Dehumidifier - when to turn it on?

[Yacht Yogi]

My dehumidifier uses quite a lot of electricity so to save costs I am going to put a time-switch on the power supply to switch it on and off. If I were to run it for just a part of each day, what would be most effective at reducing dampness, to run it at night time or in the daytime?

I would welcome your suggestions.

Cheers

 

[rotrax]

For the past five years we have used a timer on our dehumidifier. It comes on 11am and goes off at 4pm.

We also have 1x80w tube heater in the salon, plus 1x20w tube heater in the engine bay, both on 24/7.

October through to last week it cost about 60 quid from Premier Port Solent.

A fellow club member told me some years ago that the warmest part of the day, even in winter is 11 to 4, and that is when a dehumidifier would be most effective.

Seems to work-our boat, despite a very wet winter and us being unable to visit has not suffered any damp at all. She is well ventilated, which must help.

I hope this helps.

 

[[2574]]

She is well ventilated, which must help.

I hope this helps.

That's interesting, we run a dehumidifier but we don't ventilate otherwise the dehumidifier is trying to dehumidify the natural world which is unlikely to be successful! We seal our boat up as much as possible. From Lady Day to Michaelmas the dehumidifier goes home and then we fully ventilate. Otherwise we agree with your strategy.

 

[jwilson]

If it is a compressor type run it in the afternoons when warmest (more efficient when warm and many models could freeze up on a cold winters night).

If a dessicant type run it in the evenings when condensation may occur on surfaces as things cool down. Will both warm and dry the air. Dessicant types much better for cold/cold conditions but do use much more power. On a cold day a dessicant one will collect more water in 3-4 hours running than a compressor one run 12 hours or more.

 

[ditchcrawler]

I had my dessicant dehumidifier on for three months of the winter period (jan-mar) on high for 2 weeks and then on a medium setting.I used £32 worth of electricity in that period of which £19 was the standing charge.I had sealed up the vents.

 

[ctva]

All dehumidifiers as far as I know, have humidistats to automatically switch it on and off in the same way as a thermostat. We run ours from October to April and it does not use very much power as like a fridge it is on standby a fair amount of time so consuming virtually no power. Overall it probably works out at less than £2 per week including a standing charge.

If you are going to use a timer, during the day is best as dehumidifiers (non dessicant) need to be above about 5c to work. Also, close up most of the ventilation. We just have a small solar fan.

 

[sailorman]

My dehumidifier uses quite a lot of electricity so to save costs I am going to put a time-switch on the power supply to switch it on and off. If I were to run it for just a part of each day, what would be most effective at reducing dampness, to run it at night time or in the daytime?

I would welcome your suggestions.

Cheers

why do you need one this time of the year ???

 

[davidej]

What is the science behind the assertion that it works best when warm.

My intuition tells me the opposite.

Relative humidity increases as temperatures drops and a dehumidifier lowers the temperature so the RH exceeds 100% and liquid water condenses out.

Thus I would have thought it best to give it a good start by running it when the ambient temperature is least.

 

[Pye_End]

The warmer the temperature the more water in the woodwork wants to evaporate and end up in the air. Taking water out of the air then helps the air take even more water out of the woodwork.

During a dewy evening the colder air loses it's ability to hold water, and deposits it on the ground instead.

 

[nigelmercier]

All dehumidifiers as far as I know, have humidistats to automatically switch it on and off in the same way as a thermostat...

Not all do, but it is clearly the best solution. One of these would pay for itself in a couple of seasons: http://amzn.to/QTL1Y4

• High-efficiency portable dehumidifier with 10 Litres/24 hour extraction rate
• 2 Litre tank with permanent drain facility and washable dust filter
• Compact dimensions with pull-up handle
• Electronic anti-frost setting for use in cold environments
• Humidistat, allowing you to set moisture to a comfortable level and control energy usage at the same time.

 

[pmagowan]

What is the science behind the assertion that it works best when warm.

My intuition tells me the opposite.

Relative humidity increases as temperatures drops and a dehumidifier lowers the temperature so the RH exceeds 100% and liquid water condenses out.

Thus I would have thought it best to give it a good start by running it when the ambient temperature is least.

It is hard to cool cold air. The dehumidifyer works by cooling air and if the air is warm then you will get a much greater change in air temperature with the compressor and as such it will give up a lot more of its water.

 

[davidej]

It is hard to cool cold air.r.

What kind of Bo**ocks is that.

Air won't "give up "humidity until the RH exceeds 100%

can we have some proper science here

Do you guys navigate by tarot cards?

 

[pmagowan]

What kind of Bo**ocks is that.

Air won't "give up "humidity until the RH exceeds 100%

can we have some proper science here

Do you guys navigate by tarot cards?

I was presuming you could fill in the blanks and it didn't need to be written like a science paper but if you insist.

The saturated vapour pressure for any given temperature (T) and pressure (p) is roughly (1.0007 + 3.46 x 10^-6 P) x (6.1121)e (17.502T/240.97+T) From this you can clearly see the temperature and pressure relation to the saturation of the air and so should be able to work it out yourself. Bear in mind that this is a rough estimate due to the infinite variation in the relation of humidity to both temperature and pressure and so it is +/-0.2%

Or in plain language the work required by a compressor based dehumidifyer to achieve a drop in air temperature would increase as the temperature of the air decreases closer to the minimum temperature of the refrigerant where the work required would equal infinity. Therefore this type of dehumidifier will work better in warm environments where it will achive a greater drop in temperature (as per the formula above) which in turn, given no change in the total pressure in the system, will result in a larger increase in the ratio of the partial pressure of the water vapour in the air/water mixture compared to the saturated vapour pressure of the water for the given temperatures. All pressures are in hectopascals temperatures in degree celcius.

If you want the 'proper science' I advise you contact the appropriate department of your local university as for accuracy you would need a much more complex formula than the one given here.

 

[ctva]

As pmagowan says!!!

I would have said that the dehumidifier (compressor model) works by drawing moist air over the cold elements of a compressor system causing the water in the air to condensate on the elements thus removing the moisture. As the compressor will only go down so far in temperature, drawing colder air over a cold surface will not cause condensation, hence you need a minimum (what most manufacturers say) 5c to work.....I think.

 

[geem]

Waste of time running a dehumidifier if you are well ventilated!!!
Seal the boat up to run it or don't bother using it

For the past five years we have used a timer on our dehumidifier. It comes on 11am and goes off at 4pm.

We also have 1x80w tube heater in the salon, plus 1x20w tube heater in the engine bay, both on 24/7.

October through to last week it cost about 60 quid from Premier Port Solent.

A fellow club member told me some years ago that the warmest part of the day, even in winter is 11 to 4, and that is when a dehumidifier would be most effective.

Seems to work-our boat, despite a very wet winter and us being unable to visit has not suffered any damp at all. She is well ventilated, which must help.

I hope this helps.

 

[jwilson]

I was presuming you could fill in the blanks and it didn't need to be written like a science paper but if you insist.

The saturated vapour pressure for any given temperature (T) and pressure (p) is roughly (1.0007 + 3.46 x 10^-6 P) x (6.1121)e (17.502T/240.97+T) From this you can clearly see the temperature and pressure relation to the saturation of the air and so should be able to work it out yourself. Bear in mind that this is a rough estimate due to the infinite variation in the relation of humidity to both temperature and pressure and so it is +/-0.2%

Or in plain language the work required by a compressor based dehumidifyer to achieve a drop in air temperature would increase as the temperature of the air decreases closer to the minimum temperature of the refrigerant where the work required would equal infinity. Therefore this type of dehumidifier will work better in warm environments where it will achive a greater drop in temperature (as per the formula above) which in turn, given no change in the total pressure in the system, will result in a larger increase in the ratio of the partial pressure of the water vapour in the air/water mixture compared to the saturated vapour pressure of the water for the given temperatures. All pressures are in hectopascals temperatures in degree celcius.

If you want the 'proper science' I advise you contact the appropriate department of your local university as for accuracy you would need a much more complex formula than the one given here.

Thank you for the science. I can reiterate that on a practical level compressor dehumidifiers are virtually useless at low temperatures, and work well in warm/hot conditions. Once air temperature is below 5C many will freeze up entirely, a ball of ice surrounding the cold bit (evaporator??) that gradually expands till it hits the fan, stalling the fan motor, which will then either die or get hot and maybe catch fire. Some more expensive ones have an auto defrost function to stop that happening. I don't think the Delonghi one quoted above has that function. Whatever you do, avoid the XM ones sold by some chandleries. Most broke down quickly, and there were other worse problems.

Look for a Meaco DD8 or similar dessicant type for winter use, works well in any temperature.

 

[rotrax]

Waste of time running a dehumidifier if you are well ventilated!!!
Seal the boat up to run it or don't bother using it

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!

 

[Yacht Yogi]

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

 

[capt_courageous]

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

 

[nigelmercier]

Air won't "give up "humidity until the RH exceeds 100%

That would be a neat trick. Proper science?