Dew Point Q

A question came up on an on-line boating forum ... about putting a couple of tubular (60W) heaters in a boat, to try and keep things dry under the boat covers, and engine hatch over winter. Controlling them using a simple frost stat
i.e. http://www.tlc-direct.co.uk/Products/PHTH1.html
with for example a http://www.tlc-direct.co.uk/Products/TCWS10.html
I suppose the key thing will be what temperature you set them to switch on ? ... somebody mentioned the heaters need to keep above the dew point ? ...... well in practice what temp is that for UK ?
I did have a Google to look at this, but explanation did not help much http://en.wikipedia.org/wiki/Dew_point
I guess what we would need to know for a simple thermostat controlled installation (ignoring hygrometers and integrating controllers) that there should be a minimum dew point for a typical range of UK temperature & Humidity.
almost a 'set it to this' and it will always be above the dew point ?
Anybody care to shed some light on this.
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Http://www.bbc.co.uk/weather/weatherwise/factfiles/basics/precipitation_ dewandfrost.shtml
or...
http://www.faqs.org/faqs/meteorology/temp-dewpoint /
you might have to copy and paste that to get it to open....
--
Tony Sayer



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Well I followed the first one ... i.e. "Even the driest air contains some water vapour. The condensation process is the same whether we talk about dew or cloud. However, if the air is cooled sufficiently, at night for instance, some of the water vapour will condense on surfaces as dew. The temperature at which air, at a level of constant pressure, can no longer hold all the water it contains is called the dew point. However, when a sample of air is lifted up through cooling air to a level at which is condenses, we use the term condensation level instead of dew point"
The second link was akin to an undergraduate maths thesis ....
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Well it did come off this site;))...
http://www.cl.cam.ac.uk/research/dtg/weather /
refer to the line;
"If you're wondering exactly what the "Dew Point" is, look here for a simple explanation, or here for more than you could possibly want to know"
--
Tony Sayer



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It's not a fixed temperature, unfortunately, so that won't work. Leaving the heater on all the time will always reduce the relative humidity in the boat (because relative humidity drops with increasing temperature at a fixed absolute humidity). This will waste power when the outside humidity is high enough that you aren't near the dew point (relative humidity of 100%).
What you need to do is ventilate under the covers, and apply heating when the outside humidity is near 100%. Trouble is that measuring humidity accurately near the extremes is difficult, and most humidity sensors won't work there. I can't think of a good answer.
The other thing I would warn about is that messing with humidity can have a nasty effect on timber. If you do reduce on one side of a piece of timber verses the other, the timber will cup across the grain. In a house, you can get cupped floorboards which pull out the nails by running a dehumidifier in a room. In a boat, I could imagine much more dire consequences such as wrecking the hull, but I'm not a boating person.
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Andrew Gabriel
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Andrew Gabriel wrote:

120w will make so little difference to temp that its quite pointless controlling them thermostatically. If you want to go down that path, just leave em on whenever theres a risk of damp. I really cant see a degree or 2 solving anything though.
If you want dryness, you want a dehumidifier. But as Andrew says, what effect over-use of one could hahve on the timber I've no idea. IOW it must be humidistatic, and set correctly.
BTW the drying effect of heating is a bit more complex. When heated, the air RH drops, and both lower RH and higher temp increase the evaporation rate of damp sources, resulting in a higher level of damp in the air than before. As air exchanges between inside and out, the net effect of this exchange is to carry moisture out of the building. With CH this stays true even in a wet cold winter - although RH can reach 100% outside, there is still more water per cube in nice dry warm indoor air, so air exchange still dries the building.
NT
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writes:

sensors won't work there. I can't think of a

No timber at all on this boat.
In any event I don't want to reduce boat to biscuit dryness ... just enough to keep things dry & condensation free (& hence mould)
I appreciate what you have said, but I'm no closer to the answer as to what temp should I start heating at ? ... i.e. to set a frost stat ....
I don't want something complex, I thought there might be a simple answer obviously not.
have to see if I can find a combined humidistat / thermostat ... but guessing nothing in price range I'll want to pay.
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Rick Hughes wrote:

I'm not at all convinced that you should do any heating at all. Have you actually tried simply running a small fan - with simple ducting if required?
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Rod

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On Thu, 13 Nov 2008 22:31:23 +0000, Rick Hughes wrote:

Well in that case a dehumidifier would do the job. All you'd have to do is empty the condensate collection trough out as necessary (unless you could drain it to outside).

A dehumidifier is simple. It's simply about £80-100 :-)
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John Stumbles

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This was discussed on another forum ... but the consensus of the guys there was that unless you can enclose your boat in an airtight bag ... the humidifier will just keep sucking moisture out of the external mass of air ... One US company did sell a complete over bag, came up and over like the rain hood on a pram ... floor to floor rubber seals. Way too expensive though.
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On Fri, 14 Nov 2008 19:05:00 -0000, "Rick Hughes"

That's right but all you're trying to do is to keep the rh below 100% as the temperature drops. So cut the air changes and set the dehumidifier at a moderate level and it doesn't have to work too hard.
As you have noted a problem then we already know ventilation is an issue, if ventilation allowed air changes fast enough that the internal air remained in equilibrium with external air and there are no leaks then there would be no condensation. As this would probably mean a howling gale through the boat the next thing is to control air changes and have enough capacity to remove sufficient water from warmer air entering from outside and then condensing out dew as it cools later in the day.
I agree with the dehumidifier idea with a humidistat high up and draining to outside.
AJH
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AJH wrote:

This turns out not to be the case :) After all, I get condensation on my lawn most mornings this time of year...
You need to keep the dew point below the temp. of the things inside your boat. A dehumidifier is the solution that will use least power; a heater will use more power, but cost less to buy. Your call!
Andy
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Your lawn radiates its heat into outer space, and hence gets colder than the air temperature at various points overnight, thus the formation of dew. The cover will protect the boat from this to some extent (a thermal insulating cover would be ideal). Can also be caused by air colling below its dewpoint and forming mist, some of which will settle out on the ground.
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Andrew Gabriel
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Andrew Gabriel wrote:

I don't know what his boat is made of, but I'm pretty sure it too will be exposed to the cold night sky. A cover will reduce the heat loss, but it will still be there; if you get a switch from warm, moist air to cold, such as we just have, there will be condensation.
If you truly want no condensation you want to seal it, and keep the temperature of everything inside the sealed area above the dew point. Since this is likely to include some substantial bits of metal (Engine? Keel?) and possibly contact with a large, cold body of water, reducing humidity may well be the only answer.
Blowing lots of warm damp air on a foggy morning over a deckhead that has been exposed to the midnight sky is not a recipe to prevent condensation.
Andy
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Although a refrigeration dehumidifier (the affordable sort) is uselessly inefficient for unheated spaces in winter.
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On Mon, 17 Nov 2008 04:29:41 -0800 (PST), Andy Dingley

I can believe that, something is necessary to cause any "dampness" to move into the atmosphere before it can be removed. The dehumidifier provides a hundred or so Watts but in a big space...
I was basing my thoughts on typical open sheds and barns where I've never seen dew form and any wet has to be blown in.
Slightly related but as a part of a project to check woodchip supplies I've had a kg of woodchip fuel, oven dried, sitting in this room. It's gone to 10% mc wwb in a week.
AJH
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We were in Barcelona in April. The harbour was full of million dollar gin palaces having their winter bags peeled off, ready for the season
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An incandesant 100w bulb is a 96 watt heater, dew point varies with humidity and temp. There are low temp 40f dehumidifiers. A dehumidifier itself may generate enough heat if its tight to raise temp 10-15f. In a 600 sq ft basement mine raises temp 3-4f
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ransley wrote:

yes for those temps you'd need an ansorption wheel dehumidifier
NT
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On Thu, 13 Nov 2008 16:29:00 -0800 (PST), ransley

If all the light stays in the space then they are 100W heaters.
It can be a fire hazard and it may also fade or otherwise damage the local area. Remove SPAMX from email address
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