My husband and I are contracting a house in central Oklahoma, climate zone 4. We have about 2200 squ. ft. on the main floor with another 600 squ. ft. in an A-framed bonus room above the garage. We had intended to do an unvented attic, had originally priced an air source heat pump, but have decided to go with something more efficient. Our insulation guy changed our mind on the unvented attic. It looks like I can put an R-44 in cellulose (with a VERY thorough air sealing package) on the joists for the same price as I can spray R-21 of foam on my rafters. Doing an unvented attic properly (foam on exterior, proper vapor permiability) was getting a bit overwhelming, so we made a last minute switch to add ridge vents. Now we have the issue of our ducts being in the attic. Ground source geothermal would cost about $500 more after rebates than an air source heat pump, so we're definitely leaning towards the geo. A minisplit system is in the same price range. We've had an in-depth energy assessment done on our plan, and it looks like we'll have a 28,685 Btuh sensible loss load for the structure, with another 17,596 Btuh loss for the ductwork. We'll have a 12,392 Btuh total gain for the structure, another 15,405 Btuh gain for the ducts. So, the ducts being in the attic is HUGE! We live in an area that doesn't require our ducts to be suspended, so I'd like to know if it's possible to just put them on the floor, build foam board "halls" around them, and pile the cellulose on top. This would add the duct work to our thermal envelope, which only makes sense to me. I've been told it can be done, I don't know what that will cost. So far, none of my bright (mostly green) ideas have been cheap. So, can this be done? How hard will it be? Is it going to cost a fortune?

Should I just go with a mini split system? The last HVAC guy we talked to said that the mini splits have a high failure rate, and we'd be putting like 9 of them in the house. That sounds like somewhat of a disaster. Besides, I don't like the thought of having units hanging on every wall. However, if it would be more efficient, it would definitely be easier at the moment.

I would appreciate your thoughts!

With enough insulation, air sealing (from the attic) and a little air venting (to the interior), I agree that your ducts can be pretty close to "interior". I'd use geo over 9 mini splits.

We have about 2200 squ. ft. on the main floor

With all that main floor footage, did you consider a radiant slab?

The last HVAC guy we talked to said that the mini splits have a high failure rate

Is that with a particular brand or model? I'm not aware of high-failure rate across all mini splits, in fact, I've heard it's lower than HVAC units as a whole.

we'd be putting like 9 of them in the house

9 units seems excessive both for your heat load and for the sq footage of your home. How did they come to that number?

Posted By rosalynn on 23 Jun 2011 05:08 PM
The last HVAC guy we talked to said that the mini splits have a high failure rate, and we'd be putting like 9 of them in the house.

Even if the number of units were 4 instead of 9, I wouldn't want 4 of anything when 1 (or possibly 2) would do the job as well or better.  I wonder what maintenance costs and repairs would be on 4 (to 9) mini split units vs 1 (or 2) geothermal systems...

Mini split appearance looks like a through the wall air conditioner.

Mini split appearance looks like a through the wall air conditioner.

Mini-splits come in all manner of configurations, including flush-mount cassettes. They have been part of award-winning designs for years.

I just looked at a few pictures of them.  I'm sure they are lovely for those who like the commercial look of a vent in their ceiling.  I think there is a reason Fujitsu listed homes last in their description -

"Ceiling cassette systems provide both heating and cooling and are popular in commercial applications such as offices, conference rooms, restaurants, night clubs and bars, as well as new construction condominiums and homes."

I'm sure they are lovely for those who like the commercial look of a vent in their ceiling

I'm not sure how you can escape vents with air handling systems.

The mini-splits have been used for decades in Europe where design issues traditionally carry more weight than here in the US.

Posted By ICFHybrid on 24 Jun 2011 10:49 AM
I'm not sure how you can escape vents with air handling systems.

The mini-splits have been used for decades in Europe where design issues traditionally carry more weight than here in the US.

Escaping vents in a ceiling is fairly straightforward.

Got it - if other people says it's lovely then everyone else should think that too.  

Guess you believe the US courts should look to precedents in Europe too. 

I can't imagine 9 minisplits would cost less than geo, but a one, or at most a pair of multi-splits could probably work. (There's at least one Mitusbishi Hyper Heat multi-split compressor that's right-sized for your heating load (~35K @ +17F) designed for as many as 8 interior units on one compressor. There are likely others from other vendors. Interior heads will run no more than $1500/each. It depends on the layout. With a sensible cooling load of 1 ton for the whole house and less than 3 for heating I'm not sure why you need 9 of anything (multi-split heads) unless you're treating every room as a separate micro-zone, and they're all doored-off from one another. Putting indoor units in the highest heating season loss zones and letting the rest go along for the ride running a couple degrees cooler/hotter is often a comfortable/acceptable solution ( particularly in higher-R homes), but it depends on some analysis of the floor plan and where the gains/losses are.

As you've discovered, ducts in attics is a big step back in system efficiency, independently of the COP/SEER ratings of the equipment. If you go ducted, keep it all inside the insulation. There are some fairly high efficiency variable speed ASHP compressors out there that would work well in OK too, but the system cost of going ducted would be a bit higher, and net efficiency (when air handler power included) usually lower, but there are a lot of design & construction details to consider to say that for sure.

With ducts buried in air-permeable attic insulation in a vented attic the ducts need to be well sealed on every seam and joint, and the temperature of duct needs to stay above the dew point of the outside air in summer or you'll have condensation issues. If you build a foam board duct-chase do deal with it, the chase also has to be air-tight, and the R value of the foam relative to the fiber above it has to be such that the exterior surface of the foam board stays above the dew point of the exterior air as well (even at night), which can be quite a bit of foam in a high-R attic. R15-R20 would probably be safe. (4-5" of EPS, or 3-4" iso.) If you go that route, see if you can't find local sources for recycled/reclaimed foam board from commercial roofing insulation, since you 're cutting it all up and don't care about dinged corners & edges. It's typically ~25-30% the cost of virgin stock. (Craigslist etc can often find an enterprising roofer who recycles the stuff and will sell in small quantities. InsulationDepot.com will sell you a whole tractor-trailer load, if you're up for it. :-) )

Posted By geome on 24 Jun 2011 11:06 AM

Posted By ICFHybrid on 24 Jun 2011 10:49 AM
I'm not sure how you can escape vents with air handling systems.

The mini-splits have been used for decades in Europe where design issues traditionally carry more weight than here in the US.

Escaping vents in a ceiling is fairly straightforward.

Got it - if other people says it's lovely then everyone else should think that too.  

Guess you believe the US courts should look to precedents in Europe too. 

Isn't that just the flip-side of "If geome thinks it's butt-ugly, then everyone else should think that too." ???

Seriously, for those who just can't stand the appearance of an indoor wall unit, placement within the zone isn't very critical, and they're easily hidden by building valances (even fabric valances), even if it makes setting the temp with the remote a bit less reliable. 

But that's an individual's call.  Some can't stand the look of duct registers, and will pay extra to go with hydronic heating.  And some of those can't stand the look of panel radiators and are willing to spend $20K extra for radiant ceilings or floors to achieve similar or higher comfort. But some of the radiant-floor spenders are still in love with their mini-splits, wall-worts & all come cooling season.

There are ceiling cassette type interior units for some minis that work, that don't look much different than a ducted ceiling register, for those who really care. (Most of those are much higher-capacity than single-room residential zones would ever need though.)

Posted By Dana1 on 24 Jun 2011 11:56 AM
Isn't that just the flip-side of "If geome thinks it's butt-ugly, then everyone else should think that too." ???

Ahh, but I did not say that they're butt-ugly.  I only said they look commercial and (some) look like a through the wall air conditioner, which they do.  That said, I'm sure they are efficient in the right environment.  There also butt-ugly!

Posted By ICFHybrid on 24 Jun 2011 08:36 AM

We have about 2200 squ. ft. on the main floor

With all that main floor footage, did you consider a radiant slab?

The last HVAC guy we talked to said that the mini splits have a high failure rate

Is that with a particular brand or model? I'm not aware of high-failure rate across all mini splits, in fact, I've heard it's lower than HVAC units as a whole.

we'd be putting like 9 of them in the house

9 units seems excessive both for your heat load and for the sq footage of your home. How did they come to that number?

We didn't consider a radiant slab, too late now.

I think the HVAC guy that had a high failure rate was using Fujitsu.

When we got the bid for 9 units, we hadn't gotten our energy assessment, that was what they came to on their own, from our square footage.

I'd definitely avoid any HVAC contractor that...

A: Didn't do a room by room computer-based Manual-J type heat loss calculation

and

B: Recommended 9 mini-splits as a solution for anything (other than his own excess inventory problem... :-) )

Support for Mitsubishi Hyper Heat (aka "Mr Slim") minisplits is pretty good in my area. YMMV. Other bigger players in the US market are Daikin, Sanyo, LG, Samsung (and probably 3 others that I'll think of later.). Something with an HSPF of 10+ cuts the list of models down, but there's reason to believe that HSPF ratings aren't necessarily the best method of actual heating season performance in a particular climate.

The notion that installing, testing, debugging & maintaining a mini-split can be done by any yo-head with 3 hours of distributor training is just plain wrong. Yes, pre-charged straight-ahead minisplits don't take rocket-scientists to install, but the details count, and the ability to fully testing & properly adjusting the refrigerant charge of the system is key to efficiency & longevity. It can be off by quite a bit and still seem to work... until it doesn't.

With any system having multiple local vendors/contractors who have experience with the product(s), and a distributor with parts available in under is a good idea. (Waiting for parts to come from Korea when it's 18F outside isn't exactly a good move.) Whether geo or mini-split, the competence & service capabilities of the contractor is critical.

When we got the bid for 9 units, we hadn't gotten our energy assessment, that was what they came to on their own, from our square footage.

It might be worth shopping some more HVAC contractors, armed with your energy assessment. Where did the energy assessment come from? Sometimes it helps to get an assessment from someone who doesn't have an economic interest in one solution or another.