Attic to heat the house????

[1234]

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.

 

[Gary]

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

 

[m Ransley]

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

 

[[email protected]]

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?

1234 wrote:

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 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

 

[Vlad]

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

 

[Gary]

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

 

[[email protected]]

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

 

[1234]

The roof is a reflective aluminium.

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

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.


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

 

[1234]

Asshole

MOVE? It would fix that problem.

Top posting, would identify you as clueless anyway.

 

[Michael Baugh]

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.

 

[Vlad]

o
o||o

Vlad

MOVE? It would fix that problem.

Top posting, would identify you as clueless anyway.