Comments on: Heat your home with a dehumidifier

By: PJ

PJ — Sat, 15 May 2010 23:08:21 +0000

I used to own a dehumidifier but found that mine would not work at temperatures below 68 degrees without freezing up. We don’t keep our home warm enough for proper operation. It also was far from quiet, so it’s operation produced annoying noise pollution. Does anyone know of a truly quiet unit that operates well at colder temperatures? Thanks

By: Rob

Rob — Sun, 31 Jan 2010 06:31:42 +0000

Lloyd,
You seem to be suggesting that the energy disappears after being converted to work? That defies the law of conservation of energy (see http://en.wikipedia.org/wiki/Conservation_of_energy). If a motor is 80% efficient then 20% of the input energy will be converted to heat in the motor. The other 80% will do useful work, yes, but unless that useful work results in energy storage (for example, raising a mass, stretching a spring or charging a battery) the energy will eventually be converted to heat. It is a requirement of the law of conservation of energy.

For a simple example, consider an electric fan. If the motor is 80% efficient (actually fan motors are typically very inefficient), then 20% of the input electrical energy is converted to heat in the motor. The other 80% does “useful work” meaning it is transmitted through the motor shaft in the form of torque and angular displacement. This useful work will be converted to heat at the interface of the fan blades with the air (similar to how the space shuttle heats up moving through earth’s atmosphere only on a much smaller scale) or in the air itself (due to turbulence induced by the fan) or at another surface that the air hits. When you turn the fan off and the air stops moving, there is no more work. It has all been converted to heat.

The operation of the motor, compressor and fan inside a dehumidifier is more complicated but there is no need to consider the details. The law of conservation of energy requires that unless energy is stored, any work done must produce heat exactly equal to the amount of work done.

In other words, for most electrical appliances that operate inside your building envelope, the heat input into your home is exactly equal to the electrical energy consumed, regardless of any work done. A 200W fan will put exactly the same amount of heat into your home (200W) as a 200W heater. A dehumidifier is a special case since heat is output both from the electrical consumption and from the condensation of the water.

Rob

By: Lloyd

Lloyd — Thu, 28 Jan 2010 17:31:25 +0000

Hi Rob,

I don’t want to rain on your parade….but. A dehumidifier does not put heat equal to the kw used.

The dehumidifier does useful work from the kw ie: motors for the compressor and fan, they are not 100% efficient, so the only heat from the kw is that amount between useful work and 100%. Typically an electric motor will be some where between 50 and 90% efficient, the compressor like wise isn’t 100% efficient, so the only heat is the delta of efficiency. Heat is only generated in the delta, in other words if you were generating 100% heat from the kw you would have heating elements, if all kw went to heat the motors, fans and compressor would not run.

Lloyd

By: Rob

Rob — Sun, 17 Jan 2010 07:34:46 +0000

Hi Vicki,
I’m not entirely sure what your question is. Yes, a dehumidifier will make your home less humid. It’s particularly good if you live in an area that is more humid in the winter, since the dehumidifier also produces useful heat (making it essentially free to operate). Using it in the summer it will still help your humidity problem, but it will also make your home hotter, and the operating cost will be higher.

By: Vicki

Vicki — Fri, 08 Jan 2010 19:14:15 +0000

Hi…this is my problem…we installed a heat pump with 4 heads..(none in the basement)the good news is our furnace has not been on..the bad news is our basement is freezing..15c ( and warm (very) in the summer) and because we have no heat from the furnace it is humid and warm in the summer …so my thought is that the dehumidifer would most likely be a solution…any thoughts!!

By: Mary

Mary — Fri, 27 Nov 2009 21:55:36 +0000

The de-humidifier was the final piece to the puzzle. I live in the Fraser Valley in a forty plus year old BC Box. I was trying to use portable electric heaters exclusively. By November 10 my house was damp, smelly and cold; we have old plumbing and our main bathroom does not have a fan so we have accumulated moisture. My son sleeps and spends most of his time downstairs, he was uncomfortable. My husband and son were quickly loosing resolve.

I found three very inexpensive 30 litre dehumidifiers. I have used them full power for the last couple weeks; one upstairs and two downstairs. I don’t remember my house ever being this comfortable.

Thanks for all you work
Mary

By: Rob

Rob — Wed, 11 Nov 2009 00:42:00 +0000

Hi Andris,
Sorry I did not make that clearer. 8 hours is just how long I ran the dehumidifier for my test. If I ran the dehumidifier continuously (24 hours per day) I would get 6kWh for free per day. And that is $0.07 * 6 = $0.42.

By: Andris

Andris — Tue, 10 Nov 2009 10:02:35 +0000

Hello!
I don`t understand one thing about payback time. How did you save $0,42 per day? You said “effectively I get a free kWh (1000 Wh)of heat for every 4 hours of operation”. That means 2kWh for free per 8 hours. And that is 0,07 * 2 = $0,14 per day. Where is my fault?

By: Ian

Ian — Thu, 05 Nov 2009 21:03:41 +0000

Hi Adrian,

I’m no expert. My thoughts are do all of the following:
1. If the shower stall backs to an exterior wall – put in window on the wall towards the top of the ceiling to be left open while showering.
2. put in a fan in the ceiling that you can run while showering.
3. Get 2 boxes of deliquiescent (sp?) and put one in the room with condensation while the other is losing moisture when you set it outside.

Ian

By: Rob

Rob — Thu, 05 Nov 2009 08:43:05 +0000

Hi Adrian, your guess is as good as mine. All methods you described may work, but which method will result in the least energy consumption is difficult to say since there is no way to know how long each method would need to be active in order to be effective. I think trial and error is the only way you will know for sure.

By: Adrian

Adrian — Tue, 03 Nov 2009 22:05:34 +0000

Hi Rob,
May I just pick your brains a moment. We have a dampness problem in our guest room which is in the cellar. Water vapour generated by the ensuite shower room is condensing in the unheated guest room and condensing on the floor and seeping into the laminate flooring. So we need to get rid of this water vapoour. I want to do it the most energy efficient way.

I know dehumidifiers are great (I’m a chemist and love to rave about the benefits to the world of hydrogen bonding without which the humble water molecule could not generate so much energy just by condensing … oh heck while I’m at it did you know that rain in the UK generates the equivalent energy output per year as 1.3 million power stations which is one way we avoid your Richmond winter climates). Anyway back to the question.

Is is better to run a dehumidifier or a extractor fan or just leave the window open to remove the excess water vapour.

Dehumidifier runs a higher wattage solves the dampness but warms a room we don’t always need warming.
A humidity sensitive extractor fan runs lower wattage but presumably takes longer to remove dampness.
Leaving the window will only work if the water remains vapour and so presumably requires the room to be heated – and it relies on the outside humidity to be less than the inside.

Any thoughts

Adrian

By: Rob

Rob — Fri, 30 Oct 2009 20:40:35 +0000

Hi Len,
It sounds like you may have perfect conditions for using a dehumidifier as an effective heater. If you try it, I’d love to hear how it works out for you.
Rob

By: Rob

Rob — Fri, 30 Oct 2009 20:35:26 +0000

Hi Ian,
I think a reversed air conditioner may work. I’ve been considering experimenting with that as well. My primary concern would be that air conditioners are designed for a different temperature range. They are designed for pumping heat out of air at around 20-25 degrees C and into air at around 30 degrees C and up. I don’t know how well they will function pumping heat out of air at around 0 degrees C or lower. I imagine there would be frost buildup on the outside. Above 0 degrees C you could simply turn the unit off periodically and allow the frost to melt, but below 0 degrees C you may need to bring the air conditioner entirely inside frequently to defrost it. I believe air-source heat pumps designed for home heating have built in defrost cycles (which incidentally is one reason that they are less efficient than ground-source heat pumps that don’t require defrosting periodically). Portable air conditioners are unlikely to incorporate a defrost cycle since they are designed for use at temperatures well above freezing. This is all just hypothesis on my part though. I think it would be a worthwhile experiment.

By: Len

Len — Fri, 30 Oct 2009 03:09:36 +0000

Thankyou for the post… As you might have seen from my comment(s) on your oil filled heater project, I heat my house with electric power. There is one area that just gets what it can from whatever is running and that is downstairs. Last year I just dropped a space heater (small hot air burning element with fan in it) in the middle and ran it often at the full 1500w. We kept it at about 17C, I haven’t set it up yet this year and so it is only 15.6C down here right now anyway, there is more water in the air down here too (shower, laundry etc.) and so this may be a great place to try it. 700 and some odd watts may not be enough for 400 sq ft…. but then again running all the time it might.

Len

By: Ian

Ian — Wed, 28 Oct 2009 18:50:45 +0000

Hi Rob,

Thanks for the math and your insight on humidifiers as heaters.

If an air conditioner were turned around so the coils are indoors and the air intake/output are to the outside,
and a small fan blew air across the coils (now to the inside), would this would as a mini air heat pump and
if so what do you estimate the heating savings to be?

thanks.