Efficiency

[science_fox]

Which is better at drying washing? - A Heater or B De-humidifier?

I'm sure the answer is both. And then depends on "better". Neither is obviously the cheapest option. SO for some contrived reasons - a flat without outdoor drying access, and only one available socket or something, choose only A or B

I think a dedicated heated drying rail is probably the best option.... but even then will depend on variables including the weather, what else happens in that room, it's insulation and damp properties etc.

[shpalman]

Which is better at drying washing? - A Heater or B De-humidifier?

I'm sure the answer is both. And then depends on "better". Neither is obviously the cheapest option. SO for some contrived reasons - a flat without outdoor drying access, and only one available socket or something, choose only A or B

I think a dedicated heated drying rail is probably the best option.... but even then will depend on variables including the weather, what else happens in that room, it's insulation and damp properties etc.

I'd say it depends on the power rating of each, because in the simplest calculation you divide the latent heat of the evaporation of water by the power rating to get the time (per kg or mol of water depending on which units you had the latent heat in). E.g. 3 kW of power should be able to evaporate about 4.8 kg of water per hour assuming that the air doesn't get so humid that it can't take the water anymore.

A slightly more interesting calculation considers that evaporation is a thermally activated process with an energy a lot higher than the thermal energy at room temperature (obviously: water is a liquid) so if you can increase the temperature by about 13°C you'll roughly double the evaporation rate.

Heating is "efficient" in the sense that inefficiency in just about any other process means that waste heat is generated, but if heat is what you want, then it's not wasted. Dehumidifiers obviously need to dump the heat of condensation somewhere but for this application you want to keep that heat in the room.

The best thing is of course hot dry air. To demist the windows in my car I use the air conditioning and the heater at the same time.

[Allo V Psycho]

My vote has it currently as 100% for dehumidifier. We've started putting the washing on a drier in the smallest room (with the toilet lid down, obv.) and it seems to dry quickly. Use of heat just makes the existing condensation problems worse (which is why we got a dehumidifier in the first place).

Hot dry air, you say, Shpallers? Send me a can from Italy, I'll give you my address in Lancashire...

[shpalman]

My vote has it currently as 100% for dehumidifier. We've started putting the washing on a drier in the smallest room (with the toilet lid down, obv.) and it seems to dry quickly. Use of heat just makes the existing condensation problems worse (which is why we got a dehumidifier in the first place).

Hot dry air, you say, Shpallers? Send me a can from Italy, I'll give you my address in Lancashire...

Screenshot 2023-12-24 162727.png (28.9 KiB) Viewed 10231 times

[dyqik]

If you heat saturated air at 4°C or 14°C to 40°C, it'll be pretty damn dry, in terms of relative humidity (about 10% and 20% RH, respectively).

[Martin_B]

Both work by changing (reducing) the humidity. The dehumidifier by removing water directly from the air, and the heater by increasing the temperature which reduces the relative humidity.

As to which is more efficient, you could probably work out which produces the greater difference in relative humidity, but the conditions may well produce different results.

For instance, if it is cold then the humidifier may have to work hard to reduce the humidity while a few degrees increase in temperature has a greater effect. On the other hand, on a warm, humid day the dehumidifer may find it easier to drop the humidity.

I think that the heater will be the more reliable means of drying washing, but it may not be the more efficient in all cases.

[dyqik]

Clothes dryers work by heating, not by dehumidifying, which tells you the answer.

[Martin Y]

C: fan.

Unless you have a problem with the moisture building up in the house, just moving air around takes less power than moving it while also heating it or drying it.

To save me from ironing I've been known to hang shirts in a doorway and turn a desk fan on them, rather than use the tumble drier.

[nekomatic]

Clothes dryers work by heating, not by dehumidifying, which tells you the answer.

I think all that tells you is that a heater is easier and cheaper to make than a dehumidifier.

Unless you have a problem with the moisture building up in the house, just moving air around takes less power than moving it while also heating it or drying it.

This is correct but the potential for moisture building up in the house to be a problem shouldn’t be underestimated.

[Brightonian]

Or avoid washing clothes unnecessarily. For example, I used to wash shirts after one wearing but if I don't do anything strenuous I get 2-3 wearings out of them.

[dyqik]

Clothes dryers work by heating, not by dehumidifying, which tells you the answer.

I think all that tells you is that a heater is easier and cheaper to make than a dehumidifier.

Easier and cheaper = efficiency.

[Grumble]

Clothes dryers work by heating, not by dehumidifying, which tells you the answer.

I think all that tells you is that a heater is easier and cheaper to make than a dehumidifier.

Easier and cheaper = efficiency.

The question is whether it’s cheaper to run, not to make

[science_fox]

C: fan.

Unless you have a problem with the moisture building up in the house, just moving air around takes less power than moving it while also heating it or drying it.

To save me from ironing I've been known to hang shirts in a doorway and turn a desk fan on them, rather than use the tumble drier.

Like!
Thanks! - House is basically quite dry so this may well be the best option for this particular use-case.

[dyqik]

I think all that tells you is that a heater is easier and cheaper to make than a dehumidifier.

Easier and cheaper = efficiency.

The question is whether it’s cheaper to run, not to make

Condenser tumble dryers exist (i.e. dehumidifiers, but running the other way round), but only get used where venting hot air outside is prohibitively expensive to set up.

[dyqik]

Heated airers are cheaper to run than dehumidifiers.

https://www.themoneyedit.com/household- ... humidifier

[dyqik]

While that article does say that tumble dryers are more expensive to run, I would expect that to vary with the size of the dryer and load. My dryer is an American sized one, and the air that comes out has more contact time with the clothes and isn't that warm. I also have a gas hookup for a gas dryer, which would reduce the running costs by more than half.

[shpalman]

This link helpfully explains that "[condenser dryers] are less energy efficient as vented, as air is being converted to water, using a lot of energy. They cause up to 15 percent more than vented dryers"

But wiki says that about refrigerations-type dehumidifiers:

This process works most effectively at higher ambient temperatures with a high dew point temperature. In cold climates, the process is less effective. Highest efficiency is reached above 20 °C (68 °F) and 45% relative humidity. This relative humidity value is higher if the temperature of the air is lower.

You can also get those plastic boxes which you put calcium chloride into, and they suck a bit of water out of the air, but I wouldn't know if they make enough of a difference because I never have in mind how much water there is in humid nor nor how long it takes the thing to fill up. I have one in my bathroom during the winter anyway. (In the summer I open the skylight).

So I think we've converged on heating, but make sure there's a way to get rid of the damp air.

[nekomatic]

But wiki says that about refrigerations-type dehumidifiers:

This process works most effectively at higher ambient temperatures with a high dew point temperature. In cold climates, the process is less effective. Highest efficiency is reached above 20 °C (68 °F) and 45% relative humidity. This relative humidity value is higher if the temperature of the air is lower.

In the room where we dry our laundry it is 20 °C and (until the laundry is dry) well over 45% RH, and the compressor dehumidifier appears to work effectively.

I’m open to persuasion by thermodynamic analysis, but it seems unlikely to me (since I don’t want the moisture from the laundry condensing elsewhere in my house) that it’s more energy efficient to force the drying equilibrium by heating air up and throwing the heat away with the moist air, rather than removing just the moisture from the air and keeping the heat.

[IvanV]

A dehumidifier is a heat pump, it causes local chilling to effect condensation. I would have thought the thermodynamic argument would favour the heat-pump and hence dehumidifier.

When I had a major flood in my previous house, and it took several weeks to dry out, I hired a large-scale dehumidifier. I could have just turned the central heating up. A question is, where does the evaporated water go? If it increases the local humidity, that will reduce the drying effect. I had to attend regularly to tip out the water the dehumidifier collected.

[dyqik]

A dehumidifier is a heat pump, it causes local chilling to effect condensation. I would have thought the thermodynamic argument would favour the heat-pump and hence dehumidifier.

When I had a major flood in my previous house, and it took several weeks to dry out, I hired a large-scale dehumidifier. I could have just turned the central heating up. A question is, where does the evaporated water go? If it increases the local humidity, that will reduce the drying effect. I had to attend regularly to tip out the water the dehumidifier collected.

That "where does the evaporated water go?" And "If it increases the local humidity" is the difference between a tumble dryer, heated room/space and a dehumidifier, and why it's not necessarily that simple. Relative humidity is a function of temperature, as well as water content. 100% RH air at 14°C holds the same amount of water as 20% RH at 42°C.

Running a dehumidifier to get to the same RH as heating the air takes a decent amount of energy (you have to pump away the latent heat of the water removed and the heat capacity of the air required to cool it below the dew point*), and air has a pretty low heat capacity. In addition, evaporation is faster at higher temperatures, so you have to run the dehumidifier for much longer.

Heat pump assisted tumble driers now exist, by the way, if you want to increase the efficiency of heating air. You can also use air-air heat exchangers to pull some of the heat from the output air and use it to preheat the incoming air.

* You also have to supply the latent heat of evaporation for the water in the clothes, in both methods.

[FlammableFlower]

Whilst we much prefer outdoor line-drying, when the weather is crap we were lucky to inherit a compressor dehumidifier that also has a fan heater built in - so you can operate it in a choice of:
Fan only (no heat)
Fan heater
Dehumidifier fan (no heat)
Dehumidifier fan heater

The dehumidifier dial alleges to have a choice of percentage settings (that I assume are %RH) down to 20% then Continuous operation. I generally set it to 40% just dehumidifier. We've got 3 adjustable clothes horses and I set them in a line with the dehumidifier aimed to blow the output dry air down through the clothing, my theory is that this will help draw the moisture out of the clothes which goes into the air then drawn back in to the dehumidifier. If things aren't drying fast enough, I'll get it heating too. It's a big thing though, with a large trap.

[dyqik]

I suspect it also depends on your house heating system and climactic conditions.

Right now my basement is 15°C and 35% RH. Outside is 5°C and 95% RH.

Forced hot air heating means that humidity is distributed around my house pretty well, and the cold areas don't get damp. We usually have to run humidifiers in winter to reduce nose bleeds, coughs etc. from low humidity. This is rather different to a UK radiator heater home where cold rooms tend to get damp.

[IvanV]

Saturated air at typical room temperature contains about 20g/m3 of water. So a room of 25m3 - modestly sized bedroom common in British houses - contains 500g of water, max. In practice, we are only going to condense some of that. The latent heat of vaporisation of water is 2.3MJ/kg (3.6MJ = 1 kWh). To condense that, we have to get it to give up that heat, and we do that with a heat pump. That is not going to add up to a lot of energy. Though you also need a fan to suck the air through the dehumidifier.

To take an example, the Avalla X-125 dehumidifier is specfically marketed for drying clothes. It has a power of 210W, and claims to be suitable for rooms of up to 30m2, or say about 70m3, which is a sizable room by British standards. It has a 2.5l tank for collected water. It is a typical medium sized dehumidifier, similar in size and condensing power to other room-scale dehumidifiers.

When you heat a room of 25m3, then you heat the whole room, not just the air, but the walls and the contents. Depending on the room, it could be like heating up a small pie in a large oven, a lot of the energy goes to waste heating things you aren't interested in heating. What kind of a power of heater are you going to put into a room to heat it up to a nice warm temperature? Maybe a lot more than than 210W dehumidifier. Though clothes drying at a higher temperature is faster, so we have to account for running the dehumidifier for longer. The balance of advantage may depend upon the size of the room.

Maybe in practice, if you point a fan heater at your rack of drying, then the local effect of the moving warm air directly on your clothes will succeeded in drying them quickly, especially if you move them around so that each one gets its time in the blast, without needing to heat the whole room. But they have a power typically of 1 to 2kW, so you'd have to achieve the drying 5-10 times faster to use less energy than the dehumidifier.

Certainly evaporation is going to proceed faster at a high temperature. So a warm room will dry your clothes faster. Ideally it should be a small room with racks. We saw that in the case of a drying clothes in a small chamber, doing it with heat and ventilation is more efficient than condensing the water. But in a practical situation of drying clothes in a sizable room, it might use less energy to do it more slowly with a dehumidifier of much lower power than a room heater.

[Martin Y]

Ventilation must be a significant factor. A room with a couple of changes of air per hour will replace the moist air with ambient several times over while a batch of laundry dries.

[IvanV]

Ventilation must be a significant factor. A room with a couple of changes of air per hour will replace the moist air with ambient several times over while a batch of laundry dries.

Whether we are drying by heat or dehumidification, clearly we need a room without too much ventilation to do it efficiently.