Do I need it?
Last Post 18 Apr 2015 09:09 PM by sailawayrb. 16 Replies.
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gosolarUser is Offline
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14 Apr 2015 11:59 AM
Back in'91 on Long Island, I installed in floor on 2 levels under the sub floor, 2800 sq ft worked great, still in use.

So I like it.


Now building a home 1 1/2 story, 1500 sq ft true passive solar, sips 6.5 walls 8.5 roof facing south on a slab.

this is in N W Ga about an hour N of Atlanta

I have a wood stove I plan on installing as backup, and intend to use a 9 or 12K mini split for AC (and heat if I need it.)


My question is should I or would you add the pex in the slab for about $250 just in case or am over thinking this.
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15 Apr 2015 03:00 PM
It is a question of comfort. I have friend who has been radiating floors in San Diego for 25 years. This would be one of the rare instances where an elevated tube would be in order. I would also use a combi water heater to drive it.

No argument can be made for the efficiency nor how often it would feel warm to the touch, though we both know that is not the most important part of comfort.
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15 Apr 2015 05:26 PM
Posted By gosolar on 14 Apr 2015 11:59 AM
Back in'91 on Long Island, I installed in floor on 2 levels under the sub floor, 2800 sq ft worked great, still in use.

So I like it.


Now building a home 1 1/2 story, 1500 sq ft true passive solar, sips 6.5 walls 8.5 roof facing south on a slab.

this is in N W Ga about an hour N of Atlanta

I have a wood stove I plan on installing as backup, and intend to use a 9 or 12K mini split for AC (and heat if I need it.)


My question is should I or would you add the pex in the slab for about $250 just in case or am over thinking this.

Put the PEX in the floor, and heat the floor (and your domestic hot water) with a Sanden EcoCute when they become available in your area.  Hot water heating costs may exceed space heating costs otherwise.

Don't over-do the south facing glass or you'll be running the mini-split in cooling mode half the winter in your location.  With ~R20-R25 whole-wall SIPs it won't take much at your fairly modest 99% outside design temps.

With a 2800' you'll probably need 2-2.5 tons of cooling in the summer, so for a 2-level house you're probably better off with a 2-head 2-ton multi-split  (one head per floor) which would have way-overkill heating capacity, or a pair of 1-ton mini-splits.  During the heating season you'd be able to cover any shortfall from the Sanden with the mini-split or wood stove.  During the heating season you'd probably only have to run the lower-floor mini-split, but during the cooling season the upper level unit will have a much higher duty-cycle, and you may well need both at the 1% outside design condition.

Georgia Power is teed up for a big fall given their level of investment in likely-to-become-stranded generating assets (not the least of which is the new Vogtle nuke.) The price of PV solar and batteries are crashing at an accelerating rate, and if/when the full cost of those redundant assets gets rate-based the retail power rates are going to go sky high about the time PV hit's a buck-a-watt installed price, with a lifecycle cost well under the current electricity rates.  They're one of the vertically integrated utilities on my "most likely to fail spectacularly" list.  If my worst case scenarios play out it'll make the failing Hawaiian utility woes look like a cake walk. We'll see how quickly they (and the GA regulators) can adapt their business models, but the way it looks right now they may be better off stopping the Vogtle project right now, rather than running it for a few years and being stuck with the decommissioning costs of a large now-hot nuke that will never operate on the lifecycle at which it was designed, since just the refueling costs are substantially than the lifecycle cost of PV going forward. Third party ownership of rooftop PV just passed unanimously in the GA legislature. This is real competition, competition with an exponential growth rate and logarithmic declining costs, something that the incumbent monopoly has never had to deal with, and it won't be pretty as it begins to erode their kwh sales revenue, and they have to make up for it elsewhere.  It's still awaiting signature, but it's VERY tough to veto a unanimous bill that is guaranteed to be overridden.

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15 Apr 2015 05:33 PM
Makes you want to move to GA.

http://homeenergypros.lbl.gov/profiles/blogs/designing-a-perfect-residential-wall-atlanta-georgia
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15 Apr 2015 06:12 PM
That "perfect wall" is about R21/U0.048 after factoring in the thermal bridging, which is the IRC 2012 code-MINIMUM for Minneapolis per TABLE N1102.1.3:

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_11_sec002.htm

MN codes are still at IRC 2006 levels though.

The higher elevation zones in GA are pretty nice, but I personally can't deal with the level of torrid humidity they get at lower GA elevations in summer. I'd have to readjust my internal thermostat or something to move there. YMMV.
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15 Apr 2015 06:28 PM
Me too. They can have all the rH they want. I will be closer to the mountains for sure. More sun, less rain. I may turn into a solar guru in my next iteration...
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gosolarUser is Offline
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15 Apr 2015 06:28 PM
Posted By BadgerBoilerMN on 15 Apr 2015 03:00 PM
  This would be one of the rare instances where an elevated tube would be in order.

What is that, can you explain or link to info?
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15 Apr 2015 06:42 PM
Posted By Dana1 on 15 Apr 2015 06:12 PM

The higher elevation zones in GA are pretty nice, but I personally can't deal with the level of torrid humidity they get at lower GA elevations in summer. I'd have to readjust my internal thermostat or something to move there. YMMV.

We spent a week rv'g in Perry Ga a few years back day temps 110, humidity a zillion %, terrible.

This spot is on a hill top 10 miles south of Ellijay, forgot exact elevation but it's up there, I'll be clearing the trees for solar envelope in few weeks.

going to have a deck off the second floor and another up 8' over that one I call it the crows nest the views are really nice to east and north in the afternoon shade.
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16 Apr 2015 08:44 AM
Depending on floor covering, design water temperature, slab depth etc. PEX placed on chairs, elevated off the bottom of the slab will respond slightly faster, both heating and cooling off than a slab with PEX stapled at the bottom-99% of installed radiant slab PEX.
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BadgerBoilerMNUser is Offline
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16 Apr 2015 08:44 AM
Dang dbl post.
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gosolarUser is Offline
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16 Apr 2015 05:35 PM
Since it's passive with S glass no floor covering to speak of, the slab will be 4", if I do it the pex it can be zip tied to the 12x12 mesh, that mesh on chairs.

that what I was thinking
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17 Apr 2015 09:14 AM
Good on ya.
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newbostonconstUser is Offline
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17 Apr 2015 11:30 AM
Just be warned that the 4" slab could take 8 hours to heat up, so in the fall and spring you will likely get alot of overshoot. I have 1.5" slab in a 2x6 house and get overshoot.

Are you insulating the slab?
"Never argue with an idiot. They will only bring you down to their level and beat you with experience." George Carlins
gosolarUser is Offline
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17 Apr 2015 12:02 PM
yes the slab will be fully insulated with 2"

All the research I did said 4" works best, but they also said not to exceed 4"
sailawayrbUser is Offline
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18 Apr 2015 02:15 PM
More insulation is always best for operational performance, but more insulation is also harder on the pocket book. So only a ROI analysis that considers your climate will determine what is economically best. 4” seems a tad high to me for your building location from a ROI perspective.

Temp overshoot problems result from not having an adequate controller. A properly setup controller that uses indoor temp, predictive outdoor temp, and slab temp feedback is not going to exhibit any significant undershoot or overshoot. Having a good controller becomes more critical as the amount of passive solar heating and heated indoor thermal mass increases. It is cookbook design stuff, but you still have to do it to avoid problems.
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gosolarUser is Offline
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18 Apr 2015 03:46 PM
What do you mean by "controller"

please explain
sailawayrbUser is Offline
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18 Apr 2015 09:09 PM
I would describe a controller as the algorithm and electrical/mechanical device that turns a heating system ON and OFF to regulate the indoor temperature as desired. It can be as simple as a dumb thermostat controller that only uses indoor temp feedback to a much more complicated device that uses multiple predictive feedback. Here’s an example of a more complicated PLC controller that we use in our larger residential and commercial designs:

Borst Programmable Logic Controllers
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