Dehumidification and Minisplits
I’m in Austin TX. We have significant humidity, including in “shoulder” seasons where cooling is not really needed but dehumidification is. Probably the worst times are when the day is hot and humid, but the night is cold, condensation on windows is a major problem all over the city. No one likes running the AC to super chill just to dehumidify.
So the smart recommendation is a dedicated dehumidifier, like the Ultra-Aire series. Ok, costs a bit more than AC only but everyone says they are great.
My question is about combining dehumidification with getting ducting out of the attic. Seems a contradiction between using mini-split systems (ductless or very short run ducted), since now one doesn’t have a ducting system into which to place the dehumidifier.
Yes, best solution would be to encapsulate the attic. Then one could pick and choose, perhaps running a small ducting system just for the dehumidifier. In our place, though, many years of additions has left a very cut up attic, mitigating against encapsulation from below (spray foam). A chainsaw renovation might work, but that would require adding siding on the whole house (currently stucco on block), as well as the whole re-roof, estimates for that range up above $100k (Austin, man, stop moving here, people :).
So, boiling this down. Are there any knowledge or resources about:
– required ducting extent and design for dehumidification only (not heating/cooling air distribution). Does diffusion of moisture work differently than diffusion of temperature sufficiently to mean that single point (or two points) of dehumidification could be sufficient?
– Would there ever be a reason to choose a central ducted AC/Heating system, with the ducts in a hot attic, just to include dedicated central dehumidification?
– I’m also interested in knowing which ducted mini-splits have great latent capacity? Can “dry-mode” remove sufficient moisture without super chilling a room?
I keep thinking: mini-splits come from Asia (ok, so reached their zenith in Asia), how do people in Asia manage dedicated dehumidification? Then I wonder if they have areas with humid “shoulder seasons” needing high latent capacity but low sensible capacity. Maybe not in Singapore, but surely in Japan, Korea, and China? Quick googling shows some discussion in HK (most english, most north) and has people using portable units in small apartments, not without frustration.
Maybe something more exotic, like desiccant wheels using the heat in the hot attic to recharge?
https://www.novelaire.com/desiccant-wheels-31505.html
https://www.novelaire.com/dehumidifiers/commercial/recirculating.html
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Replies
James,
Properly sized ductless minisplits should do a good job of dehumidification, even in Austin. Homes that have high summertime humidity are often leaky, so sealing air leaks in your home's thermal envelope will often help keep the indoor air dry.
Another possible cause of high indoor relative humidity (RH) is overventilation. Make sure that your mechanical ventilation system isn't ventilating at too high a rate.
If air sealing work and adjustments to your ventilation system aren't enough to solve your indoor RH problem, a simple $250 stand-alone dehumidifier may be all you need.
For more information, see All About Dehumidifiers.
Thanks Martin, appreciate the link to that thread.
We have a standalone dehu. I use it. It's noisy and has to be emptied. It doesn't shift the humidity in the house. I roll that bastard around (forgive my Australian language), it doesn't really seem to help much. It's hard to know whether this is because of 1) a leaky envelope, 2) poor source control, or 3) lack of air distribution. I think Tim O'Brien (from Ultra Aire, but giving a solid assessment of the study you cite) made some good points. When I overcool the house humidity removal is much better,.
So I'm return to my question: do we have any research and/or modeling about the extent of mixing/air distribution needed to dehumidify? It seems that we don't. Seems that something analogous to Manual J + Manual D, which focuses on heat load and BTUs delivered through air, could be useful :) Currently even reputable AC contractors don't seem to have the tools to discuss latent loads systematically.
In a tight house with a "normal" or "typical" sensible cooling load DRY mode will be more than adequate for managing the latent loads without needing to overcool in a location like Austin. If the house leaks air like a tennis racket or is over-ventilated, not so much.
If the house has super low on the sensible loads even when it's hot out it might still need additional dehumidification during the low load shoulder seasons, but unless the house was specifically designed for ultra low loads it's unlikely that it couldn't be managed with DRY mode.
The Daikin Quaternity mini-splits have a clever proprietary coil & valving in the ductless head that gives them the ability to supply essentially latent-only cooling, when needed, with independently programmable relative humidity and temperature settings, and would be a pretty good choice in locations where sub-zero F temperatures are rare:
http://www.daikinac.com/content/residential/single-zone/quaternity/control-humidity/
Dana - very interesting info. Do the Mitsubishi DUCTED minis offer a dehumidification option?
We have the full size ducted Mitsubishi (SVZ). We're using the Mitsu thermostat interface and Honeywell thermostats, and they support humidification via low speed fan.
Also, we heard from a friend that works at Mitsubishi, and for the SVZ indoor units paired with the MXZ outdoor units, they're suggesting a new setting that reduces evaporator temp to improve dehumidifcation. We made that change 3 weeks ago.
Our contractor didn't set up the low speed fan, but I just had to add one wire between the thermostat and the tstat interface to enable it. We were able to knock about 9-10% RH off with the combination of the 2 changes. We regularly see 40-42%RH now..
Note that there is a difference between the "dry" mode that some systems have, where the unit still cools but the balance between latent and sensible is tipped more to latent by adjusting airflow and coil temperature, and true dehumidification. The Daikin Quaternity is the only unit I know that does true dehumidification, where it has two coils and one is acting as the evaporator and the other as the condenser so there is no net cooling. It can also provide straight cooling or heating if needed.
> diffusion of moisture
You need to ensure that for any room, moisture out >= moisture in. The latter depends on air sealing, delta-P (from wind or otherwise), outdoor dew point, other moisture sources, etc. The former is pretty much dew point and air flow rate. A slight positive pressure on the entire building provides control over where infiltration occurs (next to your dehumidifier is good, in a closed door bedroom is bad).
In general, yes - dehumidification needs far less CFM than heat/cool and an open interior door or a small supply duct (see DOAS) provides plenty of distribution.
I have had a couple of dehumidifiers that sounded like they were running, but weren't doing much. One was dead, the other needed a cleaning.
Thanks all. The Quaternity units seem like a good approach. Perhaps we could replace one of our planned ducted mini-splits with that as a wall unit. I always like the idea of redundancy anyway.
Does the admonition to get ducts out of unconditioned attics apply to dedicated dehu or DOAS ducting? I'm thinking yes, but less so than heating/cooling ducting?
Reasons to think it applies less:
- smaller ducts and less need for reaching every room, so small duct exposure overall.
- supply side is at much lower delta T in the attic (dehu supply is slightly above internal temps, dehu runs when less hot, unless ducts above *lots* of insulation)
Reasons to think the admonition still applies:
- duct leakage in the attic would still depressurize the house
- both supply and return ducting would be exposed to high attic temps, so you'd still have lots of heat gain/loss through the ducts (although less than heating/cooling ducting).
I'm thinking the right strategy is to tighten where we can, get the mini-splits in place, run them for a summer and two shoulder seasons, together with exhaust-only ventilation. Then monitor comfort and humidity, and add a DOAS aka ventilating dehu with minimal ducting.
Or replace one of the ducted mini-splits with the Quaternity unit, if I can get buy-in on that :)
I have to DISAGREE with ALL of you experts.
I built a PERSIST designed house, on west side of Houston Texas, 1280 square feet, in a FLOODWAY 6 feet off the ground where I was told by our County Engineer that I couldn't build (a middle finger to them) all the staggered joints, 2 layers of wrap, 1 house wrap, 1 commercial wrap, 4" of insulation on walls, 6"on roof, 2" on floor still working on it, might be finished in another 10 years. Used lots of recycled/reclaimed materials, Building this way creates many issues, the house is way to tight
and now needs controlled holes cut in it for fresh air, issues with attaching the porches to
the main structure, thousands of screws from 3' all the way up to 12" & the list goes on.
I had a Mitsubishi Diamond Star AC company quote me for a 3 ton system ($9700.00)
I don't think he had or has a clue as to this type of house
I self installed (NO WARRANTY) a 2 ton Mitsubishi, (cost $3000.00) and this morning it was 64 degrees, 68% RH that is to damn cold for me, the system DOES NOT REMOVE HUMIDITY EFFICIENTLY, yes it does remove SOME but. Building with this design you have to be VERY careful about your humidity on the interior & 60 to 68 % isn't going to work for the house or me. No blower door test, no blueprints, no professionals involved in the building of this house other than our Engineer who specked out certain areas, ie 16" piers, so overkill i went by belling out the bottoms to 36", upgraded steel piers supports to fiberglass to contend with rust issues, 2 piece ridge beams (5-1/2" x 15"), Douglas fir rafters (4" x 12"), beetle kill pine from Colorado for exposed ceiling.
So from what I am reading on the internet & experiencing first hand you should plan on having a Dehumidifier in your budget plans, of which I have just purchased, and will most likely upgrade to a larger capacity unit when the heat gets here.
Gotta go, working on a house
I'm with you. Proper air circulation or removal is a much better way to remove humidity. Put a dehumidifier and and a fan in front of a puddle and see which puddle disappears first.
Put a fan exhausting in one window and slightly open another window on the other end of the house, the amount of air the fan removes will have to come in through the slightly opened window. If that slightly opened window area is smaller then the fan's area then it's velocity will have to increase in order to equal the fan's volume flow rate and therefore it's temperature will drop, therefore cooling the interior and the increased air velocity will remove the humidity. Just like blowing on your arm. Keep it simple, let nature do the job, learn some fluid dynamics and thermodynamics.
My reply #19 below was intended to be in response to this posting.
Did you run a load calc?
Without running any numbers, my rough guess is your system may be twice the size it should be, which is why you are having RH issues. Homes of your description should be 1000sf-1500sf per ton of cooling.
High levels of insulation where most the load comes from air leakage are a bit more challenging. Have a blower door done.
Get a floor dehumidifier for $200 and you should be good to go.
*Edited to reflect that you don't have blower door data. I don't think blower door numbers are actually all that important, but a fan, ir camera, and smoke are damn good at finding holes. You can DIY blower directed air sealing.
Well Matt.
I have a dehumidifier on the way.
The HVAC guy (Diamond company = supposed to know what the hell he's doing) No load calculation done, said 3 ton.
I had a 8000BTU window unit in a window for a couple of weeks, it kept it cool, but was still humid, only holes are the 2 sliding doors, the windows, and any penetrations i cut in for the wood stove, kitchen exhaust vent, and bathroom venting, & hole for mini-split lines.
zip siding not installed properly (used as structural bracing) house wrap, 2 layers of 2" insulation, commercial wrap, 1 x 4 furring strips, old corrugated metal siding. grace ice&water
wrapped over top first layer of home wrap, 6" of insulation on roof, all joints staggered.
Personally now that it is water tight this is to damn tight.
I am seeing lots of information out there about this same problem, one guy has a 50 years old brick house that has so much wetness that floors are buckling.
Guess i will put a screen in a window and keep the window open to solve my fresh air intake problem & let in heat so the mini split will do it's job.
Opening the window won't fix your humidity problem, it will just make it worse. Warm outside air holds lots of humidity and raises the RH in the house as the warm outside air cools. You really just need a dehumidifier.
There are lots of holes that let air in. You are probably pretty good there, but know they are holes that let air that you haven't thought of.
Keep reading here and you will find the experts know what they are talking about. The details are really important though and not always intuitive so it takes a lot of reading.
This is another good site: https://www.energyvanguard.com/blog
Re-activating this link . . . I have a similar problem to James's: humidity control in a tight house (0.55 ACH@50 PA) in a warm climate (Virginia). I am holding the humidity in the range of 57% to 60% with a portable dehumidifier in my mechanical room, but there are problems:
1. The 70 pint dehumidifier is noisy. NOISY!
2. It runs almost continuously, and makes about 40 pints of water per day (running at 74 deg F and 50% RH in the mechanical room). I suspect the unit may die after a few seasons of this.
3. I have too little spare capacity. Right now there are only 2 people in the house, which is 3 BR and 3000 ft2. If I had a full house, I probably could not keep the RH below 60%
4. I would really like to run my RH at 50%, if I can.
So I am considering a Daikin Quaternity, as James was, or a ducted Santa Fe dehumidifier.
Two questions:
1) Has anyone tried the Daikin Quaternity used primarily for dehumidification? If so, does it keep up - keep you dry without overcooling - in the shoulder season?
2) Does anyone have experience with an Ultra-aire / Santa Fe dehumidifier inside the building envelope? Ultra-Aire does not list a noise rating in sones, so I would like to get some idea of how noisy a dedicated, ducted dehumidifier would be.
If you have a humidity problem then you probably want a dehumidifier father than another mini split. assuming that this is unavoidable moisture rather than the result of something goofy.
I think Martin mentioned over ventilation in the original post , there’s obviously a huge rabbit hole there and I’d hate to speculate about where you’re moisture problem is coming from, but it might be worth getting a quality building performance contractor at your House to dig Into the issue.
An email to ultra aire/santa fe can help you select the right product to fit your noise concerns. They do have a reputation for being quiet. And I know they do have at least one ductless option, if that’s a problem.
In houses less affected by ambient conditions, traditional air conditioning approaches are going to be less effective. That’s because traditional ac assumes a relatively steady relationship between sensible and latent heat, that doesn’t exist in a heavily insulated and airtight house. Splitting up sensible and latent cooling makes the most sense.
That being said; call the tech support line for your current mini split, tell them your issue and ask if there are any settings you can change to increase the sensible heat ratio of your mini split.
Out of the box, most mini splits optimize energy efficiency by running a warmer indoor coil as they get closer to cooling Set points which reduces the rate of latent heat removal. Some units have a feature where this can be switch to a slightly different logic.
That energy efficiency prioritization is why they have a reputation for being bad At dehumidification, but it’s not really a fair critique 100% of the time. Maybe the best option is to add a ddh but I’d rather fix my problems without having to pay for another piece of equipment, if it’s possible.
Mark,
I had an Ultra Aire 98H in my last house. It did a great job of managing humidify while providing outdoor air. (And you need ventilation in a tight house.) The unit was quite noisy, but I was able to locate it in a room that was fairly easy to isolate from the main living area. In your case, you might need to add some insulation and air sealing to your mechanical room to mitigate sound transmission.
I have a Daikin ducted minisplit in my current home. In dry mode it reduces indoor humidity pretty quickly even though this place is much leakier than my last place. That said, dry mode seems to work best during the shoulder seasons.
I'm curious, with the Ultra Aire 98H normally moving about 300 CFM, how does one use it as part of a ventilation strategy that needs far less (perhaps 60 CFM)?
I had a damper on the air intake that restricted the CFMs (note 80 CFM marking on duct).
Seems to me that operating it so far from the listed CFM would have a significant negative effect on performance and efficiency. For example, higher delta-T and freezing up or turning off to avoid freezing up.
There is some energy wasted by the oversized fans. Their power draw will only drop a tiny bit when restricted. The core will be more efficient due to a larger effective surface area to cfm.
I'm also interested if anyone has first-hand experience with the Daikin Quaternity units. I find it's very common to need dehumidification without cooling. My experience with stand-alone dehumidifiers is that it's like asking a dorm fridge to do the job of a walk-in fridge.
Ventilation doesn't change the dew point of the air, no amount of ventilation will lower the dew point of the interior air below that of the incoming air. If the exterior air dew point is higher than will be comfortable at the desired interior temperature you will need dehumidification. Period. Full stop.
This is particularly a problem in climates that have cold winters and humid summers, like the northeast of the United States. The below-ground temperature remains close to the year-round average temperature. In the summer, the temperature in basements is often below the dew point of the outside air. When that happens, outside air will cause condensation when it hits the cool wall. Ventilation does not improve the situation, it makes it worse. And the more you ventilate the worse the situation is.
In order to avoid unhealthy and unpleasant conditions, the solution is to seal off outside air as much as possible, and to dehumidify. As others have noted, HVAC systems commonly assume a fixed ration of sensible to latent cooling, and are ill-suited to situations where no sensible cooling is required. In the northeast, there are times of the year where optimal comfort requires both dehumidification and heating. For the most part the dehumidification options on the market right now are unsatisfactory. I'm excited about the Daikin units but I'd like to hear some real-world feedback.
> seal off outside air as much as possible
Note that a high indoor dew point can be the result of interior moisture sources. So some amount of ventilation can be helpful.
> HVAC systems commonly assume a fixed ration of sensible to latent cooling
I'd go further - most have a SHR that isn't fixed and moves in the wrong direction.
Would be interesting to see a cost analysis of Daikin Quaternity vs lower cost heat pump + more use of a dehumidifier.
....."no amount of ventilation will lower the dew point of the interior air below that of the incoming air." Do you even understand what the dew point is and how it is achieved? I would love to hear your explanation...
I'd love to hear your explanation of how dew point is "achieved." But I digress.
The dew point is a measure of the moisture content of air. In contrast to the other popular measure, relative humidity, which, as its name suggests is relative to temperature, dew point is an absolute measure. Air with the same dew point will always have the same moisture content. The only way to change the dew point of air is to add or remove moisture.
The dew point is expressed as a temperature, the temperature at which the air would have 100% relative humidity.
You digress...."The dew point is a measure of the moisture content of air" no that's humidity...... with all the info available on the internet i would have thought you would get it right...Try again....maybe if you know the other name for dew point it may help you ...wet bulb......or maybe look up a sling thermometer and see how that works.....you may have heard the term morning dew.....when the temperature starts to rise at which point condensation can occur.....
Honest question, how often do you get punched in the face? I imagine it's pretty regularly. Or do you act differently in person than online?
Trevor, how often you butt into other people's conversations with your arrogant opinions? If you ever met me face to face you would not consider such actions because if you did, you would be in a whole lot of hurt....Be nice.
Sorry if you feel educating people, guiding them or having them learn for themselves bothers you. Those who understand my replies usually get a laugh out of them or understand the simplicity......I like people to think for themselves......with some good common sense......you know......give a man a fish or teach him to fish.....a lot of bad fish being given away.....
No point in trying to argue with someone like you. I invite anyone to read the previous replies and draw their own conclusions.
At the risk of feeding the troll, please people, pay no attention to anything Tom says. His advice is bad, dangerously bad.
Thanks Trevor. I could put up with someone who was a bit snippy if they were a knowledgeable person who just didn't suffer fools gladly. Tom seems to be a guy who didn't absorb middle school science.
Realize DC, you attacked my comment first.Now you are speculating about me without any knowledge of me or my education or experience. FYI, second generation construction, over 40 years in all the trades, mechanical engineer, licensed in plumbing, heating and gas and operate a solar energy company... worked on thousands of homes and commercial businesses.....what you got?.....grow up!
James Howison, I live in Austin, TX also. user-6976637, I also lived for 4 years in Houston. The climate in Austin and Houston are significantly different. The humidity in Houston is much higher, so Houston experience may not translate directly to Austin.
The first thing I would do is actually measure the humidity. I get an indoor humidity around 45-55%. You mentioned Australia. A lot of people move to Austin from other climates, so it is possible you are used to a different level of indoor humidity. What is your target temperature and humidity?
I don't know anybody in Austin with a dehumidifier. Maybe in a basement (which are very uncommon), but definitely not in the living quarters.
If water is condensing on the inside of windows during the summer, then something is definitely wrong. The AC unit should be sized so it runs almost all the time to keep your house at your desired temperature. If the unit is too big, it won't run long enough. Then it won't remove water from the air. This means when it was 106 last weekend your AC should have been running A LOT and there definitely should not have been a humidity problem in your house. Now yesterday it was unseasonably cool and rainy, so the humidity might have been a problem yesterday.
Another problem could be if you have the "fan" mode turned on. If the fan is on constantly, then the fan will blow across the wet AC coils, which raises the indoor humidity.
Many older houses in Austin (like mine) are on a pier-and-beam foundation. If that is the case, crawl under the house and see if you have water under there. It should be dry.
In drier climates people are able to open their windows at night to cool down the house. I find that does not work well here because often if the temperature outside is cool enough to lower the temperature, the humidity is too high. So keep your windows shut in our current season.
Finally, I don't have a minisplit, just a traditional central AC unit. It's possible something is wrong with your unit.
Thanks Mark! We ended up getting the Ultra-Aire 98H and we love it. It's in the attic, and very quiet anyway. It manages humidity perfectly and does ventilation (and MERV 13 filtration, apparently useful for COVID). It's particularly great on wet but cooler days. I've had a Kill-a-watt hooked up to it and it has used about $100 over 8 months ... I've used the wirelesstag.net sensors to monitor humidity around the house and everywhere is consistently 40-55% rel humidity. And the house is not particularly tight. We never have to over cool, and we find that our comfort ranges are *much* closer together when humidity is managed.
In a similar, though less nuanced situation, Martin Holladay responded by saying: “If you want dehumidification (but you have no need for cooling) during the summer months, the most efficient way to do that is with a dehumidifier. With a ductless minisplit, any dehumidification that occurs is a side effect of the cooling process. A dehumidifier will be more efficient.”
For pertinent information, read this article:
All About Dehumidifiers. It includes a case study in Houston, TX, conducted by Joseph Lstiburek, Armin Rudd, and Kohta Ueno of Building Science Corporation.
Kiley,
I am glad you bought up this topic. I read that article and designed my house with that solution in mind. I now have a portable dehumidifier (with condensate running to a floor drain).
But I am dissatisfied. With all due respect to Messrs. Rudd, et al, if you try living with a portable dehumidifier inside the building envelope, running 24/7, you will find it NOISY. And it is an energy hog: I estimate that I am spending $1.10 per day on power for the dehum, plus $0.80 per day on the heat pump power that is required to shift from inside to outside the heat generated by the dehum plus the latent heat of the dehumidifier condensate. Running this dehum has doubled my total electricity consumption.
And, finally, I have only two people in a house designed for six. If I had a full house, I would need to run 3 dehums, I guesstimate, because I would need to increase my supply air ventilation in proportion to the bodies ( I run with CO2 levels in the range of 600 - 900 ppm, and would like to keep it in that range).
So I agree with DC Contrarian: "My experience with stand-alone dehumidifiers is that it's like asking a dorm fridge to do the job of a walk-in fridge."
It all depends on climate. When I lived in New Hampshire dehumidification was not a headache, but here in Virginia it definitely is.
Thanks for the kind words. I think that deliberate humidity control is the next frontier.
I was born in the 1960's, and what is considered a premium HVAC system has changed a lot in my lifetime. I can remember radiators with balancing valves, then thermostatic balancing valves. For a while as many zones as possible was the fad. I can remember humidifiers attached to furnaces.
What we finally seem to have figured out is that if you seal the house up good then it's easier to keep comfortable. But the idea of comfort is based on a single number, temperature, when humidity can have a big impact as well. I personally like to keep the humidity down, I find it means I can be comfortable with the thermostat at 80F in the summer, but it also means sometimes I run the heat and the dehumidifier at the same time in the shoulder seasons. It's not complicated to have a minisplit have the capability to operate as a dehumidifier, but so far only Daikin seems interested in it.
I fully expect that in the future it will be the norm to have a thermostat and a humidistat to control the HVAC.
This may not be as relevant to the super humid areas, but my solution to the noisy dehumidifier is to run it in the basement, in the middle of the night. This has several advantages:
It creates heat when the outside temperature is coolest, making my heatpump more efficient at removing the excess heat.
It creates noise when no one is the basement to care.
It uses electricity when demand is lowest, and electricity prices are cheapest.
I have a timer, and the unit runs from 2:00 AM to 6:00 AM, set to 'continuous'. This is enough in my area to keep the house at around 50% humidity, which is quite acceptable for myself. The basement is well connected enough that reducing humidity here, does reduce it in the whole house quite adequately.
Would be interesting to see more data about how well dehumidification can be stored/time-shifted/released.
How many of you have house plants that can absorb or emit humidity...after all it is how the planet works on a grand scale.
I wonder if I could data log the humidity in my house and then do ABA testing over a long period of time to collect this data during the shoulder season we the HVAC turned off...
@ James Howison You said you ended up installing a UltraAire. What model and what is the area sq footage you are dehumidifying? What setup did you go with for your ducting on the UltraAire? I have mini splits and looking to put a dehumidifier in the attic with a a dedicated return grille located in the ceiling in the middle of the house along with a fresh air intake to the unit as well. I like the Santa Fe Advance90 which will give me two supply outlets with one going to far side bedroom of the house and the other exhausting to the attic. I have bight humidity in the attic due to invented spray foam attic and peel and stick (vapor impermeable) in place of felt under my roof shingles.