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Inside treatment of a passive solar retrofit

Post in 'The Green Room' started by precaud, Oct 5, 2008.

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  1. precaud

    precaud Minister of Fire

    Joined:
    Jan 20, 2006
    Messages:
    2,288
    Loc:
    Sunny New Mexico
    Place: Santa Fe, NM (lots of sun!)
    Time: now.
    Subject: 1930-built single story home with full daylight basement under, massive (16" thick) uninsulated masonry walls (two layers of 'pentile', a hollow clay brick, stucco outside, plaster inside.) Equally massive interior construction.
    Project: Convert south-facing masonry porch and into a passive solar sunspace.
    Solar orientation: Front of house faces 16º east of true south. Porch and SW bedroom stepped back 5 feet each (dumb design, but that's what we have to work with.)

    The porch was an eyesore, and always kept half of the house front in shade. So rather than enclose it, I decided to tear it out and install polycarb across the corners. This will allow me to do the same in the SW corner next year if this project works out.

    See the attached pics before and after the major work was done.

    Working with non-right angles made this project a real PITA. What I thought would take me a couple weekends turned into two months. I suspect most retrofits like this are full of compromise and tradeoffs.

    So, the front windows face 16º E of S, and the new polycarb panel (8' wide x 9' tall) faces 15º W of S. Not perfect but not bad. NREL solar radiation data for this area says that the polycarb panel will give me about 110,000 btus per day to work with on an average December day. Again, not bad. Free heat.

    In the beginning, the thought was to just to paint the inside walls black and open the front door when the sun is shining and close it when it's not - essentially the "mass behind glass" concept. But since then I've read about the advantages of low-mass sunspaces and am wondering if I can/should apply that concept.

    So, the questions are: how best to finish the interior of the sunspace, and then get the heat into the house.

    Sunspace dimensions:
    north wall is 78" with 36" x 80" door
    east wall is 60"
    polycarb is 8' wide, 9' high

    How would you deal with this? Is the door going to be sufficient venting? I'd like to avoid putting more holes in the wall if possible. Any input is welcomed.

    Attached Files:

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  2. myzamboni

    myzamboni Minister of Fire

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    Why not model it like the sunboxes I've seen with a 8" port at the top feeding warm air to the house and an 8" port down by the floor pulling in cooler air from the house? Maybe make them with a slideable cover plate so you can control the amount of heat coming in?

    [​IMG]
  3. precaud

    precaud Minister of Fire

    Joined:
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    2,288
    Loc:
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    I thought I'd give an in-process update on my sunspace. But first, the link myzamboni provided illustrates the old-school, "mass behind glass" approach. The theory behind it is to use the mass to store the heat collected in daytime which then releases it into the house at night. The problem is, it takes a long time to heat up that mass and it then releases more to the outside at night than it gives to the inside of the house.

    The alternate approach is to create a high-efficiency, low-mass sunspace; get the thermal mass inside the living space by insulating the inside of the sunspace, hang black screen or shadecloth inside of the glazing to collect the heat , and force the air to pass through the screen as it circulates. The advantages are, that it heats up and starts producing useful heat very quickly (30 minutes after sunrise), puts all of the heat it creates into the living space, doesn't lose it's heat at night, and has a higher operating temperature which enhances the natural convection.

    And that's the approach I've taken: two layers of black aluminum window screen hung 8" from the glazing at the top and 15" at the bottom, a false floor with a 10" gap for incoming air to force it to pass thru the screen, R30 fiberglass in the ceiling and R13 on the inside walls covered with black Polyethylene sheeting, and a small fan to force the circulation a bit (necessary because of the long intake duct.)

    The results have been fantastic so far. On sunny days, from 8am to 4:30pm this thing provides an amazing amount of heat. For example, yesterday when the sun broke the horizon at 7:10am, it was 24F outside, 66F in the house, and 36F in the sunspace. By 7:45 it was 75F so the door was opened. By 9:25am it was 95F. Temps peak out at 108F in early afternoon, so with average intake temps at the floor of 64F, it's giving a 44F temperature rise. By 4:30pm the sunspace is back down below 85 so the door is closed. It is 74F in the house!

    It's clear this will be reducing my wood use drastically. It's mid-November, night temps in the upper teens/low 20's, and only 4 times have I had to light a fire upstairs to take the morning chill off. And it is most pleasant to come up to a warm house after work.

    I'm still experimenting to find the optimum intake duct area. And no doubt the efficiency can be improved even more if I frame in a duct at the top to pull more of the air through the top of the screen. I'll post any further refinements as they come.
  4. EatenByLimestone

    EatenByLimestone Minister of Fire

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    Dude, somebody stole your roof!!!!!


    Matt
  5. precaud

    precaud Minister of Fire

    Joined:
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    Loc:
    Sunny New Mexico
    I wish they'd come back and clean up the mess :)
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