Are dehumidifiers really heaters in disguises?
walta100
| Posted in General Questions on
I think so.
Seems to me dehumidifiers lower the relative humidity more by heating the than by removing water from the air.
For those that will surely disagree, let’s do the math. IF we have a dehumidifier runing 24/7 that removes 70 pints per day drawing 7 amp.
12 volts X 7 amps = 840Watts
840 Watts x 24 = 20160 Watthours
20160/ 1000= 20.16 k Watt Hours
20.16 kWH X 3141 = 68,826 BTUs per day
Each pint of water = 144 BTUs
70 X 144 =10,080 BTUs of water down the drain.
(10808/68826)X100= 14% of the energy becomes water and goes down the drain the other 86% of the energy stays in the room as heat.
Walta
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Replies
My garage says yes, yes they are
They're highly efficient space heaters, COP in the 3-4 range.
COP doesn't apply in this case, since it's a closed system. You can't, for example, use a heat pump to pump heat scavenged from a room back into that same room and expect to have any more energy heating the room than the total input energy feeding the heat pump. The only reason COP improves heating efficiency for a heat pump is because the heat pump is able to scavenge heat from "somewhere else", and bring it into the space being heated. In a closed space, the heat pump is both cooling down AND heating up the SAME space at the same time, so the only net increase in room temperature is coming from the energy consumed by the heat pump for it's operation, which will be equal to the total input electrical energy minus the energy that went towards condensing water out of the air (which is much more complex to calculate due to the variables involved).
Bill
See my reply #3. It's not a closed system, you're extracting water and disposing of it.
I'm going to check your math:
120V times 7A= 840 Watts
840 Watts is 2900 BTU/hr.
70 pints per day is roughly 3 pints per hour, To condense a pint of water requires that roughly 1000 BTU's be removed. That heat also gets dumped into the exhaust stream, another 3,000 BTU/hr.
That 3 pints per hour gets dumped down a drain. It leaves the dehumidifier probably 30F colder than when it came in. The heat that it lost also gets exhausted, that's another 90 BTU/hr.
So the dehumidifier in this example is putting 6000 BTU/hr into the room. It's consuming electricity equivalent to 2900 BTU/hr so it has a COP of around 2.2. That's actually low.
Deleted
You've double dipped on your numbers. You've taken the 840W of electrical energy and added 3000BTU (energy required to condense the water) to it. But the energy to condense that water came from the 840W of electricity supplied to the dehumidifier.
I mean, think about it for a second. You've got an electrical appliance, fully contained within the conditioned space, and it's consuming ~3000BTU/h of energy and putting out 6000BTU/h? Where is that extra 3000BTU/h coming from?
I don't think it's a coincidence that the rated electrical input and energy required to condense the rated water removal are almost the same. I suspect the water removal rating is a theoretical maximum derived from the energy input, and in reality is going to be less than that. The dehumidifier is trading latent heat for sensible heat, minus the efficiency losses.
It's not energy required to condense the water. The water releases heat when it is condensed.
Think about an air conditioner. That has a positive COP, it extracts heat, both latent and sensible, from the air, and dumps it outside. Usually the amount of heat extracted is several times the electrical input. A dehumidifier is exactly the same, except it's not dumping the heat outdoors but indoors. In the cooling phase it's able to extract several times the electrical input. Then it puts that heat back into the air. The sensible heat evens out, but not the latent heat. The water has been extracted, the heat that yields is put back into the room.
The heat isn't going anywhere. Yes, warm air is emitted into the room but a coil inside the unit is being cooled down. The only way you could be warming the room with a COP > 1 is if the cooling coil was outside. So, the warming from the unit running will be equal to the energy input.
BUT, there is an additional heating factor. When water vapor condenses into liquid it releases heat (2,260 joules/litre). So this will raise the COP of your dehumidifier to 1.8 or so, depending on the efficiency of the unit.
All of the electricity consumed goes into heat in the room. In addition, all of the latent heat of the water removed goes into the room. There's also a small effect that the water vapor is cooled and then sent down the drain -- for a dehumidifier with a drain -- so the heat removed from the water to cool it also goes into the room.
If you look at the electricity consumed vs water removed for actual dehumidifiers you'll find the COP is more like in the 3 to 4 range.
"the energy to condense that water came from the 840W of electricity supplied to the dehumidifier."
If a cold coil simply 'existed' in a room, it would still cause some dehumidification, ramping down as it warmed up (not sustained, since it would seek equilibrium). The energy from the dehumidifier is used to keep the coil cold. So it's pumping the heat entering the cold coil into the room. That heat comes from the latent heat of the room but also the sensible heat. Pumping sensible heat from the room back into the room is obviously doing nothing but generating additional waste heat. Pumping latent heat from the room back into the room as sensible is also causing waste heat from the pumping action, but it's additionally adding the heat entering the coil from the condensing vapor.
"Pumping sensible heat from the room back into the room is obviously doing nothing but generating additional waste heat."
And I'm guessing this is why dehumidifiers will be more efficient at humidity removal when the humidity level is higher. More of the sensible heat exiting the dehumidifier came from latent heat vs already existing sensible heat.
I haven't yet figure out, maybe DC knows: is it more the dewpoint or more the RH that determines efficiency? It seems RH must play some role, if not the dominant one. Consider 70F dewpoint at 70F sensible room air (100% RH). That is a whole lot of latent energy and a minimal amount of sensible. 70F dewpoint and 80F sensible air has the same available latent energy but more sensible, which in turn means that extra sensible energy is warming up the cold coil needlessly, thereby increasing waste.
I've put up a spreadsheet at https://docs.google.com/spreadsheets/d/1LONsi_1Fb6u4PsDQjzrbHn7ToX7yQHHEvr5OxExVwQc/edit?usp=sharing
that anyone can use to do sensible heat ratio calculations.
Dehumidifier efficiency is going to be the inverse of sensible heat ratio -- the more of the cooling that goes into latent heat, the better.
Let's assume the dehu cools to 45F. At 70F, 100% RH, the SHR is 37%. At 80F, dewpoint of 70F (73% RH) the SHR is 44%. At 60F, 100% RH, the SHR is 42%. At 70F, 50% RH the SHR is 78%.
Deleted
With all these calcs in mind, why are there no split system (outdoor condensing unit / indoor air handler) dehumidifiers for residential use on the market? Most dehumidification work gets done in the cooling season in the U.S. and Canada, I reckon.
A Mr. Cool-style, pre-charged DIY system could sell pretty well, in my opinion.
Because the goal of a dehumidifier is to remove moisture from the air, with a minimum amount of temperature change to the air. The goal of a heat pump is to heat or cool the air, with dehumidification a byproduct of the cooling process. A heat pump in cooling mode doubles as a "split system dehumidifier" in thise case.
Normal dehumidifiers are nothing more than an air conditioner with the condenser and evaporator coils internally arranged in series, so the air passes over both coils. Water condenses out on the cold coil, which is where dehumidification occurs, due to the coil being below the dew point of the air in the space being dehumidified. The heat that was removed from the air is put back into the air again by the condenser coil, and the energy losses in the system also leave as heat, which goes into the room. While a dehumidifier is not intentionally a heater, it does add heat to the room as a byproduct of it's operation, which is way to say the energy lost by the machine during the demudification process heats up the room. Some of the input energy went into condensing the water though, so you don't see ALL of the input energy going towards heating the room.
Bill
The Daiken Quaternity minisplit works as you describe. It has separate coils for heating and cooling, and can heat or cool, or dehumidify without doing either.
It's intriguing, but I haven't seen a lot of real-world experience.
Yes I read the words "Daiken Quaternity" with some regularity on this forum but I almost never see anybody actually owning one or reporting how well the dehumidify function works.
That would cost a lot for the energy you'd save. You've described a ductless heat pump.
"Each pint of water = 144 BTUs"
Where did this come from? A pint of water doesn't equal any amount of BTUs. If you're implying that removing 1 pint of water from the air represents 144BTU of latent heat energy, I think that number is wrong. Using a latent heat calculator, a pint of water represents about 280Wh, or 950BTU. If we use those numbers, almost all the energy is going towards condensing the water, but it doesn't go down the drain, it's converted to sensible heat that stays in the room. In reality, it's probably not quite that good, but it's probably closer to that than your 14% figure.
The implication is usually that the energy DID go down the drain, since typical installations of dehumidifiers that are expected to run for extended periods of time will be plumed into a drain, and will discharge the water out that drain line.
BTW, from practical experience here, I have a dehumidifier that runs in a basement room all year. It runs a lot more during the summer months, but it does cycle suprisingly frequently in the winter too. It DOES heat the room up, but not significantly, and not as much as you'd expect from the amount of energy it consumes. I've never bothered to try to really measure how much it heats the room, but my subjective experience with it is that it's not what a heater of comparable wattage would be.
Bill
Water going down the drain is not really the same as energy going down the drain though is it? Yes technically there is some energy leaving the room with the water, but the sensible heat of the vapor has been converted (transfered via the cold coil) into sensible heat.
“Water going down the drain is not really the same as energy going down the drain though is it? Yes technically there is some energy leaving the room with the water, but the sensible heat of the vapor has been converted (transfered via the cold coil) into sensible heat.”
If the water that is leaving the room happens to still be colder than the air in the room and lets say it 70 pounds of water and it is 30° cooler than the air that’s 2100BTUs down the drain.
Walta
Walta,
I think we must be thinking about this differently. I am talking about the actual energy content of the water. When the water goes from a vapor to a liquid, the energy it takes to do so (heat of vaporization) is put into the air via the dehumidifier as sensible heat-- it is not sent down the drain in liquid form.
I am guessing you mean that if the occupants desire cooling, sending that cool water down the drain is a 'waste' because it has some capacity to absorb sensible heat from the room, taking strain off the AC system?
Except, the way it's being described, energy came up from the drain rather than went down.
My practical experience is the same. In DC's model, the 840W dehumidifier would be heating up the room more than a 1500W baseboard heater. That sure never seemed to be the case when I used one.
"the way it's being described, energy came up from the drain rather than went down."
Can you expand on this? Who's description (and what post) are you referring to?
If you mean the notion that the latent heat is being added to the room as sensible, I don't know what that has to do with a drain?
I got the from this quick Google search and I agree 970BTUs is the correct number.
Couldn't it be broken into two categories:
1) The heat put out that was once latent. If this was the ONLY heat a dehumidifier put out, it would still heat up the room, but it would be 100% efficient at removing humidity. No change in room enthalpy would have occurred.
2) the waste heat lost to the room from the dehumidifier being imperfect at doing its thing-- pumping refridgerant, running a fan, a few electronics, etc. If this was the ONLY heat the dehumidifier put out, it would be zero percent efficient at removing humidity, but 100% efficient as a space heater.
What's not yet clear to me from this discussion is whether in case 1 (100% efficient) if the sensible heat energy put into the room would match the electrical input energy. Trevor, I think, is saying they match. So a perfectly efficient dehumidifier using 800 watts would put out 800 watts of heat, but ALL that heat was already in the room as latent heat.
So does a real dehumidifier that is not 100% efficient put out more heat energy than it uses in power (800 watts of electric in, 800+ watts out?) or does it simply split the 800 watts into the categories of waste heat and latent heat. E.g. a 50% efficient dehumidifier using 800 watts of power puts out only 800 watts of heat energy, but 50% was in the room as latent and 50% came from waste.
Ok, riffing off DC's input I'll try to answer my own question.
In case 1, if the ONLY heat the dehumidifier was putting out was once latent, then it's perfectly efficient electrically, i.e. it's using no power at all. Any power it uses is to run the motions of the machine and is not from the latent heat itself.
Thus the latent heat put out as sensible is in addition to the electrical input energy. The total heat output would be a combination of the two.
I've often wondered if using a dehumidifier in winter when showering doesn't make more sense than venting. Need the heat, why not use it?
It would. As does a condensing dryer.
Also, a heat pump water heater provides a certain amount of "free" heating because it dehumidifies the air, which doesn't result in sensible cooling that needs to be made up (in heating season).
Wait, what? It does result in sensible cooling. Try telling all the people who installed a heat pump water heater in their basement that it didn't get colder, it actually got warmer. The heat going into the water tank comes from the ambient environment, so how could it be putting heat both into the water and the ambient environment?
I think DC means the dehumidification process (as an isolated phenomenon) doesn't steal sensible heat from the room. But from my limited experience with a heat pump water heater in cold climates, there isn't much dehumidification happening in winter.
I once saw a friend post that they were being efficient by cooling their hot pot of soup in 'nature's refrigerator' (the snow). I couldn't help but think they were actually just tossing desirable heat into the snow. That said, maybe they needed it cooled quickly and didn't want to warm up the inside of the fridge too much...
Refrigerator also provides free heat in winter.
No, it doesn't. I feel like you're just trolling now. It's just moving heat from one part of the house to another (inside the fridge), and it constantly leaks back out. The only heat being added is the losses in the refrigeration cycle, which is way less than the energy supplied to the fridge. The only way a refrigerator is providing free heat is if you've got it plugged into your neightbour's house by a long extension cord.
The way I view a refrigerator is (like Trevor says) not really 'free heat' but assuming we want a cold box as a given, all the heat we put into that box is then pumped into the room, which is better than letting that heat bleed off into the snow.
So it's only free when compared to a cold box that pumps its heat outside the room. Putting the soup in the fridge would be a net win (but I was suggesting my friend maybe didn't want to have a hot pot of soup warming up the veggies and cheeses in the fridge for spoilage reasons.
It seems I made a few errors and it turns out are actually better than restive heaters at warming a room.
Lets do the numbers again. DC is correct that condensing the water will add the BTUs of heat to the room and I was subtracting BTUs. Also, our nameless friend Blank page is correct that I got the wrong number for the laten heat of water 950 BTUs is a better number.
120 volts X 7 amps = 840Watts
840 Watts x 24 = 20160 Watthours
20160/ 1000= 20.16 k Watt Hours
20.16 kWH X 3141 = 68,826 BTUs per day
Each pint of water condensed add = 950 BTUs
70 X 950 =66500 BTUs extracted from the water added to the air.
68826 + 66500= 135326 BTUs added to the room.
Making the dehumidifier 196% efficient as a heater if the water remains in the room long enough to reach room temp.
Should I change the title to read “dehumidifiers are better at warming a room than a heater”?
Walta
They would be better than a resistance heater, but if you used it as a heater in the dead of winter, you may want to have some Ponaris or similar nearby. The assumption that it continues to achieve the full 70 pints would likely be inaccurate as humidity level drops as well.
I agree Tim the conditions where it is possible for a 70-pint dehumidifier to collect 70 pints are almost nonextant and totally unsustainable in the wild.
Walta
Blanks says “The only heat being added is the losses in the refrigeration cycle, which is way less than the energy supplied to the fridge. “
Since we agree energy can’t be created or destroyed 100% of the energy provided to the refrigerator will become heat inside the room at the rate of 3.142 BTUs per watt hour when the refrigerator get turned off the temperatures equalize.
Walta
This is a good lead in - I was just watching/listening to the BS and Beer on Moisture Management from a month ago. They were talking about how modern high efficiency AC is achieving that by running higher temperature coils, basically sacrificing latent capacity for higher efficiency sensible capacity. Then some builders are spec'ing dedicated dehumidification to make up for that. It would be interesting to evaluate the efficiency of that as a system and compare to a lower efficiency single unit that has the latent capacity.
As far as I know, a lot of the mini split systems that exist are either in dehumidification mode or cooling mode. But none would modulate the system to meet a certain humidity level AND temperature. Maybe I'm wrong here?
Totally agree, my FH09 has a 30-33 seer rating, but I need to run a dehumidifier a lot of the time which is who knows how efficient. I've measured the discharge air temps, under high loads it can go as low as 7-8degC but under lower loads sits at 13-14degC. Would much rather low airflow at 8degC even if it cycles. The seer rating need to be done at a fixed latent ratio, or provided at different latent capacities if adjustable.
FYI, some of the mits multisplits allow you to set the cooling coil target temperature via dipswitch's (effectively adjusting the latent ratio). It seems that some of the multisplits modulate as necessary to maintain a fixed leaving temperature, while my 1:1 modulates varying the discharge air temp based on the load. I would prefer the way the multisplit works in AC, but the 1:1 in heating.
A space humidity sensor that has some control authority over the discharge air temp target would do wonders. Under low loads if my AC is on, it's likely due to humidity not temperature in my Southern Ontario Canada climate. Drying mode doesn't work that great, so I end up giving it a ridiculously low cooling setpoint and then fixing the fan on min speed to force the discharge temp to stay low without overcooling the room to quickly.
Let's expand on that situation. Say your total cooling load is 8000, roughly the capacity of the FH09. For my estimate, a latent load of 1200 seems reasonable, people in the house is a big factor here. Your FH09 only has capacity for 720btu/hr latent based on its specs, so the other 480 is coming from your dehumidifier. If you and Walta bought the same dehumidifier, your 480 latent load is adding 980btu/hr of sensible load to your home, and in effect, your FH09. So to achieve your 8000 btu cooling load, you have to operate at 8980 btu. So you are using an extra 0.031kwh there. Plus the power used by the dehumidifier, which would be another 0.134kwh.
Operating at 8000 btu/hr and 31 SEER, you are at 0.258kwh. Adding the dehumidification process in, your total kwh usage is 0.423kwh, which means your system efficiency would actually be 18.9 SEER. That's a big hit. My math is messy and a bit scattered, sorry.
Take a look at the spreadsheet I linked to in post #24. You'll see that the three things that determine how much latent heat is removed are room air temperature, room humidity, and coil temperature (for simplicity I assume that exiting air temperature is coil temperature, it may not be but that doesn't really change the analysis). What the reported latent heat capacity of an air conditioner is really telling you is the coil temperature.
Lowering the coil temperature increases dehumidification, but reduces SEER. But if you plug in numbers you'll see that there's diminishing returns -- as you get colder dehumidification increases, but there's only so much humidity in the air, and sensible cooling also increases as you get colder. So for air at 75F, 60% RH (dew point 60F), the maximum dehumidification is at 35F, where the sensible heat ratio is 56% (ie the cooling is 44% latent, 56% sensible). At 60F -- the dewpoint -- you get no latent cooling. At 50F you get a SHR of 61%.
A couple of observations:
1. You don't get a whole lot more dehumidification at 35F than you do at 50F. But the SEER is dependent upon the temperature difference between inside and outside. Let's say your outside coil is at 120F, the difference between 35F and 50F is going to move the SEER by about 20%.
2. There is a limit to how much dehumidification cooling alone can provide. If your actual ratio of sensible to latent is lower than the lowest SHR you can achieve, your not going to get enough dehumidification. In that case you need to introduce more heat to get the sensible load up. Using a dehumidifier seems like a good way of doing that!
In the case of the FH09, it's rated SHR was 92% at the 30ish SEER. I guess you could find a balance point. Most likely in the Midwest here you could find a system that can take care of both sensible and latent generally. And the hot days following a rain storm, like today, fire up the standalone dehumidifier.
The "rated SHR" is only useful for comparing to other equipment, it really has no predictive power as to what SHR you will see in operation, that is dependent on the temperature and humidity.
Got it! That makes sense.
Not a minisplit, but Chiltrix has what they call "psychrologix" which adjusts the cooling temperature to target both temperature and humidity.
https://www.chiltrix.com/chiller-controller/
Yeah, one of the reasons Chiltrix is on top of my list for systems as we plan our house build. A control system like that is what should be more common. From a company perspective, it would probably reduce their warranty call backs as well. I've seen a number of posts here about people dissatisfied due to humidity issues. Seems like a simple enough algo.
Tyler said" I think we must be thinking about this differently. I am talking about the actual energy content of the water. When the water goes from a vapor to a liquid, the energy it takes to do so (heat of vaporization)"
Yes you are thinking of it backward the heat if vaporization is absorbed when water changes from liquid to vaper (boils) and is released when it condenses back to a liquid.
Walta
Walta,
What's backwards about what I said? You may want to re-read the relevant posts and make sure we're talking about the same stuff here.
If vapor is condensed into liquid, the heat of vaporization is released, like you say. Agreement to this point. It is transferred into the cold coil and then sent into the room via the dehumidifier. Thus that heat does NOT go down the drain. No? How can this be disputed?
Re-reading, I see that the phrase 'energy it takes to do so' is probably what you are referring to, since it implies an endothermic process. Poor language on my part, but the surrounding context should make it clear that I am referring to energy that is released, i.e. exothermic. Ultimately, I fail to see what is meant by claiming the energy is going down the drain, when clearly it is being released into the room.
Interesting discussion. What would be really useful when all the debate dies down, would be for someone so summarize in practical terms what all this leads to in terms of the choices we make.
If you have a cold and clammy basement/crawl, installing a dehumidifier will dry it and also warm it up. win/win.
Akos,
Thanks. I'm going to stop reading now.