My Pex tubing Solar hot water Collector

[TheyCallMeDave]

This looks like a great forum.

Heres the project im going to start tomorrow :

I going to use 3/4"x300' coiled red Pex Tubing which measures 32"
diameter x 7" high . I will set this in a box which i will construct
out of wooden 2x12's with a 1/2" thick plywood on the backside
(treated) for a 36"x36" od Box. . I will lay 4" of dense pink
Foamboard in the bottom . A 32" square sheetmetal plate painted with
flat high temp. low gloss Black paint (primed first) will be layed on
top of the bottom Foamboard . The coiled Pex tubing will be wrapped in
heavy duty tin foil sheet to prevent UV light from seeing the Pex ,
and this foil will be pulled tight around the coil of Pex and also
painted high temp flat black. ... then the coil will sit inside the
box in direct contact with the sheetmetal plate. A double pane window
in frame measuring 36" sq. od (without any inside bars) will be
fastened on top of the box for a greenhouse effect. The box will sit
against the southside of the house on the ground at a 30 degree angle
from vertical in a location where where it will recieve full sun. I
will cut the hot water CPVC pipe above the water heater and route to
the Solar Panel using 3/4" CPVC (insulated) connecting to the 3/4" Pex
tubing stubbed out of the side of the Box using Pex compression
fittings . The return CPVC pipe (insulated) will run back to the top
of the water heaters hot water supply pipe. (Since my hot water
reqirements are very minimal, I opted not to heat the entire water
heater) .

Since stagnation temps/pressure in the box are of concern, i plan on
securing a section of shiny sheetmetal over the face of the Box for
capacity control during the summer months.

Specs on the Pex are 100 psi at 200 f. Specs on the CPVC pipe are 100
psi at 180 f. May aim is to deliver hot water in the 100-120 f. range
thru experimenting with how much to cover the front of the Box with
the shiny sheetmetal plate .

Total estimated cost for this project is $300 ($100 for the Window ,
$120 for the coiled Pex, and $80 for the wood and CPVC piping with
fittings .

[TheyCallMeDave]

Note: No pump, no storage tank , no elaborate controls, no cost-prohibitive copper tubing, simple design for a person who just has minimal hot water useage . When a hot water faucet is opened in the house, water will be pushed thru the Solar Collector Pex Tubing . No worries about freezeups .. its very rare we have a hard freeze here in FLorida where i live but Pex Tubing is good at handling freezing according to the mfg'r .

When the project is completed and in operation, I can post my findings .

[Daox]

Sounds like a great idea to me Dave.

Another perhaps easier idea would be to simply put the pex piping into the attic. My attic routinely gets up over 100F on a sunny day. This would eliminate the box, window and foiling over the pex pipe and make it even cheaper. I've really been wanting to try the idea myself, but I got a bunch of solar hot water panels for free. The'll provide more than enough hot water.

[TheyCallMeDave]

Originally Posted by Daox

Sounds like a great idea to me Dave.

Another perhaps easier idea would be to simply put the pex piping into the attic. My attic routinely gets up over 100F on a sunny day. This would eliminate the box, window and foiling over the pex pipe and make it even cheaper. I've really been wanting to try the idea myself, but I got a bunch of solar hot water panels for free. The'll provide more than enough hot water.

Wow...i like the idea of having the Pex in the attic . It would certainly be hot in the summer months as my attic gets to 130 f., but, in the winter months i dont think my attic reaches 90 f. ...but i could be wrong. I kinda wish it were winter now so i could check it out. Thanks ...will have to consider that.

[raydias]

even if the attic in winter only reaches 90 it would preheat the water that comes from the city/well or whatever to that temp. this would still save on energy costs of heating the water.

just a thought

[TheyCallMeDave]

Originally Posted by raydias

even if the attic in winter only reaches 90 it would preheat the water that comes from the city/well or whatever to that temp. this would still save on energy costs of heating the water.

just a thought

True, but my goal is to never use the electric water heater again . Although a 90 f. shower is doable for me but i prefer 100 f. I do like the attic idea though and i can take my showers late in the afternoon if need be while the attic is still warm in the winter. But my project will guarantee hot enough water in the winter months.

[GaryGary]

Hi,
Interesting idea -- kind of like a small batch heater, but simpler to build.

The ID of 3/4 pex is 0.671 inches, so 300 ft of 3/4 pex holds:
(0.671)^2*0.785*12*300/231 = 5.5 gallons -- so, best make that shower fairly quick

I'd use polyiso insulation rather than the pink xps -- its very easy to melt the xps -- I've made a lot of xps thermal sculptures
I think that the 4 inches is probably overkill -- no matter what you do to the back, the front is still only R2, so once you get the back up to (say) R8, nearly all the heat is being lost out the front.

I did a PEX collector: The $1000 Solar Water Heating System
This worked fine, but even with a tilt of 70 degrees (nearly vertical), and only single glazing, there were times when the collector temperature got up to 230F. You will really have to keep an eye on the stagnation temperature in the summer and fall. In my collector, the PEX was not pressurized, so 230F once in a while was OK, but in your case the PEX will be under full mains pressure, and you will have to keep the temperature lower.

If you have not bought your PEX yet, PEXUniverse.com has good prices and free shipping -- $115 (I think) for 300 ft of 3/4.

Gary

[TheyCallMeDave]

Originally Posted by GaryGary

Hi,
Interesting idea -- kind of like a small batch heater, but simpler to build.

The ID of 3/4 pex is 0.671 inches, so 300 ft of 3/4 pex holds:
(0.671)^2*0.785*12*300/231 = 5.5 gallons -- so, best make that shower fairly quick

I'd use polyiso insulation rather than the pink xps -- its very easy to melt the xps -- I've made a lot of xps thermal sculptures
I think that the 4 inches is probably overkill -- no matter what you do to the back, the front is still only R2, so once you get the back up to (say) R8, nearly all the heat is being lost out the front.

I did a PEX collector: The $1000 Solar Water Heating System
This worked fine, but even with a tilt of 70 degrees (nearly vertical), and only single glazing, there were times when the collector temperature got up to 230F. You will really have to keep an eye on the stagnation temperature in the summer and fall. In my collector, the PEX was not pressurized, so 230F once in a while was OK, but in your case the PEX will be under full mains pressure, and you will have to keep the temperature lower.

If you have not bought your PEX yet, PEXUniverse.com has good prices and free shipping -- $115 (I think) for 300 ft of 3/4.

Gary

Hey Gary , I picked up some 1/2" thick Polyiso with aluminum covering yesterday at Lowes -- Im going to lay down a full inch for the bottom then put a thick sheetmetal plate painted high temp. Flat Black on top of that ., then the Pex Coil will lay on top of that wrapped in h.d. tin foil also painted high temp. Flat Black. Once placed in the box, the Pex Coil will have 2" on all sides to expand (if need be) . I had Lowes order me a Double pane Low E Window (by Pella) having no strips inside the panes, so, i expect that to work exceedingly well. In fact, I plan on placing a sheetmetal plate over the top of the window and regulate it based on the seasons --- in summer months, it will probably be fully covered to prevent full suns rays and just rely on conduction warming the Pex water .

5-6 gallons of 140 f. water will more than satisfy my low hot water useage each time ...and even at that ill have to dilute it with the cold water faucet.

Any problems with your Pex expanding and contracting ? If so, how much ?

For the time being, im going to introduce the Collector water directly into the hot water supply pipe off the water heater for useage...but, I may change it over to a THermosyphon operation (heating the Water Heater ) since the Collector will be on the ground and the water heater is 8' higher.

Ill take a look at your Pex collector install. Thanks.

[TheyCallMeDave]

P.S. I paid $120 incl. shipping for my 3/4 x 300' roll on Ebay . I think it came from The Pex Store Inc. Its Pex-B rated at 100 psi at 200 f.

By the way, what kind of pressure increase are you getting across your Collector ???

[GaryGary]

Hi Dave,
I've not noticed a lot of expansion of the PEX coil, but I've not really tried to measure it.
I think that 2 inches will be plenty.

One thing I wonder about is how good the heat transfer to the PEX coil from the solar chamber will be, but I guess the easiest way to find out is to give it a try -- if it turns out to be not as good as you would like, you can think about it then.

Gary