Passive Solar project.

Wayco Wayne

I thought on it last night while I was sleeping and woke to find that I think he would need a gradual change in elevation so the change in water density could induce movement and heat transfer. Can you imagine sloping 140 feet of pex with that much accuracy. At 1/4 inch per ft. that would be 35 inches and the slab is only 9 inches deep. To slope it withing 9 inches it would need a slope of .06 inches per foot. I think the Mad scientist Homeowner is going to nee a small circ. on the project. JMHO. :)WW

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Wayco Wayne

A customer is building

a Passive solr addition on the South side of his house. He will have a concrete slab that is 29' x 5" and is 9" thick. He wants the heat in the slab to transfer heat well so there is minimal temperature difference from end to end. He wants to install 1" pex in the slab that he will fill with water because he believes the heat will transfer faster through the concrete that way. He does not want a circulator unless time proves he needs it because he wants no moving parts. He wants the heat to travel through the pex by thermosyphon currents, and by doing so even the heat throughout the slab. I dunno, I thought that's why you put rebar throughout the slab. Would the water really help his cause?? Would it help to have the pex higher or lower in the slab or even different heights?? What think you Wallies. I like these Mad Scientists types but wish I knew more about engineering so I could help more. WW

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heatboy

You're kidding, right?

Thermosiphon currents? How many feet of 1" is going to be in the slab and once (if) this flow is induced, where is it going to flow to? A Storage tank? Geez, a Grundfos 15-58 set on speed one uses less than 40 watts. More info, please.

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Wayco Wayne

More info

He is going to have us run both ends of the loop to a resevoir tank. I figured about 200 ft of tubing in the slab. I suggested a small circ connected to a PV panel so a high solar gain day would run the circ in proportion. He said he doesn't want any moving parts if he can help it. I am wondering if concrete wouldn't pass the heat along fast enough by itself to avoid warm and cold spots.

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Dan Peel

Sure......

May the force be with you......
Physics sure isn't going to help.
Dan

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Eric

I liked the force part...

If I understand you he wants to add convection within the slab?? why??

Or is the slab his "collector" and he wants to transfer heat from slab to his storage tank with pex tubing?

Or is the slab his passive storage device? (battery to store solar gain)?

Were is stored heat going?

Dont quite get the 29'x5" x 9" thick part???

Concret has an r Value of approx 0.2 per inch. so the 29' deminsion would be about R=70 if 5' then R=1 Using a guesstimated surface temp hot side and cold side. You can calc a heat loss rate from one end of slab to other and then can easily calculate the temperature gradient through the slab (ie it will be linear because your concrete is uniform) k value is constant.

We do this temperature gradient calc all the time up north here to verify our freezing point is on the warm side of vapor barrier.

ScottMP

It sounds like

The floor is the collector. Even if he install a storage tank and piped it for gravity circulation, I would think the floor would be the hottest area and the tank would not "Thermosiphon".

Unless the room temperature is up to 100, I can't see to much more than water warming going on here.

Scott

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Bob Gagnon plumbing and heating

i would

run the pex high in the slab to collect the heat, and run a series of pex in the north side of the house, the coldest side, with a pump, to get some of the heat to that area. in the spring and fall this might work pretty good. then in the winter, when you need more than just passive heat, you could circulate hot water into the loop from your boiler for year round comfort. i wish you luck and urge you to work with this guy because we really need to use solar more, and i think water is the way to collect, move and store it. i only have sliders and a lot of windows on my south side and the passive gain is HUGE! go for it. bob