Double coil air-handler / dehumidifier?
I’ve been giving a lot of thought to Joe Lstiburek’s most recent post over at Building Science.com (http://www.buildingscience.com/documents/insights/bsi-028-energy-flow-across-enclosures) and I’m especially struck by the illustration #6 showing a hotel room with a freestanding de-humidifier sitting next to a PTAC through-wall heat pump. Here in NC we’ve been building well-insulated buildings with good shading on the windows and low SHGC glass and we’re coping with the resultant imbalance between continuing need for de-humidification to remove latent heat with decreasing need for sensible cooling. This is compounded by our home-owners who like to brag about how little they use their AC and how low their energy bills are.
We’ve responded by switching to zoned-bypass heat pumps that mix the supply air with the return air so the air is much cooler going into the coil and more moisture can be extracted from it and we combine that with a smart thermostat that responds to the interior humidity of the house by surreptitiously dropping the thermostat setting five degrees to cause the HVAC equipment to cycle to de-humidify the air. I also incorporate some basic building science into my home-owner’s manual and walk-through in hopes of education my clients about the value of leaving the HVAC on but setting the thermostat to a higher temperature than normal in the summer rather than turning it off altogether (which allows the system to surreptitiously lower the t-stat on humid days).
I did a low-cost house a while back where we circulated well water through the floor loops for cooling controlled by a motorized valve hooked up to a thermostat and draining into a drip irrigation system. It worked, up to a point, (long story but this is one of the times I was glad to be a builder rather than a consultant) but it did nothing for humidity, so we got a couple of de-humidifiers, but the way they work is by using a cooling coil to condense the moisture from the air and then blowing that air over a heating coil and sending hot dry air back into the room. Which was not what the customer wanted. Eventually we discovered the Windchaser portable de-humidifier that circulates that hot dry air over a drain pan and sends the resultant hot humid air out of the house through a four inch dryer vent while circulating cool dry air back into the house from the cooling coil (and the client married me, continuing that long story).
So, staring at that picture of the de-humidifier sitting next to the through-wall heat pump made me think that a great solution for high performance homes in our climate would be a heat pump that had both a cooling coil AND a heating coil in the air handler that would be smart enough to divert enough of the heat from the outside coil to the re-heat coil so the cooling coil would extract the moisture from the air and the re-heat coil would send some of that heat back into the building to keep it from getting over-cooled and the rest would go to the outdoor coil to keep the house from getting over-heated.
I’m assuming that this is already incorporated into a mini-split system made in Asia somewhere. Any tips on where I could find such a thing? Is such a double-coil system available off the shelf in America? Does anyone here have experience with “side-arm de-humidifiers”? My HVAC contractor, who I really respect, considers those to be big trouble for service and I’m not sure why.
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Replies
Michael said: "I’m assuming that this is already incorporated into a mini-split system made in Asia somewhere. Any tips on where I could find such a thing?"
Yes. Here's one from Daikin, the Quaternity minisplit: http://www.daikinac.com/residential/productsUnits20.asp?sec=products&page=55
Michael and Others,
We have been study grouping Joe's document over at JLC
http://forums.jlconline.com/forums/showthread.php?t=50548
It is better if you log in .. in order to see and share attachments.
Michael,
Daikin may be a good match for high end homes .... it is pricey.
Daikin has a lot of good features...variable refrigerant,multi zones with one outside unit,
Very compact air handlers...multiple thermostats...very quiet..pumping condensate,
the outside unit is not much bigger than a suitcase.
Michael,
here is a multi-zone system...
http://www.daikinac.com/residential/productsUnits3.asp?sec=products&page=55
I still wonder IF even with a system like Daikin...
Might be times when a stand alone dehumidifer is the best tool?
John - the multi-zone systems do not have a dedicated dehumidification feature, whereas the Daikin Quaternity does. Therefore, when there is high indoor relative humidity, but no sensible load, the unit will not kick on. This happens a lot in the swing seasons in NC and VA and lots of other places.
The Daikin Quaternity can apparently operate in dehumidification-only mode. Of course, a separate, dedicated dehumidifier is also an option.
Some Passive House folks are also working on small PEX ground loops that flow through a heat exchanger in-line with the incoming air stream in order to pre-cool, dehumidify and/or pre-heat the incoming air.
Here are some links to the PEX ground loop (mini-geothermal) air tempering systems John is referring to that are in use in Europe (in a much less humid climate than mid-Atlantic US):
http://www.sole-ewt.de/index-e.html
http://www.comfosystems.com/,13.html
http://www.luefta.com/luefta_zubehoerkomponenten.html
Thanks, John S.
I did not realize the Quaternity had that special feature...
How do you distribute the air with a single non-ducted mini-split?
I believe most of the mini-split style systems will run in DRY mode removing moisture without changing the sensible temp. The unit inside can consist of a box hanging on the wall, possibly covered by a picture. Depending on the manufacturer they also may offer floor standing units or ceiling cassettes.
The Quaternity looks like the deal, I've e-mailed them about setting up a meeting at the IBS later this month. I haven't seen info on cost though, We just did a mini-split in a guest house and it wasn't all that outrageous, but it was a guest house part of a pretty expensive project. I like the idea of using the mini ducted systems in the bedrooms over the closet door and maybe a wall hung in the great room. Curious that they heat the air before running it through the cooling coil rather than cool the air to dry it and then run it over the heating coil, I wonder if that's a mistake from the art department or actually the way they do it and if so why?
Likewise the brine direct cooling systems are intreguing but I wonder why the use of brine rather than propylene glycol water mix? Seems a pretty corrosive choice. I love the idea of running a big loop on the dirt under the house before backfilling with gravel for the slab and using that for dehumifification and winter pre-warming. Sort of a seasonal exchange type idea.
But I keep going back to those geothermal trenches at the SIPs test houses in Oakridge that hit 117 degrees at six feet down in the middle of August. "the air is cold in my fridge unless I put a fresh pot of venison stew in there or leave the door open" even if I have two thousand feet of pipe under my house if the soil is dry and I pump heat into it all summer it won't do much for me after a month in NC. My opinion is that in this climate and with the unbalanced heat to cooling loads of the homes we are building wells are the only way to go for getting cooling out of the ground.
From the Daikin literature: "The evaporator of the indoor unit is divided into two sections and solenoid valves are provided in between. In cooling mode, the expansion valve between the condenser and the evaporator works to send low-pressure refrigerant to the indoor side, but in Quaternity Dry Operation, the expansion valve is kept open and a part of the indoor unit is used as a condenser by working the solenoid valve."
I haven't seen a wall unit that was small enough to be covered by a painting yet but they are small and quiet. and very versatile if somewhat expensive per number of locations with a max of four locations per outdoor unit. Possibly a hard sell to my clientele that expects inconspicuous ducts at a modest cost but great for passive house type folks and hard core Prius drivers. Too bad the Americans don't have something like this yet as an option for a good central unit.
John B do you have experience with ducted "side-arm dehumidifiers"? Thanks all for your insight.
Rewarming after overchilling to remove moisture, using heat removed - isn't this what Climatemaster's "Climadry" option provides in their Tranquility GSHP machines? If I read their info correctly, they do it with a recirculation loop of return water through an added coil downstream of the air chiller coil.
If your looking for a unit that hangs behind a picture check out LG's Art Cool line of products.