Offsetting the dehumidifier heat with a heat pump water heater
A dehumidifier produces heat. A heat pump water heater consumes heat. They’re a pretty good match for each other when you put them in the same room, because they kind of neutralize each other out, in terms of heat production and consumption.
In my basement I have an LG APHWC501M heat pump water heater, which I love. In the heat pump world, this is a very quiet device, not noticeable in other rooms of the house (this will of course vary from one house to another, depending on construction, but in my house I only hear it in the mechanical room).
Right next to the water heater, I have an Aprilaire E080 dehumidifier, not ducted, though I am planing on ducting it to different rooms in the basement once I actually construct those rooms (the basement is currently unfinished). It dehumidifies well, and it’s not too noisy, but it’s definitely noisier than the LG water heater. I’ll probably do some things to mitigate the sound, but the sound is not really the point of this post. I’m just providing those details for anyone who might be curious.
When the water heater is running, it reduces the temperature and humidity of the room relatively quickly by a degree or two. The water heater will typically run for 30-60 minutes after someone takes a shower. Without a dehumidifier running, the temperature recovers pretty quickly too, almost to the same temperature before the water heater ran; back to about 90% of the previous temperature, roughly, in the initial recovery, and then back to nearly 100% over a slightly longer time frame. It definitely cools the space temporarily and intermittently, and the overall effect will depend a lot on how much hot water is used. The higher the hot water consumption, the lower the temperature… but the room temperature still mostly recovers pretty quickly each time.
When the dehumidifier is running, the humidity gradually drops — the drop is not as dramatic as the drop caused by the water heater — and the heat in the room increases a bit.
I’ve attached a data visualization showing the temperature and humidity over the last 24 hours in my basement. I used the hot water only once yesterday, to take a shower a little before 8:00pm, and you can see a dramatic dip in both temperature and humidity, due to the heat pump water heater. I say “dramatic,” but the temperature drop is about 1 degree F, and the humidity drop is maybe 2%.
The less dramatic peaks and valleys in the humidity are from the dehumidifier. You’ll notice a corresponding rise in the temperature when the dehumidifier runs too.
BUT, I don’t want this graph to be over-interpreted. Here are some things to know about this data set:
1. This was a hot day, reaching about 90 degrees F, which was about 10 degrees hotter than the day before, so the outside temperature was increasing.
2. My basement is unconditioned right now. There is no heating or cooling, so the temperature in the basement is impacted by the outside temperature.
3. This is a basement, so the ground insulates it pretty well, and keeps temperature fluctuations much lower than the upstairs, but hot days still increase the basement temperature.
4. Notice that the total temperature increase over the last 24 hours, even with the hot day and even with the dehumidifier running, is still only about 1.4 degrees F, which is not a big deal.
5. I have an older house with an uninsulated slab, which means the humidity is always an issue, unless I do something about it, which is why I run a dehumidifier.
All that said, coming back to the main point of this post: a heat pump water heater and a dehumidifier are a good match for each other. They are continually trading heat back and forth, which is a pretty good arrangement. It’s not a one-to-one trade, because they run at different times and for different reasons, so I won’t say that they produce true equilibrium, but still, they complement each other well. Putting them both in the same room (or ducting the dehumidifier exhaust to the room) is a good strategy.
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I think this strategy is ideal. On a recent project I ducted the exhaust from and April Aire whole home dehu into the utility room where the HPWH lives.
Currently the house isn’t occupied so I don’t have much real time data but in the limited testing I have done so far it appears that these pieces of equipment compliment one another well.
During initial start up of the water heater, it ran for a couple hours to bring the 80gal tank volume up to temp. During this time the room temp dropped from 21°c to 17°c. (70°f to 63°f)
After allowing the room to equalize on its own, I turned on the dehu, after an hour or so the room temp had risen from 21°c to 25°c (70°f to 77°).
With the dehu running, I then ran hot water long enough to trigger the HPWH and during the run cycle of approximately 30 minutes, the room temp completely equalized at the original 21°c.
I agree with you that this isn’t a perfect trade off as these units will at times run on different schedules and for different reasons but considering that showering etc produces humidity there is a good chance that the whole home dehu will be triggered even if the bulk of the moisture is being carried away by the ventilation system.
On thing I have considered was the viability of using a relay that would activate the dehu whenever the HPWH was in operation. Electrical setups like this are not my strong suit so not sure if this idea would work. Perhaps someone with more knowledge will weigh in on this.
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I can see how adding heat from a whole house dehumidifer to the room with the HPWH would help, but if it's just a stand alone dehumidifer sitting in the same room, I'm less sure. They are not "trading heat" as you say. The dehumidifer is turning latent heat into sensible heat, and the HPWH is simply moving heat (both sensible and latent) from the air to the water.
I don't know enough about HPHW's to know whether a higher sensible temperature coupled with a lower latent load increases it's efficiency, vs a lower sensible load with higher laten. If that is true, then I guess it might help a little as you predict. But the enthalpy of the room is not increased by adding the dehumidifer beyond the waste heat added from the dehumidier (but you are paying for that in your electric bill).
But if the heat is from other rooms... that's a different story.
"But the enthalpy of the room is not increased by adding the dehumidifer beyond the waste heat added from the dehumidier (but you are paying for that in your electric bill)."
The latent heat from the extracted humidity gets added to the room. Standalone dehumidifiers are efficient space heaters, they typically have a COP of 3 or more.
The dehumidifier has a humidistat, so it will only run when dehumidification is needed, which means that the water heater isn't removing enough moisture. If the water heater were doing the job then the dehumidifier would never turn on. So it's not really an either/or. Together they make a good pair.
"The latent heat from the extracted humidity gets added to the room."
Right, I wrote that the *enthalpy* does not change. The latent heat is turned into sensible. No enthalpy is added.
I am not sure how much of an efficiency boost this is for the HPWH. The latent load still turns the water hot, but perhaps less efficiently. Not sure. There is not more energy to extract from the system though.
I agree with that fact that the dehumidifier will dehumidify when needed, unlike the HPWH. I'm not arguing that one shouldn't put a dehumidifier in the same room as a HPWH. I'm simply questioning if there is much improvement in performance of the HPWH. Certainly there may be an improvement in the overall conditioning of the room.
I don’t know what is the ideal, most efficient ambient temperature for this particular heat pump water heater. I’m sure some temperature ranges are more efficient than others. The literature for the heat pump water heater claims that it can operate in ambient temperature ranging from 23F to 120F, but I didn’t see an ideal range.
Still, they do seem like a good pairing, even if they’re not a perfect match made in heaven, so to speak.
I’m on the slow road to a comprehensive energy retrofit. It’s true that the dehumidifier is currently just sitting in the same room — the basement — as the heat pump water heater. The eventual plan is to divide the basement into separate rooms, and at that point I’ll duct the dehumidifier for the basement, and for parts of the floor above. The top floor will need to be served by a second ducted dehumidifier, which will be in the conditioned attic space. In a new house I wouldn’t want to have two separate dehumidification systems like that, but there is no easy way to get ductwork from the basement to the top floors without ruining the house.
But maybe splitting them up is actually ideal... if I ignore the extra energy required. The humidity levels of the different floors are not the same from floor to floor. I suppose there are other, better ways to dehumidify floors or rooms with different levels of humidity — a zoned system, perhaps — but in any case, I don’t really have the option of a single ducting system, so I have to come up with other ways.