When to Use Standalone Dehumidifier
Hi,
I’m in the early design stage of what’s planned to be a certified passive house. The site is in the Catskills (Hurley, NY), on the border of climate zone 5A/6A (I think technically in 6A). The house will be approximately 2,000 SF, two floors, with no basement. Energy will be all-electric, with a roof solar array (plus the grid).
I’m planning to have mini splits and a Zehnder ERV. I think there will be a significant portion of the shoulder seasons when humidity is relatively high but the AC won’t be running much – though with a tight build, infiltration will hopefully be low.
I’ve read a lot of articles/posts here and watched the recent B&S show on the subject, but haven’t quite been able to grasp – how do I know if a separate dehumidifier (built-in like an Ultra Aire – not a fan of portable units!) is desirable? It seems like this would be pretty cut and dry (so to speak) in a place like Jacksonville, but less so somewhere like the Catskills. Would it make sense to build without separate dehumidification, and retrofit later if it seems necessary – or is there some quantitative or qualitative way to decide that ahead of time? My hunch here is a separate dehumidifier isn’t necessary (rely on ERV + mini split dehumidify mode?), but I’d love to hear from the experts here.
Thanks!
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Given your proximity to the great lakes, I'm going to say 99% chance you're going to need it. I'm on the other side of those great lakes, and without a dehumidifier it would be 65-80%RH for long stretches. I ran a 50pint dehumidifier pretty much 24 hours a day for a couple of months and couldn't always keep it below 60%. It's probably going to be the summer when you need it the most. The outdoor dewpoint drops in the shoulder seasons.
If I were you, I'd design in a spot (per floor level) where you've got a 2x6 wall and access to power and a drain. Then you can try without a dehumidifier, and if you need it there are a couple of brands of in-wall dehumidifiers that would be easy retrofits if you've made those provisions. They aren't cheap, but neither is any whole home dehumidifier setup. If you want to go economical, set aside floor space with drain and electrical access, then you can place portable units there. Much cheaper, but they do tend to be noisier.
If you look at the dew points here, you'll see that Jul-Aug are the real problem months, plus maybe half of Jun and Sep as well.
https://en.wikipedia.org/wiki/Catskill_Mountains
I've done a number of PHs in the Hudson Valley area as the HVAC engineer. So far, most haven't gotten dedicated dehumidifiers nor have I heard about high humidity problems. But I like Trevor's idea of being ready for a wall unit. It will depend a lot on the house and how well it handles summer passively (ie its cooling load and demand). With really good shading you might very well need it since the AC won't run much and we have those rather high dewpoints in summer. But some folks have...hmm, how to say this nicely...concerns...about doing good (exterior) shading, and those buildings end up using AC more, so they're getting their dehumidification along with that.
Great, thank you both - good points raised, and I'll keep the shading point in mind, and try to at least reserve some space for a dehumidifier.
Cramer, I just looked up your firm (saw you on the BS+B show last week as well) - I think there's a very good chance we'll be working together at some point soon, at least indirectly, based on the design firm I'm working with :)
Looking forward to it!
It can all be calculated, but the summary is that in humid climates with typical AC units and no dehumidifier, you will have periods with uncomfortable indoor humidity. Many people (including me) just live with it. So dehumidification is desirable; necessary is up to you.
A dehumidifier should only be run when you need dehumidification and don't need cooling. If you need cooling that is the preferred way of reducing humidity. The way to extract moisture out of the air is to run it over a cold surface, cooling the air and causing moisture in it to condense out. The difference between a dehumidifier and an air conditioner is that the dehu then rewarms the air to roughly the incoming temperature. If you need cooling it makes no sense to rewarm air after you just paid for the electricity to cool it down. Note that when I say "you don't need cooling" I'm including the case where you need some cooling, but not enough to remove as much humidity as you need.
So how much humidity is too much? Part of it is personal preference -- I prefer dry air. But if you have condensation happening anywhere in your house, that is too much humidity. It leads to mold and a lot of associated problems. A lot of times you'll get condensation that you can't readily see, but you can see the effects: mold and mildew. This is particularly common in places that are cooler, like basements. Also along any plumbing that uses cold water, and air conditioning equipment.
Let me add that traditionally HVAC equipment has all been temperature-controlled, but we're just starting to see equipment that targets both temperature and humidity. Within a few years I think that will become the norm, it gives much more satisfying climate control.
I know opinions are like.....you know, but I had two Mitsubishi "Diamond Premier" or something contractors come out to price a 6k BTU minisplit for a man-shed. I told them humidity was my main concern, and one said "These come with a humidity only option, it will be fine" and the other certified tech said "the dehumidify only mode is junior varsity air conditioning, it won't dehumidify enough".
I'm just a homeowner with an interest in building, but wanted to pass this along to you since you mentioned minisplits and humidity concerns.