solar heater on flat tar roof

 I would like to install a solar air heater atop a flat tar roof that
pitches slightly facing north. I'd thought about using the top of the
roof as the back of my thermal enclosure by building a "sandbox" type
of structure and stretching green house plastic across it. Then, blow
forced air through it. All in all I could make a 20' x 60' structure
for about $300.

I am worried about 4 things.:

1. Roof damage. I would remove the plastic in the spring, but if I
missed a few days...
2. Off odors. Would chemicals etc. bake out of the tar???
3. Hail, ice, snow damage to the plastic.
4. The roof tilts slightly to the north (2 degree). Would it be worth
my while to build a structure that faces south.

Does anybody have experience with solar heaters on tar roofs???

Thanks

 

   You would not want the temperature to get anywhere near the melting point of
tar (whatever that is)  A temperature switch connected to your fans should
prevent that from happening.


   If they do, it will reduce the life of your roof.


   I can't picture how you will handle snow loads.


   Two degrees does not sound like a big deal.


   No but considerable experience with tar roofs in a hot climate.  The sun and
the heat gradually break down the tar until it gradually becomes brittle and
then erodes away.  I made my roof last indefinitely by sweeping away the loose
ballast, recoating with liquid tar, and then replacing the ballast.  Doing that
every two years or so seemed to replenish the plasticizers and replace any
eroded tar.  YMMV

Vaughn

 I was thinking I could handle the snow loads with the framework made
from drop ceiling frames turned upside down. It will take a little
experimentation to decide how often to support it underneath. I've got
a ton of them sitting around, so it's free.

This project seems so easy and cheap! I'm excited.

I tried to come up with a dirty rule of thumb to figure the btu's of a
solar air heater. From many sites, it looks like about 100 btu's per
sq ft is a somewhere in the neighborhood. Granted, this is a ROUGH
estimate and it will fluctuate with sun, materials, etc, but one must
start somewhere. Given that, a 20' x 60' structure would throw off
about 120,000 btu's! That rocks!

I think I'll start with a 4 x 8 and take some measurements. Then, if
all goes well, build a large one.

jb

 Having lived for the last 10 years with the spawn of the devil known as a flat
tar
roof, my advice to you is to not even look at it too strongly, for that'll form a
leak for sure.

You will damage the roof by walking on it, attaching anything to it, causing any
heat
buildup or even thinking about it too hard.  yes, the tar odor will bake out and
make
you sick.  Tar odor was a constant problem with my building because fresh air
intakes
had to be on the roof.  Even pulling in free air, there was the taint of tar,
especially after the monthly leak patching projects.

I had to penetrate that roof in several places for restaurant vent hoods,
refrigeration and the like.  Every single penetration has leaked at some point.
I've
tried every remedy that my roofers and I could come up with.  Nothing is
permanent.
The universal advice was "rubber roof", a $3/sq ft option that I could not
afford on
a 7,000 sq ft building.

In the strongest terms possible, I suggest forgetting about doing anything to
that
roof.

The only way I ever figured out to make use of the tremendous heat buildup on my
roof
was to scatter out a couple hundred feet of industrial quality black rubber hose
and
use that to heat water for the restaurant.  That worked well, though the hose
quickly
melted into the tar and became one with the roof.

John

wrote:


John De Armond
See my website for my current email address
http://www.neon-john.com
http://www.johndearmond.com  <-- best little blog on the net!
Tellico Plains, Occupied TN
Hell is truth seen too late. -Hobbs

  

Using the top as the back? I can't picture that geometry. Greenhouse UV
polyethylene films have 4 and 5 year guarantees. They might last longer
in a vertical orientation or covered with shadecloth in summertime.
Vertical polycarbonate ($1.50 vs 5 cents/ft^2) might last 20 years,
with more framing (in 4' vs 40'-wide rolls) and a lot less IR loss.


You might heat water as well, with a car radiator and its 12V fans.


Wintertime solar heating requires height. A 10' tall x 60' long structure
could collect an enormous amount of heat.
 

I'd leave it in place all year.


There might be a vertical absorber inside the greenhouse, eg a few layers
of black fiberglass window screen with house air to the south and solar-
heated air to the north of the screen.


Plastic film greenhouses survive these well if they have steep slopes
or center supports to shed or hold up the snow.


Yes. As an air heater, it might be 2' or 4' deep and 32' long and 8-10' high,
with some sort of ballast foundation vs roof penetrations. Or put a 14-30'
wide plastic film greenhouse or half-greenhouse (a quarter cylinder) with
steel pipe bows on 4' centers over the whole roof. Cheap, at $1/ft^2.

Here's one way to heat the house and water (view in a fixed font):

    upper                                      g
    greenhouse                                  l
   |           |                                 a
   |           |                                  z
   ~           ~                                   i     south -->
   |           |                                    n
   |  vertical |       motorized   /                 g
   |    duct   |          damper  /
   |           |                 /
   |           |            day /
   |    |      |               /
   |    v      |              /
   |           |             /   night                    roof
---|            -------------............----------------------------------
   |           .            r            .
   |           . d    room  a           d.
   |           .  a    air  d f         a. f
   |           .   m   out  i           m.
   |    ==>    .    p       a a ==>     p. a <== room air in
   |           .     e      t           e.  
   |           .      r     o n         r. n
   |           .            r            .
    -------------------------------------
               |            |
               | duct to    |
               | room floor |
               |            |
               |            |
               ~            ~

To collect heat, open the motorized damper and run the radiator fan.
To distribute heat, close the motorized damper and run the room fan.
The passive dampers could be plastic film over hardware cloth.

The motorized damper could be polyiso foamboard with an auto windshield
wiper motor and limit switches or Honeywell's $50 6161B1000 damper actuator,
which only uses 2 watts as it moves up to 45 in-lb. The room air outlet would
also have a passive damper that opens out of the page into another vertical
duct or closet to move warm air down into the house.

Here's a simpler way, with 2 motorized dampers hinged at points H,
shown in a horizontal position:

            upper                            g
            greenhouse                        l
           |           |                       a
           |           |                        z
           ~           ~                         i     south -->
           |           |                          n
           |  vertical |                           g
           |    duct   |                    
           |           |                                
           |     |     |                            
           |     v     |                                
           |           |                              
           |           |                                   roof
-----------Hinletdamper|outletdamperH------------------------------
           =\          r           /=            |
           =           a f          =            |
  room     =           d            =            |
   air ==> =           i a  ==>     =            |
           =           a            =     |      |
           =           t n          =     v      |  
           =           o            =            |
           =           r            =            |
            ------------------------|            |
                                    |            |
                                    | duct to    |
                                    | room floor |
                                    |            |
                                    |            |
                                    ~            ~

To collect heat without heating the room, lower the inlet and outlet dampers
to vertical positions to cover the openings (=) below H and block room air
and run the radator fans and a pump to an unpressurized heat storage tank.

To distribute heat, raise both dampers and run the fan and pump.

With 140 F water, we could make pressurized hot water for showers with
a $60 1"x300' 13-gallon piece of pressurized black PE pipe in the heat
storage tank and a simple graywater heat exchanger (eg 2 uninsulated
55 gallon plastic drums) to add heat to the house.

Nick