Humidity levels?

[stephen_h]

I have just bought a dessicating dehumidifier for our boat. It has the option of only running
at a set humidity level. It seems to be quite heavy on the electric so don't want it on more than
needed. At the moment I have it on 75% which seemed okay on Sunday but it was sunny then.
Any thoughts on what humidity level to set it to?

 

[Travelling Westerly]

I set my Meaco DDL8 at 55% or 60%. Keeps me and boat warm and dry.

 

[Wing Mark]

I have just bought a dessicating dehumidifier for our boat. It has the option of only running
at a set humidity level. It seems to be quite heavy on the electric so don't want it on more than
needed. At the moment I have it on 75% which seemed okay on Sunday but it was sunny then.
Any thoughts on what humidity level to set it to?

That is a very good question, the answer might not be simple!

If you set it to say 50% and it gets the boat quite dry, will it then not have much work to do, or will more water get in?
If there's a given amount of water to be pulled out, is it more efficient to extract it when the air is warm, or when it's cooler?

I use a dehumidifier in my workshop, it seems to be more effective to aim to get the place as dry as reasonable while the air is still warm from the day.
The relative humidity will go up overnight as the place cools, but not enough to get condensation anywhere that matters.

 

[sarabande]

If you set it to say 50% and it gets the boat quite dry, will it then not have much work to do, or will more water get in?

Depends on the residual moisture content of the timber and furnishings, any pools of water e.g. bilges, and the amount of ventilation and the Rel Hum and rate of any incoming air, the volume of the air to be treated and the power of the dehumidifier



If there's a given amount of water to be pulled out, is it more efficient to extract it when the air is warm, or when it's cooler?

Warm air holds more water vapour than cold, so......



I use a dehumidifier in my workshop, it seems to be more effective to aim to get the place as dry as reasonable while the air is still warm from the day.

Depends on what you mean by effective, and how dry and when you want low RH.



The relative humidity will go up overnight as the place cools, but not enough to get condensation anywhere that matters.

Depends on the temperature and RH of the workshop, the amount of ventilation, and the temp and RH of incoming air. Also some surfaces are more prone to condensation than others.

 

[geem]

If I was you I would set it to achieve 50% RH each afternoon on a timer. You will get more dehum when it's warmer inside the boat. This is likely to be in the afternoon. There is little point in running overnight when temperatures are lower. Burning power for little benefit

 

[[2574]]

If I was you I would set it to achieve 50% RH each afternoon on a timer. You will get more dehum when it's warmer inside the boat. This is likely to be in the afternoon. There is little point in running overnight when temperatures are lower. Burning power for little benefit

Is that true of a dessicant dehumidifier?

 

[geem]

Is that true of a dessicant dehumidifier?

No. I missed that bit?.
Dessicant type will run at low temperatures very well.

 

[Wing Mark]

No. I missed that bit?.
Dessicant type will run at low temperatures very well.

But they can only extract what is in the air, and saturated air at 10degC contains a lot less water than saturated air at 20 degC.
So they may have to work harder to remove the same mass of water from the system at a lower temp.
It may be keeping the air dry, but not getting the boat dry.

 

[geem]

But they can only extract what is in the air, and saturated air at 10degC contains a lot less water than saturated air at 20 degC.
So they may have to work harder to remove the same mass of water from the system at a lower temp.
It may be keeping the air dry, but not getting the boat dry.

But if you compare dessicant type with compressor type, the dessicant type will out perform a compressor type at low temperatures. If you are living aboard where you heat the boat up to a comfortable temperature the compressor type will perform best. Once you have dry air in the boat it will draw dampness from all surfaces and porous material. Vapour pressure is immensely powerful.

 

[Wing Mark]

But if you compare dessicant type with compressor type, the dessicant type will out perform a compressor type at low temperatures. If you are living aboard where you heat the boat up to a comfortable temperature the compressor type will perform best. Once you have dry air in the boat it will draw dampness from all surfaces and porous material. Vapour pressure is immensely powerful.

Indeed.
If you are heating the boat regularly, you may not need any dehumidifier, same as many/most people don't feel they need one at home.
But sometimes it's a good tool.

 

[johnalison]

My Meaco job has three humidity settings. I normally leave it at its wettest setting and the lowest fan setting. This seems perfectly adequate.

It is important to seal off all vents and gaps unless you want to dehumify the whole country. I usually find the electricity consumption is high for a week or two but then drops to almost nothing.

 

[thinwater]

Run it at night. Yes, cold air holds less water than warm air at the same RH, BUT if it is the same air (sealed boat), it will be holding the same amount of water it did during the day, resulting in higher RH at night. This is why you get dew and condensation at night. Obviously.

During the day the greenhouse effect will warm the boat and more water will come out of the wood and cloth. Then run at night to squeeze it out. The absorbent is more effective (better equilibrium) at night. In the case of compressor or Peltier effect DHs, the plates are colder and more effective at night.

Finally, Petlier DHs will freeze up if run at night if you run them when it is cold, and the best defrost cycle is to have them off during the day. Even when it is quite cold at night, the cabin will rise above freezing during the day and they will thaw (I've done this in climates colder than England).

The goal is not a specific number, but to avoid condensation. 45-55% during the day is the general recommendation of HVAC standards groups (Google it). Any higher and you get condensation behind stuff and night and mold.