Attic to heat the house????
Question:
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.
Response:
> I was just thinking of painting my metal roof black.
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: http://users.montanadsl.net/~reysa/SolarBarn2.pdf 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 > 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.
Here is a link for similar scheme that might be of interest to you: http://www.heliosenergies.com/oldindex.htm 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. > Any ideas on what I could put up there to hold the heat? I though tabout > rocks but that might be too heavy.
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 > 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.
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
Response:
Attics that are unheated should be well vented outdoors or condensation can form, then mold then rott. You are better off adding insulation.
Response:
>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? 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? > Any ideas on what I could put up there to hold the heat? I though tabout > rocks but that might be too heavy. >Water containers might be the most effective.
On the ground, their heat loss can warm the house vs the attic. >Another scheme would be to duct the heat to storage in the crawl space >or basement (where storage weight is not a problem).
Sounds good to me. >Either way, you have to think about how to transfer the heat to the >storage containers.
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 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.
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. Nick
Response:
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. Vlad – Hide quoted text — Show quoted text ->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? >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? >> Any ideas on what I could put up there to hold the heat? I though tabout >> rocks but that might be too heavy. >Water containers might be the most effective. >On the ground, their heat loss can warm the house vs the attic. >Another scheme would be to duct the heat to storage in the crawl space >or basement (where storage weight is not a problem). >Sounds good to me. >Either way, you have to think about how to transfer the heat to the >storage containers. >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 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. >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. >Nick
Response:
> 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. > Vlad
MOVE? It would fix that problem. Top posting, would identify you as clueless anyway.
Response:
>>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 (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? Conclusions: 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? Gary – Hide quoted text — Show quoted text -> Nick
Response:
> temperature of the actic during the Winter season.Rarely above 70 F.
70F isn’t a bad temp… we have the thermostat at 55-60F in the winter. we really need to insulate this place. To what does the low temp get? If you store that 70F heat in thermal mass (water filled soft bottles?) you should be able to make it through the night. >Top posting, would identify you as clueless anyway.
Almost as bad as people who don’t trim quotage from their posts. — be safe. flip Ich habe keine Ahnung was das bedeutet, oder vielleicht doch?
Response:
- Hide quoted text — Show quoted text ->>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 0.9×250 in the numerator, which makes efic = (225-60)/250 = 0.66. – Hide quoted text — Show quoted text ->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+214×0.83 = 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 20×225 | | R1/20 | 30 —www— Tt = 30+20×225/20 = 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+0.2×660 = 222 F attic air temp??? - efic = 660/(20×250) = 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+0.04×1833 = 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
Response:
The roof is a reflective aluminium. – Hide quoted text — Show quoted text -> I was just thinking of painting my metal roof black. > 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: > http://users.montanadsl.net/~reysa/SolarBarn2.pdf > 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 > 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. > Here is a link for similar scheme that might be of interest to you: > http://www.heliosenergies.com/oldindex.htm > 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. > Any ideas on what I could put up there to hold the heat? I though tabout > rocks but that might be too heavy. > 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 > 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. > 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
Response:
Asshole – Hide quoted text — Show quoted text -> 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. > Vlad > MOVE? It would fix that problem. > Top posting, would identify you as clueless anyway.
Response:
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.
– Hide quoted text — Show quoted text -> Attics that are unheated should be well vented outdoors or condensation > can form, then mold then rott. You are better off adding insulation.
Response:
o o||o Vlad – Hide quoted text — Show quoted text -> 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. > Vlad >MOVE? It would fix that problem. >Top posting, would identify you as clueless anyway.
Response:
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