Paul's DIY Solar Water Heating System

This is Paul's version of a $1K style solar water heating system. 

Some of the new features in Paul's design:

- The collector housing (box) is made from metal instead of wood.

- A selective finish is applied to the absorber for better performance.

- A thermally conductive copper adhesive bond absorber fins to riser tubes.

- Fins are formed of heavy gage aluminum using a press.

- Barrel storage tank with PEX custom wound heat exchanger coil.

More information on other $1K Solar Water Heating Systems here...

More examples of other $1K Solar Water Heating Systems here...



The Collector

Here are some pictures of my solar DHW heater. My solar collector is based on Gary's $1K design with changes described below.

Collector Box

I made some things a little differently. I used 20G steel studs for the sides.  I used .030 aluminum for the absorber plate, and I installed the absorber plate with the copper tubing facing out.


When buying the steel studs for the sides and top, ask for 20 gage 3 5/8 inch metal "runner".  The runner has no pre-drilled holes, whereas the studs do have pre-drilled hoes which would have to be plugged.

The corners have a tab about 1" long bent at 90 degrees, then pop riveted, the runner is attached to the OSB with some self drilling screws, then sealed with silicone.


The metal runner is painted with 2 coats of Rust-Oleum semi flat enamel.


The collector measures 45 3/4 by 96 inches.

Click on pictures for full size

Metal side rails with painted OSB bottom

Absorber with fins and inslation.



Collector box corner detail.

Glazing attachment detail.


The OSB back has one sheet of 1" polyiso insulation glued to it, then there are strips of 1/2" polyiso cut 4" X 86" to space the copper pipe/collector plates.


The collector absorber has 8 lengths of copper pipe, the plates on the left and right sides have one side shortened by about 2" to make everything fit. The copper pipe is attached with U shaped pipe hanger screwed through the foam to the OSB.

Mounting the Glazing:
On the top and bottom there are 2 strips of 1" polyiso to space up the foam wiggle strips. On either side I used foam tape, it has one adhesive side (it is made for mounting pickup truck caps). On the sides I used sheet metal screws with rubber washers. On the top and bottom I used 6" long flat head screws, had to order them from Mc Master Carr. I used wood strips on the top of the glazing to apply even pressure to the glazing.


The collector is mounted to the roof with angle brackets, there is a strip of rubber and silicone seal underneath.




I used a sheet of 1 Polyisocyanurate insulation and strips of Polyiso under the absorber and tubing.


The .030 aluminum was sheared to size 18" X 6", the (6" dim shrinks to 5 1/2" after bending). The aluminum was then sanded on one side with a DA sander to roughen the surface, then cleaned with solvent.

The absorber plates were formed on a bending break; I designed and built the dies myself.


Masking tape was placed on either side of the channel and then sprayed with Permatex copper gasket.
The Permatex was allowed to dry for a few minutes. The copper pipe was cleaned with Scotchbrite and contact cleaner prior to installing the collector plates (fins).

As you can see in the pictures the aluminum is a very good fit around the copper pipe, the Permatex provides a thin conductive adhesive bond.  As a test, I made a small test piece -- a 4" long section of collector.  On the test piece, the copper pipe seems to be bonded well. I think I will leave it outside for a while and see how it is affected by the outdoor environment..

I used special solar selective coating collector paint form Thermalox Paint from Dampny Co.


I am fortunate the Dampny Co is not too far from where I live, I stopped by and picked up 2 spray cans of solar selective paint.
According to the spec sheet I figured I would need 2 cans per collector but one can had just enough paint for the collector. The paint was easy to apply, a thin coating is recommended.  So, the coverage is about 30 sqft per spray can of Thermalox.

The special paint was not much more expensive (about $12/can) than regular high temp paint so I figured with all the work I put into the collector the extra cost was justified.

Instead of silicone for bonding the absorber to the copper tubing I used Permatex Copper Spray-A-Gasket Hi-Temp Adhesive Sealant.

I chose the Permatex because my absorber plates fit the copper tubing very tightly, I figured the copper sealer would give a bit better heat transfer; I did not think I needed the gap filling ability of silicone.


Click on pictures for full size


Near perfect fit of fin to tubes.



Up to the Roof


To get the collector up on the roof I tied a rope with a pulley on the end of it to my truck bumper on the other side of the house. I tied a rope to the collector, ran that up to the pulley and back down and wrapped  a few turns around a pipe driven into the ground. I pulled on the rope and pushed the collector up the ramp while my daughter kept tension on the end of the rope.

Tank, Pump, and Heat Exchanger

The storage tank is made form 2 55 gal drums, it is a design similar to the one on John Canivan's site,

Each drum has two 100' coils of 1/2 PEX run in parallel.


The pump is a Taco 00R-IFC 3 speed pump.  The check valve was removed to allow drainback. I am running the pump on medium speed as low would not overcome the head ( about 25').


The barrels are insulated with foil/bubblewrap insulation glued on with great stuff foam, then wrapped with a layer of R13 fiberglass insulation. I plan to improve the insulation, I am thinking of enclosing the sides of the tank with polyiso foam board and filling the voids with fiberglass and more fiberglass on top.

Each tank has a digital thermometer, I found them on Ebay for about $12 ea, only downside is they read in Centigrade.
The PEX heat exchanger seems to work quite well, I have a thermometer in the outlet of the heat exchanger, it is made form a kitchen meat thermometer inserted into a copper fitting. The temp coming out of the heat exchanger is about 5F cooler than the temp in the tank.

The controller is just a simple temp controller with a thermocouple attached to the collector, it is set to turn the pump on at 120*F. I am in the process of building a differential controller from some plans I have found on line.

While I don't have any data logging I have been recording tank temps, the highest temp I have seen is 120F on a bright sunny day, the tank usually runs around 95*F to 115*F depending on conditions.



The heat exchanger uses two coils
of half inch PEX in parallel.

The two barrel storage system
with insulation.



Preliminary Performance Test

A few weeks ago I did some testing of the panel, I used an insulated cooler holding 12 gl, the pump is a computer cooling pump running about 1.50gl/min. I started at 9:30AM with 60* water and by 3:30PM the water in the cooler was up to 165*, it was a sunny day with a temps in the 70s. I was quite happy with the initial results. Not bad for a $200 collector.


There is still a lot more work to be done, I plan to build another collector, get a differential controller, mount the collectors on the roof and tie them into the heat exchanger tank I am using for the wood stove water heater.

Here is picture of my meat thermometer temp gauge, that is installed in the outlet of the PEX heat exchanger.
It is made from a 3/16 compression fitting with a piece of brass tubing to adapt the thermometer diameter to the 3/16 fitting. I used some Loctite retaining compound to make a good seal.




Paul will answer email questions at: nonsense91 AT comcast DOT net    (replace AT with @, and DOT with a period)



My two cents (possibly totally wrong :) on some of the design changes that Paul made:

Metal Collector Case: There is a lot of interest in metal collector cases, especially from people in areas where wood may not have a good life.  So, its good to have a metal case design documented for those who want to go that way.  If you have done a metal case design, please send it in. 


Selective Paint:  It seems likely that the selective paint will result in some performance improvement.  The tests on commercial collectors that use factory applied selective finishes do show better performance in testing than black painted absorbers. 

The description of the Dampny Selective finish says that is is partially selective -- they claim that when properly applied, the Dampny paint has a 0.5 emissivity in the long IR.  This compares to regular black paint at 0.9+ IR emissivity, and to commercially applied selective finishes with an emissivity of about 0.05.   So, if the Dampny claim is correct, it is about half way between a good commercially applied selective finish and plain black paint.

It also claims to be a good  and durable finish for high temperature applications.

So, it seems that its likely to result in an improvement, and at the very worst should not be any worse than regular black paint.  About the only negatives I see are that it costs more than BBQ paint, and you have to order it a wait for it.


I have a can of the Dampny paint (Thermalox) paint in hand and plan to test it against plain black paint in a new set of small panel tests.  -- my expectation is that it will do somewhat better than plain black paint.


Copper Gasket Cement:  I have to say up front that I am a pretty strong believer in silicone to bond the riser tubes to the aluminum fins for many reasons outlined here.  I particularly like that the silicone provides a strong, pliable, very long lasting, and high adhesion and high temperature thermal and mechanical bond between the fin and tube -- for parts that are going to see years of serious thermal cycling and possibly condensation, this seems like a good thing.

That said, the Copper Gasket Cement may have some advantages and seems worth a try.  I will try to see I can do a small panel test with the Copper Gasket Cement as well.  Maybe Paul can be talked into an inspection after a year or so.


Fin Forming:  Paul's fins are works of art, and I am envious :)  But, they do require equipment not available to some of us.  Fortunately there are a wealth of good ways to make or buy fins....  So, you don't have to save up your pennies to buy a big press.



Gary July 28, 2010