Dehumidifier or not?

[jdc]

Heat will do fine instead of a dehumidifier since warm air with exactly the same amount of water (ie measured grammes per cubic metre) will have much lowered RH.

This graph shows the effect:

So if the ambient air is at 10 °C and 90% RH - which is about typical for winter here in Cornwall - and you heat you house or boat to 18 °C, the RH inside will be around 52%, which is a very comfortable and probably ideal value. So with heating, be it electric, fires, wood burners, central heating or whatever don't need the dehumidifier. Conversely I could heat my boat and not bother with the dehumidifier, but actually that's much less energy efficient when I'm not living aboard.

It also shows the absurdity of the assertion that 'low humidity will shrink wood': putting the heater on for a while (so RH falls from 90% to 50%) will inevitably crack all the panelling will it?

 

[CLB]

It costs much more to heat a boat, than dehumidify it.

 

[TernVI]

It costs much more to heat a boat, than dehumidify it.

If you want to own a boat with a heater, the delta cost of firing it up when you visit the boat to dry it out is not huge.
Whereas the cost of using a dehumidifier includes the cost of having mains power.

Running an eberbasto for a couple of hours every fortnight through the winter while you check over the boat and do a few odd jobs is a valid way forwards and doesn't cost much. £20 of diesel through the winter, against putting the boat in a marina, maybe a grand?

Depends on your personal circumstances.

Whatever, just don't leave the boat fermenting unchecked for 4 or 5 months.

 

[ip485]

I think the problem with heat alone is that we know hot air can "carry" far more moisture than cold. As many perhaps spend a day or a few days on their boat and then close it up and leave, inevitably it colls quickly back to ambient temperature and the moisture comes out as water - we all know about the water on the window sill in the morning. There in is the problem, war m up the boat, more water will be held in the air, nut will then end up running down any colder surface as soon as the temperature falls.

 

[Stemar]

You also have to heat up every nook and cranny, or the warm, moist air will wander off into those nooks and cool. Instant condensation.

 

[JumbleDuck]

You also have to heat up every nook and cranny, or the warm, moist air will wander off into those nooks and cool. Instant condensation.

If it's the air which was already in the boat, heating it up and cooling it down won't cause condensation because the same amount of water as before is there.

 

[ip485]

If it's the air which was already in the boat, heating it up and cooling it down won't cause condensation because the same amount of water as before is there.

I am not sure that is quite right. As air heats up it holds more water. So if you are on the boat water will come from people breathing, boiling kettles, evaporation from any water in the boat (bilges etc). So perhaps with no one on board the amount of water vapour will not change a lot if the boat is sealed and the air heated, but realistically this isnt what happens.

 

[TernVI]

Raising the temperature of your boat, the water in it will evaporate and diffuse away through any vents or porous materials.
The problems come when there is excess water inside, like bilge water, which can evaporate only to condense elsewhere, or sources of water vapour, like people breathing, cooking etc.
It's similar with houses. But houses, historically are mostly built of porous materials.

 

[ip485]

Without meaning to be pedantic I dont think the water vapour will simply diffuse away will it? As the air temperature is raised the air holds more water vapour. The air is effectively a stable mass if the temperature remains stable - a bit like clouds are stable on the day without any wind or thermal inclines. Air against any colder surfaces (typically the hull, cold water and diesel tanks etc will produce condensation on these surfaces as the air is cooled, the more so to start as the hull and tanks take much longer to warm up than the air. As the boat is allowed to cool the air can no longer "hold" the water vapour so this will drop out qucikly on any surfaces that are colder (again typically the inside of the hull, tanks etc. Dehumdifiying the warm air will mean there is less moisture to condense out as the boat cools and if the humidity is low enough, then none at all. The same is true of dehumidifying the cold air, as the boat is warmed the humidity will remain low because it was already low and condensation is far less likely to form.

It is the change in temperature that results in most condensation, and boats left during the week or beyond typically warm up quite quickly on sunny days and then cool rapidly at night. The humidity rises during the day and the water "drops out" at night, hence the soft furnishings feeling damp because they never dry out properly during the colder months.

I think the science is pretty straight forward when you think about it, and it is why a very well ventilated boat will remain dryer than a poorply ventilated boat because the warm air is being replaced by cold "fresh" air with a lower humidity much more quickly before it has dropped too much condensation. I think it is why the dryer the boat is inside, and the less damp areas for the air to pickup moisture, the dryer it will all stay.

However, I cant see there is any substitute for efficient dehumdifiers that will produce a very stable and much lower level of humidity so there isnt the water vapour content present in the first place to form condensation. Probably if you want to save money the dehumidifier is best coming on in the evening for a few hours as the boat starts to cool and again in the morning. I just run mine 24/7 as it doest use a lot of power and maintain the humidity at around 60%, compared with the 80% it would otherwise be much of the time in the winter, and every so often I lower the humidity even more typically when on board and I can get in dwon to nearly 40% which really dries it up and makes it feel really fresh and dry. Works for me anyway.

 

[Whitlock]

...The suggestion that damage to woodwork would inevitably result shows that your knowledge of the subject is limited.

Then I will not try to persuade you further except to say that, instead of attempting to totally dry out, which is the discussion that I was responding to, it is wise to maintain normal humidity while heating and ventilating.
If you don't believe what I am saying, you may benefit from reading this How to Protect Your Furniture Against the Damage from Humidity.
There are many other sources of information on the Internet but it's up to you whether you want to inform yourself about the negative effects of desiccation.

 

[TernVI]

Without meaning to be pedantic I dont think the water vapour will simply diffuse away will it? ..

Yes it will, if the concentration is higher inside than outside, it will diffuse towrds the lower partial pressure.
It's a gas, it will generally obey the gas laws.

 

[thinwater]

deleted

 

[lynallbel]

Down boat today and the meaco has used 4 pence worth of leccy since last weekend, plus a tiny bit of water residue left in the sink, so it is doing something but not much as the boat is quite a dry one.
Out of curio a few years back i did run the dehumudufier at home for a few days, it had less than a teaspoon of water in the tank.

 

[davebradf0rd]

I've found the best way to keep the inside of my boat dry is a combination of three things. A heater on a low setting (not to eat too much electricity) with a small fan blow air around the bow cabin (can get moldy) and a dehumidifier in the galley where I can tip the water away. Fan & dehumidifier run on USB & a 800w fan heater

 

[peter gibbs]

Finally got to the boat this weekend and discovered it was really quite damp. Having heard the tales of dehumidifiers bursting into flames we stopped using one on the last boat and te to have a damp problem.
So what ist he current thinking dehumidifier or not? If yes, which one is recommended?
A further question, new boat is a cat and the only seacock which can conveniently be used for a drain is the galley sink in the starboard hull. I rather think that a dehumidifier would b better situated on the bridgdeck between the two hulls with a pipe to the sink. I am concerned that this would leave the hulls to get damp so am considering a greenhouse type heater in each hull. Schoolboy physics suggests to me that the warmer air should rise and be dealt with by the dehumidifier. Wishful thinking, or could it work? If not, any other suggestions?
Thanks.

Dampness per se is surely no problem unless lots of fabrics are left on board over winter. Even then an arrangement funnelling air throughout by selected vents will protect. I take fabrics ashore now - its worth being cautious given the cost of replacement. Another solution is to acquire large airtight bags and seal fabrics on boat, bagged up all winter with dessicant.

I also wrap an old sleeping blanket round the engine to boost frost protection, on top of antifreeze winterisation of course.

Finally, a low cost aid is our old friend stay dry nappies, loaded with absorbent granules to mop up stray drips.

PWG

 

[JumbleDuck]

I am not sure that is quite right. As air heats up it holds more water. So if you are on the boat water will come from people breathing, boiling kettles, evaporation from any water in the boat (bilges etc). So perhaps with no one on board the amount of water vapour will not change a lot if the boat is sealed and the air heated, but realistically this isnt what happens.

Sorry, I meant "If nothing else changes". If the heated air absorbs more water - from people, bilges or leaks - then of course you are quite right.

 

[CLB]

This could run and run.

Why don't we all agree that we do what we are happy to do, whether that be heat, ventilate or dehumidify, and if any of us finds damp in the future, we will have to have a rethink. No-one here is going to change anyone else's mind.

 

[pessimist]

This could run and run.

Why don't we all agree that we do what we are happy to do, whether that be heat, ventilate or dehumidify, and if any of us finds damp in the future, we will have to have a rethink. No-one here is going to change anyone else's mind.

Prolly not, but it did help me make up my mind.

 

[CLB]

Prolly not, but it did help me make up my mind.

I hope you have chosen to dehumidify, otherwise you'll be disappointed

 

[Whitlock]

This could run and run.

Why don't we all agree that we do what we are happy to do, whether that be heat, ventilate or dehumidify, and if any of us finds damp in the future, we will have to have a rethink. No-one here is going to change anyone else's mind.

Quite right. Agreed.