A dehumidifier IS a heat pump. Instead of pumping heat out of the ground, it pumps heat out of water vapour, condensing the water in the process. This does require a lot of energy input in the form of electricity, but that energy isn’t lost. You get it back in the form of heat. The heat energy you get out is equal to the electrical energy you put in, plus the energy removed from the water vapour.

According to my findings you get about 1.5 units of heat energy out from 1 unit of electrical energy in plus 0.5 units of energy from condensation. It is not perpetual motion but simple energy conversion in which – as we all know – energy cannot be created or destroyed. That is, in a nutshell, the 1st law of thermodynamics which is also known as the law of conservation of energy.

Of course, if you let the output water warm up to room temperature before disposal, you lose this benefit.

The HRV does have a condensation pump and removes condensation so I assume it would perform similarly to the dehumidifier. Though as it only recovers some of the heat from the air via the exchanger it still loses more than not exchanging any air. I’ll still run it when I see that my neighbor isn’t burning or the wind is going the other way but sounds like I’ll only need to run it to swap out the stale air instead of for humidity/condensation which still occurs on my triple pane metal frame windows.

I’ve recently been charting the RH/temp indoor/outdoor to determine when I can even effectively run the HRV and drop the indoor RH. If the RH is 100% outside and the temperature is 11 celcius or greater (daytime vancouver rain in fall/ early winter) I can’t lower my RH into the 30-50% comfort zone given my indoor temp of 22C ( I have a 7 mo baby and my wife won’t wear slippers forget sweaters and a toque ! )

The condensation of the water is bought with a lot of energy – and it has to be more energy than the condensation gives you back. Otherwise you’d have invented a perpetuum mobile and that is – as we all know – impossible.

We dry our clothes on a clothes line outside in the summer, but it’s just not feasible in the winter given the high humidity, low temperature, rain, and lack of direct sunlight. I agree that clothes feel/smell fresher when dried outside. Things improve a little if after air-drying our clothing inside, I dump them into the drier for a few minutes. It softens them up and seems to make them smell fresher.

It’s been raining here and my indoor hygrometer measures 58%. Just too a shower and vented the vapor rather than using the fan and it went to 61%. I take short cool showers, a habit my father taught me from his time in the navy, so a long hot shower might be the ticket to get 80%+

Drying clothes indoors though would add quite a bit of moisture but I’ve never had much luck with that. The clothes just don’t seem to be as fresh as compared to drying outdoors. That wasn’t with a dehumidifier though. And to be fair, my old outdoors clothesline was in a windy area in a covered carport (no car at the time). It only took about an hour and a half outdoors compared to a day indoors.

63% makes much more sense but if you are reading 80%+ indoors and your instrument is accurate, real world beats scientific theory. I would say that your experience would be atypical and one of the exceptional cases where a dehumidifier does make sense to help heat the house – at least if we have comfort as a factor.

Congrats on the baby! If 21C meant a healthier baby, I’d be running that temperature 24/7 lol.

@Rusty I’ve had the same thought when looking looking at the warm clouds coming out of the dryer vent in cold weather. My own proposal, which would be practical in new installs only I think, would be to have a heat exchanger that could be decoupled to the vent tubing. It might be somewhat like a radiator which could give the water vapor time to condense and release the heat. Or it might have fins to draw heat from the pipe.

Thanks for your comments. They led me to question my own observations and results. I just did a bit of experimentation and calculation to try to clear things up for myself.

It is about 13C outside here now and has been raining all day so I presume it is 100% RH. That would indicate an absolute humidity of about 11 g/m^3 outside based on an online calculator I found here: http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/relhum.html. Inside it is about 21C (just had a baby 3 days ago… cut me some slack ). My hygrometer currently indicates 69% RH. That would indicate an absolute humidity of about 13 g/m^3 inside. I’m not currently running a dehumidifier. The increased absolute humidity indoors is presumably due to respiration, perspiration, cooking, showers, washing, etc. I don’t think we do much more of any of these than an average household. However, we hang dry our washing indoors rather than use a clothes drier and we avoid using ventilation fans when cooking or in the bathroom (why waste the energy in all that water vapor when you can recapture it with a dehumidifier).

Now lets take the case of a typical winter day. Richmond winters see outdoor temperatures averaging around 6C much like Washington but relative humidities averaging around 90% (yes… it’s really wet here). That indicates an absolute humidity of about 6.5 g/m^3. Assuming an additional 2 g/m^3 inside (as I determined above), the absolute humidity inside would be 8.5 g/m^3. At 16C indoor temperature, the relative humidity would be about 63%. This is lower than the 80% I recalled above, but I can’t argue with science . 80% would be correct for outside temperatures around 10C which are common here during the daylight hours (when I would be looking at the hygrometer) of winter.

Anyway, now let’s suppose I run my dehumidifier which I determined was condensing about 400g of water per hour. The question is how much should I expect that to change the RH indoors? The volume of my home is about 500 m^3. It is an older home, not particularly well sealed, so I would estimate around 1 air change per hour due to air infiltration (my understanding is that 0.35 air changes per hour is the recommended minimum for good air quality). 400g/h / 500m^3/h = 0.8 g/m^3 of water condensed. Thus I would expect my dehumidifier to reduce the absolute humidity to 8.5-0.8=7.8 g/m^3. The result would be inside relative humidity dropping from 63% to 58%. Again, that is on average. My observation that the humidity rarely falls below 60% is accurate given that I would be observing during the day time when outdoor temperatures are higher.

Admittedly there are some big uncertainties on some of the numbers in the above calculations (in particular the number of air changes per hour which is difficult to measure and will vary due to wind), but it seems clear that the observations I described are easily within the realm of possibility.

You are clearly running a very humid house! Lots of cooking or showering?

However, the dehumidifier you are running is not heating your house from a human perspective. At 90% humidity at 16C, the apparent temperature is 16C whereas at 20% it would be 13C. But it still does make sense to run a dehumidifier if your indoor RH is above 60% – though I think that is atypical during the winter.

I’m curious about the output directly from the dehumidifier, does it feel warmer? It seems that it would be, the same way that a swamp cooler’s output feels cooler in a humidification situation. I do find that higher than 60% does lead to unacceptable condensation.

As the human body is constantly sweating and humidifying outward breaths, lower humidity levels will make us feel colder.

If the temperature is 21C and you are running a dehumidifier to the low end of the comfort range (20%), the felt temperature will be 18C; whereas, if you run a humidifier to the higher end of the comfort range (60%), the felt temperature will be 21C.

With 20% humidity, you would need to raise the thermostat to about 23C in order to achieve a felt temperature of 21C. That’s a 5C difference! I would wager that it is much less expensive to run a humidifier to 60% than it is to raise the thermostat 5C.

In short, you would do best to heat your home with a humidifier and cool it with a dehumidifier.

There are lots of assumptions in it though:

-Wet laundry has 2kg of water in it [1]
-Latent heat of vaporisation (at 15ºC) = 2500kJ/kg [1]
-Specific heat capacity of air is 1.2 kJ/m3/ºC [1]
-Air changes per hour with window open = 4 [estimated based on 1]
-Air changes per hour with window closed = 0.5 [1]
-The laundry is put in a small room (23.2m3) over the radiator and next to the dehumidifier. [2]
-My dehumidifier uses 2kwh and gives me 2kg of water. COP = 1.7 [2]
-Gas is 4p/kwh
-Electric is 12p/kwh
-It takes 12 hours to dry the clothes with the window open and the radiator on [2]
-The temperature difference between inside and outside is 10ºC
-Gas boiler is 75% efficient

Laundry on radiator (common to both scenarios)
2kg of water in laundry is evaporated by radiator E = 2 * 2500 / 3600 = 1.38 kwh
Cost = 1.38 * 4 = 5.52p

With dehumidifier
2kg of water is collected by dehumidifier and it uses 2kwh
Cost = 2 * 12 = 24p
Heat from condensation = 2 * 2500 / 3600 = 1.38 kwh
Heat generated from dehumidifier = 3.38kwh
With the window closed, energy lost to ventilation is:
E = Volume * air changes per hour * time * Cp * ?T
E = 23.2 * 0.5 * 12 * 1.2 * 10 / 3600 = -0.46kwh
Total heat generated = 2.92kwh
Total cost = 5.52 + 24 = 29.52p

With window open
The air changes per hour increases to 4, energy lost to ventilation is:
E = Volume * air changes per hour * time * Cp * ?T
E = 23.2 * 4 * 12 * 1.2 * 10 / 3600 = -3.71kwh
To compare like with like the radiator must match the 2.92kwh of heat generation from the dehumidifier. The thermostat would ensure this.
Total radiator heat used = 2.92 + 3.71 = 6.63kwh
Cost of heat = 6.63 / 0.75 * 4 = 35.36p
Total Cost = 5.52 + 35.36 = 40.88p

So the dehumidifier is slightly better, but it is very close. Depending on assumptions you make. Please feel free to point out the mistakes I will probably have made.

1. Without dehumidifier: cool moist air enters the home and room temperature moist air exits the home.

2. With dehumidifier: cool moist air enters the home and room temperature dry air plus liquid water exits the home.

In both scenarios the air and water vapor coming into the home contain the same amount of energy (I think we agree on that). But in Scenario 2, the air and liquid water exiting the home contain less energy than the air and water vapor exiting the home in Scenario 1 (I’m not sure we agree on that… if not, look up “enthalpy of condensation” in wikipedia). Since Scenario 2 has less energy loss from the home, where do you suppose the “saved” energy goes? The first law of thermodynamics says energy must be conserved. The answer is that the energy (the enthalpy of condensation) is released inside the home in the form of heat.