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.
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.
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.
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.
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.
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?
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.
quite obviously not, unless you mean the difference between inside and outside of an unoccupied building, probably about 4 deg c difference today
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.
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.
The OP wanted to dry it out faster. He needs to lower the humidity inside the house to speed up drying.
Today is exceptional. Its currently > To evaporate 1kg requires about 2260kJ.. that's about 2kW for about 40
The same. Its energy not power.
Thats what I said.. the slight increase from a dehumidifier is enough to do it quite quickly.
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.
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
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.
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.
Go away and do some revision then you can come back and tell us the answer.
cheers Jacob
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.
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).
Not so sure about this now... :)
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.
He's not starting work until April and isn't moving in until August.
Ok, is that why 25m2 of skimming that I put on 1 hour before my previous post, is today almost completly (70%) dry?
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?
Ventilation does it easily, without heat, even in winter
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.
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