Dehumidifier advice

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CraigB

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Dec 19, 2006, 11:01:10 AM12/19/06
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I am seeking some assistance in selecting a suitable dehumidifier. This
will be
my first such purchase and I'm getting a little lost!!

I have recently purchased a house which I will not move into until
building work
has been completed - which may take until August next year. My surveyor
has found
signs of damp in the house, which we shall be remedying - but possibly
not starting
work until April. The house has accomodation over 3 floors with damp
affecting
downstairs and one room of the first floor.

The damp downstairs is not particularly noticeable, but on the 1st
floor
wallpaper is coming off and plaster flaking.

I have a number of concerns in selecting a dehumidifier:

1. The house is unoccupied and I would like the dehumidifier to run
unattended,
draining into a sink which may not be in the same room as the unit.
What sort of
pump (if any) is fitted to the continuous drain option on something
like the Ebac
6200 ?

2. The central heating is not currently operating. I see defrost
options on some
units ? Once the house is sorted I will reuse the unit in an unheated
double garage.

3. How realistic are the performance numbers quoted/ability to
dehumidify large
areas ? I'm trying to compare the Ebac 6200 with a Mitsubishi
MJE16VX-E1

Thanks for any assistance you can provide

Craig

Ian Stirling

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Dec 19, 2006, 12:28:57 PM12/19/06
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CraigB <Cr...@run-by.me.uk> wrote:
> I am seeking some assistance in selecting a suitable dehumidifier. This
> will be
> my first such purchase and I'm getting a little lost!!
>
> I have recently purchased a house which I will not move into until
> building work
> has been completed - which may take until August next year. My surveyor
> has found
> signs of damp in the house, which we shall be remedying - but possibly
> not starting
<snip>

> 2. The central heating is not currently operating. I see defrost
> options on some
> units ? Once the house is sorted I will reuse the unit in an unheated
> double garage.

Don't bother.
It will not extract any significant water at low temperatures, even with
a defrost unit.
There is little water in the air when the house is cold.

Can you possibly get the central heating working?
Having it on at a low level - say 15C, will help significantly to dry
out the house by itself.

normanwisdom

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Dec 19, 2006, 2:22:55 PM12/19/06
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Yes don't bother. Leave some windows open a bit and open internal
doors. It'll be dry by August.

cheers
Jacob

Mark S.

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Dec 19, 2006, 3:03:21 PM12/19/06
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I'll go against the other two posts, lol.

I've been working on my house far too long now. I've had my
dehumidifer on since the day I got metered electric.

I also have an oil filled radiator, well two at the minute due to
plastering etc.

It will work just fine either on it's own or with the aid of a heater.
All I wanted was to keep the house reasonable and keep some of the
damp off.

Can you not visit the property once a week to empty it as it won't
work at it's best in one place. I moved mine around each room giving
it time to take some of the damp away. Stuff in the house (work
clothes) are damp if you leave them but the house as a whole won't be
as bad as if you just left it.


Mark S.

hzatph

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Dec 19, 2006, 4:05:43 PM12/19/06
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"CraigB" <Cr...@Run-By.Me.uk> wrote in message
news:1166544070....@a3g2000cwd.googlegroups.com...

You can get drain kits for some models.

We bought the largest one form B&Q about three years ago - it failed so got
another (B&Q swapped it) which failed about 18 months later. Old Ebac still
working but had to go back under warranty for repair too. Dehumidifiers are
not very reliable in my experience.


Andy Dingley <dingbat@codesmiths.com>

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Dec 19, 2006, 4:35:24 PM12/19/06
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Ian Stirling wrote:

> Don't bother.

I'd broadly agree with that, for the OP as stated.

> It will not extract any significant water at low temperatures, even with
> a defrost unit.

That's the crucial thing with refrigeration dehumidifiers. None of them
will work when cold (and the electrical consumption can be steep then
too), even a hired builder's machine.

> There is little water in the air when the house is cold.

In a USA winter maybe, but a UK winter has plenty of moisture around.
We'll be at 80% RH for much of it and that's still an appreciable
absolute humidity

Fix whatever is putting moisture into the house, and allow as much
ventilation as possible. Visit it when the weather changes and either
open vents or close them. Make the most of what little warmth you can
get hold of. Don't think about electric dehumidifiers in a typical
unheated building until the end of March.

Phil L

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Dec 19, 2006, 4:54:27 PM12/19/06
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As others have stated, don't bother, open each upstairs window an inch and
screw the stays down if they are wooden frames, leave a gap in any curtains
to allow air to flow freely between rooms, if you intend replacing fires
etc, take them out now and use the chimneys as ventilation too.
open any hit and miss vents that are there and unblock any that are boarded
over or blocked in any way, ven ifthey are only between rooms or in
cupboards - you want as much airflow as you can get - a well ventilated
building is a dry building, regardless of temperature, and providing you've
fixed whatever it was/is that was causing the damp in the first
place...winter is actually better for drying buildings out in this way
because the cold draws the moisture out of the building.


Ian Stirling

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Dec 19, 2006, 5:18:10 PM12/19/06
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"Andy Dingley <din...@codesmiths.com>" <din...@codesmiths.com> wrote:
>
> Ian Stirling wrote:
>
>> Don't bother.
>
> I'd broadly agree with that, for the OP as stated.
>
>> It will not extract any significant water at low temperatures, even with
>> a defrost unit.
>
> That's the crucial thing with refrigeration dehumidifiers. None of them
> will work when cold (and the electrical consumption can be steep then
> too), even a hired builder's machine.
>
>> There is little water in the air when the house is cold.
>
> In a USA winter maybe, but a UK winter has plenty of moisture around.
> We'll be at 80% RH for much of it and that's still an appreciable
> absolute humidity

There is an appreciable relative humidity, and a fair absolute humidity, but
you can't efficiently extract it, which was more what I was going for.

dennis@home

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Dec 19, 2006, 5:39:02 PM12/19/06
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"Phil L" <neverc...@hotmail.com> wrote in message
news:ncZhh.18965$k74....@text.news.blueyonder.co.uk...

> As others have stated, don't bother, open each upstairs window an inch and
> screw the stays down if they are wooden frames, leave a gap in any
> curtains to allow air to flow freely between rooms, if you intend
> replacing fires etc, take them out now and use the chimneys as ventilation
> too.
> open any hit and miss vents that are there and unblock any that are
> boarded over or blocked in any way, ven ifthey are only between rooms or
> in cupboards - you want as much airflow as you can get - a well ventilated
> building is a dry building, regardless of temperature, and providing
> you've fixed whatever it was/is that was causing the damp in the first
> place...winter is actually better for drying buildings out in this way
> because the cold draws the moisture out of the building.
>

Except on days like today when its as foggy as it comes.

The Ł70 dehumidifier in Sainsburys works quite well but the permanent drain
is gravity only, no pump.


Pete C

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Dec 19, 2006, 6:00:28 PM12/19/06
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Hi,

For best performance at low temperature look for one with 'hot gas
defrost'.

Bear in mind when it's cold the damp will evaporate more slowly from
the walls, as the air can absorb less and less humidity the colder it
gets.

Units are usually specced at 30C and 80% humidity, so in a cold house
they will collect a lot lot less.

The continious drain facility is usually a cap on the back which can
be removed and a hose fitted.

cheers,
Pete.

Harry Bloomfield

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Dec 19, 2006, 6:31:50 PM12/19/06
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CraigB presented the following explanation :

> I have a number of concerns in selecting a dehumidifier:

They do not perform very well in a cold/cool atmosphere. They work best
drawing in warm moisture laden air, presenting the airflow with a cold
surface upon which the moisture can condense. If the air drawn in is
already cold, they do not work so well. Walk into a warm room, from a
cold outdoors wearing glasses and they steam up, the same effect as a
dehumidifier.

Providing some heating should be your first step.

--

Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk


meow...@care2.com

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Dec 19, 2006, 6:46:11 PM12/19/06
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CraigB wrote:

As everyone says, this is not the best first step, unless the damp is
very bad and there is immediate risk of rotting.

Refrigeration type dehumidifiers are specced at 30C, and at below 10C
theyre not worth running. (I have tried.)

Re the damp patch upstairs I'd look at rainwater goods and pointing,
and see if you've got render or some other coating on the exterior,
which tends to make things worse.

Re damp downstairs, we havent yet heard any real evidence tht there is
a problem to begin with. A surveyor diagnosing damp doesnt mean
squiddly these days. So I'd start by telling us what symptoms of damp
you have, and meanwhile do the basics like checking rainwater goods,
ensuring exterior soil is below the floor level and so on.

Finally standing empty and unheated does tend to make houses damp. Once
warmed up they usually dry out again.

As far as dehumids go, continuous drain models dont normally have any
condensate pump, a humidistat is an important feature, and for use
below 10C you need a desiccant wheel type, not the cheaper more common
refrigeration type.


NT

Andy Dingley <dingbat@codesmiths.com>

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Dec 19, 2006, 7:47:00 PM12/19/06
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Ian Stirling wrote:

> There is an appreciable relative humidity, and a fair absolute humidity, but
> you can't efficiently extract it, which was more what I was going for.

Efficiency is falling because ambient is approaching 0°C and the
refrigerator stops usefully chilling the air, not because there's any
"shortage" of moisture to be had. Storing dry paper or steel tools in
these conditions certainly attracts it! It's still extractable
chemically, if you have a volume small enough to address with calcium
chloride pots.

Over in rec.woodworking, it's an article of faith that winters are bone
dry. But then they're 'merkins (and not Florida 'merkins, it seems).

dennis@home

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Dec 20, 2006, 4:45:21 AM12/20/06
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<meow...@care2.com> wrote in message
news:1166571971....@73g2000cwn.googlegroups.com...

> CraigB wrote:
>
>> I am seeking some assistance in selecting a suitable dehumidifier. This
>> will be
>> my first such purchase and I'm getting a little lost!!
>>

>> Craig


>
> As everyone says, this is not the best first step, unless the damp is
> very bad and there is immediate risk of rotting.
>
> Refrigeration type dehumidifiers are specced at 30C, and at below 10C
> theyre not worth running. (I have tried.)

They work fine as long as the house is well insulated and draft proof.
The refrigeration dehumidifiers actually raise the tempreture and they
function as low output fan heaters.

The best way in the winter is to remove as much heat loss as possible, use a
heater and a dehumidifier.
Ventelation just wastes what heat you have.


Andy Dingley <dingbat@codesmiths.com>

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Dec 20, 2006, 6:12:39 AM12/20/06
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dennis@home wrote:

> The refrigeration dehumidifiers actually raise the tempreture and they
> function as low output fan heaters.

No they don't (to any useful level) -- do the maths.

Andrew Gabriel

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Dec 20, 2006, 8:54:27 AM12/20/06
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In article <1166613159.6...@t46g2000cwa.googlegroups.com>,

Well, mine pumps out 400W, subject to its operating duty cycle.
This is excellent for heating and drying an airing cupboard
(which has no hot plumbing in it). The heating effect would
just notice in a room too, but not in a whole house.

I would add another no vote to the dehumidifier idea though.
I did use one in an old house which was empty for a while,
and it caused some damage. After a few weeks, I noticed it
was drying out all the timber on the side facing the room
in which it operated. This caused the floorboards curl up
as the top dried and shrunk relative to the bottom, and the
floorboards in the room above to curl down for the same
reason the other way around. It took a year for them to
flatten out after I switched it off. And yes, it was on a
humidistat control too, but that doesn't stop significant
differences in humidity between one room and other (and
all the doors were left wide open). A small amount of
heating and some ventilation works very well, particularly
in winter when the cold outside air can't hold much water
even when humidity is high. Heating that air from outside
significantly reduces its relative humidity, and makes it
capable of drying out a house very effectively.

--
Andrew Gabriel

dennis@home

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Dec 20, 2006, 9:48:35 AM12/20/06
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<din...@codesmiths.com> wrote in message
news:1166613159.6...@t46g2000cwa.googlegroups.com...

Any heat is useful if you are well insulated.
Do the physics.. I'll help if you like..

Any water that evaporates within the building requires X kJ of energy.
The dehumidifier extracts X kJ of energy when it condenses said water.
They are heat pumps so are probably 50% efficient so you get another X/2 kJ
of heat from the electricity used.
So you get 1.5X kJ of useful heat from the dehumidifier.

If you heat and ventilate then it takes X kJ of energy to evaporate the
water and you then let it out into the world.
I.e. you retain zero heat.

As for the maths.. my very little one chucks out about 200W, some of the
bigger ones chuck out more.
This is not useful?

BTW you only need to keep the rh below about 60% to stop rust on steel.


Andy Dingley <dingbat@codesmiths.com>

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Dec 20, 2006, 10:58:52 AM12/20/06
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Andrew Gabriel wrote:

> >> The refrigeration dehumidifiers actually raise the tempreture and they
> >> function as low output fan heaters.
> >
> > No they don't (to any useful level) -- do the maths.
>
> Well, mine pumps out 400W, subject to its operating duty cycle.

It's not a question of how much they pump out, it's how much they cost
doing it.

Maybe it's 40W, maybe it's 400W - doesn't matter. If we re-state the
problem as "Electrically heat an empty house until it's warm enough to
use a refrigeration dehumidifier" then you can do it just as easily,
and just as efficiently, by putting a fan heater with a thermostat in
there too. kWh are heat and they cost exactly the same whether you buy
them through a fridge or through a heater.

Now are you prepared to pay the cost of electrically heating an empty
house to a usable temperature? That's the crucial question - how much,
not how.

meow...@care2.com

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Dec 20, 2006, 12:04:56 PM12/20/06
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Andrew Gabriel wrote:
> In article <1166613159.6...@t46g2000cwa.googlegroups.com>,
> "Andy Dingley <din...@codesmiths.com>" <din...@codesmiths.com> writes:
> > dennis@home wrote:

> I would add another no vote to the dehumidifier idea though.
> I did use one in an old house which was empty for a while,
> and it caused some damage. After a few weeks, I noticed it
> was drying out all the timber on the side facing the room
> in which it operated. This caused the floorboards curl up
> as the top dried and shrunk relative to the bottom, and the
> floorboards in the room above to curl down for the same
> reason the other way around. It took a year for them to
> flatten out after I switched it off. And yes, it was on a
> humidistat control too, but that doesn't stop significant
> differences in humidity between one room and other (and
> all the doors were left wide open).

This is one of the reasons a humidistat is needed. Yours will have been
set to too low an RH. Drying needs to occur slowly to avoid this
problem with woodwork. Zones with concrete floors and no woodwork can
be dried out rapidly.


> A small amount of
> heating and some ventilation works very well, particularly
> in winter when the cold outside air can't hold much water
> even when humidity is high. Heating that air from outside
> significantly reduces its relative humidity, and makes it
> capable of drying out a house very effectively.

Works well when the house is occupied. Pricey for an empty house
though.


NT

dennis@home

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Dec 20, 2006, 2:25:47 PM12/20/06
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<din...@codesmiths.com> wrote in message
news:1166630331.8...@f1g2000cwa.googlegroups.com...

>
> Andrew Gabriel wrote:
>
>> >> The refrigeration dehumidifiers actually raise the tempreture and they
>> >> function as low output fan heaters.
>> >
>> > No they don't (to any useful level) -- do the maths.
>>
>> Well, mine pumps out 400W, subject to its operating duty cycle.
>
> It's not a question of how much they pump out, it's how much they cost
> doing it.
>
> Maybe it's 40W, maybe it's 400W - doesn't matter. If we re-state the
> problem as "Electrically heat an empty house until it's warm enough to
> use a refrigeration dehumidifier" then you can do it just as easily,
> and just as efficiently, by putting a fan heater with a thermostat in
> there too. kWh are heat and they cost exactly the same whether you buy
> them through a fridge or through a heater.

Read what I posted again.
They don't cost the same as a heater.
They are more efficent than using a heat pump with a ground heat source at
drying out a house.
The real question is how well does the house hold the heat?
It can hold the heat a lot better if you are using a dehumidifier as no
ventelation is needed.
Any other method requires lots of ventilation which costs money.

> Now are you prepared to pay the cost of electrically heating an empty
> house to a usable temperature? That's the crucial question - how much,
> not how.


How much does it cost to heat and ventilate a house so it dries out?


Pete C

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Dec 20, 2006, 2:30:20 PM12/20/06
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On 20 Dec 2006 03:12:39 -0800, "Andy Dingley <din...@codesmiths.com>"
<din...@codesmiths.com> wrote:

They recover some latent heat as they dehumidify. Mine uses about 1kWh
per kilogram of water condensed.

Latent heat of evaporation at room temp is about 2.6MJ/kg.

So in recovering 1kg of water it also recovers 2.6MJ of latent heat.

2.6MJ is equivalent to 0.72kWh (1kWh = 3.6MJ)

So in using 1kWh it outputs 1kWh of heat and recovers another 0.72kWh
of latent heat.

So the effective heat output is 1.72kWh for 1kWh input.

I'm sure someone will correct me if I'm wrong :)

cheers,
Pete.

Phil L

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Dec 20, 2006, 2:56:19 PM12/20/06
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You don't need heat to dry out a building, I often 100% replaster old houses
with no heat source and they dry out completely within a week or two with
just ventilation, not bad bearing in mind the walls have been soaked with
PVA, then backing plaster applied, then soaked again to kill the suction and
then skimmed...for the first few days, the condensation is running down the
windows but after they've been left open for a week, there's no more
condensation and no dark patches of plaster, everything's dusty dry...no
heat required and certainly no dehimidifier.


Ian Stirling

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Dec 20, 2006, 3:18:24 PM12/20/06
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Pete C <pete...@gmail.com> wrote:
> On 20 Dec 2006 03:12:39 -0800, "Andy Dingley <din...@codesmiths.com>"
> <din...@codesmiths.com> wrote:
>
>>
>>dennis@home wrote:
>>
>>> The refrigeration dehumidifiers actually raise the tempreture and they
>>> function as low output fan heaters.
>>
>>No they don't (to any useful level) -- do the maths.
>
> They recover some latent heat as they dehumidify. Mine uses about 1kWh
> per kilogram of water condensed.
<snip>

> I'm sure someone will correct me if I'm wrong :)

You're not wrong - at that temperature.
At low temperatures, it gets worse, as it can't consense efficiently.
At very low temps, the efficiency drops dramatically.
As the output drops by half, the heat gain due to latent heat drops by
half too.
If it possibly can be done - hooking up the CH, even to one rad, will
heat the house lots cheaper.

dennis@home

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Dec 20, 2006, 4:43:55 PM12/20/06
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"Phil L" <neverc...@hotmail.com> wrote in message
news:Dzgih.19377$k74....@text.news.blueyonder.co.uk...

> You don't need heat to dry out a building, I often 100% replaster old
> houses with no heat source and they dry out completely within a week or
> two with just ventilation, not bad bearing in mind the walls have been
> soaked with PVA, then backing plaster applied, then soaked again to kill
> the suction and then skimmed...for the first few days, the condensation is
> running down the windows but after they've been left open for a week,
> there's no more condensation and no dark patches of plaster, everything's
> dusty dry...no heat required and certainly no dehimidifier.
>

Actually that is wrong.
You need heat to evaporate water.
To evaporate 1kg requires about 2260kJ.. that's about 2kW for about 40
minutes.
It has to come from somewhere or the water will just stay there.

BTW its how evaporative coolers work as evaporating the water cools it down.


Pete C

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Dec 20, 2006, 4:49:06 PM12/20/06
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On Wed, 20 Dec 2006 19:25:47 -0000, "dennis@home"
<den...@killspam.kicks-ass.net> wrote:

>
>How much does it cost to heat and ventilate a house so it dries out?

Hi,

I think for a gas central heated home it's always cheaper to ventilate
(eg use a shower extractor) than use a dehumdifier.

For incoming air at 0C and 100%RH and outgoing air at 20C and 80%RH,
it will take 0.7kWh to shift 1L of water by ventilating the humid air
and heating the incoming air to replace it.

So even if a dehumidifier gets 50% of input energy back from latent
heat, it's still going to cost much more than ventilation.

cheers,
Pete.

normanwisdom

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Dec 20, 2006, 5:37:55 PM12/20/06
to

The heat comes from the drier ventilating air which gets cooled down as
it picks up moisture, and by solar gain through glass. You don't need
any active heat input.

dennis@home

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Dec 20, 2006, 6:09:55 PM12/20/06
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"normanwisdom" <owd...@googlemail.com> wrote in message
news:1166654275....@73g2000cwn.googlegroups.com...


> The heat comes from the drier ventilating air which gets cooled down as
> it picks up moisture, and by solar gain through glass. You don't need
> any active heat input.
>

In the winter?


Phil L

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Dec 20, 2006, 7:23:45 PM12/20/06
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dennis@home wrote:
> "Phil L" <neverc...@hotmail.com> wrote in message
> news:Dzgih.19377$k74....@text.news.blueyonder.co.uk...
>
>> You don't need heat to dry out a building, I often 100% replaster old
>> houses with no heat source and they dry out completely within a week
>> or two with just ventilation, not bad bearing in mind the walls have
>> been soaked with PVA, then backing plaster applied, then soaked
>> again to kill the suction and then skimmed...for the first few days,
>> the condensation is running down the windows but after they've been
>> left open for a week, there's no more condensation and no dark
>> patches of plaster, everything's dusty dry...no heat required and
>> certainly no dehimidifier.
>
> Actually that is wrong.

It doesn't matter dennis...I mean, the water, plaster, heat, cold,
condensation, evaporation or anything else don't know the rules as they are
laid down on paper, all I know is that ventilation, even now - today - is
drying out dozens upon dozens of gallons of water out of an empty house, no
electric or gas.

> You need heat to evaporate water.

quite obviously not, unless you mean the difference between inside and
outside of an unoccupied building, probably about 4 deg c difference today

> To evaporate 1kg requires about 2260kJ.. that's about 2kW for about 40
> minutes.
> It has to come from somewhere or the water will just stay there.

But if you have 400 minutes, how much does it need? and 4000 minutes? and
40,000 minutes? - even a slight temperature increase of a degree is ample,
providing the ventilation is adequate....there's 40,000 minutes in a month
BTW, perfectly adequate to dry any building out, with plenty of ventilation.

normanwisdom

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Dec 21, 2006, 3:55:28 AM12/21/06
to

Yes - more so in winter as the absolute humidity of the air is lower.
An additional small rise in temp when the air finds its way into a
building increases it's drying power. You get some heat gain through
glass even on a cloudy day in winter.

dennis@home

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Dec 21, 2006, 4:04:53 AM12/21/06
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"Phil L" <neverc...@hotmail.com> wrote in message
news:lukih.19512$k74....@text.news.blueyonder.co.uk...

> dennis@home wrote:
>> "Phil L" <neverc...@hotmail.com> wrote in message
>> news:Dzgih.19377$k74....@text.news.blueyonder.co.uk...
>>
>>> You don't need heat to dry out a building, I often 100% replaster old
>>> houses with no heat source and they dry out completely within a week
>>> or two with just ventilation, not bad bearing in mind the walls have
>>> been soaked with PVA, then backing plaster applied, then soaked
>>> again to kill the suction and then skimmed...for the first few days,
>>> the condensation is running down the windows but after they've been
>>> left open for a week, there's no more condensation and no dark
>>> patches of plaster, everything's dusty dry...no heat required and
>>> certainly no dehimidifier.
>>
>> Actually that is wrong.
>
> It doesn't matter dennis...I mean, the water, plaster, heat, cold,
> condensation, evaporation or anything else don't know the rules as they
> are laid down on paper, all I know is that ventilation, even now - today -
> is drying out dozens upon dozens of gallons of water out of an empty
> house, no electric or gas.

The OP wanted to dry it out faster.
He needs to lower the humidity inside the house to speed up drying.


>> You need heat to evaporate water.
>
> quite obviously not, unless you mean the difference between inside and
> outside of an unoccupied building, probably about 4 deg c difference
> today

Today is exceptional.
Its currently <1C and 100+% humidity.
If you leave all the windows open in a fog like this it will be wetter than
if you close them.

>> To evaporate 1kg requires about 2260kJ.. that's about 2kW for about 40
>> minutes.
>> It has to come from somewhere or the water will just stay there.
>
> But if you have 400 minutes, how much does it need? and 4000 minutes? and
> 40,000 minutes? -

The same.
Its energy not power.

> even a slight temperature increase of a degree is ample,

Thats what I said.. the slight increase from a dehumidifier is enough to do
it quite quickly.

> providing the ventilation is adequate....there's 40,000 minutes in a month
> BTW, perfectly adequate to dry any building out, with plenty of
> ventilation.

It takes 6+ months to dry out a new build and thats with heating and
ventilation.
If you want to dry it faster then ventilation alone is not going to do it
especially in winter.


dennis@home

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Dec 21, 2006, 4:24:32 AM12/21/06
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"normanwisdom" <owd...@googlemail.com> wrote in message
news:1166691328....@42g2000cwt.googlegroups.com...

Turn your heating off, open all your windows and chuck a couple of buckets
of water on your walls.
Tell us when it dries.
I'll come and have a look for your post next month.


normanwisdom

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Dec 21, 2006, 4:56:27 AM12/21/06
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dennis@home wrote:
> "normanwisdom" <owd...@googlemail.com> wrote in message
> news:1166691328....@42g2000cwt.googlegroups.com...
> >
> > dennis@home wrote:
> >> "normanwisdom" <owd...@googlemail.com> wrote in message
> >> news:1166654275....@73g2000cwn.googlegroups.com...
> >>
> >>
> >> > The heat comes from the drier ventilating air which gets cooled down as
> >> > it picks up moisture, and by solar gain through glass. You don't need
> >> > any active heat input.
> >> >
> >>
> >> In the winter?
> >
> > Yes - more so in winter as the absolute humidity of the air is lower.
> > An additional small rise in temp when the air finds its way into a
> > building increases it's drying power. You get some heat gain through
> > glass even on a cloudy day in winter.
> >
>
> Turn your heating off, open all your windows and chuck a couple of buckets
> of water on your walls.
> Tell us when it dries.
> I'll come and have a look for your post next month.

You need a bit of elementary physics revision - look up "absolute" and
"relative" "humidity" and a few other things such as "latent heat"
etc. You obviously weren't paying attention during physics lessons.
In summer you can get the opposite effect when warm humid air enters
cold building and you get condensation. Really noticeable in spring
when you get those first warm days and unheated buildings, sheds etc
suddenly become really damp esp if there is a large mass of cold
masonry. or cast iron machinery etc.
cheers
Jacob

dennis@home

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Dec 21, 2006, 5:25:30 AM12/21/06
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"normanwisdom" <owd...@googlemail.com> wrote in message
news:1166694987.2...@79g2000cws.googlegroups.com...

> You need a bit of elementary physics revision - look up "absolute" and
> "relative" "humidity" and a few other things such as "latent heat"
> etc. You obviously weren't paying attention during physics lessons.

Do you want to bet?

> In summer you can get the opposite effect when warm humid air enters
> cold building and you get condensation. Really noticeable in spring
> when you get those first warm days and unheated buildings, sheds etc
> suddenly become really damp esp if there is a large mass of cold
> masonry. or cast iron machinery etc.
> cheers
> Jacob
>

I think we all know about condensation.
But as the air is already humid it has been supplied with energy.
Where is the energy coming from to evaporate the water when you are trying
to dry out damp in the winter?
Its the middle of winter and as damp as hell ATM like it is for much of
winter in the UK (maybe you live in the channel isles?).
Without energy input not much is going to happen.
Its just a case of where that energy is coming from and how much you reuse
and how much you blow out the windows.

AFAICS the ones that say dehumidifiers don't do the job are the ones that
haven't tried it because they don't do the job.
The rest of us with one know differently.

BTW don't overdo the heat or all the timber will warp and the plaster will
shrink and crack.


The Natural Philosopher

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Dec 21, 2006, 5:45:27 AM12/21/06
to
To de humdify you simply need a source of air that is at lower RH than
what you want to dehumidfy.

If you chill air till the moisture drops out, its dry and cold, and as
it warms up it will absorb moisture.

If you simply take air and heat it, that has the same effect, but the
temeperature is higher.

Your choice.


normanwisdom

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Dec 21, 2006, 6:30:41 AM12/21/06
to

> Where is the energy coming from to evaporate the water when you are trying
> to dry out damp in the winter?

Go away and do some revision then you can come back and tell us the
answer.

cheers
Jacob

dennis@home

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Dec 21, 2006, 7:50:15 AM12/21/06
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"normanwisdom" <owd...@googlemail.com> wrote in message
news:1166700641.2...@a3g2000cwd.googlegroups.com...

Why do I need to do revision I have stated where it is coming from with a
dehumidifier.
I expect that you can do the same with what you stated?

While you are at it you can explain how much of the heat is blown out the
windows before it has a chance to evaporate the water.
I can see great problems in heating the air and getting it fully saturated
before it is expelled.
Not doing so reduces the efficiency by a significant amount.


dennis@home

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Dec 21, 2006, 8:08:25 AM12/21/06
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"The Natural Philosopher" <a@b.c> wrote in message
news:116669779...@despina.uk.clara.net...

> To de humdify you simply need a source of air that is at lower RH than
> what you want to dehumidfy.
>
> If you chill air till the moisture drops out, its dry and cold, and as it
> warms up it will absorb moisture.

With a dehumidifier the air isn't cooled (well not on exit).
It goes through the chiller (the fridge evaporator bit) where the water
condenses out and then through the heated part of the heat pump (the
condensor part of a fridge).
Its warmer drier air that comes out.
Its warmer from the heat extracted in condensing the water (about 2MJ per
kg) and by the electricity driving the unit.
If you keep this heat in the room it will just get hotter and hotter (and
drier).

Pete C

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Dec 21, 2006, 1:28:31 PM12/21/06
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On Wed, 20 Dec 2006 21:49:06 +0000, Pete C <pete...@gmail.com>
wrote:

>On Wed, 20 Dec 2006 19:25:47 -0000, "dennis@home"
><den...@killspam.kicks-ass.net> wrote:
>
>>
>>How much does it cost to heat and ventilate a house so it dries out?
>
>Hi,
>
>I think for a gas central heated home it's always cheaper to ventilate
>(eg use a shower extractor) than use a dehumdifier.

Not so sure about this now... :)

>For incoming air at 0C and 100%RH and outgoing air at 20C and 80%RH,
>it will take 0.7kWh to shift 1L of water by ventilating the humid air
>and heating the incoming air to replace it.

For an average dehumidifier that uses 1.25kWh to remove 1L of water,
this adds 1.25kWh of heat and recovers equivalent of 0.72kWh of latent
heat, so the total heat added to the house is ~2KWh.

For incoming air at 0C and 100%RH and outgoing air at 20C and 65% RH,
it takes about 0.9kWh to shift 1L of water by ventilating the humid
air and heating the incoming air.

However to heat the house in the same way as the dehumidifier needs an
extra 2kWh, so the total is 2.9kWh.

If a gas CH boiler is only 66% efficient, there needs to be 4.4kWh of
gas used to heat and dehumidify the same as the dehumidifier.

So 3.5x as much gas as electric is needed, which isn't so clear cut as
being cheaper.

So a lot depends on the efficiency of the CH boiler vs. the
dehumidifier.

cheers,
Pete.

Phil L

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Dec 21, 2006, 4:34:43 PM12/21/06
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dennis@home wrote:
> "Phil L" <neverc...@hotmail.com> wrote in message
> news:lukih.19512$k74....@text.news.blueyonder.co.uk...
>> dennis@home wrote:
>>> "Phil L" <neverc...@hotmail.com> wrote in message
>>> news:Dzgih.19377$k74....@text.news.blueyonder.co.uk...
>>>
>>>> You don't need heat to dry out a building, I often 100% replaster
>>>> old houses with no heat source and they dry out completely within
>>>> a week or two with just ventilation, not bad bearing in mind the
>>>> walls have been soaked with PVA, then backing plaster applied,
>>>> then soaked again to kill the suction and then skimmed...for the
>>>> first few days, the condensation is running down the windows but
>>>> after they've been left open for a week, there's no more
>>>> condensation and no dark patches of plaster, everything's dusty
>>>> dry...no heat required and certainly no dehimidifier.
>>>
>>> Actually that is wrong.
>>
>> It doesn't matter dennis...I mean, the water, plaster, heat, cold,
>> condensation, evaporation or anything else don't know the rules as
>> they are laid down on paper, all I know is that ventilation, even
>> now - today - is drying out dozens upon dozens of gallons of water
>> out of an empty house, no electric or gas.
>
> The OP wanted to dry it out faster.
> He needs to lower the humidity inside the house to speed up drying.
>
>
He's not starting work until April and isn't moving in until August.

>>> You need heat to evaporate water.
>>
>> quite obviously not, unless you mean the difference between inside
>> and outside of an unoccupied building, probably about 4 deg c difference
>> today
>
> Today is exceptional.
> Its currently <1C and 100+% humidity.
> If you leave all the windows open in a fog like this it will be
> wetter than if you close them.

Ok, is that why 25m2 of skimming that I put on 1 hour before my previous
post, is today almost completly (70%) dry?


>
>>> To evaporate 1kg requires about 2260kJ.. that's about 2kW for about
>>> 40 minutes.
>>> It has to come from somewhere or the water will just stay there.
>>
>> But if you have 400 minutes, how much does it need? and 4000
>> minutes? and 40,000 minutes? -
>
> The same.
> Its energy not power.
>
>> even a slight temperature increase of a degree is ample,
>
> Thats what I said.. the slight increase from a dehumidifier is enough
> to do it quite quickly.
>


No, you are completely wrong.
My car leaks in water through the back door - it's been like this since I
bought it but I'm too idle to do anything about it - the result of this is
that the rear carpet is almost always wet, sometimes drenched...the car
steams up on the inside, that is to say, an unheated car, outside in the
frost gives up moisture which condenses on the glass of the interior...also
my greenhouse, which is glass and unheated, also has condensation running
down the windows on cold days, how does that water inside the car and
greenhouse evapourate, even though there is no source of heat?

>> providing the ventilation is adequate....there's 40,000 minutes in a
>> month BTW, perfectly adequate to dry any building out, with plenty of
>> ventilation.
>
> It takes 6+ months to dry out a new build and thats with heating and
> ventilation.
> If you want to dry it faster then ventilation alone is not going to
> do it especially in winter.

Ventilation does it easily, without heat, even in winter


dennis@home

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Dec 21, 2006, 4:57:25 PM12/21/06
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"Phil L" <neverc...@hotmail.com> wrote in message
news:T5Dih.19957$k74....@text.news.blueyonder.co.uk...

> No, you are completely wrong.
> My car leaks in water through the back door - it's been like this since I
> bought it but I'm too idle to do anything about it - the result of this is
> that the rear carpet is almost always wet, sometimes drenched...the car
> steams up on the inside, that is to say, an unheated car, outside in the
> frost gives up moisture which condenses on the glass of the
> interior...also my greenhouse, which is glass and unheated, also has
> condensation running down the windows on cold days, how does that water
> inside the car and greenhouse evapourate, even though there is no source
> of heat?

Its physics Jim, you cana change the laws of physics.
Oh I forgot you don't believe in physics.

PS I hope your plaster isn't 70% dry after an hour as it needs water to set,
just like concrete.

Phil L

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Dec 21, 2006, 5:14:16 PM12/21/06
to
dennis@home wrote:
> "Phil L" <neverc...@hotmail.com> wrote in message
> news:T5Dih.19957$k74....@text.news.blueyonder.co.uk...
>
>> No, you are completely wrong.
>> My car leaks in water through the back door - it's been like this
>> since I bought it but I'm too idle to do anything about it - the
>> result of this is that the rear carpet is almost always wet,
>> sometimes drenched...the car steams up on the inside, that is to
>> say, an unheated car, outside in the frost gives up moisture which
>> condenses on the glass of the interior...also my greenhouse, which
>> is glass and unheated, also has condensation running down the
>> windows on cold days, how does that water inside the car and
>> greenhouse evapourate, even though there is no source of heat?
>
> Its physics Jim, you cana change the laws of physics.
> Oh I forgot you don't believe in physics.

I do believe in physics dennis and if you are correct with your formulae and
predictions re condensation and evaporation, then you'd better tell the
bigwigs down at 'physics house' that they've got it all wrong, they are
totally way off.

>
> PS I hope your plaster isn't 70% dry after an hour as it needs water
> to set, just like concrete.

No, my plaster was 70% dry today, I put it on yesterday, that's the day
before today...tomorrow will see today's plaster similarly dried out while
yesterday's will be completely dry, and all by magic too, considering
there's no electricity or gas to the property and it's only a degree or two
warmer than outdoors, although all the windows are open....you don't
honestly believe that housebuilders wait six months after plastering before
painting?

And you still haven't explained where all the condensation comes from, which
is running down the inside of the greenhouse windows every morning when I
get up..


Andrew Gabriel

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Dec 21, 2006, 5:30:54 PM12/21/06
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In article <T5Dih.19957$k74....@text.news.blueyonder.co.uk>,

"Phil L" <neverc...@hotmail.com> writes:
> dennis@home wrote:
>> "Phil L" <neverc...@hotmail.com> wrote in message
>> news:lukih.19512$k74....@text.news.blueyonder.co.uk...
>>> dennis@home wrote:
>>>> You need heat to evaporate water.
>>>
>>> quite obviously not, unless you mean the difference between inside
>>> and outside of an unoccupied building, probably about 4 deg c difference
>>> today

Guys, you don't need to argue, because you're both right.

Moisture does require heat (or rather energy) to evaporate.
So consider the wet wall. A bit of moisture evaporates from
the wall and in doing do it steals some heat from the wall,
so the wall gets colder. Now that the wall is colder than the
surrounding air, it will extract heat from the air, which
continues to provide the energy to evaporate more water.
This is the same principle as a wet bulb thermometer, which
will be a few degrees colder than a dry bulb thermometer
because the moisture is stealing heat from it in order to
evaporate.

However, the example in this case wasn't simply a wet wall,
it was a newly plastered wall. This changes things because
plaster gives off a lot of energy when it sets, which heats
the wall up and considerably speeds up the drying process.
The most obvious case of this is if you mix up some plaster
which was too old and sets in a few minutes in the bucket.
All that energy is given off much quicker in a confined
space, which is why that bucket of plaster gets hot.

--
Andrew Gabriel

dennis@home

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Dec 21, 2006, 5:40:46 PM12/21/06
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"Phil L" <neverc...@hotmail.com> wrote in message
news:YGDih.19989$k74....@text.news.blueyonder.co.uk...

Well plaster setting is exothermic.
It doesn't need to dry much for the surface to appear to be dry.
Walls are painted with emultion which is water + oil so it wets the wall
anyway.

> And you still haven't explained where all the condensation comes from,
> which is running down the inside of the greenhouse windows every morning
> when I get up..

The same place as the fog and dew.


Phil L

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Dec 21, 2006, 5:44:29 PM12/21/06
to

Thanks for the explanation...although I knew that plaster gave off a small
amount of heat...also there's the added heat of people being in there during
the day, don't people give off around 200watts each or summat?

Either way, a ventilated hoose is a dry hoose. (providing it's not raining
in!)


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