Manual J, layup selection and HVAC recommendations
I am looking at building a home in NW Arkansas and hoping to bounce some ideas off the group here. We are still a ways away but trying to look at options based on what I’ve learned reading hundreds of pages here.
NW AR is CZ 4 but the AR code treats the insulation requirements as CZ3 – R30 roofs and R13 walls. My plan is to build to 2012 CZ4 standards as a PGH. I’m thinking to use 2×4 with cellulose fill covered by 1½” ZipR sheathing. R50 cellulose in vented attic. Double pane .30 windows or better. Does this make sense for the climate?
I have two alternatives on floor plans depending on slope of the land we find. If a flat lot, then slab on grade with R10 perimeter insulation and perhaps insulation under the slab. The single story craftsman house plan my dear wife likes is about 1875 ft2 with numerous corners and quite a bit of glass, mostly on the south. Very few windows on east and west. Using loadcalc.net I get 23,000 btu/hr heat loss. This seems high assuming tight construction and the insulation levels noted. The plan is mostly 10 foot ceiling also. I’ve used gross measurements for windows and doors- Is that right? My plan would be to meet the needed heat and cooling with two ducted mini splits (Fujitsu?). DW not having anything to do with ductless. The layout is conducive to hiding the ducted units in false ceilings in two different closets and using false ceilings in short hallways for a couple short runs – perhaps three rooms for each unit. Does this sound reasonable or are there better ways.
The other alternative plan is a day light basement ranch on a hillside lot. The plan is about 1650 ft2 above grade and 800 ft2 finished in the basement. The remaining basement space would be unconditioned. Quite a few windows, large sliders on south side both main level and down, mostly 9’ ceilings. The heat loss calcs come out at 33,000 btu/hour. Does that sound right? I’m thinking to use a ducted mini split below the floor running ducts in floor trusses in we go this direction. Does this make sense ?– I see LG has larger capacity units. Can’t say that I understand the issues around static pressure – not an HVAC guy and don’t play one on TV. Would a more traditional heat pump be better for this house?
Thanks for helping me learn more!
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The higher than expected heat load is almost certainly to be due to the expanse of south facing glass and extra corners, as you have surmised. See what the up-charge is for higher performance windows, say U0.25-U0.28, and compare that to the cost going with a higher-R wall, or maybe do both.
It's probably worth bumping up to 2x6 24" o.c. w/cellulose on the wall, keeping the 1.5' ZIP-R, which will bring the heat load numbers down some, but won't much affect the cooling load numbers. The whole-wall R of the 2x4 16" o.c. cellulose + 1.5" ZIP-R is R16-ish, but you'd be R20-21-ish if you went with 2x6 24" o.c.
That's a wall good enough to thing about Net Zero Energy in zone 3, but maybe a bit thin for zone 4.
R10 slab-edge or foundation wall insulation is bare code-minimum for zone 4. R12-R15 wouldn't be crazy. Even the unfinished not-directly conditioned portion of the basement it's still considered conditioned space and needs to be insulated. Even an inch of EPS under the slab would go a long way toward keeping the slab temp above the summertime dew points, which will keep the basement from SMELLING like a basement. From a long term energy savings point of view it could use more, but won't need more than two inches.
For analyzing mini-split solutions, run separate heat load totals by how you intend do zone it, at both the 99% outside design temp, as well as at 47F. Ideally a mini-split's minimum output would be below the +47F heat load for the zone it is supporting, and would have 10-50% of excess capacity at your 99% outside design temp. Your outside design temps in NW Arkansas are pretty close to the HSPF test +17F capacity & efficiency point, which makes it pretty easy to analyze. (In Fort Smith the 99% design temp is +19F, in Fayetteville it's +13F.)
If by design you can get the design load on the single story Craftsman type house down to the high teens at +17F, a single 1.5 ton Fujitsu -18RLFCD would get you there. That would be better than a pair of them, since the +47F minimum modulated output on that series is 3100 BTU/hr independent of size. A pair of them would have a min-output of 6200 BTU/hr and would beging cycling on/off at about 50-55F outside temps at design load of 23,000 BTU/hr, but with a design load of 18,000 BTU/hr and a SINGLE xxRLFCD it would still be mostly modulating right up to 60F or higher, wherever the heating/cooling balance point is.
The load numbers on the 1-story seem somewhat high for a slightly-better-than-code house, but the numbers for the walk-out seem quite a bit high until you account for the heat losses of the un-finished not-fully conditioned portion of the basement. The heat load on the walk-out basement is likely to be about half that of the load of the fully above grade floor and it may well be within the output of the -7RLF mini-duct cassette which would have to be be a zone on a 2-3 ton multi-split, but it still might be more efficient with separate minisplits.
A 2-ton traditional heat pump might work OK for the 1-story, but not for the walkout since duct dampered zoning cripples efficiency, among other problems. A modulating 2-tonner like a Carrier GreenSpeed would have mini-split type efficiency and comfort and would cover a 23K heat load @ +15F, but is usually more expensive than mini-split solutions.
Thanks Dana. I appreciate the layup advice. Can you clarify if for window footage in my heat loss calcs should be gross opening or just the actual glass dimensions?
As for the 2 mini splits, my thought for the craftsman ranch was that the plan layout, not necessarily the load, dictated the need for two ducted units. No good way to run ducts across the house while staying inside the envelope.
The walkout plan has the same issues on the main floor. I' m concerned over too much ducting if I only used one. If I need two for the main floor and then one for the basement caused my question of whether a traditional heat pump would work better. Your comments on zone control makes the heat pump option not sound very good. Would one ducted unit under the main floor be able to cover 1600 ft2 or would that be too much ducting for a mini?
Any other thoughts?