Mini Split Questions
Last Post 27 Jun 2018 09:52 PM by Dana1. 6 Replies.
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RobtUser is Offline
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13 Jun 2018 03:42 PM

We are moving along with a post fire rebuild in the Sonoma Valley. Foundations are largely complete and the first sticks went up this week on the guest house. My questions at this point are sizing and other considerations for ducted mini splits.

The overall construction is 1. Main house 2,200 sf with sealed attic and sealed crawl space. 2. Building 2 comprising 1,000 sf one bed apartment + 1,000 sf of unconditioned garage / workshop. The apartment will use the existing slab overlaid with 2x4’s and two layers of ply. Foam boards in between the 2x4’s. Sealed attic over the apartment only. All walls will be 2x6 16” OC overall R21.

We are shooting for a max of 1.5 ACH on all living spaces. Each space will have an HRV

I have had Manual  J load calcs performed by a reputable consultant. Heating loads are higher than cooling loads. The house calculates to 24,000 btu. The apartment to 11,500btu. Both seem on the high side to me based on my reading on this site and elsewhere but may be it doesn’t really matter as long as the units are sized appropriately.

The basic assumptions seem reasonable:

Outside design temp 32 degrees

Inside design temp 68 degrees

However of the Main House 24,000 btu total heat load 37% is lost through the floor / crawl space which is assumed to have a U factor of 1.18. Does this seem like a lot? I am told the program (wrightsoft) allows a ground temp to be factored in. If this load is correct then if I put a couple of inches of rigid foam down could I make a big improvement to the load?

The plan is to use two ducted minisplits in the house, one to handle master bed/ bath and great room; the other to handle two bedrooms and guest bath. This will allow for short ducts.

In the apartment one central ducted minisplit should be adequate and again have short ducts.

So far I have talked to one recommended contractor who uses Fujitsu and Daikin. He wants to increase the loads substantially to avoid ‘complaints’ but will install whatever we want as long as we release him from lack of performance. His major concern seems to be that people don’t know how to operate these systems and will complain at the time it takes to ramp up. As long as we are prepared to run the system more or less constantly I don’t see the problem.

 I would appreciate thoughts on the load calcs  and the proposed set up generally. Any suggestions on equipment specs that I should be looking for? Preferences for Fujitsu vs Mitsubishi etc. Any other considerations?    

Many thanks!

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Dana1User is Offline
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13 Jun 2018 07:11 PM
The floor losses DO seem like a lot. What is the floor stackup, subfloor + finish floor?

I assume the crawlspace walls insulated?

IRC 2015 calls out R5 (minimum) continuous insulation on crawlspace walls for DOE US climate zone 3 (you are in 3C, which is different from the Title 24 zone map). Taking that to R8 (2" of EPS) isn't insane. That's usually less material than insulating the crawlspace floor. The deep subsoil temps in your area are around 60F anyway, so the heat loss is primarily going to be at the above-grade and near-grade crawlspace walls. With the walls insulated and the house kept a 70F the crawlspace won't drop below 65F or so, a 5F difference. So for 2100 square feet of U1.18 floor (probably the highest possible high estimate) the floor losses would be:

U1.18 x 2100' x 5F= 12,390 BTU/hr, about half your load.

With R8 on the crawlspace floor that floor losses drop by almost an order of magnitude to about 2000 BTU/hr.

A single 1.5 ton Fujitsu 18RLFCD can deliver 25,600 BTU/hr @ +47F, 21,600 BTU/hr @ +17F, and can probably cover the entire load @ 32F even without insulating the crawlspace floor, but with insulation you'd have margin. The RLCD series cassettes don't really need super short ducts, the way most mini-duct cassettes do, and would be the right choice.

A pair of them would have cycling issues, since the minimum modulated output of each at +47F is 3100 BTU/hr (6200 BTU/hr total.) Your whole house load won't necessarily be as high 6200 BTU/hr @ 47F with the sun shining.
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13 Jun 2018 10:08 PM
Thinking about it, I can't fathom how the floor can really be U1.18. A 3/4" plywood subfloor on it's own is about R0.93, just the undisturbed horizontal air film in the crawlspace ceiling is worth another R0.61. So even without finish flooring or the air film on top of the finish floor you're at R1.54, which would be (1/R=) U0.65. With any kind of finish floor and counting the air-film on top of the floor you would be at less than U0.5. So if they were using U1.18 for the floor's U-factor those losses are exaggerate by at least 2x, and probably 3x.

BTW: Using reclaimed roofing EPS (usually 1.25lbs density "Type VIII" EPS, at R4.17/inch) can take the sting out of a couple inches of 2000 square feet of crawlspace floor insulation. It's typically priced at only 1/4 to 1/3 that of virgin-stock EPS. This is better stuff the 1lb density "Type-I" EPS usually sold at box stores, dense enough to be considered "walkable roof" when installed under membrane roofs. The key is finding a reclaimer near you with 65-70 sheets of 2" EPS in stock. (Nationwide Foam surely has it in at least one depot or another at any point in time, but the shipping adds up too.) From a financial rationality point of view ~R5 is usually rational on a lifecycle basis using virgin stock foam in climate zone 3 so R8-ish using reclaimed goods isn't a stretch. For a starting point on the financial rationality aspects of various "whole-assembly R" values (the net performance after factoring in thermal bridging of framing, etc, not center cavity R) , see the zone 3 row of Table 2 on page 10 of this document

https://buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf
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14 Jun 2018 06:26 PM
Thanks Dana for your usual helpful responses.

Yes crawl space walls are insulated w 2” of foam board. But these walls do not daylight as there is a deck with concrete slab all around the house, so this is really a 3’ basement not a typical crawl space.

The floor will be wood in the great room, carpet in bedrooms and tile in kitchen and baths all over a wood sub floor.

We will probably add a rat slab which would add a small R value to the floor. Since the program tells us we are losing 37% of our heat through the floor I asked our consultant to model foam under the floor and he said it only saved a couple of thousand btu’s and wouldn’t be cost effective.

Based on your second post and my limited intuition it would seem something is wrong here 1. with the overall load and 2. With assumption of adding under floor foam. What can it be? Program error? Operator error?
Assuming that the total load is correct at around 24,000 btus then we could use a single 1.5 ton as you suggest, duct runs could probably be kept to 30’ and relatively straight. The house divides roughly 1/3 2/3 – would it be preferable to have two smaller units with much shorter ducts?

Another thought on sizing – our temperatures are generally mild but when the do drop it is typically for a few hours overnight; it is very rare for us to have sustained daytime temps below 32F. I assume thermal mass in a tight house would help to maintain a reasonable temperature?

Thanks for the advice and for the recycled foam suggestion, I am exploring that.
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14 Jun 2018 09:56 PM
The problem is the U1.18 floor. That simply CAN NOT be right! 37% of 24,000 BTU/hr is about 9000 BTU/hr, which is what you might expect out of a slab-on-grade with no slab insulation, only slab-edge insulation. Reputation notwithstanding, this was probably a screw-up by the consultant.

Simply changing that U-factor to something more realistic takes more than 5000 BTU/hr off the load numbers which means your more likely load is on the order of 19,000 BTU/hr, not 24,000 BTU/hr, even without insulating the crawlspace floor. A half inch of carpet + carpet underlayment adds about R1.4 to the R0.93 subfloor, and with air films that comes up to over R3, or U0.33, which takes 2/3 off the floor loss numbers for those rooms!

Radiant flooring folks tend to pad the R-values a bit (for margin on heating design) but this list isn't too out of whack:

http://www.radiantprofessionalsalliance.org/Pages/FloorCoveringR-ValueChart.aspx

If you add up the flooring stackup from subfloor to finish floor, add 2x R0.6 for air films for the total R, the floor's U-factor will then be 1/R(total).

With reclaimed EPS or XPS (but not polyiso, which can become waterlogged) foam under a rat slab the expense of a couple inches of foam is "worth it". If using virgin stock a single inch is still worth it on a lifecycle basis, but not the short or medium term. If it allows you to use one mini-split instead of two it pays for itself up front!

The Fujitsu slim duct air handlers have something like twice the drive of most of the competition. You can run them 30' or more without a problem if the duct designer isn't an idiot who loves to create duct-scuptures with lots of twists & turns. Hard piped duct will usually work better than flex for the long runs unles the flex it stretch tight and well supported, but short runs of flex off the trunks for noise isolation and ease of connection won't hurt. A star-configuration of fully stretched flex from a short-fat plenum cans sometimes work well:

https://www.energyvanguard.com/sites/default/files/styles/panopoly_image_original/public/flex-duct-done-right-mike-macfarland.jpeg?itok=UixhWuCo

On your 1000' apartment, what is the purpose of the 2x4s on top of the pre-existing slab? A continuous layer of 1.5-2" EPS and a poly vapor barrier under the doubled-up plywood works fine without the 2x4s, with the first layer of plywood Tapconned to the slab, the second glued & nailed to the first. The seams of the first layer need to overlap the seams of the foam sheets by a foot, and the seams of the second layer overlap the seams of the first by a foot. Over torquing the Tapcons near the seams in the foam can create some minor compression, but they can be backed off for a more level floor. Any density EPS will work except under load-bearing walls (which have to be engineered if resting on a foam layer), but it's not a problem for partition walls. When the R4/inch foam is being thermally bridged by the R1/inch wood the overall heat losses out the floor go way up, and you'd be able to feel temperature striping of the 2x4s near the exterior walls during cold snaps. Doing it without the 2x4s and a continuous layer of foam is cheaper, easier, and higher performance.

The Fujitsu units are nice because they can be mounted vertically in an extremely shallow utility-closet. All others need to be mounted horizontally, which makes service access a PITA. Check out this retrofit installation of an 18RLFCD that Larry Waters of A-1 Guaranteed Heating & Air, Inc(Vallejo CA) did on a 1200' 1950s vintage house in Berkeley that was going all-electric :

https://uploads.disquscdn.com/images/2ffa6e108a7ded9f51130ff14126239b275b1244b7d53138beb63b4182d68f13.jpg?w=800&h=1200


https://uploads.disquscdn.com/images/7843213f27734395e6ede8ea696552a8eafd3a2dd7f62c2b61241bb23189a293.jpg?w=800&h=1200

(A blog piece on that house appeared here a couple months ago: https://www.greentechmedia.com/articles/read/what-does-it-take-to-electrify-everything-in-your-home#gs.g_4G6P8 )

That approach only steals 5-7 square feet of floor area, and you don't have to dive into a crawlspace to unclog the condensate drain or swap filters, etc. You can't do that with a Mitsubishi or Daikin, and the duct runs would need to be shorter too.

This was a house half the size of yours but probaly crummier insulation and probably not nearly as air tight as your target numbers.

So just for yuks let's say your heat load at 32F is about 18,000 BTU/hr (it's probably less), with a 70F indoor design temperature. Doing a linear approximation, that's a 38F difference and a 18K load so the load increases by 18,000/38= ~475 BTU/hr for every incremental degree below 32F. We know the 18RLFCD is good for 25,600 BTU/hr @ 47F, 21,600 BTU/hr @ +17F. So at your colder night temps it's good for at least 21.6K. 21,600/475=45F, so you wouldn't start to lose any ground until it's under (70F-45F= ) 25F outdoors. While temps that low aren't unheard of, they don't stay below 25F for long enough to really matter.

Did the load calculations include the 230 BTU/hr per sleeping human deduction? Did the include the 24/7 plug loads like refrigerators, etc?

It just seems VERY unlikely that your house has a heating constant of 24,000/38F= 631 BTU/hr per degree F. That's higher than the measured performance of my sub-code 2x4 framed 1.5 story 2400' antique + 1500' of insulated (but not actively heated) basement. With new tight 2x6 construction and U0.32 or lower windows w/R38 or more in the attic a 2200' one story should come in 500 BTU/hr per degree difference, maybe even under 450BTU/degree-hour.
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26 Jun 2018 01:10 AM
Dana – thanks again for your very helpful input. I am getting back to this now after an unplanned hiatus last week. Still trying to get my head around it all.
You agree with my gut feeling that the 9,000 btu load o f the floor is high. You give some values for the floor assembly itself but with a sealed crawl space isn’t the relevant number the sum of 1. The loss to the ground 2. The loss through the crawl space walls?
You say “9000 BTU/hr, which is what you might expect out of a slab-on-grade with no slab insulation, only slab-edge insulation”. Unless we add insulation on the ground isn’t that pretty much what we have?
Even with a low delta T of 68 degrees design temp and 60 degree ground temp and a U value of 1.18 my back of the envelope calculation comes up with a very big number. Is a 1.18 U value realistic for loss into the ground? If this is correct then adding some rigid foam should surely make a massive difference?
Another question. Should we be looking at one or two units in the house? One would be cheaper but require longer duct runs and with layout of the house as well as allowing for usage o f different areas it seems to me we would be better off with two? What do you think?

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27 Jun 2018 09:52 PM
An unvented crawlspace has the insulating value of the air in the crawlspace, including the additional ~R0.6 for the air film directly under the subfloor, whereas a slab-on-grade is thermally coupled conductivly to the soil (there is no air film under the slab, only on top). They are definitely NOT the same!

The U-factor of ANY floor stackup with air films on both sides is going to be substantially lower than U1.18. Even a slab on grade will usually be lower than that:

The thermal conductivity of the soil varies by it's composition and moisture content (a lot), but it is at least somewhat insulative. (For a primer on soil-R, see: https://inspectapedia.com/insulation/R-Value_of_Soil.php ) Even a solid 4" concrete slab with 150# concrete is about R0.30 (U3.3) if you don't count the air films or soil-R. The air film above the floor when the room is warmer than the subsoil is about R0.25, so you're looking at about R0.55 (U1.8) ) for a bare slab-on-grade, even when completely discounting the soil-R.

Even if you only count the R-value of just the first 3" of soil you're looking at at LEAST R0.30 assuming the soil isn't wet. That would bring the slab-on-grade up to R0.85 (=U1.18, the value used for the whole floor). It's usually legitimate to assume even more soil-R.

Additional mini-splits usually means a higher total minimum modulation, which means the temperature at which one might start cycling on/off (killing efficiency) is much lower. With a single it will modulate. If one assumes a 20K total load at a temperature difference of 40F (32F outside, 72F inside) the load changes about 500 BTU/hr per degree of temperature difference. Taking the Fujitsu xxRLFCD series minimum modulation of 3100 BTU/hr, with a single unit it would still be modulating at about 3100/500= 6F temperature difference. With two of them sharing identical loads they would stop modulating at a temperature difference of 12F, or at about 60F. Given that your average nighttime highs are is barely cooler than 60F even in January it means they would be only modulating continuously for a few weeks mid-winter, and cycle quite a bit during the shoulder seasons.

https://weatherspark.com/m/628/1/Average-Weather-in-January-in-Sonoma-California-United-States#Sections-Temperature

That's not exactly a disaster, but in practice one of them will be much more lightly loaded than the other, and would likely start cycling somewhere in the 40s F, which makes it less efficient and less comfortable. If you can do it with just one, that's the right way to go.

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