To circulate air, fan points in or fan points out?

[DaveC426913]

TL;DR Summary

For the general case, where one would want to discourage moisture during storage, is it more effective to have a fan that blow air into the spae, or a fan that sucks air out?

Got a friend with a boat he's storing over the winter. He wants to discoruage misture and mold build up by keeping air circulating. He puts a box fan in the companionway, (and presumably opens the fore hatch).

All other things being equal, is it better to point the fan into the companionway, or out of the companionway?

My suspicion is that it is better to have the fan pointing in, because air entrainment works in his favour, whereas, if it's pointing outward, air entrainment works against him (because much of the air being moved comes from outside anyway).

(I also suspect the best thing he can do is make a cowl to enclose the fan within the companionway.)

 

[russ_watters]

Storing where? In a covered boathouse? In the water? A shed? And what location/climate? It sounds like you're talking about ventilating the inside of the boat, not the thing it is stored in?

A couple of thoughts:

Humidity, temperature and therefore corrosion and mold growth potential are low in the winter. If it's not in water, you mainly just want it dry before storage.

If a fan blows into something it will be positively pressurized and air will flow out of every orifice. And vice versa. That probably doesn't matter I'd you aren't doing any pretreatment to the air.

 

[jack action]

I don't know how much of a difference it would make, but I would prefer having the fan pull the air out.

As @russ_watters pointed out, this would create a lower pressure inside, thus favoring the evaporation of water inside.

Plus, assuming the fan starts and stops, you pull out the old inside moist air almost instantly and let the pure new dry air get in slowly through every orifice, vs. pressurizing the inside moist air almost instantly — favoring condensation — and having a mix of new dry air and the old humid air slowly leak out the various orifices.

 

[DaveC426913]

Yes, he has a fan in the companionway of his boat, so blowing into or out of the boat itself.

The fan sits about six inches in front of a 1.5 foot square hole in a panel in his companionway.

I concluded that, even more than the direction if air flow, something that would help is to duct the fan - put a cowl around it, so that air is forced though the boat, rather than just wasting a lot of energy circulating air outside. Yes?

 

[Baluncore]

The fan will create turbulence and heat the air slightly. That will lower the relative humidity. Push colder dry air in through a duct, to low down in the hull, maintaining a slight positive internal pressure. That will push moisture out of any gaps, back the way it could come in.

Wetter air has lower density, so it will rise. The vents should be high up in the cabin. That will prevent condensation on the cold ceiling at night.

Throw an old carpet and a tarpaulin over the boat, to shed wind, rain and snow, to keep it insulated and warm on cold clear nights. A warm boat will evaporate moisture into colder, dry air, that warms, rises, and is vented.

 

[Lnewqban]

I concluded that, even more than the direction if air flow, something that would help is to duct the fan - put a cowl around it, so that air is forced though the boat, rather than just wasting a lot of energy circulating air outside. Yes?

Yes.
What is the easiest path for the air to enter or leave the space, the fore hatch or the companionway and around the box fan?

 

[sophiecentaur]

When laying up a boat, you need to open as many voids as possible and lift seat cushions to get air everywhere you can.. If you don't do that there will be many places where 'fresh' air will not reach.

As for the direction of the fan, 'blow' tends to be more directional than 'suck' so blowing air into the cabin may carry the effect further into the cabin. Use smoke to see which direction the air goes for the prevailing wind and choose the fan direction to be in step with the natural flow.

The direction of the wind can make a huge difference to how much air gets in. The same applies to kitchen extractor fans which can be disappointing if the wind hits the external wall. Guaranteed effective airflow requires a fair bit of fan power / diameter.

I used a de-humidifier one winter because a lot of water had got into the boat due to a loose keel. I just let it run and run because I didn't pay for electricity. Sooo much water came out into the sink drain.

 

[jack action]

Just a thought: I have never seen a bathroom fan — which is also used to remove humid air — blowing into the bathroom. They always "remove" the air from the room.

 

[DaveE]

Since you'll have low flow and pressures, I don't think it matters. The system will be close to symmetrical. There are a bunch of design rules for "real" forced air ventilation but they just aren't very important in this sort of situation.

 

[DaveC426913]

Just a thought: I have never seen a bathroom fan — which is also used to remove humid air — blowing into the bathroom. They always "remove" the air from the room.

Perhaps. But, unless we know otherwise, that might not necessarily be for reasons of efficiency.

Consider how uncomfortable it would be to be wet and have to stand under a cold draft in a small room like that. Human comfort could easily be the deciding factor in that case.*

The boat is a different scenario with different needs. Not saying you're wrong; just saying it's a risky comparison.

* same reason you reverse the direction of ceiling fans in the winter. In the summer you want a breeze - in the winter you don't. A human comfort thing, not an efficiency thing.

 

[DaveC426913]

Since you'll have low flow and pressures, I don't think it matters. The system will be close to symmetrical. There are a bunch of design rules for "real" forced air ventilation but they just aren't very important in this sort of situation.

Yeah, that sortta what I was thinking, which is why I asked here. I think the best possible thing he can do is duct the fan.

 

[sophiecentaur]

Just a thought: I have never seen a bathroom fan — which is also used to remove humid air — blowing into the bathroom. They always "remove" the air from the room.

Chill factor is the right term.

 

[DaveC426913]

Sorry all, I could have posted this right up front for your reference. This is from the poster:

This was my suggestion as to his best, simplest option:

 

[DaveC426913]

Chill factor is the right term.

 

[DaveE]

Yeah, that sortta what I was thinking, which is why I asked here. I think the best possible thing he can do is duct the fan.

Yes, but even that may not be necessary for a well built fiberglass boat. The important thing is to get all of the water out. Drain ALL of the bilge water, remove any sponge-like wet stuff (carpet, foam, etc.), keep more water from getting in, and provide a bit of airflow.

Did he have a problem last year? Why?

 

[Baluncore]

Just a thought: I have never seen a bathroom fan — which is also used to remove humid air — blowing into the bathroom. They always "remove" the air from the room.

Warm and moist, air rises to the ceiling of a bathroom. That is especially true during a hot shower. To prevent mould growth on the ceiling, that low density air must be removed, hence the extractor fan in the ceiling, or high on an outer wall. Air entry can be through the bathroom door, cracks, or a window vent. You do not want hot air from the bathroom flowing from the bathroom, through the door, into the rest of the house.

 

[russ_watters]

You do not want hot air from the bathroom flowing from the bathroom, through the door, into the rest of the house.

Note, that's also the case when using the bathroom for its other main purpose.