Dew Point Q

[Rick Hughes]

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.

for example a 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

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.

[tony sayer]

dewandfrost.shtml

or...

might have to copy and paste that to get it to open....

[Andrew Gabriel]

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.

[meow2222]

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

[Rick Hughes]

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.

[Rick Hughes]

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 ....

[Rod]

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?

[ransley]

p://en.wikipedia.org/wiki/Dew_point

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

[tony sayer]

In article , Rick Hughes scribeth thus

Well it did come off this site;))...

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"

[meow2222]

te:

ttp://en.wikipedia.org/wiki/Dew_point

yes for those temps you'd need an ansorption wheel dehumidifier

NT

[Andy Dingley]

No such (useful) critter, as it varies too much.

A better rough guide is to keep the temperature of likely condensing surfaces slightly above the temperature of the warmest incoming air (i.e. a constant temperature difference, not a constant temperature). You ought to find that leaving a constant heater power switched on, possibly through a timeswitch, is adequate to achieve this without any active controls at all. Can't estimate the power though, without knowing volumes, areas and insulation quality.

Condensation doesn't happen randomly, it happens when moist air at one temperature (at which it can carry that moisture) is cooled down to a temperature at which the RH would exceed 100%. So long as the air remains above this first temperature, you're OK. You might even drop slightly below it, by an amount dependent on how far the incoming air is from saturation.

Hoadley's "Understanding Wood" is a fine book and amongst other things has an excellent chapter on humidity-related issues.

[John Stumbles]

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 :-)

[Rick Hughes]

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.

[Jim Michaels]

much

>>> I guess what we would need to know for a simple thermostat controlled

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

[AJH]

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

[Andy Champ]

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

[Andy Dingley]

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

[Andrew Gabriel]

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.

[Andy Dingley]

Although a refrigeration dehumidifier (the affordable sort) is uselessly inefficient for unheated spaces in winter.

[Andy Champ]

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