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Attic to heat the house????

Discussion in 'Home Power and Microgeneration' started by 1234, Sep 22, 2004.

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  1. 1234

    1234 Guest

    I was just thinking of painting my metal roof black. I was thinking that
    the sun would warm up the attic alot during the day than at night I
    could use a small fan to pull the heat down into the living space.
    Any ideas on what I could put up there to hold the heat? I though tabout
    rocks but that might be too heavy. In the summer when I dont want the
    heat I will just install some soler powered vents.
    The other question would be moisture. I suppose pulling heat down would
    require that I replaced the air with inside air thus causing
    condensation on the inside of the top roof.
  2. Gary

    Gary Guest

    If the roof is already a dark color, then you won't get a lot more
    heat by painting it black. Black will absorb about 0.95 of the
    incident solar radiation, and dark green (for example) will absorb
    about 0.9.
    You could increase the efficiency of your metal roofing solar
    collector a lot by putting a layer of transparent corrugated
    polycarbonate over it (costs about $1 per sqft). You can see the kind
    of temperature increase that this brings by going to this doc:
    The transparent cover greatly reduces the losses to the outside.
    But, it might work pretty well even without the cover -- I'd try it
    that way first.

    I was thinking that
    Here is a link for similar scheme that might be of interest to you:
    They sell a kit with the fan and ducting, but its fairly pricey.
    There might also be some ideas you can use in your scheme.
    Water containers might be the most effective.
    Water holds 1 BTU/lb, rocks hold about 0.2 BTU/lb (so, 100 lb of water
    will store the same amount of heat as 500 lbs or rock).
    You could also just duct the heat to the house during the day. If you
    can heat the house up to the highest temperature that is comfortable
    during the day (maybe the high 70's), then you are storing heat in the
    thermal mass of the house -- you get the heat back as the house cools
    down. In this way, you meet the heating requirements of your house
    during the day, and store some for after sunset.

    Another scheme would be to duct the heat to storage in the crawl space
    or basement (where storage weight is not a problem).
    Either way, you have to think about how to transfer the heat to the
    storage containers.

    But, before going to the work of incorporating storage, I would see
    how much heat you can collect to determine if storage is warranted.

    In the summer when I dont want the

    I would be very curious to know how well your scheme works -- if you
    go ahead with it please post the results.

    Good Luck -- Gary
  3. m Ransley

    m Ransley Guest

    Attics that are unheated should be well vented outdoors or condensation
    can form, then mold then rott. You are better off adding insulation.
  4. Guest

    And remove the roof first, for more efficiency. What's the solar collection
    efficiency of: a) a black metal roof, and b) the roof with a polycarb cover,
    and c) the cover without the roof, in full sun (250 Btu/h-ft^2) on a 30 F
    day, with 120 F air in the attic?

    How about d) a 1' wide x 10' vertical strip of black metal roof with
    5 Btu/h-F-ft fin-tube pipe containing 120 F water under the ridge,
    and e) consisting of (d) with a polycarb cover but no metal roof?
    On the ground, their heat loss can warm the house vs the attic.
    Sounds good to me.
    Warm air from the attic, with a large air duct (eg 4'x4') and a ceiling
    fan at the top of a stairwell to pull air up from the basement? Making the
    total container surface at least 10X the glazing area would keep the air-water
    temp diff low and the storage efficiency high.

    The attic airflow needs to stop when the sun goes down. The dew point of
    70 F (530 R) air at 50% RH is 530/(1-530ln(0.5)/9621)-460 = 50.5 F. With
    120 F air in the attic, condensation is likely to appear first when the R1
    glazing is 50.5 F, when (Ta+120)/2 = 50.5 at an outdoor temp Ta = -19 F,
    or warmer, if there's a strong wind.

  5. Vlad

    Vlad Guest

    I have a black roof with mainly East/West exposure ( not the best). I
    have measure the temperature of the actic during the Winter
    season.Rarely above 70 F.
    But again, I am probably to close to the North Pole for this to be of
    any value. Too cold and the Sun doesn't raise much above the horizon
    during the Winter.
    Very good source of heat for the Hot water tank during the Summer.

  6. Gary

    Gary Guest

    I took a cut at your a), b) and c), but with Tattic reduced to 90F (on
    the theory that the fan would turn on when the attic temperature gets
    to 90F)?

    c) One layer of clear Polycarbonate glazing as the roof:

    Uglaz = 1.2 (for single glazed collector including radiation)
    Rglaz = 0.83

    Qloss = (Tattic - Tamb)/ Rglaz = (90F-30F)/0.83 = 72 BTU/hr-ft^2

    efic = (250 -72)/ 250 = 71% Wow!

    b) One layer of clear Polycarb over dark metal roofing --
    heat transfer by conduction through metal only (no ducts):
    Rglaz = 0.83
    Rattic = 0.65 (R from metal to attic -- one still air film)

    Qabsorbed = (0.9 trans)(0.95 absorb) (250 BTU/ft^2-hr) =
    = 214 BTU/hr-ft^2 absorbed by metal roof plate

    Qattic = (Tcol - Tattic)/Rattic -- heat to attic
    Qloss = (Tcol - Tamb)/Rglaz -- loss out glazing

    Qattic + Qloss = Qabsorbed

    (Tcol-90F)/0.65 + (Tcol-30F)/0.83 = 214
    This gives Tcol = 142F

    Qattic = (142F - 90F)/0.65 = 80 BTU/hr-ft^2 to attic

    efic = 80/250 = 32%

    Less than half as good as the clear PC only, but it might be
    attractive if you already have a metal roof?
    It could be made a bit better with some mild forced convection on the
    inside surface of metal.

    On (say) a 2000 sqft house with (1.4/2)(2000) = 1400sqft of South
    facing roof it could be a whole lot of heat.

    c) Dark colored metal roof only (no glazing):
    Assume a 7.5mph wind -- Rout = 0.25
    Rattic = 0.65 (as above)

    Qabsorbed = (0.95 absorb)(250BTU/hr-ft^2) = 238 BTU/hr-ft^2

    Qattic = (Tcol - Tattic)/Rattic
    Qloss = (Tcol - Tamb)/Rout

    Qattic + Qloss = Qabsorbed

    (Tcol-90F)/0.65 + (Tcol-30F)/0.25 = 238
    This give Tcol of just about 90F
    Since the collector and attic are equal in temperature,
    no heat is transfered to attic, and efic = 0??

    If you change to no wind -- Rout = 0.65 -- then:

    (Tcol-90F)/0.65 + (Tcol-30F)/0.65 = 238

    This gives Tcol = 138F

    Qin = (138F-90F)/0.65 = 74 BTU/hr-ft^2
    efic = 74/238 = 31% -- so wind makes a lot of difference!?

    But, still, the bare roof might do pretty well in the shoulder months,
    and when there is not much wind?

    Does this all seem about right? Did I go astray above?

    A roof from one layer of Polycarb would be both thermally and dollar
    efficient -- you just need to embrace the transparent look?
    On your PC roof, how do you collect the heat once in the attic?
    How do you distribute it to the house?
    Will the building code folks OK this kind of roof?

    A dark metal roof with a layer of PC on the outside has fair thermal
    performance, and pretty good dollar performance?

    A dark metal roof only (no glazing) has poor thermal performance, but
    will collect some heat on some days, and costs almost nothing?

  7. Guest

    I woulda used R1...
    And 0.9x250 in the numerator, which makes efic = (225-60)/250 = 0.66.
    Another way to do this:

    |--|-->|-----X-www--- 90
    --- | 0.65
    214 | breaking the circuit at X and replacing
    0.83 | the part to the left with an equivalent,
    30 ---www--- Tt = 30+214x0.83 = 207.6. Rt = 0.83.

    Q -->
    --www-------X-www--- 90
    | 0.83 0.65
    --- 207.6 Q = (207.6-90)/(0.83+0.65) = 79.45 Btu/h-ft^2.
    I woulda said U = 2+V/2 = 5.75, so Rout = 0.174.
    Q -->
    --www-------X-www--- 90
    | 0.25 0.65
    --- 89.5 Q = (89.5-90)/(0.25+0.65) = -0.6 Btu/h-ft^2.
    You might hang a layer of dark shadecloth inside. Then again, daylight is
    nice, from skylights in the attic floor. People often mistake my polycarb
    roof for a metal roof.
    A fan or blower, as in Soldier's Grove. A 600 ft^2 attic collecting
    99K Btu/h (29 kW) with a 10K cfm fan might have 80 F air entering
    the attic and 80+99K/10K = 89.9 F air leaving. Grainger's $120 86 W
    4C853 48" ceiling fan might move 21K cfm up a stairwell into an attic
    and push warm attic air down from the peak to the basement heat store
    via a LARGE duct, eg 2 rooms with 4'x4' ceiling and floor grates. The
    fan might have a 4'x4' motorized cover that opens when it is running.
    The duct near the attic peak might have a one-way plastic film damper
    or another motorized foamboard cover.
    Warm air rises...
    Mine did, under the BOCA code. It's held up well for the last 7 years,
    altho it may eventually leak a bit, since the 4'x12' panels are attached
    to the purlins with lots of hex head screws with neoprene washers.

    A 1' slice of polycarb roof with a 20' slant height under a 5 Btu/h-F foot
    of 90 F fin tube might look like this:

    |--|-->|-----X-www--- 90
    --- | 0.2
    20x225 |
    R1/20 |
    30 ---www--- Tt = 30+20x225/20 = 255. Rt = 0.05.

    Q -->
    --www-------X-www--- 90
    | 0.05 0.2
    --- 255 Q = (255-90)/(0.05+0.2) = 660 Btu/h-ft,
    | with a 90+0.2x660 = 222 F attic air temp???

    efic = 660/(20x250) = 0.132, or 0.22 with two fin tubes. Not very efficient,
    but cheap and easy. Less labor, no sheathing, no tarpaper, no shingles.

    With a fan and a $200 800 Btu/h-F 2'x2' all-copper SHW 2347 Magicaire
    air-water heat exchanger under a 32' ridge, we have

    Q -->
    --www-------X-www--- 90
    | 0.05 0.04
    --- 255 Q = (255-90)/(0.05+0.04) = 1833 Btu/h-ft,
    | with 90+0.04x1833 = 163 F attic air?

    A $128 32' 160 Btu/h-F attic ridge fin-tube or an 800 Btu/h-F fan-coil unit
    might collect 21K or 58.7K Btu/h in full sun in December, enough to make
    hot water for showers, with a foamboard hat and a PV/DC circulation pump
    with battery backup to avoid freezing.

  8. 1234

    1234 Guest

    The roof is a reflective aluminium.
  9. 1234

    1234 Guest

  10. Any bit of heat that gets into the attic needs to be immediately
    removed in areas that get snow.
    And even if no snow, the moisture that comes along with the
    heat can be a problem.
  11. Vlad

    Vlad Guest



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