clothes drying

[Jon Fairbairn]

I'm thinking of converting a small room in my house (it's roughly a 2.5m cube) to use as a clothes drying room/storage room. So I've read

and the page it links to about dehumidifiers, and the thread on dehumidifier selection that that links to.

I'm reckoning on a 200W-ish dehumidifier (Ebac 2650e, say, which is 207 Watts on full), but in the thread mentioned above, this post

does some maths comparing dehumidifiers with ventilation -- it's not clear cut when you get your heat from gas (as I do) and heat recovery ventilation would push in the direction of ventilation, so an alternative would be to use a Vent-axia HR25H . The HR25 consumes only 25W on boost (I don't know how much of that heat goes outside and how much comes in; if I'd been designing the thing I'd cool the motor with the incoming air after the heat exchanger, but I don't know if they did that). The room would be kept up to temperature by a TRV controlled radiator.

Now, I don't know how to do the calculations. How much water can 55m³/hr of warmed-up outside air shift? What happens to the RH when you warm air up? And so on. I don't suppose it's hard to find out, but it would take quite a while to get it all down pat, so I'm hoping someone here already knows how to compare the energy costs of the two options. The data on the vent axia site seems fairly complete, but I can't see from Ebac's how much air their unit shifts.

This is in Cambridge, so the outside RH in winter (I'll dry outdoors the rest of the year) averages around 80% with the temperature averaging in the region of 5-10°C.

can anyone help?

[Harry Bloomfield]

Jon Fairbairn used his keyboard to write :

Consider not only a dehumidifier, but a cheap fan to stir the air around the room. It is not only dry air which dries clothes, but air flow passing the clothes. A good out door drying day needs low humidity and a good breeze, the temperature is secondary.

[Andrew Gabriel]

I just read that, and it's filled up with lots of waffle and bullshit since I last looked. Go back to an older version...

and the page it links to about dehumidifiers, and the thread

It works best at higher temperatures than TRV's normally go to. I run it at 30C, being the limit of what my dehumidifier can handle, but if you are using a radiator, I would run it at full output during drying for most effectiveness (biggest difference in humidity of the outdoor and indoor temperatures). This will require higher radiator output than you might otherwise expect in such a room.

It's going to be very difficult to work out. You can easily calculate the amount of water it might absorb by checking humidity tables, but you can't know how effectively the damp material is going to pass moisture into the air, so you don't know what humidity you are going to reach in the cupboard. Indeed, this will vary with the amount of exposed damp material, amount of air circulation (forced circulation helps a lot), the temperature the radiator manages to get the air up to, and how damp the material is (which will change throughout the process).

[meow2222]

A dehumidifier running 10% of the time eats 20.7w, and your fan eats

25w, and would need to run all the time. The fan chucks heat out - less when you use recovery, but its still throwing heat out. So the dh wins on energy use.

But the bigger issue is that the fan wont dry the air anywhere near as much, so drying will take far longer.

If you install a ceiling fan, overnight drying can be cut down to an hour or less.

Finally, the link andrew gave is the one with the questionable info in it. I've asked him to back up what he says, but its yet to happen. There's simply no reason why dehumidifiers should be the one domestic appliance that requires mutliple layers of fault protection, unlike any other appliance. If he can back up what he claims I'd welcome it, but its hard to see how.

NT

[Jon Fairbairn]

Would it? The HR25H has a humidistat, and if your 10% duty cycle for the dehumidifier is based on humidistat control (where does the number come from, BTW?), I'd say it was fairer to assume that the HR25 would be on full the same proportion of the time and drop to the low setting (1.9W) the rest of the time. So (1*25+9*1.9)/10 = 4.2 watts on average.

high = 55m³/h, low = 20m³/h, so a complete air change on full would take about 17mins, so a 10% duty cycle would mean a cycle took three hours, so I'd have to expect some multiple of that for the clothes to dry...

but is that win big enough to compensate for the relative efficiency of electric v gas heat? I'm not yet convinced. The HR25H description says something about retaining 84% of the temperature difference, so 0°C outside and 20° inside (not all that common round here) would mean the air came in at 17°C (modulo weasel words). Heating a roomful of dry air back up to 20 would take 60kJ (or one kW minute). Did I get that right? How much more heat does the air being damp take?

I'm not sure I mind that. Currently I often dry indoors on clothes racks, and while it takes too long, it's not desperately too long (but it does make the windows steam up).

Even if the room temperature were up at 32°C and the fan kept the RH up at 90%, it would take an Ebac 2650 1.5 hours to shift 2l of water, so less than an hour would surprise me. [I haven't been able to weigh a load of washing accurately enough to say how much water there is in one at the end, but it's something like that; cockeyed.com reckons on 4l for a US machine, but they're typically bigger and less good at spinning]. Another thing I don't know is what the performance of dehumidifiers does at lower temperatures. To heat the room all the way up to 32°C would mean losing a fair amount of energy through the walls and ceiling (even if I insulate them properly, which hasn't happened yet: tuit deficit plus some awkward bits of ceiling and a reluctance to lose space in an already small house. Horizontal part of ceiling is OK, though!)

But I'm not sure a ceiling fan's worth it. Ceiling fans don't seem to be cheap, and waiting overnight doesn't bother me.

Well, if I were intending to install the dh in a small cupboard I'd be a bit concerned to keep the running temperature within safe limits, but as I'm talking about a room, if the dh needs extra protection for running in a room, it shouldn't be on sale!

[Dave Liquorice]

I'd go for the KISS heat recovery system. At least all the moisture then gets dumped outside rather than a tank that you forget to empty. The fan with humidistat is also fit and forget apart filter cleaning. There may be problems with conducted noise from a dehumidfier sat on the floor. Dehumidifiers are much more complex things as well, more to go wrong.

[Andrew Gabriel]

In my drying cupboard, I have a 400W dehumidifier. I set the humidistat about half way (never calibrated to see what that actually corresponds to), and with a full load of washing hanging in the cupboard, it will run continuously for the best part of an hour. Then it starts cycling on and off as there's not enough moisture left in the materials to keep the humidity high enough, and most things at that stage are dry except where they are in contact with the hanging rail or folded over and hence not fully exposed to the air flow on all sides. (Towels and heavy denims take longer.)

If I forget about it, a couple of hours later it seems to be permanently off by the humidistat.

I haven't accurately measured it, but I'm guess I get around a litre out of my dehumidifier for a load of washing with no towels or denims. That's after a 1400 RPM spin at the end of the wash cycle. It's noticably more with towels/denims, perhaps

1.5 litres.

My drying cupboard goes up to 30C max. If the combination of the dehumidifier heat and heat from the centralheating pump and pipework in there takes it above that, I automatically cut off the dehumidifier to protect it against overheating itself. The dehumidifier gets it close to 30C by itself, but it actually requires the heating on to take it over, and it rarely is when I'm drying stuff in the cupboard.

I'm sure forced air circulation helps, but I just rely on the fan in the dehumidifier, which is quite powerful.

[meow2222]

No, because the fan is way less effective at drying the air. IME a fan can run continuously and not produce as much drying as a dh running

10% of the time.

I dont see how that makes sense. The win is a cost win, full stop. Gas vs elec only changes its value.

Sure - but the energy calculations do. Your fan eats the same average power for 10x as long.

I dont get that much water out per wash. Perhaps some of it evaporates into the surrounding rooms.

falls fairly quickly. In practice you wont get near the mfr quoted extraction rate, since odds are its specced at 30C and youre operating at 20.

theres no need

=A317 at Argos last time I looked

Even in a cupboard 200w wont get you into trouble. if you go upto a

400-500w machine, thats another story.

Another difference is that dhs are consistently very quiet, whereas fans vary, and often arent.

Fan purchase cost looks lower, but you have to knock a hole in the wall to install it and run wiring...

NT

[Jon Fairbairn]

If you are talking about a plain extractor fan, you have to ask where the air that replaces what's blown out is coming from, how it gets heated up and how damp it is by the time it gets to the drying space. I was hoping to do some calculations for this, but I still haven't got my head around calculations with humid air.

No, because both systems throw heat out. In the case where the heat is coming from electricity, it's thrown out before it gets to you. Here's some rough numbers.

Typical combined cycle gas power station efficiency = 60% average uk electricity transmission loss = 7%

so net result = 56% efficiency ie 44% of energy in gas (burnt at power station) is lost outside your house

burn gas in boiler => SEDBUK rating of my boiler = 88.6% so lose

12% there, so can lose up to 32% of heat to outside before losing as much as getting the energy from electricity.

I remain unconvinced of that. I'd like to see some calculations; since the air coming in is heated up, it's RH goes down, but I haven't worked out by how much yet, and I don't know by how much a dh can reduce it in one pass.

That would apply whichever method were used to get rid of the water.

My first experiment with a 2650e produced very close to 1l of water after about eight hours (no towels). Second go was more like 1.5l for a load including towels. I don't know what the duty cycle of the dehumidifier was; the fan was running constantly (because I told it to). I'll have to see if I can borrow an energy meter to find out how much is used in a run like that.

But by how much?

Thanks for that. I'd been looking at TLC and they were all rather a lot more than that. Unfortunately I'd have to reposition the ceiling rose to fit one :-(

The HR25H that a friend has is very quiet. Quieter than the

2650e dehumidifier (both when comparing full to full and when comparing low to low).

No. The HR25H is ludicrously expensive! Maybe they'll come down if more people use them. Where you have to have an extractor, they're a definite win.

[meow2222]

from outdoors, by the central heating, and it has the same water content as outdoor air, but since water vapour capacity increases with temperature this is a lower RH figure than outdoor air.

OK I see what you're saying, I was just considering what happens in the house. However if you want to take all that into account its more complex. There are energy losses at every point in the chain: drilling & operating the well, national distribution, power plants, etc, all these and more take energy to implement and maintain.

A portable dh can create a static nuisance if left running too long when outdoors is pouring with rain all day long, so the 2 do produce quite different conditions..

no, because the RH is quite different.

I dont remember...

Thats not usually too hard, as long as youre not moving it far.

models of both vary, the difference is that with fans they often stray into unacceptable.

is this a 4" fan or a bigun? The large ones do seem excessively priced, and the 2 manufactureres I asked couldnt give a sensible answer as to why. Just a small market I guess.

NT