Will a mini split dehumidify the air in the house?  If yes will it dehumidify in both heating and cooling mode?

yes, no, yes.

A minisplit is no different from any other air conditioner - it cools air by passing it over a cold surface. If the air is humid, the cold surface will chill the air enough so that it gives up water in the form of condensate on the cold surface, just like a glass of ice water.

It is not possible to condense water out of air while simultaneously heating it.

Other technologies exist to dehumidify air without cooling it, desiccants, to name one, but that is beyond the scope of the question.

Thanks Curt.

A mini-split may not dehumidify the air while in the heating mode, but the relative humidity in a home can go down when the air temperature is raised although there may not be any change in the amount of water vapor in the air.  Am I correct?

Yes and the relatively humidity is what matters for most purposes (mold, comfort, etc).

Raising the temp is not the difficulty, I can do that easily enough, though I don't really want to. The last 3 winters have been unusually humid ones - though the unusual is probably now the usual with climate change. I have a dehumidifier, but it only works so so, and it is noisy. Sunny dry days my house ventilation system works great. Wet humid days, which seem to be the majority of our winter days now, it is a battle to get and keep the humidity down.

I guess the only way I am going to get some warm dry air into the house is by burning wood - which keeps taking me back to my outside wood burning bread oven/smokehouse/grill/supplemental heat. Though maybe rather than figuring a way transfer the heat to water to hook into my radiant system, I should just pipe the dry air into the house. Sure would be less complicated.

-Rosalinda

Only the Daikin Quaternity series has settable dehumidistat control that dehumidifies in both heating & cooling mode. (They have proprietary coil & valving design that allows that to happen in heating mode!) The Quaternity series mini-splits let you set both a humidity upper bound and a temperature setpoint, and it controls both simultaneously.

All others dehumidify in a less-controlled fashion in the standard cooling mode (and yes, it's fine, even in tropical swamps), and many/most also have a "dehumidify" mode that runs at low blower speed optimizing dehumidification, ignoring the sensible cooling temperature setpoint while in that mode, but with no dehumidistat they just stay in that mode until you switch it into either heating or cooling mode.

But even on wet winter days, HRV/ERV ventilation rates will take the indoor humidity down. Even when it's soaked, FOGGY, and RAINING the outdoor dew point is no higher than the outdoor temp. At a very healthy and comfortable 70F/40% RH the dew point of your interior air is about 45F, so if it's 45F or cooler outdoors, you can bring the interior HR down to 40%, even if it's raining. When it's 50F outside and 70F indoors you can still get to ~50% RH via ventilation alone.


To know whether and when ventilation is going to do you some good, look at the outdoor dew point data from nearby weatherstations (wunderground.com, weatherspark.com and many other sources.) As long as the outdoor dew point in your neighborhood is 50F or below, boosting the ventilation rate can get you down to the "healthy" 30-50% RH range recommended by health professionals. (ASHRAE sez 65% for an upper bound, but that's above the dust-mite reproduction threshold, and it doesn't take much to get mold going when it's that humid indoors. Your building won't fall apart from rot at 65% RH, but humans are more susceptible to fungal infections an dust-mite/mold allergy symptoms at that humidity.)

But DO check out the Daikin Quaternity series- there's a price premium to be paid there, but they are VERY nice and very efficient units, near the top of the pack by most measures:

http://www.daikinac.com/residential/productsUnits20.asp?sec=products

A woodstove doesn't remove moisture from the house any more than a standard mini-split in heating mode does, unless it's forcing some infiltration-ventilation by drawing it's combustion air from indoors. Otherwise, simply raising the temp always lowers the RELATIVE humidity, independent of the heating source, but doesn't change the DEW POINT or absolute humidity- the moisture is still there.

Thanks Dana,
The Daikin looks great, but that is way more system than I need.

What burning wood (and it will be burned outside) lets me do is bring in warm DRY air, and then use my ventilation system to extract the humid air. I keep the house around 60 to 65 as that is the most comfortable temp for us. We also like it cooler at night.

Unfortunately, the outside humidity has been very high - for example tonight it is 92% according to Wunderground. Tomorrow it is predicted to be go from a low of 51% to a high of 71%. The humidity in the house just does not drop to my satisfaction when it is cloudy outside. I use the spot ventilation fans (bathroom fan, stove fan) when I am adding large amount of humidity to the house regardless of outside temps/humidity with make up air from outside, but it only works so so. If we get several cloudy/wet days (of which we have had way too many), I have to use my dehumidifier also. A source of dry warm air might just do the trick to get the inside humidity below the 50% mark, something I have only been able to do on dry sunny days. Summers are not a problem, it is the winter issue I have yet to get a handle on.

-Rosalinda

I wish Daikin or Mitsubishi would make an aesthetically appealing wall unit. LG is the only mini-split manufacturer that actually makes nice looking wall units. The others with their white plastic looking wall units leave much to be desired in the aesthetic department.



LG ART COOL



DAIKIN

Posted By Dana1 on 20 Feb 2013 04:24 PM
Only the Daikin Quaternity series has settable dehumidistat control that dehumidifies in both heating & cooling mode. (They have proprietary coil & valving design that allows that to happen in heating mode!) The Quaternity series mini-splits let you set both a humidity upper bound and a temperature setpoint, and it controls both simultaneously.

Between Daikin or Mitsubishi, which do you think are more reliable?

If you own stock or are a dealer of either, please give full disclosure.

Do you think LG mini-splits are reliable?

I have not seen any reliability testing of Daikin, Mitsubishi, LG or Fujitsu.  From what I am told, Daikin has a very low failure rate.  Dealers that I talked to gave high prices for Daikin and lowest prices for LG.  One distributor I know suggested that we use Fujitsu instead of Mitsubishi since Fujitsu makes a lot of parts for other companies and thus sells at a lower price.

I wonder what other people have heard or know about mini-splits because in some multi-storey homes it is getting more difficult to find space for large ductwork.  In some cases, we make all of the walls taller to accommodate ductwork but since I am into alternative construction, the extra heights can add to the overall cost of the house.

Posted By Rosalinda on 21 Feb 2013 02:35 AM
Thanks Dana,
The Daikin looks great, but that is way more system than I need.

What burning wood (and it will be burned outside) lets me do is bring in warm DRY air, and then use my ventilation system to extract the humid air. I keep the house around 60 to 65 as that is the most comfortable temp for us. We also like it cooler at night.

Unfortunately, the outside humidity has been very high - for example tonight it is 92% according to Wunderground. Tomorrow it is predicted to be go from a low of 51% to a high of 71%. The humidity in the house just does not drop to my satisfaction when it is cloudy outside. I use the spot ventilation fans (bathroom fan, stove fan) when I am adding large amount of humidity to the house regardless of outside temps/humidity with make up air from outside, but it only works so so. If we get several cloudy/wet days (of which we have had way too many), I have to use my dehumidifier also. A source of dry warm air might just do the trick to get the inside humidity below the 50% mark, something I have only been able to do on dry sunny days. Summers are not a problem, it is the winter issue I have yet to get a handle on.

-Rosalinda

They make a 3/4 ton Quaternity (internet price about 3 grand or sometimes less) - it's just a single-head minisplit with a clever split-coil and valving system on the interior head. What are your manual-J heating & cooling loads? (And outside design temps?)


You seem to be missing some fundamentals about relative vs. absolute humidity levels and what they really mean. Air with 92% relative humidity at 40F has a dew-point of 37F, which translates to 43% RH when that air is warmed to 60F, or 37% RH @ 65F.  Stop looking at the outdoor relative humidity- it's a meaningless number, since it's RELATIVE to the outdoor temp, not your indoor temp. The outdoor dew point is one measure of it's absolute humidity (or you could use wet-bulb temp, which is easier to measure directly, but not typically reported by web-weather sites). If the dew point is 45F or less you can dehumidify to under 50%RH @ 65F by increasing ventilation rates.

And when it's under 45F outside, the dew point can be no higher than 45F.  When the dew point rised to the outdoor temperature, the outdoor relative humidity is 100%, and it's a pea-soup fog with rain condition.

So it's always outdoor dew point, dew point, dew point, forget the outdoor temperature and relative humidity reading (though you could work backward from there with a psychrometric chart or a psychrometric calculator.)  Wunderground and Weatherspark report & record dew point as well as RH & temp, so use it!

If the reported dew point is under 45F, increasing ventilation rates can bring your indoor RH into the healthy range @ 60-65F indoor temps.

That's all you need to pay attention to!

Hopefully this is a bit clearer?

Posted By Alton on 21 Feb 2013 07:20 AM
I have not seen any reliability testing of Daikin, Mitsubishi, LG or Fujitsu.  From what I am told, Daikin has a very low failure rate.  Dealers that I talked to gave high prices for Daikin and lowest prices for LG.  One distributor I know suggested that we use Fujitsu instead of Mitsubishi since Fujitsu makes a lot of parts for other companies and thus sells at a lower price.

I wonder what other people have heard or know about mini-splits because in some multi-storey homes it is getting more difficult to find space for large ductwork.  In some cases, we make all of the walls taller to accommodate ductwork but since I am into alternative construction, the extra heights can add to the overall cost of the house.

That is highly local-market specific.  When setting up my mother's place in western WA quotes for 1-ton Fujitsus came in $500-1000 higher than for 1-1.5 ton Mitsubishi units of comparable HSPF and SEER. (We went with a 1.5 ton FE18 Mitsu in the end, and it's worked out pretty much as expected.)  The local Daikin installer didn't even show up or offer a quote- too busy, I guess, or maybe they were uncomfortable with her slummy neighborhood.

In MA the quotes for equivalent Fujistu & Mitsubishi are a bit more competitive, but the Daikin Quaternity usually comes in a several hundred higher per ton, but it's a more feature-rich unit too.  Were I in the market for a minisplit for my own home I'd be inclined toward the Quaternity precisely for it's ablity to control to a dehumidification setpoint, since the bulk of the cooling load at my house is latent load, and the existing central air gets used maybe 10 days in a typical summer, while the dehumidifier in the basement works overtime, burning on the order of 350-600kwh between May and September, raising the temp in the basement.  Clearly YMMV.

Thanks Dana, as always you provide an education along with solutions - much appreciated.

My current heating system is providing all the heating I need and since it, and the house, are new, I don't think I will need to replace it in the foreseeable future. My Manual J total heat loss is 21,702. I don't need summer cooling as our nights cool down to the 60s, so there are only 7 days or less when we could use it. I thought with a real small (tiny?) unit, I could get a delta of around 5 degrees in the winter and 10 degrees in the summer along with quiet dehumidification. I will take a look at those dew points, and see what effect using my ventilation system has on those cloudy days.

-Rosalinda

Posted By Lbear on 21 Feb 2013 03:06 AM

Between Daikin or Mitsubishi, which do you think are more reliable?

When I bought my Daikin in 2008 the distributor said there had been only one failed unit in the US up to that point. It was returned to Daikin's US plant for inspection where they discovered the installer had poked the refrigerant line through drywall w/o sealing the end. The compressor failed due to drywall residue in it!

There is a fairly sophisticated control board in the Daikin. After the drywall crew put a screw through the refrigerant line embedded in the foam in the garage wall (we had not put a nail guard plate over the line) the unit refused to make any attempt at starting. It showed a low pressure error code on the controller (thermostat) in the garage.

Today's dew point was 29 Degrees F, outside temps in the low 30s, cloudy with a few snow showers, inside temp 61, indoor humidity 65%. I ran the ventilation system for 5 hours, and there was about a 1% decrease in humidity. No activity added high amounts of humidity (such as cooking or showers)

On a sunny day, using the same system and run time, the humidity will usually drop 10 to 15%

I will keep trying and monitoring results.

-Rosalinda

Do you like it to be 61F inside?

What kind of a ventilation system is it again?

That's an awful lot of water inside for below freezing outside.

ICF - yes we like the house around 60 degrees and cooler at night.

I have a 100 cfm bathroom fan, and a 3 speed range hood fan with a high speed of 600 cfm on the main floor, and a Panasonic FV-13VKS2, 1 to 150 cfm ventilation fan in the ground floor utility room.

Summer time is not a problem. It is winter humidity that has been a challenge. I use the bathroom fan and range hood fan as needed. I use an open window on the south side of the house on the main floor and the ventilation fan running in the utility room on the ground floor. This works well on sunny days. On cloudy days the humidity just does not seem to drop like it should. That is when I run my dehumidifier. The biggest problem with the dehumidifier is that it is very noisy and only gets the humidity down to around 60 to 65%. Since the outside air does not seem to be dry enough, I was looking for a way to get dry make up air into the house, preferably warm dry air. That is why I started this thread....

-Rosalinda

30F air coming into the house when you run that 600 cfm fan should dry things right up. You said you ventilated for five hours. That's as much as 5 or 10 complete air changes, isn't it? How many humans are in there? Big pets? Lots of house plants? Is it possible that it is condensing on some surface inside that you can't see?

60F during winter? Wow, that seems cold for interior temperature settings. Average winter setting for a residential home is 68F - 72F.

Getting out of a shower at 60F doesn't seem like a fun experience. Once you get older, 60F interior temps will not be well tolerated.