Heat your home with a dehumidifier
2009-10-25 by Rob - Categories: Experiments, Home heatingA moisture problem
We (my wife and I that is) keep the temperature in our home relatively low in winter. As I’m writing this, it’s a balmy 16 degrees C in my living room. My lovely wife is wearing a toque and she’s about to put on another sweater because she’s feeling “a bit of a chill”, but she’s a trooper and wouldn’t have it any other way. That’s how she was raised. In Richmond, BC, where we live, winters are… well… wet. It’s pretty much a case of 100% relative humidity outside 24/7 and the water table is at ground level… well… truthfully sometimes it’s a few feet above ground level, but that’s why we have the pumps.
Combine 100% relative humidity outside with low temperatures inside and as you might expect, we occasionally have issues with condensation, especially on windows. “Experts” generally don’t recommend keeping interior temperatures below about 17 degrees C for exactly this reason. I don’t care much for expert opinions (experience has convinced me that I’m more expert than most of them), but I also don’t care much for condensation.
A solution with a bonus: free energy
The solution (without simply raising the temperature of our home), is a dehumidifier. While I purchased it for its intended purpose (to reduce humidity levels) I now realize that it also makes a very effective heater. Ah… but doesn’t it cost money and energy to operate a dehumidifier? Well… actually… NO! At least not in the winter, when we’re heating our home with electricity anyway. In fact, a dehumidifier is MORE efficient than an ordinary electric heater, which is already 100% efficient. Yes, a dehumidifier is more than 100% efficient at heating your home. That is to say the amount of heat a dehumidifier will release into your home is greater than the amount of electrical energy it will consume. The reason is simple: a dehumidifier removes energy from water vapor in the air in order to condense it to a liquid. This energy is released into your home.
It’s all about enthalpy
There is a property of any substance known as the enthalpy of vaporization. “Enthalpy” really just means energy. The enthalpy of vaporization of a substance is a measure of how much energy it takes to convert a given mass of the substance from a liquid to a gas. It also indicates how much energy is released when a given mass of the substance is condensed from a gas to a liquid. The enthalpy of vaporization of water is 2257 kJ/kg.
What is the efficiency?
How efficient is a dehumidifier at heating your home? Let’s figure it out together. I mean that literally. As I write this, I haven’t actually figured it out yet myself. I’m flying by the seat of my pants here, people; I’m a scientist gone rogue. But luckily I’m also a scientist who recently acquired a portable dehumidifier. I plugged it into a Kill-A-Watt meter several hours ago to measure exactly how much electrical energy (indicated in kWh by the Kill-A-Watt meter) it consumed. It’s been running for about 8 hours and it has consumed 3.87 kWh of electricity. Thus, based on the first law of thermodynamcis I know it has put at least 3.87 kWh of heat into my home.
However I have also determined with a simple digital scale that it has condensed 3.23 kg of water in that same time. How much additional energy did it release into my home as a result of that? That’s where the enthalpy of vaporization comes in. 3.23 kg multiplied by the enthalpy of vaporization of water (2257 kJ/kg) gives 7290 kJ of energy. A kWh is equivalent to 3600 kJ so 7290 kJ is equivalent to 2.025 kWh.
Thus, the total amount of heat released into my home by the dehumidifier over the last 8 hours is equivalent to the 3.87 kWh of electricity consumed, plus the 2.025 kWh of energy released by the condensation of water. The “efficiency” is equal to the energy output divided by the energy input or in this case (3.87 + 2.025)/3.87 = 1.52 or 152% efficiency. An efficiency over 100% is more appropriately referred to as a “coefficient of performance” since technically, it is impossible to achieve greater than 100% efficiency (having more than 100% efficiency in energy conversion would defy the first law of thermodynamics). So if you ever measure more than 100% efficiency, as I just did, what it really means is that you have moved energy from one place to another rather than simply converted energy from one form to another. Such is the case with a dehumidifier which removes energy from water vapor and releases it into the home in the form of heat, condensing the water to liquid in the process. But whatever the terminology you want to use, the fact remains that I can release 1.52 kWh of heat into my home for every 1 kWh of electricity my dehumidifier consumes.
What’s the payback time?
I paid about $250 CAD for my dehumidifier. It consumes about 480W of electricity (3.87 kWh in 8 h) and outputs about 730W of heat (480W*1.52) into my home. I can buy a decent electric heater that will output 730W for about $50. So the difference in price is about $200. Let’s calculate the difference in the cost to operate. A 730W electric heater consumes exactly 730W of electricity. The dehumidifier only consumes 480W of electricity to produce the same 730W of heat. The difference (730-480) is 250W. Effectively I get a free kWh (1000 Wh)of heat for every 4 hours of operation. I currently pay about $0.07 per kWh for electricty, so I save about $0.42 per day when operating the dehumidifier in place of a heater. My heating season runs from October through March, or around 180 days of the year. Therefore, I can save about 180*$0.42 = $75 per year by operating the dehumidifier in place of a heater. That will take a little over 2.5 years to pay back the difference in price of $200.
Will this work for anyone?
In a word, “No”. The human body is most comfortable at a relative humidity between 20% and 60%. I can run my dehumidifier continuously in winter and not expect to ever drop below 20% relative humidity inside my home. The same may not be true for homeowners in other locations maintaining their homes at higher temperatures than I do. Heating with a dehumidifier works for me because of the high relative humidity in Richmond, even in the winter, and because of the low temperature at which I keep the interior of my home. It could work well for anyone who lives in a similar environment and keeps their home at a low temperature. But if you live where temperatures are usually below 0 degrees C outside in winter then you likely have a much lower relatively humidity. In that case, a dehumidifier will not be able to condense nearly as much water for a given amount of input energy and its operation may bring the relative humidity below a comfortable level.
Clothes dryer vs a rack and a dehumidifier
If you’re considering hanging your wet clothing to dry inside your home, vs using your drier, then you should know that a dehumidifier will be far more efficient than a clothes dryer. In the case of a clothes dryer, electrical energy is used to vaporize the water in your clothing and the water vapor (and all the energy you’ve put into it) is expelled from your home through your drier vent. There is a net loss of energy from your home. If instead you use a dehumidifier, the heat already in your home is used to vaporize (evaporate) the water in your clothing. This energy is recaptured by the dehumidifier when the water vapor is condensed to liquid. Unlike the drier, the dehumidifier doesn’t expel any energy from your home.
Heat pump vs dehumidifier
A dehumidifier is effectively a heat pump. Rather than extracting heat from the ground or the outside air, a dehumidifier extracts heat from water vapor contained in a home’s inside air. In my home, for reasons given above, I can run my dehumidifier continuously without reducing the relative humidity in my home below a comfortable level and I’ve found the coefficient of performance (COP) is about 1.52. A typical air source heat pump has a COP of around 4 assuming an outside temperature of around 0 degrees C (a typical Richmond winter). A typical ground source heat pump has a COP of around 7 assuming a ground temperature of around 10 degrees C (a typical Richmond ground temperature). So clearly, a heat pump (either air or ground source) is much more efficient. If I had a heat pump, I would be consuming more energy than otherwise by operating my dehumidifier. That said, I feel secure in the knowledge that I can run my single dehumidifier continously and consume less energy to heat my home than if I were running an electric heater. I’ll save the installation of a heat pump for another day… perhaps.
Can you heat your whole home this way?
No. If I were to install more portable dehumidifiers to provide all the heat my home requires (to maintain a balmy 16 degrees C all winter long) I would almost certainly bring the relative humidity below comfortable levels, and the COP would drop below the measured value of 1.52 simply because there isn’t enough water vapor in the air to be condensed. So the idea of using a dehumidifier to heat one’s home is clearly not scalable. At best a dehumidifier may provide suplemental heat. I think I might get away with using two portable dehumidifiers continuously which would each save me about $75 per year based on the calculations above. That’s about $150 per year in total. Currently, that’s about 10% of my home’s annual heating bill.
2009-10-28 at 10:50 am
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.
2009-10-29 at 7:09 pm
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
2009-10-30 at 12:35 pm
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.
2009-10-30 at 12:40 pm
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
2009-11-03 at 2:05 pm
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
2009-11-05 at 12:43 am
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.
2009-11-05 at 1:03 pm
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
2009-11-10 at 2:02 am
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?
2009-11-10 at 4:42 pm
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.
2009-11-27 at 1:55 pm
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
2010-01-08 at 11:14 am
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!!
2010-01-16 at 11:34 pm
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.
2010-01-28 at 9:31 am
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
2010-01-30 at 10:31 pm
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
2010-02-21 at 6:37 am
Rob and all,
if you want to use it for heating purposes, you can dramatically increase (double, triple) the COP of a dehumidifier or airco (in fact they are the same thing: both are heat pumps) by installing it in or connected to your solar attic: the warmer the air in, the warmer the air out
venting your humid rooms to the attic will also improve the condensation effects: warm air can contain more moisture than cold air.
pump the warm, dry air back to the room. As this air is warmer, it will dehumidify better
win win win
you can also dramatically increase the efficiency of your solar attic by putting in a simple baffle (which will reduce heat losses during the night) and stocking the surplus energy in a stack of plastic water bottles for release at night
Please go take a look at the littlehouses forum owned by Laren Corie, a very experienced man in passive solar house design
all these ideas and more are Laren’s, you can find them on the forum
frank