sir, I am simply correcting your misinformation. If people adopted your approach they would be wasting ££ running a dehumidifier. The purpose of the dehumidifier is to drop the moisture content of the space such that you lower the dew point on the internal surfaces. By lowering the dew point you reduce the frequency of condensation occurring. If you are running a dehumidifier in a sealed box then you will eventually reduce the humidity levels in that box. if you keep adding damp air to the box by virtue of openings to outside, you will struggle to lower the moisture content in the box. Why spend ££ achieving little?
Mould on internal surfaces can occur even at relatively low internal RH levels particular if the internal surfaces are poorly insulated. I have found success in natural ventilation to my boat by virtue of 5 large dorades open over the winter. The boat has remained dry and mould free. I have also used a dehumidifier with the dorade caps installed but I have never tried to do both. Since this subject is my field of engineering I feel reasonable qualified to talk fact.

 

[geem]

Thanks for the replies, everyone. I am running a compressor type dehumidifier. It sounds like most people prefer the daytime running. I will try a little experiment to see which method produces the most water in the tank. As to why it's needed this time of year, it is something to do with rain, wet crew, waves and wet spinnakers. You should have been out there on Easter Sunday!

Cheers

The warmer the air in the boat, the more moisture a compressor type will remove. At low air temperature the compressor type will freeze the coil and likely spend as much time in defrost mode as actually removing moisture. Running the dehumidifier during the day is likely to be better when temperatures are higher.

 

[Rivers & creeks]

The warmer the air in the boat, the more moisture a compressor type will remove. At low air temperature the compressor type will freeze the coil and likely spend as much time in defrost mode as actually removing moisture. Running the dehumidifier during the day is likely to be better when temperatures are higher.

Thank goodness for that simple explanation, none of the scientists agree and the rest of us are for just opening another vent

 

[rotrax]

Sir, I am simply correcting your misinformation. If people adopted your approach they would be wasting ££ running a dehumidifier. The purpose of the dehumidifier is to drop the moisture content of the space such that you lower the dew point on the internal surfaces. By lowering the dew point you reduce the frequency of condensation occurring. If you are running a dehumidifier in a sealed box then you will eventually reduce the humidity levels in that box. if you keep adding damp air to the box by virtue of openings to outside, you will struggle to lower the moisture content in the box. Why spend ££ achieving little?
Mould on internal surfaces can occur even at relatively low internal RH levels particular if the internal surfaces are poorly insulated. I have found success in natural ventilation to my boat by virtue of 5 large dorades open over the winter. The boat has remained dry and mould free. I have also used a dehumidifier with the dorade caps installed but I have never tried to do both. Since this subject is my field of engineering I feel reasonable qualified to talk fact.

Thank you for your concern.

I know that technically I am wasting a bit of energy, but having seen the results of unventilated boats stored over winter in temperate climates I will pay a bit extra and carry on as I am.

A friend has a Spencer 35 which he keeps in Washington State.

He took me to Annacortis to see it in its dock this time last year.

If you ran a finger down any surface it came away with mould.

He was devastated that his pride and joy had got in such a state. Previously it had been in San Diago, different climate, no problems.

A truism I am fond of is from Mao Tse Tungs Little Red Book.

"True Knowlege is the product of direct experience."

My direct experience is that my boat survives the wet British winters dry and warm, and costs £60.00 of Premier Marina's electricity.

I am happy with that, and will not change my regime.

About half of my fellow clubmembers use a similar system, and a couple are retired eminent Scientists. The matter has been bought up before, but my experience when I had the same two tube heaters, the same compressor dehumidifier and closed Dorades showed that mould IS possible without some ventilation.

 

[lw395]

Thank you for your concern.

I know that technically I am wasting a bit of energy, but having seen the results of unventilated boats stored over winter in temperate climates I will pay a bit extra and carry on as I am.

A friend has a Spencer 35 which he keeps in Washington State.

He took me to Annacortis to see it in its dock this time last year.

If you ran a finger down any surface it came away with mould.

He was devastated that his pride and joy had got in such a state. Previously it had been in San Diago, different climate, no problems.

A truism I am fond of is from Mao Tse Tungs Little Red Book.

"True Knowlege is the product of direct experience."

My direct experience is that my boat survives the wet British winters dry and warm, and costs £60.00 of Premier Marina's electricity.

I am happy with that, and will not change my regime.

About half of my fellow clubmembers use a similar system, and a couple are retired eminent Scientists. The matter has been bought up before, but my experience when I had the same two tube heaters, the same compressor dehumidifier and closed Dorades showed that mould IS possible without some ventilation.

As you say, it works, you're happy.
I think it depends on what you are trying to achieve.
If you want to dry out a wet boat, then there is more water in the air when it's warmest, so run the dehumidifier then.
If you are trying to prevent condensation, then run it when condensation is most prone to occuring. That's when it's coldest, as air must be very dry not to leave condensation on a cold surface.
Cold surfaces like cars get dew on them at night.

Ventillation works best when there is a heat source, as otherwise humidity can enter the boat just as easily as it can leave.
Around here, the great outdoors has been pretty humid a lot of this past winter.
What we really need is intelligent ventillation which opens when indoors is more humid than out.

TBh, if the boat starts off dry and lets no rain in, then a tiny amount of heat will keep it dry.
Drying wet kites between autumn series races is a different game....
The dehumidifier is of course a heat source, and the fact that it has a fan moving the air around is good too.