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re: attic to heat the house????
24 sep 2004
gary   wrote:

>>>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)...
>> 
>> 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?
>
>i took a cut at your a), b) and c), but with tattic reduced to 90f...
>
>c) one layer of clear polycarbonate glazing as the roof:
>
>    uglaz = 1.2 (for single glazed collector including radiation)
>    rglaz = 0.83

i woulda used r1...

>    qloss = (tattic - tamb)/ rglaz = (90f-30f)/0.83 = 72 btu/hr-ft^2
>
>    efic = (250 -72)/ 250 = 71%    wow!

and 0.9x250 in the numerator, which makes efic = (225-60)/250 = 0.66.

>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%

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.
     - 
     |
     -

>c) dark colored metal roof only (no glazing):
>    assume a 7.5mph wind -- rout = 0.25

i woulda said u = 2+v/2 = 5.75, so rout = 0.174.

>    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??

                   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.
     - 
     |
     -

>    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!?

yes...

>conclusions:
>a roof from one layer of polycarb would be both thermally and dollar 
>efficient -- you just need to embrace the transparent look?

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.

>     on your pc roof, how do you collect the heat once in the attic?

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. 

>     how do you distribute it to the house?

warm air rises...

>     will the building code folks ok this kind of roof?

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.

nick




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