More Q&A Spotlight
“Ben87” wants advice on dehumidifiers. He has a 1600-sq.-ft. slab-on-grade duplex under construction in Climate Zone 4A, and he knows it will need some type of dehumidification, but he’s not sure how to spend his money. He lists two options:
Option 1: He could buy a portable dehumidifier for about $300.
Option 2: He could buy a ducted, whole-house dehumidifier, such as the Santa Fe or Aprilaire e80 , both of which cost about $1500.
As Ben sees it, the whole-house system will last longer, but his research suggests that the efficiency boost is marginal (they’re all between 2 and 2.35 l/kwh). He asks: “Will they really last 5x as long? Are there other ductable units that are better options?”
If he goes with a portable unit, he’s not sure where to put it. He wants the unit to auto-drain, which limits the location choices. His first-floor mechanical closet is a possibility, but he doesn’t think the unit would effectively dehumidify the rest of the house.
An assortment of considerations
As one reader points out, portable units work for small spaces, but a whole-house dehumidification system can handle larger amounts of humidity over longer time periods, and they are more robust and long-lasting.
Ben wonders how he would duct a larger system, as space is tight, but because the house is under construction, there is still the possibility of integrating a whole-house dehumidifier into his HVAC system.
Charlie Sullivan recommends ThermaStor brands (Santa Fe, Phoenix, Quest), even though they aren’t on Energy Star lists because of their larger size. “To qualify for Energy Star,” he explains, “it needs to be physically small, but the extra technology that ThermaStor uses makes them larger. If you compare the numbers for the best SantaFe units, they are fantastic. But the best technology is found…
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6 Comments
Dehumidification is one of the uncharted frontiers in building science. Right now there isn't any methodology that I'm aware of to predict whether a building will require dehumidification, or whether air conditioning and ventilation will keep the humidity in a comfortable zone. There also isn't great information on the capacity of dehumidifiers, particularly in terms of modeling their impact. Basically we're stuck with trial and error.
So it's not surprising that basic questions about dehumidifier effectiveness are hard to answer.
The qualitative answer is that new builds that have good insulation in climates with high dew points during shoulder seasons will benefit and likely need dehumidification. But sizing it? You can calculate how much moisture an HRV brings in but internal humidity generation depends a lot on the occupants. And is diffusion through the walls also significant?
And then figuring out what degree of distribution is needed is another question. If you have ducted central HVAC, that fan can run and mix things around, but if you don't it's hard to know what is needed. Maybe just intake on one level and supply on another, but it's hard to know.
Seems like a good candidate for a major research project with some high-fidelity simulations and some test installations.
There's no reason why there couldn't be a single device that heats, cools and dehumidifies. It's easy to imagine, because at least one already exists, the Daikin Quaternity. It's a minisplit with two coils in the head, it can be configured so that both coils heat, both coils cool, or one cools and one heats, which is what a dehumidifier does.
Now, I have no direct experience with this device -- and they seem very rare in the wild -- but there doesn't seem to be any theoretical reason why this design couldn't be extended to ducted units. And there's no reason why your indoor comfort couldn't be controlled by both a thermostat and a humidistat.
The question remains about how one would engineer such a system. The maximum pure dehumidification would be both coils running full out, one cooling and one heating. Or half the cooling of the maximum cooling -- so around half the dehumidification. Would that be enough in all cases? Or would people find that the still need supplemental dehumidification? I don't know, and what's worse is, I don't know how to find out.
The advantage I see of using a "real" dehumidifier instead of one of those is the ability to include the efficiency boosting technologies that the best dehumidifiers use: heat exchangers between inflow and outflow from the cold coil and the new multiple expansion cycle system used on the ultra-efficient Quest models. Now if the air handling box had the ability to switch the airflow to go through a heat exchanger or not, that could be a great solution--wouldn't even need to split the coils if you want dehumidification without any heating.
Dehumidifiers put off a lot of heat. So you don't want heat exchange, unless you need heating you want heat parity -- keeping the temperature the same -- which means putting back less heat than you take out and exhausting the rest.
Systems like the Quaternity should really be described as having three coils --- two indoor, one outdoor. All three can either heat or cool, at any given time two of the three have to be doing one thing and the third the other thing. So when you're in dehumidification mode the outside coil is also shedding some heat. That's more efficient than running a stand-alone dehumidifier and then running AC to remove the heat it introduces.
Sorry, but I have to disagree with Chuck and Jake: We live in zone 5a and I added an Aprilaire dehumidifier to our central HVAC system - because humidity can be an issue in Spring, Summer and Fall. During the peak of summer, the AC system keeps the temp and humidity down. On many days, however, the Aprilaire keeps the humidity down - eliminating the need to run the AC compressor as often, even if it is warm outside. This reduces our power bill. Plus the ducted Aprilaire ensures that dehumidified air is evenly distributed throughout our house.
By comparison, a standalone dehumidifier is cheap - but not the best value. Similar to a window-mounted AC unit, it will be nearly impossible to distribute dehumidified air around his house - particularly to bedrooms at night when the doors are closed. Even a central ducted airhandler won't solve this problem.
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