"Stack effect" and condensation removal

[sarabande]

I have been chasing solutions for condensation removal (boat ashore, aligned E - W).

My latest ferreting of ideas links to "stack effect" whereby a chimney or similar tube fastened to a cowl vent (retained to prevent rain access below) should exploit the temperature difference between the internal cabin temperature and external ambient and create airflow without the need for power.

The boat is presently not sealed against airflow (e.g. a ventilation grille in the hatch boards) and leaks air - but not enough to remove condensation and keep the cabin surfaces sweet and dry.

I have three x 1 metre lengths of 6" ID stainless woodburner chimney, which I could place over the present cowl vent, and support against the boom. I have tried leaving this in place in the garden to simulate the boat set up, and in sunshine there is a noticeable draught up the chimney.

That's for a patch of sunshine, but the draught quickly reduces when cloudy. I have not explored what happens at night.

My next step is to add a ventilator cowl to the top of the chimney to facilitate the draught when the wind blows. Many types available.

I am not concerned with the aesthetics of a 3m steel chimney just for'd of the main hatch (about another 3m above ground level), if I can encourage good airflow.

Chimney Cowls for Wood Burners | Brewer Cowls | Brewer Cowls

I think it likely that even in winter daytime or windy conditions, I can extract damp air from inside the hull at a beneficial rate, but wonder what happens on a windless frosty night. Will cold air (at low % Rel Hum) creep back down the chimney to condense on the cabin surfaces ? I can't visualise the atmospherics.

There's an interesting diagram about diurnal RH variation, incorporating height above ground

 

[RunAgroundHard]

... but wonder what happens on a windless frosty night. Will cold air (at low % Rel Hum) creep back down the chimney to condense on the cabin surfaces ? I can't visualise the atmospherics.

With my wood burning stoves at home, lined chimney stacks, I find cold, damp, still air, flows down the chimney. Dry and cold air does not do this. From time to time our home is unoccupied for a few days and we leave the heating on frost protection, so the house cools right down to outside temperature. Traditional stone wall, lathe and plaster, so heat is not retained after a short period. I think less likely if the air is dry based on my observations. For example, when the conditions are as described, I know I am going to have down draft issues when starting the fire, so have to preheat chimney stack. If I don't paper / kindling smoke from starting the fire blows back. Dry air and this does not happen.

That's how I experience it, hope this helps.

 

[Sandy]

What you need is a big cover over the boat with the hatches open and a chap to visit when he is working on his boat to check that things are drying out.

 

[thinwater]

With my wood burning stoves at home, lined chimney stacks, I find cold, damp, still air, flows down the chimney. Dry and cold air does not do this. From time to time our home is unoccupied for a few days and we leave the heating on frost protection, so the house cools right down to outside temperature. Traditional stone wall, lathe and plaster, so heat is not retained after a short period. I think less likely if the air is dry based on my observations. For example, when the conditions are as described, I know I am going to have down draft issues when starting the fire, so have to preheat chimney stack. If I don't paper / kindling smoke from starting the fire blows back. Dry air and this does not happen.

That's how I experience it, hope this helps.

Yes.

A common villain, with chimneys on the outside of houses, is that the air in the chimney is colder(denser) than the air in the house, so it falls. The mathematical model did not allow for the temperature difference between the stack and the building, which is the larger driver. Even if the house is not actively heated, solar heating can cause the same differential.

 

[Graham_Wright]

Please explain why the proposed method ensures no condensation.
I have read many times here that, if you leave the boat ventilated, there will be no condensation.
Why?
Outside air sucked in, temperature drops at night, water vapour condenses out.
Please refute.

 

[B27]

Please explain why the proposed method ensures no condensation.
I have read many times here that, if you leave the boat ventilated, there will be no condensation.
Why?
Outside air sucked in, temperature drops at night, water vapour condenses out.
Please refute.

Indeed.
Most boats are left over winter near the sea or river, so there's often warm damp air coming off the sea ready to cause condensation on anything cooler.

I don't think you can eliminate condensation completely without either a heat source or active control of the ventilation.

Possibly we should be thinking about when condensation is actually harmful?
Mostly that's when it's warmer, so mould grows?
Or when stuff stays damp for extended periods?