Dehumidifier sums

[AE-NMidlands]

Hello all
Having read that dehumidifiers recover the latent heat from the moisture that they condense I thought I would see if I could quantify it to work out the cost implications.

https://www.screwfix.com/p/blyss-wdh-31 ... _container claims it can remove 16l of water a day (presumably only in ideal conditions) and consumes 280 W continuously. That is 6.72 kWhr a day and I would hope for some heat released to offset it...

If the latent heat of evaporation of water is 2264 kJ/kg and 1J is 2.78×10−7 kWhr, I reckon that would be 10.07 kWhr released over the day (to which I guess you should add most of the 6.72 kWhr.)

So we have a best case of maybe 16 kWhr of heat from 7 kWhr input. Knowing how much heat steam transfers when you get scalded by it I am surprised it isn't more! Or have I gone wrong in my arithmetic?
A

[Joeboy]

Can't help with the sums. We run two dehumidifiers. One is on a hive plug and now runs in the 5 hour go faster window. The second is an ebac unit and one point of note it needs a manual push of a button to step it up into dehumid mode. Can't use it on a hive plug as it will just switch on a run at the lowest setting which is a basic air circulation setting. Not ideal. Please ensure that you don't buy one that needs manual intervention. They certainly recirc the heat. We run in total for about 7 hours per day one unit. Our meter shows a moisture level of around 48% and has done so for weeks. I wouldn't go through Winter in a well sealed home without some moisture removal facility.



This is the hive plug 5 hour unit with manual adjust knobs Pulls about 1/2 a ltr per 24 hours now. WAY down on start up volumes a couple of months back.

[Stinsy]

I have this dehumidifier:

https://www.amazon.co.uk/gp/product/B089YK765Z

The 25L/day should be taken with a pinch of salt. Useful only to compare one dehumidifier with another. Be aware that "compressor" type dehumidifiers only work well in heated spaces. They're not for garages, unheated conservatories, attics, etc.. "Desiccant" type dehumidifiers work better in cold spaces and you can double their L/day to compare them with compressor models.

Dehumidifiers are known for making a room feel cooler because your body can regulate temperature better by sweating in a dry room compared with a damp one. I've always assumed they have a net warming effect on the room due to inefficiencies. I;ve never thought about them benefiting form the phase/change.

[AE-NMidlands]

I have this dehumidifier:
https://www.amazon.co.uk/gp/product/B089YK765Z
The 25L/day should be taken with a pinch of salt. Useful only to compare one dehumidifier with another. Be aware that "compressor" type dehumidifiers only work well in heated spaces. They're not for garages, unheated conservatories, attics, etc.. "Desiccant" type dehumidifiers work better in cold spaces and you can double their L/day to compare them with compressor models.

Dehumidifiers are known for making a room feel cooler because your body can regulate temperature better by sweating in a dry room compared with a damp one. I've always assumed they have a net warming effect on the room due to inefficiencies. I;ve never thought about them benefiting form the phase/change.

Thanks, I'll look at it.
I wouldn't even consider a dessicant type because it depends on continuously buying it in - or putting heat in to dry it off if it is silica gel rather than calcium chloride or something.
I would guess they would claim the max output on a test by putting it in a warm almost-condensing atmosphere which a) has tons of vapour in it and b) will shed water almost as soon as you disturb it!
I am hoping that a drier room in winter will feel warmer, as it is possible that the air will have a lower specific heat and conduct less away from our bodies. In summer (in the UK midlands) all the windows are open all the time anyway and it is very rare that we need to do any active cooling.
I'm still hoping a physicist will come along, correct my sums and tell me you get far more heat out from the condensation. Aren't condensing tumble driers supposed to do that too?
A

[Stinsy]

My heat pump tumble drier produces a lot of heat. Very noticeable in the unheated utility room.

[Countrypaul]

I have this dehumidifier:
https://www.amazon.co.uk/gp/product/B089YK765Z
The 25L/day should be taken with a pinch of salt. Useful only to compare one dehumidifier with another. Be aware that "compressor" type dehumidifiers only work well in heated spaces. They're not for garages, unheated conservatories, attics, etc.. "Desiccant" type dehumidifiers work better in cold spaces and you can double their L/day to compare them with compressor models.

Dehumidifiers are known for making a room feel cooler because your body can regulate temperature better by sweating in a dry room compared with a damp one. I've always assumed they have a net warming effect on the room due to inefficiencies. I;ve never thought about them benefiting form the phase/change.

Thanks, I'll look at it.
I wouldn't even consider a dessicant type because it depends on continuously buying it in - or putting heat in to dry it off if it is silica gel rather than calcium chloride or something.
I would guess they would claim the max output on a test by putting it in a warm almost-condensing atmosphere which a) has tons of vapour in it and b) will shed water almost as soon as you disturb it!
I am hoping that a drier room in winter will feel warmer, as it is possible that the air will have a lower specific heat and conduct less away from our bodies. In summer (in the UK midlands) all the windows are open all the time anyway and it is very rare that we need to do any active cooling.
I'm still hoping a physicist will come along, correct my sums and tell me you get far more heat out from the condensation. Aren't condensing tumble driers supposed to do that too?
A

I'm not a physicist, I am a chemist. Your calculations appear correct, though you do not comment on the temperature of the condensate in the dehumidifier. If that is at room temperature then there is no adjustment, if the condensate is colder when it enters the storage container then you should allow for warming that back upto room temperature (assuming that is how warm it gets before disposal).

10KWh is about the same amount of heat as you would get from burning 1L of petrol.

[Marcus]

As I'm living in a caravan in west Wales with dogs I usually use a dehumidifier (about 150w compressor type) just to keep the caravan habitable. Without it everything's horribly clammy and molds start to form. It does indeed put out heat although I've never tried to quantify it's COP.

As the weather's been very mild I've not had to heat the caravan over the last week other than the heat output of the dehumidifier ( and 3 dogs, the cat and me) and I've even had to turn that off on occasion as it was getting warmer than I'm used to.

The dessicant units are expensive to buy and (based on the one I fixed/tested for someone) use a lot more power, but the air coming out was <40% rh according to my hygrometer, whereas the output of mine was above 60% (ambient cool air at >80% rh and 14°c).

[AE-NMidlands]

I'm not a physicist, I am a chemist. Your calculations appear correct, though you do not comment on the temperature of the condensate in the dehumidifier. If that is at room temperature then there is no adjustment, if the condensate is colder when it enters the storage container then you should allow for warming that back upto room temperature (assuming that is how warm it gets before disposal).

10KWh is about the same amount of heat as you would get from burning 1L of petrol.

I hadn't even considered the condensate temperature! My impression (probably from schooldays) was that m.s.delta T pales into insignificance compared with latent heats. (I remember an experiment where a constant heat source raises the temperature (of ice or something?) steadily, but when it gets to melting / phase change time the rise stops dead for absolutely ages until the last of the solid has gone.)

If the condensate is much cooler than "indoors" I suppose it would be worth piping it outside directly.
A

[Mr Gus]

We run one in our North facing bedroom, right by the radiator, dimplex compressor type that they sold at Homebase a couple of years ago for £30, I didn't post it on "st elsewhere" for folk.as.some people used to get snarky about what to me were energy efficient & gift horse priced for people at many different stops along a low energy transition.

Has proven excellent both winter & summer (as discussed on the thread) no winter window moisture, better summer persisting, & once under control only running 5-10 minutes per hour typically winter, but harder working in summer (36c at night)

..whilst on the subject of dessicant, we use the pound shop dehumidifier in cars, what's the correct disposal process for the Crystal turned fluid please?

[Oliver90owner]

whilst on the subject of dessicant, we use the pound shop dehumidifier in cars, what's the correct disposal process for the Crystal turned fluid please?

The drain.