Fujitsu in Austin
Building a house in Austin TX. 1152 sq ft, 2 bedroom, 2 bath house with an open floorpan and somewhat unique design. Imagine a big room that then has about half open living space, and then a few smaller room inside for bedrooms and bathrooms. Nearly continuous strip of high windows around most of the building, but not really other windows (except for bedroom egress.) Exposed trusses and 2x T&G roof deck. Rendering is attached.
Roof is 1/2:12 pitch and will have 5″ of polyiso, all above the roof deck, 2×2.5″ staggered, taped seams. Double lock standing seam metal roof. Around 1.5″ of continuous polyiso on the exterior walls (still need to sort out some details re:batten approach) and metal siding. Leaning towards just unfaced batts in 2×6 walls 24 oc, but a few inches of open cell spray foam is also a possibility. Slab on grade.
Manual J estimates ~15k btu cooling load and the plan is to install an 18k Fujitsu ducted mini split. My previous AC retrofit (https://www.greenbuildingadvisor.com/question/ducted-mini-split-duct-design-and-distribution-questions) has been working well so I thought why not repeat the process.
Unit would be mounted in a ceiling chase above the master bathroom, open to the rest of the house. Return will be just a big metal box mounted to the back of the unit with a 20×25 filter mounted on it. Rigid metal supply ducts with a small run serving the master bedroom and the rest going down a trunk line with registers along the way, terminating at open living space.
Any thoughts/feedback on this plan?
Also, does anyone know a good HVAC contractor in Austin, knowledgeable in these systems? Fujitsu’s contractor locator comes up blank. So far the randoms I’ve tried tell me one ton per 400 sq ft and mini splits don’t have air handlers. I can buy equipment online but I’d still prefer a pro to pull the vacuum and charge the lines, and wouldn’t mind paying a bit more for them to get the equipment and me be covered by a warranty.
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I'm in Austin and looking for an mini-split installer as well. Not set on a certain mnfr but Fujitsu's high efficiency units do seem nice. You have any luck?
I've found two interested parties, although nobody seems to do much Fujitsu down here. One guy quoted me an 18k Carrier (I think rebranded Midea?) and the other an 18k Daikin. Yet to pull the trigger, still hoping to get a Fujitsu sorted.
If I were doing a project in Austin, I would be starting with Kristof Irwin and Positive Energy (https://positiveenergy.pro) for the design. I enjoy hearing Kristof on the Building Science Podcast. He's very informative and seems like a super nice person as well.
Good designers then are able to put you in contact with good installers.
Thanks, I reached out to Kristof.
Even if the walls are open, a bathroom is not an ideal spot for hvac intake. You can have the unit there but flip it around and run the return to the hallway section in front of the dryers.
Depending on the orientation of your place, you might need more cooling around the perimeter of the living space especially if you have any south or west facing windows. The single supply in the middle of the room would probably not cover enough of the space for comfort. I would add a T there and run two ducts to either end of the house, to the dining area and to the end of the living room sitting area. You can run the ducts inside the trusses for minimal visual noise.
Unfortunately there's not a lot of places to run ductwork (that I'd be happy with). The house is on a slab and the underside of the roof and trusses are exposed. The gray shaded areas in the floorpan depict which portions of the interior rooms are covered by ceilings. The ducts will go here. The ceiling joists are only 2x6 however I can get away with a 14x6" or even 14x8" rectangular duct that extends beyond the top of the joist a bit. The sight line inside the house is such you won't see them unless you're standing on the kitchen table or something.
I don't think you'll gain anything with the open-cell in the walls (and it will probably add cost). Do you have a lot of confidence in the Manual J? 1.25 tons seems like a bit high for a well insulated small home with minimal glass, but it may be what's needed for the cooling season. I would second the idea of hiring a professional engineer since correcting the design later would be challenging.
I ended up with 2.25" of continuous wall insulation (1.5" + .75") so any cavity insulation seems almost overkill at this point.
Manual J was done using coolcalc.net. House is fairly well insulated, but there is a continuous strip of 30" tall windows around [almost] the entire building. Windows are double pane, low-e, thermally broken aluminum... so not bad, but not excellent. First time building a home so we'll see how I did on air sealing. Also it is stupid hot in the summer here in Austin. Fujitsu only offers 12k or 18k ducted units, but if I go Daikin I could do a 15k.
Found someone to quote the Fujitsu... just slightly more than the Daikin. Leaning towards the Fujitsu as I've been happy with the other one. It also lists min cooling of 3,100 BTU vs 5,800 for the Daikin.
Manual J says 15k cooling load but some folks (Dana, especially) suggest even the calculators overestimate what's needed. Fujitsu's 12k unit lists a max cooling BTU of 13,600 which is pretty darn close. On the other hand, both the 12k and 18k units list a min cooling of 3,100. Does that mean 3,100 is the lowest amount either unit will modulate? If so, what efficiency price do I pay for the extra capacity of the 18k (apart from the few hundred bucks upfront cost)?
>"Manual J says 15k cooling load but some folks (Dana, especially) suggest even the calculators overestimate what's needed. "
That's right, and it's not just me. Unless you use a blower door to fine-tune the air leakage numbers the default setting of most Manual-J tools are ridiculously high.
This guy does a pretty good walk-through of the inherent error in the popular profession Wrightsoft Manual-J tool, well worth a look for any newbies (and many pros) curious about just how upsized a Manual-J often is.
https://www.youtube.com/watch?v=EymtchcD2RM
I picked the 18k Fujitsu. Might be overkill, but given the minimum modulation was the same for the 12k it wasn't clear what the smaller unit was getting me (maybe better latent heat removal?). Either way it gets hot here and I wanted to err on the side of caution, so I got the 18k.
Final duct layout is below. Blue boxes are registers and red is a 30x24 return air filter/plenum. There is a plenum/splitter off the front of the air handler that directs 20% of the output towards the master BR/bath and 80% down the other end towards the guest BR . Registers marked "down" will be on the underside of the trunk duct, feeding into the enclosed rooms. Registers marked "up" point up into the open space of the house.
Now shopping for registers. Generally familiar with sizing based on fpm/cfm/desired throw, and looking at hart and cooley since they have published data (and that's the only one I know).
The bedroom choices seem pretty straightforward based on how much air I expect to deliver.
Less clear on what's the best choice for that last register feeding the living room/kitchen area. I'm expecting ~400 cfm at ~550 fpm at the end of that trunk run. Would a ceiling or wall register be more appropriate given it will actually be pointing up? Should I do a 2-way? 3-way? Does it really matter given it's a fairly small, very open, well insulated house?
How low the compressor will modulate says almost nothing about efficiency or dehumidification. For example, a unit that modulates 3:1 while maintaining the correct CFM/ton will outperform a same capacity unit that modulates 5:1 with excessive CFM/ton (a common problem).
>"How low the compressor will modulate says almost nothing about efficiency... "
True, but the Fujitsu 18RLFCD is still QUITE efficient at it's minimum modulation:
https://ashp.neep.org/#!/product/32101
There isn't any efficiency advantage of going with the similar Daikin
https://ashp.neep.org/#!/product/33598
At 82F the Fujitsu is still more efficient running at 3100 BTU/hr than the Daikin is at it's minimum 9600 BTU/hr at that temp, even though at the rated 18,000 BTU/hr @ 95F there is a slight advantage to the Daikin.
The additional COMFORT factor of running nearly continuously during the cooling season even during low loads tips the balance toward the Fujitsu. If the Manual-J is calculating a design load of 15K at Austin's 1% design temp of 86F the total whole house load is probably going to be less than the Daikin's minimum MOST of the time during the shoulder season, and it may still be cycling on/off sometimes even at the 1% design condition if there's any padding in the Manual-J load number. It's pretty clear that the Fujitsu won't do that.
>"...a unit that modulates 3:1 while maintaining the correct CFM/ton will outperform a same capacity unit that modulates 5:1 with excessive CFM/ton..."
Setting the blower speed on the Fujitsu to it's minimum or running it in "DRY" mode for several hours if it isn't keeping up with the latent load during light overall load would good enough for most homes.
It seems possible (even likely?) the 12k would have be enough. I also presume the 12k would be more efficient running closer to its max, vs the 18k somewhere in the middle. What was not real clear was exactly how that efficiency difference translated to dollars. Given how god awful hot the summers are here I chose to pay more upfront, some marginal amount more over time, but not worry about having the house be cold enough.
I would love to track the performance once the house is all put together and I have a full year's worth of data. What's the best way to actually measure BTU loss? Kill-a-wall on the AC to measure how much it's cooling, combined with outdoor temp data for the same time period?
If you can give up 8-10 square feet in the bathroom you could mount the 1.5 ton Fujitsu in a micro-closet/cupboard facing the hall above a big fat common return grille & filter facing the hall.
See:
https://d4c5gb8slvq7w.cloudfront.net/eyJlZGl0cyI6eyJyZXNpemUiOnsid2lkdGgiOjcwMH19LCJidWNrZXQiOiJncmVlbmJ1aWxkaW5nYWR2aXNvci5zMy50YXVudG9uY2xvdWQuY29tIiwia2V5IjoiYXBwXC91cGxvYWRzXC8yMDE5XC8wOFwvMDIwOTIzNDlcL0R1Y3RlZC1taW5pc3BsaS0xLURhbmEtRG9yc2V0dC03MDB4OTMzLmpwZyJ9
https://uploads.disquscdn.com/images/7843213f27734395e6ede8ea696552a8eafd3a2dd7f62c2b61241bb23189a293.jpg << The access door to the utility closet isn't installed yet, but the drywall is up.
https://www.greenbuildingadvisor.com/article/getting-the-right-minisplit
Making the unit easily accessible to a floor-mounted human makes servicing the cassette & filter dead-easy. If, say the condensate drain gets plugged (that never happens in TX, right? :-) ) and needs cleaning when mounted in a ceiling service cavity it's a bigger PITA to deal with.
Appreciate the feedback but no need to re-litigate the placement, it's already been installed. I did see that install as well as a handful of others, although I do wish there was a library or something. Maybe I'll do that one day.
It's in the ceiling of the master bathroom but there's still ~4' of open space above the ceiling and below the underside of the roof. The unit is fully accessible from above and I am also considering a smaller access panel below so you can easily access the important bits from below too. Not as easy as if it was at ground level, but also not too hard.
Any thoughts on the registers?
I still don't think you'll get good coverage of your living space with the single register. I would at least T at the end and have two register, one blowing into your living room and one into kitchen area. These should blow along the ceiling, not up. Make sure to check the CFM at each outlet and match it with the proper register to get enough throw to cover the size of the space.
One more question on registers....
For both bedrooms the registers in the ceiling, along one wall. What throw should I be targeting? Do I want to get the air all the way across the room? Land halfway in the middle?