Ventilating Dehumidifiers for Better Indoor Air Quality - GreenBuildingAdvisor (2024)

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12 Comments

1. 

   frontrange |Apr 20, 2022 08:46pm |#1

   Seems like this would work fine in a leaky house, but how's the air getting out of the house in a well-sealed house?

1. 

   Jon_Harrod |Apr 21, 2022 08:32am |#2

   This is an interesting question. Looking at the pressure charts in the blower door manual, even in a reasonably tight house (2-3 ACH50), 75CFM of supply air will only cause 2 or 3 pascals of pressurization. In practice, it will likely be less, because the flappers on bath and kitchen fans will show up pretty tight during a (depressurization) blower door test but will be leakier when the house is under positive pressure. The pressures will be roughly in line with what the house normally experiences from stack effect and wind. In an _extremely_ tight (i.e., passive house) building, the pressures may be higher--in that case some relief air or a balanced ventilation system may make sense.

1. 

   frontrange |Apr 21, 2022 10:13am |#3

   That makes sense. I was not thinking of exhaust fans, which of course will let air out.

2. 

   charlie_sullivan |Apr 21, 2022 10:51am |#4

   I would think that as long as you have exhaust fans with flappers, a tighter house will be less likely to have problems than a moderately leaky one. The more of that air that goes out the exhaust ports rather than through inadvertent leaks, the better--you have less chance of depositing moisture in the walls. That assumes that you have your exhaust ducts set up right so you don't have condensation in them dripping in harmful places.

2. Expert Member

   PETER G ENGLE PE |Apr 21, 2022 11:18am |#5

   This system might be fine for moderate climates and/or the shoulder seasons in very damp areas, but during very hot/cold weather, you lose the benefit of energy recovery that is provided by a balanced ERV/HRV system. So after we go to the trouble of designing out ductwork by using ductless minisplits, we now have a "solution" that includes 6" bedroom ducts, 8" ducts for the main spaces, and a substantially larger duct for the return in addition to the spaghetti network of ducts for the ERV that is used in heating/cooling weather and the relatively high cost of the minisplits themselves. Seems to me we are still missing something in this scenario.

1. 

   paulrey27 |Aug 24, 2022 03:11pm |#9

   Would you couple a dehumidifier with an ERV in your climate?

2. 

   rwsac |Aug 26, 2022 03:22pm |#10

   To further Peter's point, in a cold climate, if this device is the source of fresh air, and that air is very cold, is there a problem with condensation on the cold ductwork? I would think that even at 50% RH, if you have ducts with below freezing air in them running through the house, that might be an issue. It seems like an HRV would still be in order? I would think an ERV would not be necessary, as the dehumidifier already does a better job of removing unwanted moisture, and adding moisture to the air coming in to a tight house in winter seems to be less of a problem from what I have heard/read/experienced. Is this the recommended configuration? If so, does the HRV go before or after the dehumidifier? Does it matter?

1. 

   Jon_Harrod |Aug 27, 2022 10:26am |#11

   The duct bringing outdoor air into the dehumidifier absolutely needs to be insulated to prevent condensation. Downstream of the unit, the outdoor air has been mixed and tempered with indoor air. In a typical installation, it might be 60 cfm of outdoor air and 150 cfm of indoor air. If it's really cold outside, this ventilation will likely keep the indoor humidity well under 50%, even in a tight house. In my experience (CZ 5 & 6), condensation downstream of the unit isn't a problem. The latest IECC requires ERVs/HRVs in the coldest climate zones (7 & 8). In warmer zones, I think ventilating dehumidifiers are a pretty good "all-in-one" solution.

1. 

   rwsac |Sep 13, 2022 12:05pm |#12

   Thanks for the clarification, I didn't catch that there was an indoor air return on the unit that would pull in conditioned air to mix with the fresh. Makes sense now. I suppose that there would be a little bit of "extra" (when compared to an HRV) energy needed to condition that new fresh air, but it might be worth it to more efficiently remove the humidity.

3. 

   rgalbraith |Apr 28, 2022 11:16pm |#6

   How does the Santa Fe Ultra98 compare to the Minotair Pentacare V12? Are there other products in this category? Thanks.

4. Expert Member

   CARL SEVILLE |May 04, 2022 04:10am |#7

   One issue that is often overlooked with dehumidifiers is the heat the generate. Air exiting the unit can be significantly higher than room temperature requiring recoolpng to offset the heat gain.

1. 

   paulrey27 |Aug 24, 2022 03:09pm |#8

   That's a good point - I assume that's why Nikki's comments were to locate supply as near to the mini-split head as possible to aid in the mixing and re-tempering of air? It seems like one can take a lot of care to add these beneficial systems but it's exhaustive to keep up with all the thoughts. That is what I love about coming to GBA!