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Ducted mini-split?

Last Post 27 Feb 2013 08:42 AM by jdebree. 13 Replies.

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jdebree

Basic Member

Posts:497

26 Feb 2013 08:30 AM
I'm in the process of building a small ICF home in upstate SC, and looking ahead to the HVAC system. I've already decided to go mini-split, as the house is basically open with few separate rooms. I recently read of a ducted mini-split system, and realized that I have a good layout for that. I have a large central closet (almost the size of a small bedroom) and I could easily drop the ceiling enough to house ductwork. Distribution would be 3 or 4 ducts, and I can reach every major room this way. Is this a good way to go? Would this eliminate the big wall units, replacing them with a typical A/C vent? Are these systems efficient? A couple factors- we keep the whole house the same temperature, so don't need individual zones. The ductwork would be in the conditioned space. The house is 1200 sq ft, R-25walls; R-49 ceiling.

joe.ami

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26 Feb 2013 09:00 AM
"ducted mini split" is virtually an oxymoron (though not quite). What I can tell you is among the reasons mini's get efficiency as high as they do is by eliminating duct systems and larger blowers. So my suggestion is split or duct but not both.

Joe Hardin www.amicontracting.com We Dig Comfort! www.doityourselfgeothermal.com Dig Your Own Comfort!

toddm

Veteran Member

Posts:1152

26 Feb 2013 09:03 AM
My single indoor unit is surprisingly effective. The bath is 40 feet away around the corner and usually behind a closed door but stays within a degree or two of the house. Part of the efficiency is that it is ductless. But the indoor unit is difficult to hide. I thought I could camouflage it with bookshelves but in heat mode it blows straight down.

Dana1

Senior Member

Posts:6991

26 Feb 2013 12:09 PM
With extremely short duct runs to allow adjacent rooms to "share" a head the impact on net efficiency is minimal- less of an impact of having 2 separate heads if each is woefully oversized for the tiny loads. If you're running ducts 10s of feet the advantage begins to evaporate, and becomes comparable to fully-modulating inverter drive central air conditioning with fully-modulating air handler speed. With R25 walls and R49 roof, if you have comparably high performance windows you could probably go with point source heating- a single 1-1.5 ton mini-split head, but maybe not for cooling (depends on how much east or west facing glass you have.) Your heat loads should be quite modest- even if you're in the cooler hills with sub-25F 99% outside design temps. At 20F most 1-ton mini-splits can deliver ~15-17,000BTU/hr or more, and 1.5 ton units are ~22,000BTU/hr +. With reasonable shading and glass a 1200' house at those R values would usually come in under 1.5 tons cooling, so this is definitely mini-split territory.

jdebree

Basic Member

Posts:497

26 Feb 2013 01:34 PM
I have no windows that get west sun. Most windows are facing east and north. We are surrounded by trees, so little sun hits the house except the middle hours of the day, and the roof has 2' overhangs, so at most we'll get filtered morning sun. The windows are Marvin casements; nothing fancy, but adequate for the climate. I'll sit down and calculate the window area. I've been reading up on so-called ducted systems, and they are basically a unit mounted in the wall. Since I can drop the central closet ceiling, they can be fully concealed. I really only need two heads- one for the main house, which is one room divided into three by a slight wall and beam across the ceiling, and the master suite. Would I be better off running two units of different sizes, or connect ductwork to a single bigger unit? Total ductwork would only be a few feet, but the ducts are 90 degrees to each other. We're not very fussy about climate control. Between about 55 to 80 degrees outdoor temperature, we won't run the system at all, preferring open windows and fresh breezes. Our normal winter thermostat is about 60; summer is about 80. Edit- The windows and doors represent about 275 sq ft of area, and the windows have a U of .3.

jdebree

Basic Member

Posts:497

26 Feb 2013 02:27 PM
Posted By jdebree on 26 Feb 2013 01:34 PM I have no windows that get west sun. Most windows are facing east and north. We are surrounded by trees, so little sun hits the house except the middle hours of the day, and the roof has 2' overhangs, so at most we'll get filtered morning sun. The windows are Marvin casements; nothing fancy, but adequate for the climate. I'll sit down and calculate the window area. I've been reading up on so-called ducted systems, and they are basically a unit mounted in the wall. Since I can drop the central closet ceiling, they can be fully concealed. I really only need two heads- one for the main house, which is one room divided into three by a slight wall and beam across the ceiling, and the master suite. Would I be better off running two units of different sizes, or connect ductwork to a single bigger unit? Total ductwork would only be a few feet, but the ducts are 90 degrees to each other. We're not very fussy about climate control. Between about 55 to 80 degrees outdoor temperature, we won't run the system at all, preferring open windows and fresh breezes. Our normal winter thermostat is about 60; summer is about 80. Edit- The windows and doors represent about 275 sq ft of area, and the windows have a U of .3. Total wall area is 1343 sq ft.

Bob I

Veteran Member

Posts:1435

26 Feb 2013 02:33 PM
so you can install a ducted system or one or two single units. One of the surprises in a well insulated house is that the temperature stays constant throughout the house anyway, so the ducts may not be necesary. So if the smaller rooms open onto a larger rooom, put the unit there & it will work fine. So basically you could do it either way, but the single unit should work fine.

Bob Irving<br>RH Irving Homebuilders<br>Certified Passive House Consultant

jdebree

Basic Member

Posts:497

26 Feb 2013 03:55 PM
I'm a little concerned about the master bedroom, as the door will be closed at times, and it is on the southeast corner of the house. I could see that room getting warm in the summer. I'm sure the rest of the house will be fine, as it is open, and, despite arguments here and elsewhere, ICF does seem to yield a very even temperature in all parts of a room. Maybe all well-insulated buildings do.

Lbear

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26 Feb 2013 04:02 PM
As others have touched upon, the whole purpose of a mini-split is that it is ductless and therefore provides excellent SEER ratings. Once you introduce ducts, you lose efficiency, it's just the nature of the beast. The more ducts, the less efficient. At least with ducts in conditioned space, you are ahead of most installs. If you are going with the duct route, why not just go with a HVAC system with an air handler & two-stage heat pump? I've seen homes that have 100% mini-splits as being listed as NOT having central air, which supposedly is a financial hit on the home. Not many people want a home that they think doesn't have A/C. I've even heard stories that banks will get squirmy when they find out you are not installing central A/C. Mini splits, while common place in Europe and Asia, is still somewhat of a foreign beast here in the USA. The U.S. is about 10-15 years behind the energy curve of other nations and this ineptitude is then displayed by consumers, realtors, and loan agencies. That is why 95% of the homes being built today in the southwest still put the HVAC ductwork into an unconditioned attic.

acwizard

Basic Member

Posts:265

26 Feb 2013 04:28 PM
Ducted mini split air handlers do not have static blower capabilities to push the air down long runs of ductwork.The limitations are less than 10 feet.If you are looking for another approach then use a recessed air handler like First Company matched with a high efficiency heat pump condensing unit.

Bob I

Veteran Member

Posts:1435

26 Feb 2013 04:39 PM
" ICF does seem to yield a very even temperature in all parts of a room. Maybe all well-insulated buildings do." yes; that is a function of air tightness and insulation, has nothing to do with the type of insulation used

Bob Irving<br>RH Irving Homebuilders<br>Certified Passive House Consultant

Lee Dodge

Advanced Member

Posts:714

26 Feb 2013 04:58 PM
At least one circumstance where ducting is beneficial is when direct solar insolation is high in some rooms but not others. The comments about well insulated houses not needing ductwork needs to have this caveat attached. My house is well insulated, but I have intentional solar gain in one end of the house but not the other, and there are significant temperature differences that can be addressed with ductwork. In your case, you have a fairly large amount of window area, with the window area being 23% as large as the floor area. However, your windows are mostly facing east or north, and you are well shaded, so hopefully that will limit direct solar insolation.

Lee Dodge, <a href="http://www.ResidentialEnergyLaboratory.com">Residential Energy Laboratory,</a> in a net-zero source energy modified production house

Dana1

Senior Member

Posts:6991

26 Feb 2013 06:23 PM
Posted By jdebree on 26 Feb 2013 03:55 PM I'm a little concerned about the master bedroom, as the door will be closed at times, and it is on the southeast corner of the house. I could see that room getting warm in the summer. I'm sure the rest of the house will be fine, as it is open, and, despite arguments here and elsewhere, ICF does seem to yield a very even temperature in all parts of a room. Maybe all well-insulated buildings do. That slow burn roasting feeling in the AM will just be your cue that it's time to crawl out of bed, eh? :-) Seriously if it's the highest solar gain room in the house, it probably deserves it's own head for the cooling peaks, but it sounds very much like 2 heads could serve the whole thing. A room by room heating & cooling load calc would tell you how to best optimize it. There are 1.5 ton 2-head multi-splits out there, but sometimes a pair of 3/4 ton units is cheaper and works better. It just depends. Somehow I don't think you're sweating the resale value on anything here, since the high-R envelope won't have an easy cash-value in the generic market. You're buying comfort, and maybe putting a price on some of the externalities of your energy use. It sounds like a great place, something like 2x code, min on most measures.

jdebree

Basic Member

Posts:497

27 Feb 2013 08:42 AM
I hope to leave this house feet-first, if you know what I mean, so yes, it is way beyond code, both structurally and energy-wise. Since we will soon be retired, it needs to be as cheap and low maintenance as possible. We're building it out-of-pocket, and re-sale is nor much of a factor, quite frankly. Money spent now is an issue, though, so I need to configure a system that's gonna do the job with the best bang for the buck.